Talk:Many-worlds interpretation/Archive 1

Many-minds
"Some physicists prefer the term many-minds" - Wouldn't this be a different theory altogether? -- Oliver P. 13:31 Feb 13, 2003 (UTC)

Yes!

Occam's razor
The article currently says:


 * However, many physicists dislike the implication that there are an infinite number of non-observable alternate universes, on the basis of Occam's Razor. (Note that that both sides claim to be using Occam's Razor, but are applying it to different things.)

I don't think that this objection should be presented as a valid alternative viewpoint because, as far as I can tell, it's untenable. Consider the following two explanations for the lights we see in the night sky:


 * 1) The Universe is billions of light years across at least (possibly much larger), and contains (at least) billions of galaxies, each with billions of stars, each containing something like 10^60 fermions and untold numbers of bosons. The photons we observe were produced by all that stuff over a period of billions of years.
 * 2) The Universe is a few light years in diameter, with the solar system at the center, and at the edge are machines which produce the photons that we see.

The second explanation is just as consistent with observations as the first, and it's far more "economical in its use of natural resources," so to speak. But it's absurd to argue that Occam's razor favors it over the first explanation because it has "fewer entities." This appears to be the sort of argument that the "many physicists" referred to above are making. If it is, then they're wrong, and the description should reflect that.

If there are no objections within a week or two I'll rewrite the paragraph.

-- BenRG 05:05, 20 Sep 2003 (UTC)


 * Actually (2) doesn't fit observations at all, as it would clearly not lead to differing parallax measurements. Nor can I understand why one would claim that an infinity of invisible unknown seemingly complex machines has "fewer entities" than a system that can be summed up as "the universe is made up of many stars similar to ours". Personally I think this is a crappy example. Likewise, any attempt to wiggle out of the problem that MWI by claiming Occam doesn't apply is likewise unjustified. Everett never offered a mechanism by which "you" only see the histories down one path, which makes the wavefunction collapse re-appear IMHO. Nor does there seem to be any reason why two people would agree on the past, considering re-merging histories (the relative state of two people with differing distant pasts would seem to be degenerate). Later authors have added their take on this, but none of them seem particularily convincing either. So in the end MWI "hides" wavefunction collapse in an infinite number of unseen devices, and applying Occam seems perfectly relivant and nessesary. Maury 15:19, 19 Jun 2005 (UTC)

I'll agree, Occam's razor is the wrong thing to be pointing to. Physicists simply don't like unobservable objects. I've clipped the reference, but please feel free to do a rewrite if you think you can state things in a better way.

Thank you. Fairandbalanced 16:43, 20 Sep 2003 (UTC)

Schrodinger's cat
Could we leave Schrodinger's cat out of this one? Or at least combine Schrodinger's cat with the next "presidential election" example, which maybe we could more meaningfully rework as "Dick Cheney's heart" (he does have doesn't he?) where there is a pacemaker sensitive to gamma radiation? CSTAR 15:15, 15 Jul 2004 (UTC)

Nasty math
Are there any resources on WP or WikiBooks that will help me figure out the nasty math in the "simple example"? I know what a Hilbert Space is, what bra and ket vectors are, etc... but do we have "intro to quantum mechanics" or something? (probably belongs in WikiBooks rather than here...) and if so could we link to it? (And if not could we start it? :-]) Glenn Willen (Talk)   21:40, 4 Aug 2004 (UTC)


 * Well you can start reading quantum mechanics. Now this article on MWI has a wide range in what it requires. Some of it is at the graduate-level physics course, so don't be discouraged if you're not at that level.  Hopefully, you can get something right away from the beginning.  Feedback is appreciated!! CSTAR 21:44, 4 Aug 2004 (UTC)

Possible worlds
Many worlds is quite different from Possible worlds. CSTAR 23:30, 4 Aug 2004 (UTC)


 * I don't object to putting a link to holomovement; but discussing this in the introductory section to the article is totally misleading. CSTAR 23:45, 4 Aug 2004 (UTC)
 * Allright we got this one solved :-) Now open your mind slowly and see ! If you were to touch holomovement Togo 23:51, 4 Aug 2004 (UTC)
 * By the way I am still confident that the possible worlds and the multiple worlds are differnt views of the same issue. After all As you go down your path you may be producing many worlds but they are only so long possible and you stay united with your consciousness. From what I've read about Leibnitz thats what he was visualizing too! Togo 22:23, 5 Aug 2004 (UTC)


 * Not surprisingly, we don't agree on this either :( But we probably do agree that the difference between possible worlds and many worlds needs to be elucidated more than is done in the (generally quite good) article on possible worlds. I intend to get to that sometime soon. That article does correctly point out that possible worlds are used in models of modal logic, but does not really explain at any length what a model of modal logic is. (Fortunately, this isn't too hard).  A possible world is essentially a valuation of all atomic assertions in a specific formal language that allows for "modal" operators. Possible worlds do not have an intrinsic of branching, which is one of the characteristics of many worlds.  The real difference between the two is that the notion of entanglement or relative state plays no role in possible worlds.  We disagree, but hopefully we're not at each other's throats!!  I do agree that Bohm (who seems to be a hero of yours) is an interesting "character".  CSTAR 22:37, 5 Aug 2004 (UTC)


 * The two are by no means identical, but they are at least somewhat related. I've just changed some self-contradictory wording in the article on this point. Ben Finn 20:29, 11 May 2006 (UTC)

Schrodinger's cat
I removed this for now, although it should go back in somewhere towards the top of the article after "as simple example" and before the mathematics of reduced state. Nevertheless, it should be reworked, because perhaps it tries to deal with too many issues at once. (The cat, the radioctive emission, the observer and the rest of the universe). CSTAR 21:56, 5 Aug 2004 (UTC)

Comments from anonymous user on the inaccessibility of the article

 * This article should have a brief, general definition of the Interpretation, perhaps as a new second paragraph. If you are well acquainted with the Interpretation please insert such a definition. &mdash; stub comment inserted into body of the article, 2 Dec 2004.

Despite the efforts made to cleanup this article, I'm concerned that there has been no significant improvement in the level of accessibility to general readers. In particular, no changes have been made to the all important first paragraph, as evidenced by the retention of such enigmatic phrases as "the privileged status of observation".

My premise here is that the introductory paragraphs must deliver a basic explanation of what the MWI means &mdash; and fundamentally implies &mdash; to any given reader. For those of you with the capacity to do so CSTAR, please make this part of the article more meaningful and useful to general readers! After making the first paragraph comprehensible, please relocate the second and third paragraphs, which appear to have escaped from a physics textbook!

New intro
The theory does not attempt to explain superposition of states! That's an entirely different, well understood issue. It tries to explain, among other things. measurement. CSTAR 05:19, 2 Dec 2004 (UTC)


 * I'm going to revert it, although we should try to incorporate suggestions and improvements of Aarchiba into the reverted version; the newer (previous to the revert) version has too many incorrect statements.CSTAR 05:26, 2 Dec 2004 (UTC)

Additional comments
We should take Aarchiba's attempt to improve the article seriously, although I objected to several facts of his newer ersion:


 * Superposition as I mentiuned above
 * The phrase parallel universes at the beginning of the article, gives me at least, the creeps.
 * The important fact isn't so much the parallel universes themselves, but the superposition of the so-called parallel universes (interpreting the mathematics that follows)
 * Stuff was moved out of the intro that should have been there, such as Everett's leaving physics.

CSTAR 05:49, 2 Dec 2004 (UTC)


 * Hmm. I wasn't entirely happy with the old version, but perhaps we should include it here, for easier comparison.
 * The reason I wrote it is because I find the article very difficult to read in spite of a reasonable physics background; for a general encyclopedia we can surely do better.
 * I'll try to address the points:
 * Superposition: probably a bad choice of phrasing. The intent was to explain that it tries ot understand superposition of states not as a temporary phenomenon but as a basic property of the universe.
 * The "parallel universes" was there not because it describes the theory but because it describes what popular culture has done with the theory (and that will give many readers some idea of what's going on).
 * The history I moved to another section in the interest of having a short lead section.
 * Let's try to make a wishlist for the article. See below. --Andrew 07:03, Dec 2, 2004 (UTC)

I think the article needs:


 * A good intro that can at least tell the non-technical reader whether they're in the right place - at most two or three paragraphs (good luck!)
 * It's one of the leading interpretations of quantum mechanics
 * It views the universe as endlessly branching: all possibilities happen
 * It led to the idea of parallel universes in fiction
 * It was invented by Everett
 * A longer intro, still for the non-technical reader, that describes what problems it's intended to resolve, what problems it introduces, and maybe a simple example (no, simpler than the "simple example")
 * A history section describing who invented it, who developed it, and who based which further theories on it
 * A section comparing it with alternative interpretations
 * The hardcore technical stuff in the article now
 * Good linking to explanations sufficient to have a hope of understanding the hardcore technical explanations

Please just go ahead and edit the list; if you think something'll be controversial flag it, but otherwise it's easier to hack directly on this. --Andrew 07:03, Dec 2, 2004 (UTC)

Relative states
Is the article about the relative-states formalism or the many-worlds interpretation? The article claims they're different and that it's bad to confuse them, then seems to go off and talk about relative states (only?). Perhaps relative states formalism should have a separate article? --Andrew 07:03, Dec 2, 2004 (UTC)


 * They're really the same; that was the point of the mathematics that follows. Doesn't the article say they are the same right at the top? I wasn't aware the article said they're different somewhere else, although that statement may be in some legacy paragraph; who knows, I'll have a look (although later my connection is very slow now).


 * I also don't like the annotation on the graphic. It refers to decision; QM has nothing to do with decisions by anybody. Such language seems to suggest there is a connection between consciuousness and QM and I think I am in good company when I assert there isn't.  I will make a new graphic to replace it. CSTAR 14:41, 2 Dec 2004 (UTC)

"Rejoining" universes?
I'll preface this by saying that I am in no way a physicist and am mostly interested in the philosophical implications of this theory. Having said that, I have heard or read somewhere that there is an alternate or modified many-worlds theory in which universes that have branched apart can come back together (as it were) with the collapse of certain wave functions (or some other process unknown to me). Does this sound familiar to anyone, and if so, should it be included in the article? --Anakolouthon 21:59, 6 Dec 2004 (UTC)


 * See Michael Price's FAQ where that possibility is briefly mentioned. I would rather not put that in. CSTAR 23:12, 6 Dec 2004 (UTC)


 * The process of branching can be regarded as "dimensionality increase" of the ambient trajectory space. (Think of quantum mechanical evolution as being simulated on n-qubit registers). This is related to (a) increased entanglement and (b) increased entropy (there is a well-known relation between entanglement and entropy).  For this reason, the existence of rejoining universes would violate basic principles of thermodynamics. However, I'm at the limits of what I know here (beyond maybe?) CSTAR 23:35, 6 Dec 2004 (UTC)
 * No basic principles of thermodynamics would have to say that anything that seperates must come back together, seems logical to me considering finite energy (conservation of) and entropy limits °Togo


 * The increase of entropy more or less forbids universes interacting (by coming back into coherence) in the same way that it forbids a glass from un-breaking. But read the FAQ. --Andrew 06:54, Dec 13, 2004 (UTC)

Good Checklist

 * I agree heartily with the sentiment of improving the introduction of this page. The current page, while very descriptive and informative, does really not give the reader a sense of what MWI actually *is* until somewhere around the 4th paragraph of the "Relative State" section.

Perhaps a good first step could be to add a summary sentence at the top? Perhaps something like

"Yet when the particles are observed, they appear as particles and not as non-localized waves. To explain this phenomenon, the Copenhagen interpretation of quantum mechanics proposed a process of "collapse" from wave behavior to particle-like behavior to explain this phenomenon of observation. The Many-worlds interpretation, on the other hand, proposes that each time an observation is made, each possible state the object could have been in splits off into a separate world or reality."

Maybe the information starting with "By the time John von Neumann..." could be put in a "History" section or something?


 * I think there is some good information in here, but is is buried deeply. It would be nice to present it in a way that was quickly understandable. For example, the sentences

"One consequence is that every observation causes the universal wavefunction to decohere into two or more non-interacting branches, or "worlds". Since many observation-like events are constantly happening, there are an enormous number of simultaneously existing states."

and

"In the Copenhagen interpretation, the mathematics of quantum mechanics allows one to predict probabilities for the occurrence of various events. In the many-worlds interpretation, all these events occur simultaneously."

Are very nice. Maybe there is a clearer way to sum them up for a reader that is merely looking for a brief overview of MWI?


 * Also, do you think it would be acceptable to mention the writings of David Deutsch? In his Fabric of Reality book he offers a variant interpretation of MWI, the Multiverse, which seems to be a variant on MWI.

Lastly, Henry Sturman proposes an experiment which could be used to distinguish between MWI and Copenhagen here (see the "Proof of other universes" section). Anyone opposed to adding a link to this? (perhaps this should go on the copenhagen page as well?)

Sorry for the ramble. I'm pressed for time with schoolwork atm, or I would make some edits myself. Since this page has a lot of information and a decent-sized Talk page, I thought I would check before doing anything drastic. --Culix 01:30, 13 Dec 2004 (UTC)
 * To preface my remarks: My knowledge of QM is limited and I have not read Deutsch's book.
 * From the web page referenced, it sounds like Deutsch's book is a popularization of QM from a proselytizing MWI point of view. If we have an article on the book, we should link to that article, but as a reference text on the subject I don't think we can recommend it: the web page author correctly points out a major flaw in Deutsch's motivating example.
 * On the other hand, I don't think the web page's author is sufficiently knowledgeable about QM to warrant being cited as a reference. And in particular, I don't think his hypothetical experiment for testing MWI is worth linking to - it's pretty clear that it won't work (not that he claims to be sure it will).  Now, there have been various working physicists who have published journal articles on experiments, theoretical and otherwise, that attempt to test MWI.  Those, it would be worth referencing.  It's worth pointing out, though, that an interpretation is not necessarily testable, in the way that a theory is.  It's quite possible (even likely) that MWI will never differ from the Copenhagen interpretation in any prediction.  The best we could then hope for would be to come up with ever more elaborate situations of waveform collapse/quantum decoherence and see which interpretation is most palatable. --Andrew 04:08, Dec 13, 2004 (UTC)

I added a new second sentence which explains what it is.CSTAR 04:27, 13 Dec 2004 (UTC)


 * P.S. I'm not sure it's any better with the new sentence, but at least it tries to solve the problem of explaining MWI as quickly as possible. It's always difficult to strike a balance between accuracy and comprehensibility and frustrating to attempt that balance. Ideally one would like to avoid the level of What the Bleep Do We Know?!. CSTAR 05:06, 13 Dec 2004 (UTC)


 * I think it's an improvement, but it could still maybe be clearer. It kind of obscures the really revolutionary part of the theory:
 * It attempts to explain the process of observation of a quantum system as a quantum superposition of states of several copies of the original system.
 * The revolutionary parts of the theory are:
 * It does away with the privileged status of "observation".
 * It makes copies of the whole universe.
 * On another level, we should be able to give a vague notion what theory this is to people to whom "quantum superposition of states" is incomprehensible.
 * How about:
 * It avoids the privileged status of observation in the Copenhagen interpretation by viewing the entire universe as a quantum superposition of many states which do not measurably interact on macroscopic scales.
 * --Andrew 06:50, Dec 13, 2004 (UTC)


 * Yes that sounds good and is accurate. Please go ahead and put it in. I believe you are suggesting it as the 3rd sentence?


 * I would however, keep the von Neumann part later in the intro, despite Culix suggestions (which I think are also very good). BTW, please pardon my being somehat obsessive about this. CSTAR 15:00, 13 Dec 2004 (UTC)


 * Not putting up with inaccurate, clumsy writing (such as my earlier attempt) is the best way to get a good article - keep it up. --Andrew 18:01, Dec 13, 2004 (UTC)

Those changes look very nice, good work to you both! And no worries about being obsessive - I wholeheartedly agree with improving and tweaking this article ;)

I agree that Deutsch's book has a somewhat religious fervor to it; though this may be because he believes it "works so well!". Perhaps you are right about not using it as a reference. Hm. I would be interested to know what you don't like about Sturman's experiment. I am not very knowledgable about the field of QM, so maybe your arguments against his experiment would help illuminate something.

I like the new sentence. I also agree that it would be nice to have something for "people to whom "quantum superposition of states" is incomprehensible". What do you think about adding on to the end of the sentence, something like " - in layman's terms, there are many overlapping universes that only affect each other in small ways." ? --Culix 21:25, 13 Dec 2004 (UTC)


 * Except that only affect each other in small ways is not quite right. "quantum superposition of states" is a particular way of combining states.


 * Think of a coin, it is a system characterized by two states heads or tails. When you toss the coin, it has one of two outcomes i.e. lands up in one of the two states. The most know-nothing approach to describing this outcome is to say it is nondeterministic. We can actually say a little more.  Indeed empirically we know the outcome is probabilistic which means that with a certain probability p it comes out heads and with probability q=1 - p it comes out tails. This probabilistic formulation has an operational interpretation based on repeated trials (we can formulate this better mathematically but I won't do it).  If the coin is fair p=q =1/2.


 * A quantum mechanical coin has a whole family of superposition of states which can be viewed geometrically as the Bloch sphere. Tails |0> is the south pole and heads |1> is the north pole. CSTAR 22:17, 13 Dec 2004 (UTC)


 * Not to be too picky, but I just got back from a math conference (the Canadian Mathematical Society meeting) at which one of the plenary talks was titled "A Mathematician Flips a Coin": they did some studies with high-speed video cameras and calculations and showed that a normally flipped coin is biased to come up the way it started...   PDF --Andrew 23:52, Dec 13, 2004 (UTC)


 * Yeah, that's possible, but does that change the probabilistic interpretation? CSTAR 00:03, 14 Dec 2004 (UTC)

Fiction
I think the section on fictional works could be a little less of an attack to science fiction. Calling fictional stories "misleading" and "deficient" because they don't conform to the details of quantum theory is asking too much from fiction. The stories are scientifically inaccurate or too fanciful, but you will be misled or deficiently informed only if you confuse fiction with fact. I was the one who added Borges, BTW. I've never implied that Borges got the idea of the MWI before the scientists, only that the literary concept and the scientific theory were similar, on the surface. --Pablo D. Flores 01:10, 16 Jan 2005 (UTC)
 * I'm aware of that and the point I was trying to make was that Borges' story (as do others) are not illustrations of MWI because they don't involve superposition of states. I ceratinly didn't intend it to be an attack on Borges!CSTAR 03:52, 16 Jan 2005 (UTC)

Misleading graphic
The graphic of splitting is very misleading, not because there is an infinite branching tree, but because the origin of the tree has nothing to do (at least in any commonly accepted interpretation) with decisions being made. his graphic belongs maybe in an article decision trees. I will be happy to compose a graphic of an infinite btanching tree but without reference to "decisions".CSTAR 04:55, 5 Feb 2005 (UTC)

Non-50% probabilities
Consider a variation of the Schroedinger's Cat experiment where you only wait long enough for a 1/3 chance of the cat being dead. There are only two possible outcomes for the experiment, so when you open the box, the universe only splits two ways, and it seems to me that that makes for a 50% chance that you'll be in the branch with the dead cat. How does a probability of 1/3 come out of this? I checked the FAQ linked from the article, and quickly got bogged down in tensor calculus -- and I never had classes in anything more complicated than basic vector calc. --Carnildo 23:15, 17 Mar 2005 (UTC)

The possibility for a cat to be 1/3 dead is possible yet extremely unlikely. Chances are you will never branch off into such a universe. You have to keep in mind that i may branch of into a different world than you will. Life is just a bunch of quantum quackery. --Aero66 13:05, 18 Mar 2005 (UTC)


 * There is little point you trying to impose this on the article. Please try to edit in a collaborative way. If you feel the article misses some essential point, please bring it up here. Charles Matthews 20:04, 18 Mar 2005 (UTC)

"Life is just a bunch of quantum quackery."
Could you please explain this remark which you have seen fit to place in the main article? CSTAR 19:37, 18 Mar 2005 (UTC)

I will provide you with an example: Based on the many worlds interpretation of quantum mechanics, the future holds that i may never receive an answer from you although my dopplgangers (copies) will. The theory removes the certainty we previously had knowing who we would talk to tommorrow. If you are socially tied with other individuals as i am, you will understand why this is of concern. We should investigate other interpretations of our world until it is certain that many worlds theory stands. The remark in the article was not suicide note, but merely a given fact to attract attention. I will come to accept any discovered truths about our world. Aero66 20:01, 18 Mar 2005 (UTC)


 * In many-worlds, the different copies are correlated, so there isn't that sort of uncertainty in who you're talking to. Of course you might never receive an answer from someone, since they could disappear in your world - but the possibility of death isn't anything new.

This is not the proper forum for discussing your new ideas regarding MWI, as interesting as they may be. Original research cannot be included in Wikipedia articles, and talk pages should not be used for discussing topics irrelevant to the content of the article. -- Schaefer 17:18, 19 Mar 2005 (UTC)

I understand your concept of correlated copies, but it is important to note that each copie is a seperated being. Suppose you are replaced with Schrodingers cat in the thought expirement. Instead of having a 50/50 chance of life or death, you have a 50/50 chance of going into one door rather than the other. Door A happens to contain cake while door B contains a sandwich. If you happen to go through door A you will never taste the sandwich although your copies will. We never know which copy we are going to spend our future with. For a recap read (RE: Life is just a bunch of quantum quackery) on this discussion page. Aero66 25:01, 19 Mar 2005 (UTC)


 * Wrong. In MWI, you go through both doors.  And since the decisions that split universes are on the quantum level, the result is about a bajillion forks of the universe, in which you go through door A, go through door B, smack face-first into the barrier between the doors, say "to heck with it" and go through door C, remain befuddled with indecision until you starve to death, and many other futures, with the "go through door A"-type and "go through door B"-type futures being the most common. --Carnildo 06:05, 20 Mar 2005 (UTC)

The point that im trying to make is that you wont experiance all these outcomes. You will split into one universe while your copies will split into the other bajillion universes. Aero66 12:00, 20 Mar 2005 (UTC)


 * It is an issue in philosophy as to whether copies of yourself in other possible worlds can be you or are just copies of you (counterparts). I.e. whether you exist in other possible worlds. David Lewis (the main proponent of the real existence of possible worlds in philosophy) held the latter. Ben Finn 20:32, 11 May 2006 (UTC)

Neutrality
There are significant sections of this article that I feel are unfairly biased towards Everett's theories. The opinion that his theory adds nothing, and is untestable (since universes are non-cummunicating) is barely mentioned (living in a high-probability universe is common to a number of interpretations, and does not provide significant evidence for one or another). Subjective polls of "acceptance", while perhaps not stated to be evidence for Everett's theory, are at least implied to be significant (section 3).

Re: Neutrality
Much better after the changes. Thank you! :)

So why do we see whatever we see?
If I understood this right, every measurement is an interactions which splits the universe into different relative cases. But what determines what case the observer (me, for example) is seeing? In Schrodinger's cat experiment - what determines whether I'm in a universe where the cat died or didn't? The universe "splits", but we still see a single result. What decides which universe we are "in"?


 * You're part of the universe, too. You split. One of you sees one thing, another of you sees the other. The model doesn't give any special treatment to observers, and that's half its beauty. But anyway, a Wikipedia talk page isn't the place for this sort of dicussion. This page is for discussing the article, not its subject. If you want more explanation you can e-mail me (my address is on my user page). -- Schaefer 16:06, 24 Jun 2005 (UTC)

Well, since no one gave me any real answer, I'll ask again: the determinism only exist if we take all the universes as a whole, but for a specific human observer - me, for example - there is still only one reality, only one universe. I understand there are many "me"s, but each one of them sees something different, because he is in a different "world". Is there any explanation for why I (or any other "me") exist in the univrse I exist in, and not in any other? My consciousness still perceives only a dead cat, or a live one. Not both. What decides what specific situation I'll see? It seems there is no real determinism for the observers, and the effects of consciousness lack any physical explanation in this "interpretation".
 * Well that my friend, is the meat of the issue. No one can give you a "real answer" it's the quest for one that is the objective. DV8 2XL 21:07, 3 November 2005 (UTC)

From my Everett FAQ (in the external links section) -

Q25 Why am I in this world and not another? Why does the universe appear random?

These are really the same questions. Consider, for a moment, this analogy: Suppose Fred has his brain divided in two and transplanted into two different cloned bodies (this is a gedanken operation! [*]). Let's further suppose that each half-brain regenerates to full functionality and call the resultant individuals Fred-Left and Fred-Right. Fred-Left can ask, why did I end up as Fred-Left? Similarly Fred-Right can ask, why did I end up as Fred-Right? The only answer possible is that there was no reason. From Fred's point of view it is a subjectively random choice which individual "Fred" ends up as. To the surgeon the whole process is deterministic. To both the Freds it seems random.

Same with many-worlds. There was no reason "why" you ended up in this world, rather than another - you end up in all the quantum worlds. It is a subjectively random choice, an artefact of your brain and consciousness being split, along with the rest of the world, that makes our experiences seem random. The universe is, in effect, performing umpteen split-brain operations on us all the time. The randomness apparent in nature is a consequence of the continual splitting into mutually unobservable worlds.

(See "How do probabilities emerge within many-worlds?" for how the subjective randomness is moderated by the usual probabilistic laws of QM.)

[*] Split brain experiments were performed on epileptic patients (severing the corpus callosum, one of the pathways connecting the cerebral hemispheres, moderated epileptic attacks). Complete hemispherical separation was discontinued when testing of the patients revealed the presence of two distinct consciousnesses in the same skull. So this analogy is only partly imaginary. --Michael C Price 07:33, 19 May 2006 (UTC)

Acceptance of the many-worlds interpretation
This part is extremely unfair towards the MWI:
 * However, the physicist Asher Peres in his 1993 textbook expresses a great deal of skepticism towards MWI which is shared by many physicists. In fact, he questions whether many worlds is really an "interpretation" at all (particularly in a section with the title Everett's interpretation and other bizarre theories). Indeed, the many-worlds interpretation can be regarded as a purely formal transformation, which adds nothing to the instrumentalist (i.e. statistical) rules of the quantum mechanics. Perhaps more significantly, Peres seems to suggest that positing the existence of an infinite number of non-communicating parallel universes is worse than the problem it is supposed to solve.
 * As such, because the interpretation results from an equivalence between two mathematical formalisms, and is considered unfalsifiable (because the multiple parallel universes are non-communicating), critics consider the many-worlds interpretation metaphysical rather than a testable scientific theory. Moreover, notwithstanding the personal opinions or speculation of individual physicists (or indeed even a statistical majority of physicists), subjective polls of "acceptance" such as the above cannot be interpreted as evidence of the correctness or incorrectness of a particular theory; as such, for example, the mere fact that any particular person or percentage of people "accept" the many-worlds interpretation should not be considered evidence of its accuracy.

If we assume that there is a empirically testable difference, then MWI and CI are scientifically valid theories, that are up to be tested against each other.

Or, if we assume that there is no mathematical difference between the MWI and the Copenhagen interpretations, then identical argument can be used to attack CI and support the MWI, or to attack the MWI and support the CI. For example we can replace "critics consider the MWI metaphysical" by "critics consider the CI metaphysical" and an argument of exactly identical validity.

The arguments listed in this section (and in some of the external links) start with completely unjustified supposition than MWI is worse than CI, and use it to prove that MWI is indeed worse than CI.

The argument that support for a theory does not constitute an "evidence of its accuracy" is also flawed. If, as stated in the argument, the MWI gives exactly the same predictions as theory the CI, and accuracy is defined methamatically/empirically, then it's exactly as accurate as CI. So relative levels of acceptance of the CI and the MWI are completely moot, as accepting accuracy of either implies accepting accuracy of the other. Taw 21:36, 23 July 2005 (UTC)


 * One of the arguments you refer to this is Asher Peres' argument. I just made note of it as objectively as possible. David Deutsch has arguments in favor of it, however, I didn't put those in since the article spends a lot of time discussing the theory. The other material you quoted was put in by somebody else who insisted it ws too biaed in favor of MWI. --CSTAR 23:58, 23 July 2005 (UTC)


 * I have reworded some text that had been reverted. The conclusion now reads However others note that science is a group activity (e.g. peer review) and that polls that indicate shifts in belief of scientists can't be dismissed lightly.  Remember no one is claiming that many worlds is true just because of poll results, but the numbers do have a bearing on its credibility.  Indeed amongst scientists it quite relevant; how often is a theory dismissed by a scientist with the phrase "Oh no one takes that seriously!"--Michael C Price 17:35, 17 May 2006 (UTC)


 * [Edit conflict] I reworded your rewording. I have specific reasons (see edit summary) but I can explain the change here in more detail if you think it is necessary.--CSTAR 17:42, 17 May 2006 (UTC)
 * No need to explain, it looks OK. --Michael C Price 18:09, 17 May 2006 (UTC)

Similarity to Leibnitz's Monadism 'best of all possible worlds'
There should be some mention, a tie-in & link if you will, with Leibnitz's philosophical stance that we are currently in the best of all possible worlds, and, more in line with the theory, that all possible universes exist in a pyramid below our current universe of every different possible permutation on into infinity of alternate past possibilities, but that our 'current' one is the top of this pyramid. Not exactly the same as MWI theory, as each universe is its own 'top of the pyramid', and Leibnitz has a much more 'hard deterministic' rather than random quantum effect, view-point. Though the similarity remains nonetheless. Nagelfar 21:10, 13 November 2005 (UTC)


 * See Possible worlds and multiverse.--CSTAR 21:23, 13 November 2005 (UTC)


 * Do you mean to insinuate that the reference would be more suited at one of those articles? I visually scanned and then searched 'Leibnitz' with my browser and found no mention of him either place. What my intention for aiming a possible reference to this particular article was a linking of the exact concept, i.e. bifurcation of a separate cohesive universe(s) from a single universe. Which implies more than possiblity of or alternate stand alone 'universes.' Being that this is so particular an idea, one metaphysic the other quantum mechanic, I thought it fitting enough here. Nagelfar 01:56, 14 November 2005 (UTC)


 * MWI is discussed in this article is a very specific formulation involving quantum superpositions of alternate histories. Though it does fit better into the multiverse article, the Leibnizian idea may deserve an article in its own right. It is sufficiently different from the concept discussed here, that other than a brief mention amd a link, I don't thik it should not be put here.--CSTAR 05:18, 14 November 2005 (UTC)


 * A link is all I meant, to Monadism and a brief discription, though it may be difficult due to the complexity of the concept, it bears similarity here particularly, was all I had meant. Nagelfar 22:08, 14 November 2005 (UTC)
 * nevertheless the link should go in Possible worlds or Modal realism as it is more related to those than to MWI. DV8 2XL 22:48, 14 November 2005 (UTC)
 * Not necessarily. It depends on if you mean "world" as whole 'universe/multiverse' or not (the original etymology, "ver-alt", means 'through time/history', or "time worn progression"). The terminology contradicts if you think of an isolated world in MWI, but at the same time the concept is the same for an individual 'world' within the whole uni/multiverse under MWI. For example, if in MWI every particle must exist in every possible combination by means of every different possible world/universe, then any individual universe must have its particular arrangement to not duplicate any other universe, so that the particles can be in every potential position and in every combination / permutation possible. Putting any individual universe in the most perfect position it is capable of having as to not repeat any other; making it 'the best of all possible "permutations" or worlds', for the very fact that, it is one instance necessary to the completion of every particle being in every possible place at every given time. Maybe associating it to his concept of "Monadism" would be incorrect, I don't mean to misdirect my point. His assertion that this is "the best all of worlds" seems not to be directly related (His Monadism may relate to Chaos Theory though this is not the place for such discussion) Nagelfar 23:53, 18 November 2005 (UTC)

What good does it do?
I was wondering, what new things can MWI tell us about the world and universe we live in? I am comvinced that CI tells us things that noone else could say (when it was formed), but I fail to see what MWI can contribute with. To me, it sounds too much like a religious argument.
 * I think there's no known alternative to the many-worlds interpretation if you are looking for an intuitive and physical understanding of what's going on in quantum superpositions and entangled states. In addition, the problem of the measurement in quantum mechanics is easily solved without postulating any unknown and unphysical mechanism as the collapse of the wave packet (see the Copenhagen interpretation -- before the collapse it also involves quantum superpositions) or the consistent-history requirement (see consistent histories) or some backwards-in-time information flow (see the transactional interpretation). All other known interpretations need additional axioms. Thus, according to the principle of Occam's razor I consider the many-worlds interpretation the best one. By the many-worlds interpretation, the universe grows in a similar way as it did when scientists realized that the stars are just distant suns. This may look like an additional assumption, but in both cases it is a consequence of a better theory requiring fewer assumptions than the old one. --DenisDiderot 18:29, 21 December 2005 (UTC)

Pseudo-science?
I'm no physicists but this smells like pseudo-science to me. Why is it called an "interpretation" and not a hypothesis? Could it be that if it were to be called a hypothesis, then it would need to be either falsifiable or admitedly an unscientific belief? Neurodivergent 16:19, 21 December 2005 (UTC)
 * Everett's Many-worlds interpretation doesn't add any new hypothesis to quantum mechanics, it even manages to do away with two of the axioms (assumptions) of the standard formulation of quantum mechanics, see the section about axiomatics in the article. The concept of quantum superpositions is common to all formulations of quantum mechanics, though they are usually argued away as something purely mathematical. But physical understanding of the EPR paradox and entangled states requires acknowledging them as something real and physical, and that's exactly what the Many-worlds interpretation does, no more, no less. If there are real quantum superpositions, then there's no reason why macroscopic objects shouldn't rest in superpositions of states or even entangled states as well. This is an implication, not an assumption. --DenisDiderot 18:09, 21 December 2005 (UTC)

Irrelevent (IMO) phrase
Excuse me, how does the phrase "taking superposition at face value" used in the intro avoid misconceptions? --CSTAR 15:08, 28 January 2006 (UTC)


 * PS In case there is any doubt, I earlier made the edit which removed the phrase.--15:11, 28 January 2006 (UTC)

The phrase points out right from the start that the Many-worlds interpretation doesn't introduce any new mechanism, as many people think, e.g. some kind of mechanism that divides the universe. Instead, it considers standard quantum superpositions in quantum mechanics as real, which already implies the existence of multiple universes.--DenisDiderot 16:19, 28 January 2006 (UTC)


 * Fine. But please get rid of or replace "at face value" and be more specific. --CSTAR 22:43, 28 January 2006 (UTC)

Misleading edit summary?
Why was it a misleading edit summary?

The article itself is quite clear about the relation between superposition etc (see for instance the very detailed part on CP maps). --CSTAR 18:15, 5 February 2006 (UTC)


 * Moreover, the intro now misses the point entirely about the relation between reversibility and determinism and the apparently irreversible nature of some quantum processes.


 * I suggest you be more careful before accusing other editors of writing misleading edit summaries. --CSTAR 18:20, 5 February 2006 (UTC)


 * The word "copyedit" is inconsistent with just deleting a phrase. Above, you demanded to "please get rid of or replace 'at face value' and be more specific." Now, more than a week later, you just delete the result. I suggest you be more careful in your demands.
 * In reality, the mentioned phrase is the single idea all the difference to other interpretations can be reduced to. Thus it is a good idea to mention it right from the start. Otherwise, beginners will again get the idea that some new mechanism of branching the universe was proposed, which is very common, unfortunately. --DenisDiderot 18:27, 5 February 2006 (UTC)


 * I consider the deleted phrase
 * "The many-worlds interpretation (or MWI) is an interpretation of quantum mechanics that considers the concept of quantum superpositions a real phenomenon instead of a mathematical construct"
 * particularly important in view of objection 2 to the featured article status referring to an older version. --DenisDiderot 18:38, 5 February 2006 (UTC)
 * I think I will add my voice to CSTAR's and ask you to watch your language. You may wish to familarise yourself with WP:AGF--DV8 2XL 18:40, 5 February 2006 (UTC)
 * I really assumed good faith. That's why I conformed to the request about being more specific a week ago. --DenisDiderot 18:50, 5 February 2006 (UTC)

Addressing objection 2
Objection 2 in the featured article status is:


 * Section 8 needs an explanation of exactly what is misleading about many-worlds in science fiction, in addition to the why

The presence of the sentence in dispute in the intro hardly addresses that concern. Morever, any attempt to deal with this objection in the intro will distort the expository nature of intros in WP articles.

I repeat, it is my opinion that the intro is now awkward and confusing.--CSTAR 19:05, 5 February 2006 (UTC)


 * This is a misunderstanding. I didn't refer to phrase 2 in objection 1, but to the sentence
 * I couldn't understand what the intro was saying, so stopped there. Needs to be crafted so that a layman can gain an appreciation of what it being discussed
 * (see Featured article candidates/Many-worlds interpretation/archive1) Objection 3 plays the same tone:
 * I have to agree with jguk - even through I read about this theory earlier, and I am a fan of hard sf, this is worse then star trek technobabble. It may make sense for somebody familiar with quantum physics, but it needs explanation for laymen like most of us are, I am afraid. The lead is quite short, it can be expanded 3-4 times.
 * --DenisDiderot 19:26, 5 February 2006 (UTC)


 * These "objections" are clearly not addressed by anything that is currently in dispute. --CSTAR 19:36, 5 February 2006 (UTC)
 * I wonder why you quote the word objections. Do you respect them? I indeed think that mentioning the connection between considering superpositions a real phenomenon and proposing parallel universes is a step forward in explaining to beginners what the central idea of the interpretation is. Maybe an additional link to the double-slit experiment helps those who don't know about superpositons. --DenisDiderot 19:45, 5 February 2006 (UTC)


 * The double-slit experiment is already linked. Please don't distract from the discussion by citing the presentation style of your interlocutor. The fact is that the objections raised are not addressed by the disputed sentence in any identifiable way. --CSTAR 19:53, 5 February 2006 (UTC)
 * Is this a "fact" or is it your opinion? --DenisDiderot 20:12, 5 February 2006 (UTC)


 * Well, if it's not a fact, then you should be able to identify how your edits address the general concerns expressed in these objections.--CSTAR 20:27, 5 February 2006 (UTC)


 * I repeat: I indeed think that mentioning the connection between considering superpositions a real phenomenon and proposing parallel universes is a step forward in explaining to beginners what the central idea of the interpretation is, especially for those who have grasped the idea of superpositions (The latter is accomplished best by viewing an example as the double-slit experiment.) --DenisDiderot 20:57, 5 February 2006 (UTC)


 * Is there really an identifiable relation between:
 * I couldn't understand what the intro was saying, so stopped there. Needs to be crafted so that a layman can gain an appreciation of what it being discussed

and
 * mentioning the connection between considering superpositions a real phenomenon and proposing parallel universes 
 * How does asserting the reality of superposition help a layman gain an apprectiation of what is being discussed? --CSTAR 21:32, 5 February 2006 (UTC)
 * The layman will first have to grasp what superposition is about, which is best accomplished by delving into the double-slit or similar experiments. Then the question immediately arises what's really going on during the time when the electron (or other particle) is described as by a superposed state. Copenhagen interpretation and Many worlds interpretation disagree on this point. The former tells the layman that this is just a mathematical construct and doesn't answer his question, the latter answers him by taking the superposition at face value and describing the electron to really be present in different versions in different superposed worlds/universes. I hope this is clear enough. --DenisDiderot 21:44, 5 February 2006 (UTC)

Reply to User:DenisDiderot
That's your opinion about the meaning of copyedit. I actually retained the fact of superposition and in the last sentence of the intro was very clear about the relation between unitarity and superposition and non-determinism (which it now lacks BTW).


 * Now, more than a week later, you just delete the result.

The characterization in your reply, that I deleted the result, is most certainly not true. It suggests to me you didn't bother to read what I wrote. I carefully thought about this introductory paragraph for more than a week and came up with what seemed to me to be a better formulation, which included the fact of reversibility and superposition. As it is now, in my opinion, it is extremely awkward. My edit was consistent with the WP process of convergence and certainly is arguably copyedit. Accusing a good faith edit of "misleading summary" is unnecessarilly beligerent.

Also whether the fact of superposition is "real" or not is irrelevant. As is pointed out in the article (in some form or other) MWI is a conservative extension of standard QM.


 * --CSTAR 18:47, 5 February 2006 (UTC)

I added a sentence about the apparent irreversibility in quantum mechanics, which is a different point than the deleted phrase. Please reconsider: If the existence of multiple superposed universes is proposed, then this has to be considered a real phenomenon. But beginners won't understand that this is implied by (and implies) the familiar superpositions being real. --DenisDiderot 19:02, 5 February 2006 (UTC)


 * MWI can be a eithe

r a ontological commitment or a mathematical reduction. Whichever way it is taken is not an issue to address in explaining it.--CSTAR 19:08, 5 February 2006 (UTC)

I suggest you read Featured article candidates/Many-worlds interpretation/archive1. The phrase about existence is already an ontological claim. In my opinion, I just made this more clear to beginners. --DenisDiderot 19:13, 5 February 2006 (UTC)


 * Indeed it is an ontological claim as is currently stated, and I don't suggest changing that. MWI is most often regarded as an ontological claim. But MWI is also a mathematical reduction (see for instance Asher Peres' book) that can be regarded in a purely instrumentalist way. You are making a very big deal in your addition about the fact that superposition is real; why not then make the claim that the state function is real or that the unitary which describes the dynamics is real? Why is this necessary? In my opinion this is not the place to do discuss this problem of actualism.


 * In the guise of "making it clear to beginners" the intro now has too many words.--CSTAR 19:45, 5 February 2006 (UTC)

Please explain how you can hold the view that there are many superposed worlds without superposition being real. I consider Asher Peres' book a fringe view which certainly contradicts the very name of the interpretation and should thus be named differently. --DenisDiderot 19:54, 5 February 2006 (UTC)


 * Peres' view is hardly fringe. Similar views are expressed by Chris Fuchs. But in any case, I am not arguing in favor of that position (again please read what I wrote); I said that the itro to the article is not the place to discuss actualism.


 * MWI is simply a representation of a CP map describing non-reversible dynamics (as is shown for instance in the article).


 * In my opinion, the intro is now garbled.--CSTAR 19:59, 5 February 2006 (UTC)

I think you haven't answered my question. --DenisDiderot 20:09, 5 February 2006 (UTC)

I have answered your question, but let me repeat and elaborate my answer:


 * Mathematically, MWI is simply a representation of the map describing non-reversible dynamics of a quantum mechanical system as is shown, for instance in the article. Whether or not the elements in the superposition are real can be regarded as irrelevant. Some people do regard the claim as irrelevant or even meaningless. Whether you happen to agree with them or not, or whether you consider their views "fringe views" is irrelevant. To elaborate on this further, consider for instance, a representation of the Wiener process as a measure over sample paths; these sample paths can be regarded as alternate universes. One can legitimately argue that the sample paths are real. However, aside from this ontological question, the sample path representation is a useful mathamatical representation. Would you argue that one should rewrite the intro to that article to say the sample paths are real? I believe this addresses your question.--CSTAR 20:22, 5 February 2006 (UTC)

I think I must repeat: You can't keep the phrase about existence of many superposed worlds in the article and still deny that this implies superposition to be a real phenomenon. In the same way, if you regard the sample paths as alternate universes, then you are compelled to consider the sample paths real. That's the whole point of talking about alternate (existing) universes. By the way, your very insistent opposition to my phrase shows clearly that your edit was much more than just a copyedit, doesn't it? --DenisDiderot 20:54, 5 February 2006 (UTC)


 * Each sample path is a possible unfolding of the process. The wiener process itself is a composite structure consisting of the sample paths and a probability measure on the space of sample paths. In a similar way, MWI consists of possible unfoldings of the universe and some structure built on top of these unfoldings.


 * Re: By the way, your very insistent opposition against my phrase shows clearly that your edit was much more than just a copyedit, doesn't it?


 * How can you can possibly judge what my intentions were? I explained earlier why I thought my modification was copyedit; specifically, n my edit, I retained the role played by superposition. Do you not believe what I said? To repeat: I thought (and still think) that mentioning the problem of actualism (specifically that of superposition) in the introduction is irrelevant and confusing.
 * If you thought so, then your edit summary should have stated that and that you want this phrase to be deleted. Instead you claimed to have just copyedited it. --DenisDiderot 21:34, 5 February 2006 (UTC)
 * Your main point in this discussion is to assert the opposite. In any case, why are you bringing the issue of my edit summary up now, unless it is to prove I acted in bad faith?


 * Note that at several points in this discussion you have brought in elements unrelated to the discussion (such as my use of quotation marks or my asserting something was a fact) of a very specific question, that is:


 * whether the fact that superposition is real or not should be mentioned in the introduction to the article


 * Please, stick to the question being discussed.--CSTAR 21:22, 5 February 2006 (UTC)
 * I'll continue to address both, and I think it is possible to follow both lines of thought easily due to the nature of structuring texts in wiki. --DenisDiderot 21:31, 5 February 2006 (UTC)
 * So you're claiming I acted in bad faith. Why don't you file an RfC?
 * I don't claim this. You might also have been just negligent. At any rate, it is a minor issue. --DenisDiderot 21:52, 5 February 2006 (UTC)

I don't think that it has any relevance what MWI mathematically is. This is physics, and physics deals with real phenomena and tries to explain real processes in nature. Have we given up this goal? --DenisDiderot 21:20, 5 February 2006 (UTC)


 * Oh really? The mathematical structures are irrelevant? This is physics? No, this is not physics, this is ontology, which is not physics. Note that I'm not saying it's irrelevant, just that it's not physics. --CSTAR 21:25, 5 February 2006 (UTC)

The mathematical structure is just a tool for trying to understand what's going on in reality. It is not the goal. Do you agree? --DenisDiderot 21:31, 5 February 2006 (UTC)
 * Whether I agree or not is irrelevenat. That's POV. A popular view among respected physicists is that the formalism of QM is just an algorithm to predict outcomes of experiments.--CSTAR 21:34, 5 February 2006 (UTC)

No, this is clearly the so-called "shut up and calculate interpretation", which is ridiculed by most respected physicists. At any rate, it is contrary to the Many-worlds interpretation, since it was the whole point of Everett's theory to avoid having to "shut up". --DenisDiderot 21:48, 5 February 2006 (UTC)

Reply
Ridiculed by most respected physicists? That's a stretch.
 * I think it is clear that your "copyedit" intends to enforce shutting up about what interests beginners most, i.e. what's considered to be really going on when the electron is in a superposition in the double-slit experiment. "Shut up and calculate" is a common joke among physicists. --DenisDiderot 22:08, 5 February 2006 (UTC)


 * Whoa, now that's a mean accusation! --CSTAR 22:13, 5 February 2006 (UTC)
 * Really? Didn't you state above that exactly this question had to be considered irrelevant and should be excluded from the intro? If I'm mistaken, then ok, let's keep it in the article and everything is fine. --DenisDiderot 22:16, 5 February 2006 (UTC)


 * Do you mean it's OK so long as the introduction stays as you wrote it, otherwise you are free to throw accusations around?--CSTAR 23:16, 5 February 2006 (UTC)
 * No, of course not. Pull yourself together and be reasonable! --DenisDiderot 23:37, 5 February 2006 (UTC)

The point of Everett's theory: From the 1st para of his paper (Rev Mod. Phhs v29 1957):


 * The task of quantizing general relativity raises serious questions about the meaning of the present formulation and interpretation of quantum mechanics when applied to so fundamental a structure as the space-time geometry itself. This paper ... presents a reformulation of quantum theory in  a form suitable for application to general relativity''

--CSTAR 22:01, 5 February 2006 (UTC)


 * Yes, that's because general relativity is much more clear about reality than the Copenhagen interpretation and therefore excludes shutting up and calculate. Einstein shared this opposition in his EPR discussions. --DenisDiderot 22:08, 5 February 2006 (UTC)
 * No he wasn't cooncerned about shutting up and calculating, but I don't have the patience to copy more of the article.--CSTAR 22:13, 5 February 2006 (UTC)

See also the sentence
 * For instance, in some interpretations of quantum mechanics (such as the many-worlds interpretation) the ket vector associated to the system state is assumed to correspond to an element of physical reality, while in others it does not.

on page Interpretation of quantum mechanics. Should it be deleted as well? Are we supposed to shut up about this point all over Wikipedia? --DenisDiderot 22:31, 5 February 2006 (UTC)


 * That's a straw man argument, because I am not proposing any other changes to this article (or any article) on the point of realism. We are only discussing the suitability of emphasizing with a dedicated sentence prominently in the introduction of the article, the assertion that superposition is real. I have tried to argue the following points:


 * That the disputed sentence is unnecessary since actualism is discussed elesewhere in the article and produces a confusing introduction and moreover is not universally agreed on by reputable physicists. Your characterization of them as being ridiculed has little support in the community of practicing physicists.
 * I don't consider the sentence confusing. On the contrary, see above.
 * It's stunning how you keep ignoring the humour in "shut up and calculate!". --DenisDiderot 23:45, 5 February 2006 (UTC)
 * I have tried to show how it is possible to discuss physical theories in general (such as brownian motion) without mentioning "realism" of any construct (such as sample path) used in that theory. As I side issue, I don't happen to agree with any purely instrumentalist interpretation of quantum mechanics. See the discussion in Talk:Interpretation of quantum mechanics.

Please tell me why we are supposed to consider it desirable to avoid talking about realism in a natural science? This is very strange. Natural science is not mathematics, it is about reality and being forced to shut up about reality might please Jesuits but destroys science and, in addition, makes science inaccessible for many interested laymen! --DenisDiderot 23:37, 5 February 2006 (UTC)


 * I have explicitly avoided arguing against actualism or realism in general. There are peripeheral claims which you keep bringing in,specifically about my motivations, which are (a) false and (b) not helpful to the discussion.--CSTAR 23:16, 5 February 2006 (UTC)

I disagree on (a) as well as on (b), and your claims about realism in MWI were pretty general. You keep denoting your interlocutor's arguments as irrelevant or not helpful, each time ignoring that this is just your opinion. I think this is not helpful.

Do you intend to enforce shutting up on reality in biology as well? What about the existence of atoms in chemistry? Are biologists from now on supposed to talk about genes and the DNA molecule without attributing reality to them? If not, why do Physicists have to behave differently? --DenisDiderot 23:37, 5 February 2006 (UTC)


 * No personal accusations please. You are completely mischaracterizing my claims. Furthermore, the fact that I went to spanish jesuit high school is completely irrelevant to this argument and the fact you're bringing it up here is highly objectionable. Particularly since many spaniards and latin americans are jesuit educated even though most (like myself) are atheists.--CSTAR 23:59, 5 February 2006 (UTC)

I never claimed that you were a Jesuite. I didn't even know that you are a spaniard. It is true that proponents of the catholic church have been opposing exactly the point discussed here. They had forced Galilei to deny the reality of his "hypothesis" about the planets in the 17th century and the Jesuits continued similarly in the 18th century, when they were very powerful. -- That's when Denis Diderot lived, by the way. :) --DenisDiderot 00:23, 6 February 2006 (UTC)

Don't forget that the church didn't object to Galilei calculating the positions of the planets using his "hypothesis". They did object to attributing reality to the latter. Check the literature! It's worth while. --DenisDiderot 00:38, 6 February 2006 (UTC)

I find the atmosphere here to be a bit too hot. How about a truce, taking some time off from this article, and coming back later with a cooler head? Oleg Alexandrov (talk) 00:39, 6 February 2006 (UTC)

Actually, it was Denis Diderot who was educated by Jesuits. He managed to free himself from their propaganda. So User:CSTAR and Denis Diderot have very much in common, which is honorable. --DenisDiderot 00:44, 6 February 2006 (UTC)
 * Well I for one will be sorry to see this end; it has been one of the more entertaining debates I have seen in a long time. Props to both sides - you aquitted yourselves well. --DV8 2XL 00:54, 6 February 2006 (UTC)


 * c'est à dire Denis Diderot le vraie était disciple des jésuites, pas forcément celui là. --CSTAR 02:11, 6 February 2006 (UTC)
 * Where's your sense of humour? By the way, "disciple" is the wrong word. --DenisDiderot 08:53, 6 February 2006 (UTC)

Il se voit bien que vous ne savez, ni quand ni où, vous taire: je cite le Petit Robert 1993


 * Disciple: Persone qui reçoit l'enseignement (d'un maître) écolier, élève.

En plus, en googlant vous auriez pu voire pas mal d'examples: en voici un d'un document de l'unesco


 * des hommes, ce pour quoi ils avaient été effectivement conçus ? Et oublions le trait d’esprit qui fait de Voltaire un simple disciple des Jésuites ! Nombre de physiocrates et d’encyclopédistes étaient dans la même situation, comme d’ailleurs toute une génération qui, poussée par son hostilité envers l’Église et l’ordre établi - l’Ordre avec un O majuscule - est allée jusqu’aux frontières de la déraison ; or tous avaient grandi sous l’influence des Jésuites ... »

--CSTAR 14:37, 6 February 2006 (UTC)

This might be the opinion of some Unesco official, obviously an enemy of the Enlightenment, as you, probably, since you like degrading great thinkers of the Enlightenment. "Disciple" means in French as well as in English a follower of a thought school (led by a maître), not a pupil of a general school.

Stop your personal attacks! You shouldn't tell me when I'm supposed to be silent. Never before I have encountered such a hostility and lack of humour. --DenisDiderot 18:35, 6 February 2006 (UTC)

the intro
Hey guys. I tried to join in and pick sides in the argument going on above, and simply couldn't pick up the thread. So I won't.

But I will say this: this article has problems. The very first sentence, many worlds considers superposition a real phenomenon instead of a mathematical construct, is perfectly meaningless at best, misleading at worst. Perfectly meaningless because the terms "real phenomenon" and "mathematical construct" are pretty useless (though I would argue that they actually apply quite nicely to virtual particles). It's misleading because superposition of states has observable consequences, it is the foundation of all quantum mechanics. Any flavor of quantum mechanics worth its spit treats superpositions as real phenomena, where "real phenomenon" means something with measurable predictions.

Here's my suggestion for an intro:


 * In physics, the many worlds interpretation is a formalism for doing quantum mechanics in which one views the space of states of outcomes of the measurement of a system as a subspace of the space of entangled states of the system and the measurement apparatus together. The advantage of this is that states always evolve unitarily; there is no nonunitary collapse of the wavefunction as in the Copenhagen interpretation.  It is called "many worlds" because it involves viewing the outcome state as being in a space which includes other possible outcomes of the system entangled with corresponding states of the observer, thus viewing the observer as being in one world of many, one for each of the possible outcomes.

OK, that was pretty hard to write, and I don't really like the outcome, and don't want to suggest swapping it in. But this bit about superposition being a "real phenomenon" leaves a bad taste in my mouth. I don't like the mentioning of a "multiverse" in the intro either. I haven't followed that link, but I have a deep horror that it is going to take me to a D&D or Marvel comics page. -lethe talk [ +] 02:14, 6 February 2006 (UTC)


 * The Many-worlds interpretation considers the superposition components to be literally present (in different worlds, universes etc.), which is not the case in other interpretations. Don't you think that therefore there is a difference in how superposition is treated in the interpretations? Shouldn't we stress this? --DenisDiderot 08:57, 6 February 2006 (UTC)
 * literally present? I don't know what that phrase means, but I can't think of any sense in which the Copenhagen thinks superpositions are not present.  I definitely think we should not stress this. -lethe talk [ +] 09:30, 6 February 2006 (UTC)
 * Why do you think the words "many worlds", "multiverse", "parallel universes" are used if the superposition components are not considered to be literally present? Why are they said to exist then? The Copenhagen interpretation definitely denies the existence of many worlds, since they are considered to be somehow cut off by the wavefunction collapse. Above, you are talking about a mathematical space, but the universe is a real space. --DenisDiderot 09:41, 6 February 2006 (UTC)
 * As far as I know, parallel universes and multiverses are only used in pop sci books. So you ask why do I think they are used, and the answer is, I think they are not used.  For your second question, Copenhagen has unitary evolution and linear superpositions, which can be interrupted by collapse, but that doesn't mean they're not there.  Of course they are there.  Once again, linear superposition is the foundation of all QM.  Any theory which doesn't have superposition isn't QM.  -lethe talk [ +] 09:49, 6 February 2006 (UTC)
 * Hmm. What about the name of the article (many-worlds) then? Is it a misnomer? What do you think is literally present before the measurement? Please don't answer by "Shut up and calculate". The many-worlds interpretation is capable of providing a perfectly consistent real picture of what's going on. You may censor yourself in thinking about reality, but aren't you nevertheless interested in it? --DenisDiderot 12:46, 6 February 2006 (UTC)
 * First, yes, "many-worlds" isn't a great name. Everett himself never called it that.  I don't know if I would call it a misnomer, but it does tend to make people believe that it is a theory about multiverses, when in fact it is a theory about subspace and projections.  But we can't change the name.  Second, why would I answer "shut up and calculate"?  That's not an interpretation of quantum mechanics.  As far as I know, that's a phrase used by David Griffiths to suggest that one doesn't need an interpretation to do quantum mechanics.  That's not relevant here, so let's leave it.  Third, I can't answer what is "literally there", because I don't know what that phrase means.  Quantum mechanics is about systems (things you can measure) which act a certain way and are modelled a certain way.  I can tell you what measurements in QM look like, and I can tell you what the models look like.  You take your pick whether you consider one or both of those things to be "literally there".  Finally, why are you suggesting that I censor myself?  Because I won't answer questions about what is "literally there"?  Let's not get melodramatic. -lethe talk [ +] 13:09, 6 February 2006 (UTC)

At least you admit that you don't want to answer my questions. --DenisDiderot 19:00, 6 February 2006 (UTC)

Lead section needs work, then. Charles Matthews 13:18, 6 February 2006 (UTC)

lethe - you really should check-out the multiverse article which is based largly on Tegmark's classifications - you may find it enlightening. It fits Everett into the other 'complete alternative histories' theories.

This debate seems to be orbiting the meaning of the term Existence for which there is no universally accepted theory. Which is exacerbated by the fact that in this (and other multiverse theories)the statement "for some x Fx" is indeterminate. --DV8 2XL 14:47, 6 February 2006 (UTC)
 * Everett's theory was a relative state theory. And yes why are we discusing existence in a physics article? As I said earlier, that's ontology a perfectly legitimate endeavor (but not in this article), not physics.--CSTAR 15:06, 6 February 2006 (UTC)
 * I agree totally. Denis seems bent on making this about philosophy.  Interpretations of quantum mechanics are not a purely philosophical question (although they have an admittedly philosophical flavor).  What's "literally there" or what's "really true" have no place in this article.  By saying so, I'm not espousing a "shut-up and calculate" stance.  In summary, QM is about calculating matrix elements, interpretations is about deciding what happens in your model when measurements occur, and philosophy is about deciding "what's really there".  Only the middle one has a place in this article, therefore the claim that "many worlds treats superpositions as literally there" simply has to go.  I will now excise. -lethe talk [ +] 15:17, 6 February 2006 (UTC)

While we are at it, I have issue with the line; ''According to the principle of minimalism (Occam's razor) in scientific theories stating that of two theories explaining the same observations the theory with fewer assumptions or axioms is the better one, this speaks very much for Everett's theory. '' from the Many-worlds interpretation section. I think it should go or be reworded. What say ye all? --DV8 2XL 17:30, 6 February 2006 (UTC)
 * I agree. Generally, I get suspicious when Occam's razor is invoked. Particularly in this case, where though indeed MWI has fewer axioms, it proposes a universe with more things in it (many words versus many worlds as I believe Max Tegmark puts it). Is this really Occam's razor anyway?--CSTAR 18:02, 6 February 2006 (UTC)
 * I vanished it. And no I don't think Occam applies here anyway, and in general I think it is high time that it is returned to the study of philosophy from whence it came; Nature has only a passing familiarity with the concept. --DV8 2XL 18:46, 6 February 2006 (UTC)

DV8, see my answer to "What good does it do?" above. --DenisDiderot 18:41, 6 February 2006 (UTC)
 * See my answer above. --DV8 2XL 18:48, 6 February 2006 (UTC)

Who would like to help develop an alternative view?
I am quite new to Wiki so pardon me if I make any breach of protocols by writing here. That said, I do have a PhD in quantum theory, so not a crank.

From reading the excellent article I am starting to believe that Everett has been discredited (by me, at least!). Like many others, I was familiar with the popular (and I might say sensationalist) descriptions of MWI which talk about multiple universes co-existing, which struck me as patent nonsense. Having read the article, it now seems to me that pehaps Everett had no such thing in mind.

Everett is right that there is, according to quantum theory, a single many-body wave-function that covers everything in the universe. Most of the examples and calculations found in text books or popular science books adopt the single-particle approximation, in which one particle is assumed to have an associated wave-function; but it is just that- an approximation, one in which the effect of the rest of the universe on the particle is treated as a potential field in the Hamiltonian. For non-physicists, what this means is that all of the other particles in the universe are replaced, for the purposes of trying to make the calculations simple, by their averaged-out background effect on the particle being studied (if anyone is interested I can provide a better explanation). So there is a tendency to talk about the collape of the single-particle wavefuntion as if it were a real effect, whereas it is only an approximation. What 'really' happens in a measurement is that the particle whose properties are being measured is intracting in a complex quantum mechanical way with the zillions of other particles that make up the measuring device. So when we talk about the collapse of the wavpacket, what we should really think is as follows: the particle was in a state in which the single-particle approximation could be sensibly applied, and in which we could view the particle as having its own wave-function; now there is'a measurement' which means that the degree of interaction between the particle and the rest of the measuring device is such that the single-particle approximation no longer applies; and now, after the measurement, the interaction has simplified and we can go back to a single particle approximation, but surprise surprise the wavefunction has changed. All clear so far??

Next, let's suppose that there is a wave-function which at a given instant describes the whole of the universe. One property of wave functions is that they can be expressed as linear expansions of other functions, each of which could also be a legitimate wave function. This means that it is technically correct to say that the quantum mechanical state of the universe (ie its wavefunction), is methematically equivalent to a super-imposition of other possible states of the universe (each of which would be multiplied by an expansion co-efficient), but that is quite different from saying that there are multiple universes each existing simultaneously. Let me see if I can give an analogy to illustrate this...

...well, a quick analogy (which is not exactly water-tight but nonetheless illustrates the issues) imagine that I put 1000 pounds into a bank account, and withdraw 999 pounds. As a result I have one pound. Mathematically this is indistinguishable, in terms of the overall result, from putting 1,000,000 pounds into my account and withdrawing 999,999. Both patterns of withdrawal and deposit yield the same overall 'state' of my bank account. More generally, in the same way that the number one can be expressed as the super-imposition of 1000 and -999, or as the super-imposition of 1,000,000 and -999,999, it can also be represented as the super-imposition of many other combinations of numbers. Taking the analogy with MWI, a bank balance of one pound could be treated as a sum over all possible states of my bank balance each with an appropriate expansion coefficient. While this would be mathematically correct, it does not mean that there are infinitely many simultaneous states of my bank balance each with its own independent existence. Given this, people need to be wary of attributing a physical significance the the individual multiple states considered by Everett.

Taking this as a starting off point, does anybody fancy trading ideas about how to derive a more sensible interetation of MWI?

---

Hi! generally add items to the bottom or people won't see them. And put four ~s at the end of your posts so people know where your post ends etc. Stephen B Streater 19:13, 25 March 2006 (UTC)

Comment in overview
Hi, in the "Brief overview" section there is a sentence which I don't understand: "The novelty in DeWitt's viewpoint was that the  various complete alternative histories could be   superposed to form new quantum mechanical states." Despite rereading several times and already understanding (I hope) QM & MWI I do not follow this sentence. I think it needs reworking but as I'm not sure what the author was trying to say I'd rather leave it to them to amend. IMO there is no real or substantive novelty to deWitt's approach except in his presentation of Graham's derivation of probability within the MW paradigm; his article was reworking Everett's ideas and presenting them in a clearer, more explicit fashion (e.g. deWitt's name "many worlds" is much, much clearer than the wimpy moniker "relative state formulation" that Everett proposed, under pressure/protest from Wheeler, I understand). This is not to denigrate deWitt's contribution -- without deWitt many-worlds may have been neglected and ignored by the wider physics community for decades more.

--Michael C Price 22:46, 1 May 2006 (UTC)


 * I think I may have written that. What I was trying to say was that the "Universal" Hilbert space is a direct sum (or continuous direct sum) over a family of Hilbert spaces indexed on observation histories.--CSTAR 22:55, 1 May 2006 (UTC)


 * Yes, the actual sentence in the article (which I did write)


 * In DeWitt's formulation, the state of S after measurement is given by a quantum superposition of alternative histories of S.


 * The sentence refers to the possible state after a sequence of measurements of a single qubit system as in the example that follows.--CSTAR 23:11, 1 May 2006 (UTC)


 * I made a change in the article which hopefully makes this clearer. HOWEVER, I make no claim as to the historical accuracy of the claim.  In particular, I have always been under the impression that DeWitt's rendering was the "logical unfolding" into a Hilbert direct sum over paths of Wheeler's relative state idea. But that's an impression only, based on a possibly shoddy inference from naming conventions. --CSTAR 23:24, 1 May 2006 (UTC)


 * Hi, I see the changes you made to section 3, which clarifies matters somewhat. Now that you've explained it the original sentence I highlighted (in section 4: "Relative State", not 3: "Brief Overview" as I mistakenly said) seems redundant (as well as confusing).  I suggest deletion, which leaves the important point about alternative histories undiminished.


 * BTW, I didn't mean to imply that Wheeler dreamt up the relative state idea (although I'll have to check my sources to be sure) I think it was only the name Wheeler suggested -- it was Everett's idea. Wheeler and Everett fought over the presentation and Everett's article was something of an unhappy comprimise (Everett's longer thesis: "Theory of the Universal Wavefunction" is a clearer presentation of Everett's viewpoint.) --Michael C Price 23:47, 1 May 2006 (UTC)

Recent edits
There are numerous problems with the recent changes to this article which I reverted and were re-reverted. I am sorry to say, the following added paragraphs are simply wrong (at best) or meaningless:


 * Both sides in physics, are attempting to model (mathematically) the physical world and the various conservation laws fundamental to science, and the difficulty deals with what are known as singularities or mathematimatical discontinuities in the calculus equations (e.g. dividing by a quantity which goes to zero at some point in time) involving the law of conservation of energy taught in secondary school physics.


 * Certain facets of the necessary calculus lead to the Schrödinger equation (in all schools of interpretation) which involves the definition of energy (including matter, by Einstein's famous E=mC^2) in quantum system, and therefore it's conservation. It describes the time-dependence of quantum mechanical systems. It is of central importance to the theory of quantum mechanics, playing a role analogous to Newton's second law in classical mechanics.

MWI has nothing to do with singularities or with Einstein's E=mc^2. Quite frankly this sounds like gibberish. Unless something sensible replaces it soon, it should be reverted.--CSTAR 06:20, 11 May 2006 (UTC)


 * Other problems with the new text:1. the first line is now a sentence fragment "the many worlds interpretation is how the equations." 2. what does HUP have to do with the measurement difficulties in the Copenhagen interpretation?  Fabartus, I can appreciate that you spent a lot of time on this, but please don't be so belligerent ("YOU DON'T THROW AWAY TWO HOURS of work and expect to avoid a fight CSTAR!!!").  Why don't you make a list of the problems with the current text, post them on the talk page, and suggest your revisions here.  With very long edits, if the new edit is not an improvement, it's a lot easier to revert it than to go through it and decide what should be kept and what should be excised, and then to incorporate the result smoothly into the text.  Therefore, if you're planning a long edit, make sure it's a good edit.  The way to do so is to take it to the talk page. -lethe talk [ +] 07:27, 11 May 2006 (UTC)

Michael D. Wolok's additions
If individual claims of Michael are unjustified, they should be deleted or edited sentence by sentence. It's very unpolite and aggressive to just revert the whole edit which does contain some interesting points. --DenisDiderot 09:59, 12 May 2006 (UTC)


 * If you think there are interesting points, then please extract them. The text is mostly wrong, and mostly badly written.  Restoring the whole text wholesale (including even Wolok's sig) is unacceptable. -lethe talk [ +] 10:01, 12 May 2006 (UTC)

Putting a signature into an article is a beginner's fault. We should be more tolerant towards beginners and avoid discouraging them just because of some minor mistakes. --DenisDiderot 10:32, 12 May 2006 (UTC)


 * Of course I agree with you that we should be tolerant to newbies. In fact, that's a wikipedia policy (WP:BITE).  However, this does not mean that we have to accept every addition that every newbie (nor well-seasoned editor) wants to make to the article.  If you want to comb through Wolok's edits to find valuable points that could be added to the article, I invite you to do so.  But the text as it stands entails a degradation of the article.  We do not suffer degradations of the encyclopedia for the sake of comfort of newbies who want to flout consensus and process. -lethe talk [ +] 11:03, 12 May 2006 (UTC)

problems with Wolok's text
Comments by lethe to version diff;
 * 1) Double slit is not a paradox and doesn't need MWI for resolution. -lethe talk [ +]
 * Feynman considers the observations of the double slit experiment paradox. See his Feynman Lectures on Physics.

Feynmann said the double-slit experiment embodies all the difficulties of quantum mechanics and is quantum mechanic's sole mystery. Prior to the Copenhagen Interpretation of quantum mechanics there were only waves and particles. According to the double-slit exeriment light does not travel like a particle or a wave. That is a paradox. Now, the word, "paradox" does not mean contradiction. Paradoxes can have solutions. Neils Bohr and Warner Heisenberg noted the paradox, and came up with the concept of "wave-particle duality" and "complementarity." They would not have had to invent these expressions if there was no paradox.

Feynmann told physics students who questioned the validity of these dubious expressions to "shut up and calculate" because he admitted there was no good explantion for the results of the double-slit experiment. Until Neils Bohr and Warner Heisenberg came up with these phrases, physicists believed light either had to be a wave or a particle. The MWI of QM completely eliminates any difficulty here. The MWI of QM resolves the apparant paradox without having to resort to something never observed and totally ad hoc: "wave-particle duality" and "complementarity."

Every new physics student has a hard time swallowing "wave-particle duality" and "complementarity." To refuse to see any problem here is to cover your eyes. And its not just physics students who have a problem swallowing "wave-particle duality" and "complementarity."

If "wave-particle duality" and "complementarity" are your cup of tea, then by all means drink them down. But if you are going to present MW, you ought to present the reasons why those who accept it do so. This is a commonly stated reason by those who accept MW for their belief. It may be wrong, you can give reasons why you think it is wrong, but if you are going to present the MW viewpoint you need to present the reasons why those who accept it do so.

". . . the argument for the many worlds was won with the double-slit experiment." --David Deutcsh

Even if I disagree with Communism, if I am writing an encyclopedic article on Communism, I need to include the reasons why those who believe in it do so.

As a theory Many Worlds is meaningless if you don't understand the reasons for its acceptance.

Michael D. Wolok 02:15, 15 May 2006 (UTC)


 * Every physics textbook considers the double slit experiment. This is not an article about fundamentals of quantum mechanics, it's an article about MWI. -lethe talk [ +]
 * 1) Ditto Schrödinger's cat -lethe talk [ +]
 * This is a paradox par excellence and it's exactly what MWI is about.
 * On this matter I disagree with you. -lethe

How are we going to have an intelligent discussion when we can't even agree on something so basic as this??? Even Roger Penrose who disagrees with MW accepts this paradox, and admits MW would solve it. It is ridiculous to be arguing something so elementry. Michael D. Wolok 02:15, 15 May 2006 (UTC)

talk [ +]
 * 1) Wave-particle duality is the same phenomenon as the above two points. Getting a bit redundant, isn't it? -lethe talk [ +]
 * 2) "Returns quantum mechanics to a classical theory" is patently false -lethe talk [ +]
 * It depends on the definition of "classical".
 * Well, can you propose a definition of "classical" which allows MWI to be considered a classical theory?

Yes. "Classical theory" is not my expression. When I first heard the expression "classical theory" I thought it meant "non-relativistic." I have conversed with dozens of PhD physicists who specialize in relativity, quantum mechanics or cosmology. I frequently hear them use the expression "classical theory." I always ask them what they mean, and they all give the exact same answer: a deterministic theory or a theory which is in general accordance with General Relativity. Now I know there is still a problem with quantum gravity and relativity, but that is not a direct assualt on relativity. If quantum mechanics implied there was no such thing as quantum gravity, that might be a problem. But the search for quantum gravity is still on. Einstein had many objections to QM, quantum gravity was not one. The MWI answers all Einstein's objections.

Michael D. Wolok 02:33, 15 May 2006 (UTC)


 * a return to classical realism is what is meant--Michael C Price 09:49, 13 May 2006 (UTC)
 * I know of no such definition. The most concise definition of "classical" that I could give would "algebra of observables is commutative".  Quantum mechanics does not satisfy this definition, and using the MWI interpretation does not save it. -lethe talk [ +]
 * 1) "reconciles quantum mechanics with general relativity" is also false.  This is the problem that quantum gravity attempts to solve.  Saying that MWI does so amounts to saying that MWI solves the problems of quantum gravity, which is wrong. -lethe talk [ +]
 * There are issues in the Copenhagen interpretation, in particular the concept of the immediate simultaneous collapse of the wave function, which are ignorant of the relativity of simultaneity. They don't appear in the MWI interpretation. By the way, the problems with relativity were mentioned by Everett himself as a justification for his Relative state theory.
 * You have not addressed the point. General relativity isn't about relative simultaneity, which is admittedly a thorny issue in QM.  General relativity is about the geometry of spacetime being dynamical.  -lethe talk [ +]
 * Of course there are other problems with general relativity unsolved by MWI or Copenhagen interpretation. So Michael's claim should be rendered more precise.
 * Propose a more precise version of Wolok's text then. But do not re-add the incorrect text.  We're here to improve this article, not to be welcoming to newbies (though we'll try to do both when possible).
 * 1) "restores causality"  causality was never lost in QM (despite the EPR paradox) -lethe talk [ +]
 * "Causality" should be changed to "determinism". I think that's what Michael means.

Michael Price is correct. For some stupid reason I always equate causality with determinism Michael D. Wolok 01:20, 15 May 2006 (UTC)


 * Well, quantum mechanics is nondeterministic. MWI does not change this fact,

How can we have an intelligent discussion with someone who seems not to have the slightest understanding of the MW? According to MW, the wave-function does not collapse. All you have is Schrodinger's equation which is entirely deterministic. According to MW, God does not play dice with the universe. Stephen Hawking was asked if man has "free will." He answered, "No." He said everything is determined and gave Hugh Everett's theory. There is nothing more basic to understanding Hugh Everett's theory than knowing it is a deterministic theory. Michael D. Wolok 01:20, 15 May 2006 (UTC)


 * yes it does. MWI drops the nondeterministic probability axiom and we are left with just the deterministic, unitary evolution of the state vector.--Michael C Price 09:56, 13 May 2006 (UTC)
 * so your modification makes the article incorrect.
 * 1) Capricious, randomness?  this statement has no meaning -lethe talk [ +]
 * There is randomness in quantum mechanics. It is reduced to a more fundamental determinism in MWI.
 * It is not true. -lethe talk [ +]
 * Lethe, you're incorrect, see my point about unitary evolution being deterministic.--Michael C Price 09:56, 13 May 2006 (UTC)
 * 1) Automatically generates Feynman's path integral?  In so far as that can mean anything, it's not true, or at least not particular to MWI.   -lethe talk [ +]
 * Feynman's path integral is a consequence of the principle of superposition, which is common to all interpretations.
 * My point exactly. Feynman's path integral is common to all interpretations, which renders its relevance in this article dubious. -lethe talk [ +]

I could keep listing things here, but what's the point? The whole lot is bad. Why are you defending this text? Please read it more carefully, Denis. -lethe talk [ +] 10:00, 12 May 2006 (UTC)
 * I just consider your behaviour unpolite and aggressive. The efforts of a new non-vandalizing user should be worshipped and be subjected to a rational critique. I think you should consider more carefully what Michael tried to say.
 * I'm sorry if my edits offended you. I appreciate that it is more welcoming to take in every newbies edits, but it's also very time consuming and it is not practical especially with very long edits.  This article has long been a battleground for nonscientific input, and if people who want their edits in the article cannot be bothered to respond to comments on their talk pages or engage in any kind of productive discussion towards consensus, then it's simply not worth the effort.  Now, rather than argue about whether I've been rude, why don't you propose some ideas for inclusion in the article?  Let's keep our eyes on the ball.  -lethe talk [ +] 10:46, 12 May 2006 (UTC)

By the way, what do you consider "unencyclopedic"? Do you consider Denis Diderot's Encyclopédie unencyclopedic? --DenisDiderot 10:23, 12 May 2006 (UTC)


 * I've never read that work, so I cannot comment on its encyclopedicness. I will note that material from the 1911 Encyclopedia Britannica usually has to be rewritten as it is considered too "flowery" for wikipedia. -lethe talk [ +] 10:46, 12 May 2006 (UTC)

FWIW, I'm with Lethe on this one. In general, any big sweeping additions to this article should be discussed here first. Stephen B Streater 17:22, 12 May 2006 (UTC)

The guidelines I read said add your additions to the article first. I, also, got this advice from Wikipedia administrators. Michael D. Wolok 01:08, 15 May 2006 (UTC)


 * This is fine, but if the changes are contentious, we restore the status quo ante while a consensus emerges. Stephen B Streater 07:17, 15 May 2006 (UTC)


 * The edits in question were just plain wrong. They were not supported, and are not part of some current alternate view. They were simply OR and for that reason alone had to go. WP:BITE does not mean we must pander to every crank that thinks he has rewritten the laws of physics; it means no one has the right to ridicule or belittle them for trying. --DV8 2XL 17:29, 12 May 2006 (UTC)

I can support every claim I made. I made a few claims that needed minor editing, but that is it. Michael Price understood what I meant. Michael D. Wolok 01:08, 15 May 2006 (UTC)


 * Exactly my point. Thank you DV8.  Now, Denis, if you think there were some good points mixed in with the bad, then please, bring them up on the talk page.  Keeping the whole lot, good with bad, is no way to improve the article.  Frankly, I'm having a hard time assuming good faith on your part; it seems like you are reverting to the bad version solely to spite me for my alleged rudeness.  Let's please put the health of the article before the sensibilities of the newbie.  If I was too harsh with Wolok, I do apologize.  -lethe talk [ +] 17:38, 12 May 2006 (UTC)

Don't forget that I didn't propose to keep everything but rather to do corrections (and deletions) of the individual points. I considered Michael's remarks about Wheeler's point of view quite interesting. I didn't know that Everett was systematically discouraged by Wheeler and others from publishing his theory. I'll say more about the other points later. --DenisDiderot 22:21, 12 May 2006 (UTC)
 * Interesting as it may be it, and everything else added to Wikipedia, is subject to WP:V. Anecdotes in particular should be subject to very close scrutiny, as they are vulnerable to embellishment. --DV8 2XL 22:35, 12 May 2006 (UTC)

For whatever it's worth, I agree with lethe's position here.--CSTAR 01:39, 13 May 2006 (UTC)

PS Some comments on Michael C Price's remarks. He's right about determinism. Realism is another matter. If he means realism in the sense of EPR, then I think he may be right. However, there is another problem with making this statement, is that MWI is itself incomplete, in a way (which I think is sometimes called the basis problem.) Namely, MWI can be viewed in the following way as a kind of graded structure


 * $$ H_{t_0} \rightarrow H_{t_1} \rightarrow H_{t_2} \rightarrow \cdots $$

of isometric mappings. The math of such structures may be related to semigroups of completely positive maps. However missing from this is a tensor product representation


 * $$ H_{t_{k+1}} = H_{t_k} \otimes K_{t_k} $$

which is necessary to recover various claims made by MWI. Thus I'm not sure MWI really solves anything about realism. --CSTAR

CSTAR, could you try explaining the "MWI basis problem" in English? -- I haven't a clue as to what you're trying to say. I know you're addicted to equations but, really, don't you think that's taking it a bit too far :-) AFAIK the MWI basis "problem" is normally presented (by such as John Bell) as the complaint that the Everett splitting is an artefact of the (eigen)basis chosen.  Quite a silly objection (but then Bell had a lot of silly things to say about MWI), since all measurable entities are basis independent.--Michael C Price 00:34, 14 May 2006 (UTC)
 * Re: AFAIK the MWI basis "problem" is normally presented (by such as John Bell) as the complaint that the Everett splitting is an artefact of the (eigen)basis chosen.
 * Well, isn't that the same thing?


 * How can I (or anyone else) tell, since you haven't defined your terms? For starters what is
 * $$ H_{t_k} $$ ??
 * and
 * $$ K_{t_k} $$ ??

--Michael C Price 09:02, 14 May 2006 (UTC)
 * Anyway as far as it's being a silly objection, I don't think it is, because it means the Everett splitting is non-canonical.
 * Re:since all measurable entities are basis independent. All observables of course are coordinate independent.
 * Agreed. This is what I meant.--Michael C Price 09:02, 14 May 2006 (UTC)

This means that it is possible to assign a probability measure, canonically on R by particular kinds of measurements associated to the observable. But "measurements" (that is, irreversible quantum operations) can be made wrt to other orthogonal decompositions, precisely since there are non-commuting observables.
 * The last sentence loses me. Yes, each measurement has an eigenbasis, but so what?  In analysing a problem you can work in any basis -- it's just that some bases are easier to calculate with than others.--Michael C Price 09:02, 14 May 2006 (UTC)
 * But anyway, I am certainly not suggesting this comment should go into the article or that your statement about realism shouldn't go into the article, since it is a commonly held view. However, I want to make it clear that I don't personally subscribe to this view of MWI for the reason I just mentioned. However, this is not the venue to discuss this. Yes, I am weaseling out of an argument:)
 * PS What's this I'm addicted to equations business? I didn't know there was such a kind of addiction. I hope that you're not suggesting, by use of code language (equations = obscurantism) I'm addicted to something else:) --CSTAR 00:58, 14 May 2006 (UTC)
 * I am suggesting that the way you use equations amounts to obscurantism -- quite unintentionally, I'm sure. Taking the time to explain things in English or semi-English will make your arguments much more accessible to others.  I find that I learn from trying to express myself with the minimum of equations.--Michael C Price 09:02, 14 May 2006 (UTC)
 * Good point. But talk pages on WP proceed at a fast and furious pace. I don't think there is one Wiki interaction style on talk pages, but I see them as one of successive collaborative approximation. Admittedly what I said was not clear, but I'm trying to get something down on the page and hopefully clarify it later.--CSTAR 16:33, 14 May 2006 (UTC)
 * PS I'm referring to writing on talk pages. Of course, writing something in an article is a different matter. The only requirement for a talk page is that it be related to the writing of the article and not contain obscenities or slurs (personal, racial etc). But anyway I'll try to explain more precisely what I wrote above, in the next few days. --CSTAR 16:51, 14 May 2006 (UTC)


 * Re: What I meant in plain English (FWIW). Well this isn't plain English, and I'm not sure I can give a description in plain English.  However, in perhaps plainer language:


 * An Everett splitting is a tensor product decomposition


 * $$ H_{k+1} = H_{k} \otimes \mathbb{C}^2 $$


 * which corresponds to a binary splitting of the observation tree. Such decompositions are not canonical. --CSTAR 02:30, 14 May 2006 (UTC)
 * Define your terms please, and I might be able to comment, but at the moment I don't follow that statement at all.--Michael C Price 09:02, 14 May 2006 (UTC)


 * I rather naïvely assumed that each split was into infinitely many alternatives. Stephen B Streater 06:58, 14 May 2006 (UTC)


 * Well, just iterate a binary splitting an infinite number of times.--CSTAR 07:08, 14 May 2006 (UTC)


 * This is different though. For a start, there are only countably many eigenvectors with non-zero eigenvalues (each showing the possible result of an experiment with its associated probablility), but an infinite number of binary splits will give uncountably many worlds. And in the binary split alternative, the worlds will follow on one after another, whereas in MWI it goes straight there. Stephen B Streater 07:59, 14 May 2006 (UTC)
 * Well, that's a good point. ALthough, in my original tensor product decomposition, the factor Hilbert space $$ K_i$$ (which is specialized in the example to a two-dimensional one)is not necessarilly two dimensional. Moreover, the uncountably many worlds given by an infinite number of splits is a continuum of worlds which would correspond to a continuous observable.


 * Re: Whereas in MWI it goes straight there. I am assuming that the splittings may occur at infinitesimal times from each other. Agreed this requires some mathematical elaboration to make sense. However, I think of it analogously to the Wiener process which is a random walk with moves taking place at infinitesimal times.--CSTAR 16:29, 14 May 2006 (UTC)
 * I donno about that last passage CSTAR - you're treading on dangerous ground. --DV8 2XL 16:37, 14 May 2006 (UTC)
 * Why is it dangerous? --CSTAR 16:39, 14 May 2006 (UTC)
 * PS this view of Wiener process is not new. --CSTAR 16:41, 14 May 2006 (UTC)
 * Because every time the term 'infinitesimal' comes up, I get deeply suspicious. I prefer 'instantaniously' as it gets rid of those pesky things. --DV8 2XL 17:04, 14 May 2006 (UTC)
 * See for instance S. Albeverio, JE Fenstad, R. Hoegh-Krohn, and TL Lindstrom, Nonstandard Methods in Stochastic Analysis and Mathematical Physics, Academic Press. --CSTAR 17:09, 14 May 2006 (UTC)

Wheeler nor any other physicist discouraged Everett from writing his thesis. In fact, Wheeler put his name on Everett's Thesis. Everett left the field of physics because of the initial, almost universally negative reaction to his thesis by other physicists. Even John Wheeler, his thesis advisor, turned on him. These are facts, that are highly relevant. John Wheeler's later objection that the Many-Worlds Interpretation "carries too much metaphysical baggage," is a famous quote. Michael D. Wolok 01:08, 15 May 2006 (UTC)

Feynmann said the double-slit experiment embodies all the difficulties of quantum mechanics and is quantum mechanic's sole mystery. Prior to the Copenhagen Interpretation of quantum mechanics there were only waves and particles. According to the double-slit experiment light does not travel like a particle or a wave. That is a paradox. Now, the word, "paradox" does not mean contradiction. Paradoxes can have solutions. Neils Bohr and Warner Heisenberg noted the paradox, and came up with the concept of "wave-particle duality" and "complementarity." They would not have had to invent these expressions if there was no paradox.

Feynmann told physics students who questioned the validity of these dubious expressions to "shut up and calculate" because he admitted there was no good explanation for the results of the double-slit experiment. Until Neils Bohr and Warner Heisenberg came up with these phrases, physicists believed light either had to be a wave or a particle. The MWI of QM completely eliminates any difficulty here. The MWI of QM resolves the apparent paradox without having to resort to something never observed and totally ad hoc: "wave-particle duality" and "complementarity."

Every new physics student has a hard time swallowing "wave-particle duality" and "complementarity." To refuse to see any problem here is to cover your eyes. And its not just physics students who have a problem swallowing "wave-particle duality" and "complementarity."

If "wave-particle duality" and "complementarity" are your cup of tea, then by all means drink them down. But if you are going to present MW, you ought to present the reasons why those who accept it do so. This is a commonly stated reason by those who accept MW for their belief. It may be wrong, you can give reasons why you think it is wrong, but if you are going to present the MW viewpoint you need to present the reasons why those who accept it do so.

". . . the argument for the many worlds was won with the double-slit experiment." --David Deutcsh

Even if I disagree with Communism, if I am writing an encyclopedic article on Communism, I need to include the reasons why those who believe in it do so.

As a theory Many Worlds is meaningless if you don't understand the reasons for its acceptance.

Michael D. Wolok 02:15, 15 May 2006 (UTC)

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Ditto Schrödinger's cat -lethe talk +

This is a paradox par excellence and it's exactly what MWI is about.

On this matter I disagree with you. -lethe How are we going to have an intelligent discussion when we can't even agree on something so basic as this??? Even Roger Penrose who disagrees with MW accepts this paradox, and admits MW would solves it. It is ridiculous to be arguing something so elementary.

Michael D. Wolok 02:15, 15 May 2006 (UTC)

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Well, can you propose a definition of "classical" which allows MWI to be considered a classical theory?

Yes. "Classical theory" is not my expression. When I first heard the expression "classical theory" I thought it meant "non-relativistic." I have conversed with dozens of PhD physicists who specialize in relativity, quantum mechanics or cosmology. I frequently hear them use the expression "classical theory." I always ask them what they mean, and they all give the exact same answer: a deterministic theory or a theory which is in general accordance with General Relativity. Now I know there is still a problem with quantum gravity and relativity, but that is not a direct assault on relativity. If quantum mechanics implied there was no such thing as quantum gravity, that might be a problem. But the search for quantum gravity is still on. Einstein had many objections to QM, quantum gravity was not one. The MWI answers all Einstein's objections.

Michael D. Wolok 02:33, 15 May 2006 (UTC)

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"restores causality" causality was never lost in QM (despite the EPR paradox) -lethe talk +

"Causality" should be changed to "determinism". I think that's what Michael means.

Michael Price is correct. For some stupid reason I always equate causality with determinism

Michael D. Wolok 01:20, 15 May 2006 (UTC)

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FWIW, I'm with Lethe on this one. In general, any big sweeping additions to this article should be discussed here first. Stephen B Streater 17:22, 12 May 2006 (UTC)

The guidelines I read said add your additions to the article first. I, also, got this advice from Wikipedia administrators. Michael D. Wolok 01:08, 15 May 2006 (UTC)

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> The edits in question were just plain wrong. They were not supported,

I can support every claim I made. I made a few claims that needed minor editing, but that is it. Michael Price understood what I meant. Nobody asked me to support my claims. Support for my claims do not belong in the article. I would think there is near universal acceptance for my claims even from those who don't accept Hugh Everett's Many World's thesis.

Michael D. Wolok 01:08, 15 May 2006 (UTC)

By the way, the reason my name appeared in the article was someone advised me to put four ~ after entry. I misunderstood them and thought they meant in the article since I hadn't added anything to the discussion page. It was a simple misunderstanding. I still don't know how to operate here. I wish we had a better way of communcating with each other. I am used to posting on message boards.

Michael D. Wolok 02:48, 15 May 2006 (UTC)

Greetings.
Greetings Michael,

I just spent five hours answering all Lethe's objections. I then did some operation that inadvertantly caused my Wikipedia page to go to another website, and lost all my work.

Sometime later this week I will try to rewrite what I wrote.

In the meantime, I would appreciate it if Michael Price would email me at:          MichaelDWolok@aol.com

Michael, you might remember that I last emailed you a few days before 9/11.


 * Aha, I knew I heard from you before.... Good to hear from you again.  I agree with your points about MWI.  We'll get them onto Wiki eventually, I'm sure.  Click on my sig and you should get my newly created user talk page --Michael C Price 09:13, 15 May 2006 (UTC)


 * What would help you would be some trustworthy independent sources which you can cite. See WP:OR. Stephen B Streater 09:34, 15 May 2006 (UTC)

You might recall, I predicted that sometime in the next twenty years Islamic fundamentalists would nuke Manhattan. I think 9/11, and its consequences may have made that possiblity a lot less likely now.

I am trying to understand how Wikipedia gives Lethe final say on what appears in this article.

Cheers, Michael


 * I left some links on your talk page which might help explain how things work here. -lethe talk [ +] 01:18, 15 May 2006 (UTC)

Collapse of the wave function
In response to an earlier claim: Schrodinger's equation equation is linear and so cannot explain all the effects we see. In fact, things would tend to spread out and blur without the collapse of the wave function bringing things back into focus. So MWI does not remove the need for the wave function to collapse. It just says that it collapses into all possible values each time instead of just one. Stephen B Streater 08:09, 15 May 2006 (UTC)

That's a very strange way of expressing MWI, and one that lends itself to misinterpretation. The fact is that it is not necessary to assume wave function collapse to explain our observations -- this is the whole point of MWI. What Everett/MWI explains is how we can explain our observations without resorting to wave function collapse.--Michael C Price 12:56, 15 May 2006 (UTC)


 * I agree with MichaelCPrice. There is no collapse in MWI. The formation of an entangled state is a linear operation. See my explanation in the end of edit on the article I did today. --DenisDiderot 14:13, 15 May 2006 (UTC)


 * MWI is formally no different than QM with wave function collapse as proposed by von Neumann (although he didn't have the machinery of completely positive superoperators to express it).  At the risk of being called an equation addict here's why:


 * The dynamics of a QM system whose Hilbert space is H is given by a family
 * $$ P_{t,s} $$
 * of propagators on the (mixed) state space of the system. This means each operator Pt, s is a  quantum operation (see linked article for the definition)  on the trace class operators on H and the Pt, s is a markovian family; Markovian means that the future can be determined from the present.  The Markovian property is true for arbitrary (reversible or non-reversible) time evolution of the mixed state of the QM system. (Remark: This is not quite true. There are non-markovian "evolutions" which do arise in nature, but these aren't germane to this discussion because they aren't dealt with, in any way that I know of at least, by MWI.)


 * Mathematically this is expressed by the propagator equation for the definition of which see the article on time evolution. Now by the Stinespring factorization theorem, each element Pt, s of the propagator family can be viewed as a map obtained by a splitting of H (which does not have to be binary, as User:Stephen B Streater has pointed out) as is shown in the article. This is actually a very geometrical view of MWI.


 * So in some sense it's true that there is no "wave-function collapse" because the the completely positive maps disappear in the new formalism. It is replaced by an isometric embedding of the system state space S into a quantum superposition of many copies of S.  However, this is completely equivalent mathematically with original formalism with wave-function collapse.
 * --CSTAR 17:12, 15 May 2006 (UTC)
 * PS. I believe this is essentially the view of the somewhat maligned (and deceased) Asher Peres.--CSTAR 17:24, 15 May 2006 (UTC)

Is the double-slit experiment a paradox?
"The Copenhagen interpretation of quantum theory starts from a paradox." --Heisenberg, Physics and Philosophy, pg. 44.

Michael D. Wolok 19:25, 17 May 2006 (UTC)

Is the double-slit experiment a paradox?

Wikipedia says so under the heading: The Double-Slit experiment. It refers to the "Double-Slit Paradox." It then gives the Copenhagen answer to the paradox. It doesn't give the Many Worlds interpretation to the paradox. It therefore seems to me that the article favors the Copenhagen Interpretation over Many Worlds. It even gives the transactional interpretation of Many Worlds.

I wish someone would add the Many Worlds solution.

More Cheers, Michael

Michael D. Wolok 13:58, 15 May 2006 (UTC)

Added the MWI viewpoint to the double-slit page. --Michael C Price 02:20, 17 May 2006 (UTC)


 * My view is that it is not a paradox. How can reality be a paradox? Stephen B Streater 08:25, 17 May 2006 (UTC)

Reality, my friend, is filled with paradoxes!!!

Get a dictionary!!!! Please note the difference in meaning between the two words: "paradox" and "contradiction." A paradox is NOT a contradiction. A paradox is merely a SEEMING contradiction. Paradoxes often have solutions. Contradictions do not. It is hard to have a high-level intellectual conversation about a very difficult subject with people who don't possess a high-school level vocabulary. The double-slit experiment presents a paradox because up to the CI of Quantum Mechanics it was assumed that matter-energy is either a particle or a wave. The double-slit experiment seems to suggest that matter-energy is not a particle nor a wave. This presented physicists with a paradox that needed an answer. The CI interpretation is one answer--an answer that many physicsts feel is ad hoc. Many-Worlds presents a more natural solution, but one that requires the presumtion of many universes whose existence cannot be verified. Since these universes cannot be verified many physicists refuse to accept Many Worlds. On the other hand, no one can verify what happens beyond the Event Horizons of black holes. That part of black-hole theory isn't falsifiable. Many Worlds is a conjecture. It could be wrong. But it seems to many physicists to be the best interpretation of QM to date. It justifies the anthropic principle, and thereby solves the fine-tuning problem. Without Many-Worlds, the fine-tuning problem looms large. Many-Worlds does everything else I claimed.

If you read the Wikipedia entry under Feynman's path--integral, sum-over-histories approach to QED, he makes clear that light does not actually take every path, that his method of calculation is just a mathematical trick.


 * "he"? Where does it say light doesn't take every path?  I've had a quick look at the interpretation section of the PI entry and it seems balanced.--Michael C Price 13:52, 17 May 2006 (UTC)

If Many-Worlds is true, then Feynman's calculation method is not a mathematical trick. Feynman's sum-over-histories has often been refered to as a "mathematical curiosity." If Many-Worlds is correct is not just a mathematical curiosity, it is literial. Michael D. Wolok 13:11, 17 May 2006 (UTC)


 * Paradox is described here. The double slit experiment only appears to be a paradox to people with sub-standard intuition based on incorrect theories of physics.

I guess that would include Heisenberg!!!! "The Copenhagen interpretation of quantum theory starts from a paradox." --Heisenberg, Physics and Philosophy, pg. 44. Why don't you read the definition of PARADOX given by your link!!!! Einstein object to wave-particle duality. Feynman claimed it could not be understood. 19:25, 17 May 2006 (UTC)


 * You mean, anyone who doesn't subscribe to MWI? :-) --Michael C Price 13:52, 17 May 2006 (UTC)


 * I wouldn't base an encyclopaedia definition on the supposed lack of accurate intuition of some people. Stephen B Streater 13:37, 17 May 2006 (UTC)

Nonlinear?
The most recent edits inlcude the statement
 * MWI describes measurements as a formation of an entangled state which is a perfectly linear process without any collapse of the wave function.

I read this assertion (perhaps erroneously) as suggesting that wavefunction collapse is non-linear. However, wave-function collapse, as an operator on density matrices, can be extended uniquely to a positive linear operator on trace-class operators. It thus is reasonable to call it a linear operator.--CSTAR 19:31, 15 May 2006 (UTC)
 * Yes, with hindsight that's true. The wave function collapse is a projection and thus a linear operator. But this projector is chosen somehow at random from a large set of possible projectors, and if you include this "choice", the whole measurement operation can hardly be considered linear. You need a much more involved mathematical structure for describing this, which is not the case for MWI. There is also another important distinction from the normal time evolution in quantum mechanics by unitary operators: The latter is completely reversible while the wave function collapse is not.


 * Yes, there it is clear that wave-function collapse is non-reversible . But I don't see how noticing in "hindsight" that something is linear justifies calling it (or suggesting that it is) nonlinear.--CSTAR 20:11, 15 May 2006 (UTC)


 * It is also true that the effect on density matrices of "wave function collapse" is perfectly deterministic (and linear.) This is one reason the density matrix formalism is so useful.--CSTAR 21:01, 15 May 2006 (UTC)


 * Without hindsight, you don't know which projector is chosen, so you need a random variable to describe the process. But the set of possible events and the probability distribution of that random variable depends on the state on which it acts: Consider a system in an eigenstate $$|a\rangle$$ of the measurement operator in question. Since the result is certain, the probability distribution has a single $$\delta$$-peak on that value. The same is true for another eigenstate $$|b\rangle$$, but the peak is located at $$b$$. For the superposed state $$\alpha |a\rangle + \beta |b\rangle $$ you get still another probability distribution which does not depend linearly on the coefficients $$\alpha$$ and $$\beta$$ but quadratically on their absolute values and in a more complicated manner on their relative phase. Thus you have to deal with a complicated structure involving linear, quadratic and more complicated dependencies if you consider the whole picture and try to describe it mathematically. The density operator also depends quadratically on the coefficients, not linearly! --DenisDiderot 22:07, 15 May 2006 (UTC)
 * The density operator doesn't really reflect the wave function collapse, because it carries contributions of different amplitudes on through the calculation even though the Copenhagen interpretation assumes only one of them to be present after the collapse.
 * Another nonlinearity of the collapse is already obvious to the beginner: The renormalization of the state, e.g. if you have the state $$|\psi\rangle = \alpha |a\rangle + \beta |b\rangle $$ before the measurement and $$|a\rangle$$ and $$|b\rangle$$ are normalized, the collapse in a measurement with result a changes it to $$|\psi'\rangle = |a\rangle = \frac{1}{\alpha} P_a |\psi\rangle $$ (with the projector $$P_a$$ on the a-eigenspace), which involves a reciprocal dependency -- this can hardly be considered linear behaviour! --DenisDiderot 22:23, 15 May 2006 (UTC)


 * If you write things out in terms of density matrices, it is linear.--CSTAR 22:46, 15 May 2006 (UTC)


 * It is true that if you perform a measurement of a system observable A (where A has finite discrete spectrum for simplicity of argument) with the system in a pure state, then one can regard the outcome as a classical probability measure on pure states; moreover, the corresponding density matrix is just a convex combination of the pure states (where the weights are the probabilities).
 * However, the resulting density matrix depends linearly on the source density matrix.--CSTAR 22:36, 15 May 2006 (UTC)


 * No, that's absolutely wrong. If you have a superposition of two situations, the resulting density matrix is not a linear superposition of the two individual density matrices. For a simple example, let's consider the density matrices of some pure states $$|a\rangle, |b\rangle $$ and their superposition
 * $$ | \psi \rangle = \alpha |a \rangle + \beta |b \rangle $$
 * They read:


 * $$|a \rangle \langle a|, |b \rangle \langle b|$$ and
 * $$|\psi \rangle \langle \psi| = |\alpha|^2 |a \rangle \langle a| + \alpha^* \beta |b \rangle \langle a| + \alpha \beta^* |a \rangle \langle b| + |\beta|^2 |b \rangle \langle b| $$


 * What you have proved above is that the mapping which associates to each state
 * $$| \psi \rangle $$
 * the corresponding density matrix
 * $$|\psi \rangle \langle \psi| $$
 * is not linear. Well yeah, I never disputed that (I never even mentioned it). However, if you express evolution in terms of the density matrices, that dependence is linear. That is the claim I am making. BTW he fact that you might be able to express a mapping as a composition of nonlinear ones, does not prove it's nonlinear. --CSTAR 01:09, 16 May 2006 (UTC)


 * This is not a linear dependency on the source density matrices. The coefficients enter in a quadratic an mixed form and there are the additional matrices $$|b \rangle \langle a|, |a \rangle \langle b|$$. The same is true for the unitary time evolution whose linearity is not reflected in the density matrix formalism. Think of a time evolution that changes $$|a\rangle$$ to $$|\psi\rangle$$ and you'll see there's no linear dependency on the source density matrix in this evolution either. --DenisDiderot 23:47, 15 May 2006 (UTC)


 * Re: Your comment The same is true for the unitary time evolution whose linearity is not reflected in the density matrix formalism. If the (unitary) time evolution of a quantum mechanical system is given by the family of unitary operators U(t) on the system Hilbert space, where t denotes time, then the time evolution of density matrices for the same system is given by


 * $$ U(t) \tau U(t)^\dagger $$


 * which is linear in the density matrix &tau; .--CSTAR 02:01, 16 May 2006 (UTC)


 * I admit that the time evolution is linear in the density matrix, but don't forget that if you add two density matrices you don't get the density matrices for the quantum superposition of the two described situations. For example if you have two density matrices of pure states $$|a \rangle \langle a|, |b \rangle \langle b|$$, the sum is the density matrix of the incoherent mixture of these two states, not that of the superposition which would amount to the term mentioned above. Furthermore, if you use complex coefficients in your addition, you won't usually get a valid density matrix. So the formal linearity you mention is of a completely different character than the one quantum mechanics is normally about when we are talking of physical quantum superpositions of states or situations. In order to be more specific, I'll add "in terms of quantum superpositions" to the linearity claim in the article. --DenisDiderot 08:05, 16 May 2006 (UTC)


 * Re but don't forget that if you add two density matrices you don't get the density matrices  Pf course, but I never said this. Yes, well at last we agree on something. :) --CSTAR 14:55, 16 May 2006 (UTC)

Also the most recent edits should replace entirely the older section (that follows it) "a simple example" or be incorporated to it somehow.--CSTAR 19:31, 15 May 2006 (UTC)

Isn't the problem with the disputed sentence simply that it says "linear" where it should say "unitary"? -- BenRG 14:49, 16 May 2006 (UTC)
 * Probaby not since wavefunction collapse is arguably both linear and unitary, although not deterministic! --Michael C Price 02:01, 17 May 2006 (UTC)
 * I think "linear in terms of quantum superpositions" is clearer and more physical. --DenisDiderot 15:05, 16 May 2006 (UTC)

If everyone agrees that mythical wavefunction collapse is unitary -- it is norm-preserving -- then I'll remove the nonunitary tag from the von Neumann collapse description. Anyone object? --Michael C Price 09:04, 17 May 2006 (UTC)
 * Since the collapse of the wavefunction is supposed to be irreversible, it technically cannot be unitary. --DenisDiderot 09:10, 17 May 2006 (UTC)
 * But reversibility is not part of the definition of unitary (is it?) -- I thought unitary referred to the preservation of a vector's norm (as a consequence of the adjoint of a unitary operator being it's inverse). Hence time evolution is unitary if the norm is preserved and, as a consequence, probability is conserved.  Collapse doesn't violate conservation of probability because it includes a renormalisation.--Michael C Price 09:36, 17 May 2006 (UTC)
 * A linear operator that preserves the norm is automatically reversible:
 * You can see that quite intuitively: A linear non-invertible operator must have at least once the eigenvalue 0 (otherwise you can easily find the inverse starting from the Jordan normal form when it has only nonzero eigenvalues on the diagonal). The norm of the corresponding eigenvector wouldn't be preserved. Actually, unitary (= norm-preserving) operators only have the eigenvalues $$e^{i \phi}$$ and can always be diagonalized, so all unitary operators look like this (in a suitably chosen vector basis):
 * $$\begin{bmatrix} e^{i \phi_1} & 0 & 0 & 0 \\ 0 & e^{i \phi_2} & 0 & 0 \\ 0 & 0 & \ddots & 0 \\ 0 & 0 & 0 & e^{i \phi_n} \end{bmatrix}$$--DenisDiderot 10:09, 17 May 2006 (UTC)


 * on a purely mathematical note, the above is not quite right. isometries need not be invertible in general. they become invertible, hence unitary, only when their range is dense. although for finite dim case, isometry is same as unitary. Mct mht 11:33, 17 May 2006 (UTC)
 * Agreed, I missed that. The standard definition for unitarity is $$A^\dagger A = 1$$, which implies isometry, reversibility and the eigenvalues being phases. --DenisDiderot 17:11, 17 May 2006 (UTC)


 * so invertibility is indeed part of definition for unitarity. norm-preserving does not imply unitary. wave-function collapse is clearly not unitary. besides being mathematically obvious, physically, it increases entropy, hence irreversible. Mct mht 12:01, 17 May 2006 (UTC)

also, the statistics and outcomes of measurement can be treated simultaneously in one single quantum operation, which is a linear completely positive map. i think this was called an instrument by E. B. Davies in his book on open quantum systems. this gives a precise meaning as to what's meant by linearity. Mct mht 12:19, 17 May 2006 (UTC)

Wave function collapse
First, what would it mean to say wave-function collapse is unitary? Consider measurement of an observable A ehich has a basis of eigenvectors. There are at least two ways of describing wave function collapse associtaed to measuring A: (a) as a map from a pure state (described as a vector in a Hilbert space) to a discrte probability measure on pure states or (b) as a completely positive map (or quantum operation) on density matrices. How could this possibly be unitary? In either case, unitarity for such objects doesn't make sense a priori. Let us consider the description (b) as quantum operation. It is possible for a quantum operation P to be described by a unitary U (which is unique up to scalar) as follows: For any density matrix &tau;,


 * $$ P(\tau) = U \tau U^\dagger$$

The quantum operations described in this way are those that describe reversible evolution.

It is possible however to replace U with a (non-invertible) isometry on an infinite dimensional Hilbert space in which case it is certainly not possible to claim that P is described by a unitary. This descibes non-reversible non unitary "wave-function collpase". Note that the most general quantum operation can be represented by Kraus matrices (for which see article).--CSTAR 14:23, 17 May 2006 (UTC)

I think it's quite obvious from the above suggestions that the efforts to describe the wave-function collapse mathematically (let alone defining and experimentally showing when and why it's supposed to happen and when and why not) not mythically by an obscure concept as complementarity, are cumbersome and much more elaborate than the unitary formation of an entangled state in MWI which is just a special case of the standard unitary time evolution. Moreover, formations of entangled states are needed in quantum mechanics anyway, also in the Copenhangen interpretation. --DenisDiderot 17:05, 17 May 2006 (UTC)


 * Paradox is described here. The double slit experiment only appears to be a paradox to people with sub-standard intuition based on incorrect theories of physics.

I guess that would include Heisenberg!!!! "The Copenhagen interpretation of quantum theory starts from a paradox." --Heisenberg, Physics and Philosophy, pg. 44. Michael D. Wolok 19:11, 17 May 2006 (UTC)


 * Yes. Scientists at that time had limited intuition in sub-atomic affairs. Just as people who thought the Sun orbited the Earth thought the simple orbits described by a heliocentric model were paradoxical. Now our understanding has increased, these no longer appear paradoxical. The equations are consistent with the reality they describe. All these interpretations are attempts to find analogies using an impoverished language. Any paradoxes arise because of the paucity of the analogies, not the underlying theory. Stephen B Streater 18:50, 18 May 2006 (UTC)

Wave-Particle Duality Is A PARADOX
"The Copenhagen interpretation of quantum theory starts from a PARADOX." --Heisenberg, Physics and Philosophy, pg. 44.

`` [the double-slit experiment] has in it the heart of quantum mechanics. In reality it contains the only mystery´´ ---Richard Feynman

"...the way we have to describe Nature is generally incomprehensible to us" [p.77]. " ---Richard Feynman

"You're not going to be able to understand it" [p.9]. ---Richard Feynman

You had to know which experiment you were analyzing in order to tell if light was waves or particles. This state of confusion was called the "wave-particle duality" of light, and it was jokingly said by someone that light was waves on Mondays, Wednesdays, and Fridays; it was particles on Tuesdays, Thursdays, and Saturdays, and on Sundays, we think about it! It is the purpose of these lectures to tell you how this puzzle was finally "resolved." [p.23, note] --Richard Feynman  The word resolved is in quotes because Feynman admits the resolution really can't be understood.

'I think I can safely say that no-one understands quantum mechanics… Do not keep asking yourself, if you can possibly avoid it, "but how can it be like that?"… Nobody knows how it can be like that.' --Richard Feynman

"...the wave theory cannot explain how the detector makes equally loud clicks as the light gets dimmer. Quantum electrodynamics 'resolves' this wave-particle duality by saying that light is made of particles (as Newton originally thought), but the price of this great advancement of science is a retreat by physics to the position of being able to calculate only the probability that a photon will hit a detector, without offering a good model of how it actually happens." [p.37] --Richard Feynman

"Feynman himself must say that the light goes everywhere at once, follows all possible paths, which is something a single finite particle can't do, regardless of the probability of where it may be found by a detector (cf. p.46, about diffraction gratings). So the wave-particle duality is not "resolved."

"Nature has got it cooked up so we'll never be able to figure out how She does it: if we put instruments in to find out which way the light goes, we can find out, all right, but the wonderful interference effects disappear. But if we don't have instruments that can tell which way the light goes, the interference effects come back! Very strange, indeed!" [p.81] --Richard Feynman

In fact, both objects [i.e. electrons and photons] behave somewhat like waves, and somewhat like particles. In order to save ourselves from inventing new words such as "wavicles," we have chosen to call these objects "particles." [p.85] --Richard Feynman

I am not going to explain how the photons actually "decide" whether to bounce back to go through; that is not known. [p.24] --Richard Feynman

"The modern explanation of this PARADOX—the wave/particle duality—is that both light and particles have BOTH a wave nature and a particle nature." -- members.dancris.com/~bfraser/qm/qmconcpt.htm

How do atoms of Uranium "know" that enough other atoms have decayed to account for the statistical half-life of the isotope, and that they need to wait, perhaps for millennia, to decay? Again, it looks like they can't "know," but the individual events just happen to conform to the statistical average.

"The particle propagates in space in a distributed, probabilistic wavelike fashion but arrives at a detector as a localized, complete corpuscle. This PARADOXICAL conceptual framework has some explanations in the forms of the Copenhagen interpretation, Path Integral Formulation, or the Many Worlds Interpretation." --Wikipedia

"Now the problem facing physicists was how to explain this wave-particle duality. To paraphrase and extend Einstein's point, there were now two theories, not only of light, but also of matter. Both were indispensable, yet the two were without any logical connection."

" Einstein could not agree with Bohr. In 1954, a year before his death, he maintained: 'Like the moon has a definite position whether or not we look at the moon, the same must also hold for the atomic objects, as there is no sharp distinction possible between these and macroscopic objects. Observation cannot CREATE an element of reality like a position, there must be something contained in the complete description of physical reality which corresponds to the possibility of observing a position, already before the observation has been actually made.' "

" Indeed, Heisenberg generalized this rejection of causality from the quantum domain to the whole world of science: 'Because all experiments are subject to the laws of quantum mechanics, quantum mechanics definitely shows the invalidity of the causal laws.' "

Heisenberg later in life said, "I remember discussions with Bohr which went through many hours till very late at night and ended almost in despair; and when at the end I went alone for a walk in the neighbouring park I repeated to myself again and again the question: Can nature possibly be as absurd as it seemed to us in these atomic experiments?"

Einstein wrote, "a physical reality independent of substantiation and perception. It can be completely comprehended by a theoretical construction which describes phenomena in space and time. The laws of nature imply complete causality."

Einstein seems to imply the quantum mechanics violates causality. Causality means every cause has an effect, but in QM there cause for the exact time a particle decays or for it mode of decay.

"The point raised by von Neumann served to highlight what might be called the 'boundary problem.' Where should scientists draw the line between the measuring apparatus, which is part of the macro world, and the object being measured, which is part of the micro world? The logic of von Neumann is that the Copenhagen interpretation arbitrarily gives a privileged position to the observer in the construction of reality. All matter is made up of atoms and therefore subject to the laws of quantum mechanics and yet, somehow, the observer or measuring apparatus has a privileged position. No matter how much Bohr talked about the role of the observer, he never explained how such a beast could play this unique role."

"the assessment of the Copenhagen interpretation made by Murray Gell-Mann, when writing about the task of understanding quantum mechanics, is pertinent. Bohr and Heisenberg, says Gell-Mann, 'brainwashed a whole generation of physicists into thinking that the job was done 60 years ago'."

'The Heisenberg-Bohr tranquillizing philosophy - or religion? - is so delicately contrived that, for the time being, it provides a gentle pillow for the true believer from which he cannot very easily be aroused. So let him lie there.' -- Albert Einstein

http://www.prometheus.demon.co.uk/02/02kumar.htm

Michael D. Wolok 19:47, 17 May 2006 (UTC)


 * I guess wave-particle duality is a paradox in the sense of Banach-Tarski. Which is to say, not at all.  Rather, it's just a surprising and counterintuitive notion.  -lethe talk [ +] 21:23, 17 May 2006 (UTC)

Please look up the definition of "PARADOX." A paradox is not a contradiction. Moreover, Einstein, Schrodinger, Feynman, Penrose and others didn't think the Copenhagen Interpretation does a good job of answering the PARADOX. Michael D. Wolok 22:42, 17 May 2006 (UTC)


 * I have looked it up. There is a phrase "veridical paradox".  The Banach-Tarski paradox is a veridical paradox.  In other words, it's not at all paradoxical, just surprising.  Same goes for wave-particle duality. -lethe talk [ +]


 * Sorry for barging in here, but "a surprising and counterintuitive notion" is a paradox. . What other definition are we thinking of? -Will Beback 19:39, 18 May 2006 (UTC)


 * Something which may have been paradoxical half a century ago when these notions were first considered may be well understood now. The modern understanding is that the equations are correct and that attempts to describe small things as either uniquely wave-like or uniquely particle-like are inaccurate. In short, "Wave-particle duality WAS a paradox". This is also consistent with the quotes given, and so, logically, these quotes cannot be used to disprove my statement. Stephen B Streater 19:47, 18 May 2006 (UTC)
 * Paradoxes may be true, indeed they mostly are true. What makes them a paradox is their counterintuitive nature. It is impossible to "solve" a paradox, though it can be explained. What definition of "paradox" are you using? -Will Beback 21:22, 18 May 2006 (UTC)
 * "A paradox is an apparently true statement or group of statements that leads to a contradiction or a situation which defies intuition". Intuition improves over time as peoples' understanding improves. There is no concept of "solving" a paradox here, but rather a lack of misunderstanding as Physics advances. Stephen B Streater 21:31, 18 May 2006 (UTC)


 * How has the paradox disappeared? Are there references which say that the duality of light was once a pradox, but is no longer? Acocrding to whom has our intuition changed in the last 100 years? -Will Beback 21:41, 18 May 2006 (UTC)


 * I am not the one making the claim in the article. According to whom has our intuition not changed in the last 100 years? Stephen B Streater 11:03, 19 May 2006 (UTC)


 * Reply to Beback: my dictionary defines a paradox as something which is self-contradictory. Wave-particle duality is not a paradox by the definition, it's merely a surprising result of quantum mechanics.  Now there are other definitions of paradox which include surprising results., so you can call that a paradox.  People in the past who called it paradoxes probably thought that it actually seems self-contradictory to be wavelike and particlelike.  Luckily, we know that's not the case today.  The paradox has indeed disappeared. -lethe talk [ +] 13:19, 19 May 2006 (UTC)

I just spent the last two hours adding my response, but Wikipedia claim it could add my response because it lost my data.
 * You can add your replies in bits to ameliorate this problem. Stephen B Streater 06:17, 21 May 2006 (UTC)

The paradox has not disappeared. The paradox remains a paradox. It may not be a true contradiction. In order for it not to be a contradiction you have to accept the principle of "complementarity" which many physicists find problamtical or you have to accept the Many Worlds Interpretation of QM. Just because ***YOU*** don't find the principle of "complementarity" problematical does not mean wave-particle duality is not a paradox. Heisenberg himself called Wave-Particle Duality a paradox.

"People in the past who called it paradoxes probably thought that it actually seems self-contradictory to be wavelike and particlelike."

What an ignorant statement. Richard Feynman thought it really wasn't understandable. Nothing has changed that would have made Wave-Particle dualiy more understandable to Richard Feynman. Sometimes light acts like wave!!! It doesn't act wave-like. Sometimes it acts EXACTLY like a wave. Sometimes it acts exactly like a particle, not particle-like!!! Light NEVER acts wavelike or particlelike. How it acts depends upon how you choose to observe it. According to the CI interpretation it doesn't even exist between measurements as anything real. GET REAL! Between measurements light is neither a particle nor a wave, it is not wave-like, nor is it particle-like. This is at the very least paradoxical.


 * From the latest Merriam-Webster Collegiate dictionary:
 * Main Entry: par·a·dox Function: noun Pronunciation: 'par-&-"däks Etymology: from Greek paradoxon CONTRARY TO EXPECTATION 1 : a tenet contrary to received opinion
 * Clear QM is reveived opinion and so is not a paradox - according to the very first definition given below and duplicated here. Stephen B Streater 06:17, 21 May 2006 (UTC)


 * I was asked to take a look at this discussion. A general statement in an attempt to be helpful here. It is not up to us, as Wikipedians, to decide whether or not Wave-Particle Duality "is" or "is not" a paradox.  There is apparently some disagreement among experts (though I note that so far we have not seen any quotes from experts who say that this is not a paradox, or the paradox has been solved, or etc., etc., all of which sound perfectly sensible and possible to me, but we need sources I think).  If there is disagreement, we should simply report on it in some fashion.  It seems safe enough to report this neutrally, that many prominent physicists such as Feynman and Heisenberg have referred to this as a paradox.


 * Of course I also agree that a paradox is not the same thing as a contradiction, and I should add that I have no expertise in physics, and no intention to get involved in a content dispute, other than to offer the general guidance that Wikipedia itself need not concern itself with answering questions that baffle experts: we merely report.--Jimbo Wales 23:27, 22 May 2006 (UTC)


 * What a pleasant surprise to see you here. Simply reporting makes life a lot easier than it could have been. Stephen B Streater 06:16, 23 May 2006 (UTC)


 * Here's a quote from an expert saying EPR is not a paradox. Ray Streater is consulted on the Nobel Prize for Physics and his list of publications is here. I think he counts as a reliable source. If people are happy that this reference to paradox is the one we're talking about, I'll amend the article to reflect the contribution from Jimbo Wlaes. Stephen B Streater 09:12, 24 May 2006 (UTC)
 * Well, EPR is the "paradox" of measurement at one location "affecting" the outcome of measurement at a spacelike separated location, while wave-particle duality is the "paradox" that a quantum system can be a wave or a particle (i.e. you can choose to measure either one of the two noncommuting observables position and momentum). Well, while I'd bet that Streater père would be a guy who would agree that wave-particle duality is not a paradox (does it violate OR if you ask him to put up a webpage addressing that?), I am not sure that the nonparadoxical nature of EPR implies the nonparadoxical nature of wave-particle duality.  Am I missing something?  PS I enjoyed reading about cannabis and schizophrenia. -lethe talk [ +] 11:45, 24 May 2006 (UTC)


 * Good point about EPR - I should have read that more closely. He's expressed quite a few opinions on his pages, so I'll look more closely to see if he's thought this was important enough to comment on already. I'm slightly wondering about whether the whole debate centres on the meaning of the word "paradox" rather than any underlying thing about the nature of Quantum Theory. My main point was that understanding move on - Zeno's paradox of the arrow, for example, is based a misconception which is now well undersood. Stephen B Streater 12:43, 24 May 2006 (UTC)


 * Yes, I was thinking the same thing: this is really a semantic debate, as evidenced by the many demands to "check your dictionary". Paradox is just a word, and with the right definition, sure, wave-particle duality is a paradox, as is EPR.  The EPR paradox was thought by Einstein to violate the principles of special relativity, so he thought it was a true paradox, in the sense that contradictory things are both true (hence the name).  He was of course wrong, understanding moved on, QM is compatible with SR.  But with the right definition of "paradox", then sure, we can still call it a paradox.  The same goes for wave-particle duality (and Zeno, and Banach-Tarski, etc etc etc).  If Wolok wants to invoke a dictionary definition which allows to call it a paradox, then who am I to argue?  Semantic debates are never interesting and the can cause you to get off the point, so let's return to the point which tipped off this long discussion and caused Wolok to email Max Tegmark and Wikipedia's own god-king in a desperate search for support: while wave-particle duality is strange, it is not actually contradictory, and therefore does not require any resolution.  Therefore citing the resolution of wave-particle duality as a point in favor as a point in favor of MWI is vacuous.  But if Wolok wants to go over to the article wave-particle duality and list quotes which say that it's a paradox (à la Heisenberg) or just that it's hard to understand (à la Feynman), he is welcome to do so.  I don't see any need to go looking for quotes that say it is not a paradox.  We don't need to find quotes which says it is not paradoxical, we can simply get the "resolution" from any undergraduate or even layman's quantum theory book. -lethe talk [ +] 13:08, 24 May 2006 (UTC)
 * It's quite simple. Quantum mechanics is a paradox until you adopt an interpretation that resolves the underlying issues to your satisfaction.  Schrodinger's cat, Einstein's EPR, Wigner's friend -- these were all proposed as paradoxes.  Whether each gedanken experiment is a paradox is a subjective thing.  If you are happy with your interpretation then they are not paradoxes -- relative to you.  IMO MWI resolves all the quantum paradoxes -- that's why I'm a MWIer.  Obviously most non-MWIers won't agree, and I wouldn't expect them to.--Michael C Price 13:45, 24 May 2006 (UTC)

Feynman's sum-over-histories
Feynman's sum-over-histories is true for MW interpretations of QM, but it is not automatically generated by the CI. If you accept the CI, Feynman's sum-over-histories is just a mathematical quirk. According to the CI light does not actually take every path. According to MW it does. See how Stephen Hawking and Hartle use Feynman's sum-over-histories to create Quantum Cosmology, their No Boundary Proposal which is completely analagous to Hugh Everett's M-Ws.
 * Feynman's sum-over-histories or path integral approach to quantum mechanics/fields theory naturally lends itself to many-worlds, but you can't derive Feynman's approach from many-worlds. MW is a metatheory whereas sum-over-histories or PI is a physical theory.  MW makes no pronouncement about the weighting of each path, whereas the novel thing about the path integral approach is the weighting is just (with some qualifications) an imaginary phase angle, where the angle is proportional to the classical action.--Michael C Price 11:18, 18 May 2006 (UTC)

MWI is a theory
I don't see how the following statement (call it P) is justified:


 * MWI is also regarded as a theory by those MWI proponents who argue that all other, non-MWI interpretations, are actually inconsistent, illogical or unscientific in their handling of measurements.

How can one conclude something about MWI (that is, that MWI is a theory) based on facts about something else (that is, other interpretations are inconsistent). It may happen to be a coincidence that the statement is true, "it just so happens that P is true", but if so, it needs a citation.--CSTAR 15:18, 18 May 2006 (UTC)

Beause interpretation often has the implication of choice between alternatives. If there is no (credible) alternative then there is no choice; such a view becomes less one of interpretation and more one of fact. Which is why detractors of MW always view it as an interpretation and its proponents are less likely to view it as such. You may or may not agree with this reasoning, but it's certainly a fact that many MWers do, and that is all the quoted text is stating. --Michael C Price 15:53, 18 May 2006 (UTC)


 * Well OK, but I think we need to put in a citation at some point. I would have thought that the valid conclusion was that other interpretations were worthless or unscientific (as an example of the latter, I suppose we culd propose the "Angels do manipulations behind the scenes"; this is a kind of intelligent falling interpretation).--CSTAR 16:04, 18 May 2006 (UTC)

The sentence that follows P does act as a citation of sorts, since a metatheory is also a theory, viz:
 * Hugh Everett, MWI's originator, argued that his proposal was a metatheory, since it made statements about other interpretations of quantum theory.

Perhaps theory could be expanded to read theory or at least a metatheory in P, although this seems a bit cumbersome or redundant. --Michael C Price 16:25, 18 May 2006 (UTC)

MWI is not a theory. It is an interpretation of a theory which gives exactly the same predictions as all other (correct) interpretations of the theory and uses the same equations to make those predictions. Stephen B Streater 18:45, 18 May 2006 (UTC)


 * Even if your statement is true (which is debatable), the quoted statement is still true since it is a statement about the beliefs of those who hold a contrary opinion to you. Read it carefully.--Michael C Price 19:02, 18 May 2006 (UTC)


 * Read the title of this section. Carefully. Stephen B Streater 19:41, 18 May 2006 (UTC)


 * That's my fault. I put the current section title in this talk page, just to have a title for a new section. As a matter of fact, I am mostly agnostic on MWI (or rather, believe that it is just a mathematical formalism equivalent to many formalisms of wave function collapse, as I tried to explain above) As the article itself is written though I think it is currently NPOV as it should be. Do you see problems in it?--CSTAR 19:47, 18 May 2006 (UTC)


 * I haven't seen all the changes to the article, but the edits I have seen are much more NPOV than the talk pages, so I haven't seen any reason to edit the article itself. Stephen B Streater 19:54, 18 May 2006 (UTC)


 * MWI is more than just a mathematical formalism, because it avoids having to define under which circumstances and why the wavefunction collapse is physically supposed to happen, which is a very hard physical problem involving a lot of additional assumptions. Physically it's not equivalent at all to CI while mathematically, all non-contradictory theories on whatever subject are equivalent. --DenisDiderot 20:01, 18 May 2006 (UTC)
 * Re: all non-contradictory theories on whatever subject are equivalent.
 * Well of course, if you talk of "equivalence" in the sense of material implication yes. But other than this, what you said is certainly not true. There is a concept in model theory called conservative extension which relates formal theories T_1 and T_2 in terms of their models. I claim MWI is a conservative extension of any one of a number of theories of wavefunction collapse. --CSTAR 20:08, 18 May 2006 (UTC)


 * I'm sure that MWI is -- in the sense of model theory which I know from nonstandard analysis -- not equivalent to CI because it requires fewer physical assumptions that all have to be translated into mathematical axioms, if the description is to be complete. But usually the latter is not done, because it's hard to formulate rules like "the wavefunction collapses when our knowledge changes" or even "the wavefunction collapses when our potential knowledge changes" mathematically. Moreover, I mentioned in the article Hartle's discovery about Born's probability axiom being implied by MWI, which is not the case for CI. This is clearly a case of two non-equivalent mathematical models. --DenisDiderot 20:23, 18 May 2006 (UTC)
 * If you're so sure about this, then there's little point in my trying to convince you otherwise.--CSTAR 20:27, 18 May 2006 (UTC)
 * Stop metadiscussions (discussions about the discussion) right from the start! Stick to the subject! If you can disprove the arguments I mentioned, I'm happy to learn more. If not, it's fair enough to say so. --DenisDiderot 20:36, 18 May 2006 (UTC)
 * Exactly!--Michael C Price 23:25, 18 May 2006 (UTC)
 * In natural science the number of assumptions/axioms you need for a theory is decisive for the credibility of the theory. In mathematics on the other hand, all theories are only conjunctive constructs (answers to "What if...?" questions), so they don't need any credibility anyway. Nevertheless, even mathematicians favor concise axiom systems over cumbersome ones.


 * Speaking of mathematics: CI is not a mathematical model at all, because the following concepts are yet to be defined mathematically, among others:
 * complementarity
 * "our knowledge" that changes
 * the "our" in "our knowledge" (human beings, animals as well?, neurons?, beings with DNA?, conscious beings?)
 * "potential knowledge"
 * --DenisDiderot 06:32, 19 May 2006 (UTC)

Reply
 * I was speaking of a formalization of of CI based on the quantum operation model of wave-function collapse. As far as the three bulletted items, they are defined mathematically. Complementarity for instance can be quantified in any number of ways.  As is well-known (Omnès, 1999) the "origin of complementarity lies in the noncommutativity of operators" describing observables in quantum mechanics. Knowledge and potential knowledge as well in terms of some information theoretic notion.   See Chris Fuchs article qon quantum mechanics as information.
 * This handles only one aspect of the maldefined concept of "complementarity". Bohr used it also to describe the quantum and the supposed classical realm in a measurement. --DenisDiderot 15:55, 19 May 2006 (UTC)
 * Re :In mathematics on the other hand, all theories are only conjunctive constructs (answers to "What if...?" questions), so they don't need any credibility anyway.  That's one view of mathematics. But even with that view, any formal theory can (and should) be accompanied by a formal semantics. It is in that formal semantical sense that one can say two theories are equivalent (in the sense they have the same model theory) or other relations between theories (such as conservative extension).--CSTAR 13:44, 19 May 2006 (UTC)


 * As far as the three bulletted items, they are defined mathematically. Complementarity for instance What about the other bulleted items, though? They need defining in the CI -- which they don't in the MWI.  Trying do define when an observer triggers wavefunction collapse is a hopeless task.  Other issues with CI -- or any collapse variant of QM -- include the handling of imperfect and approximate measurements; do they imperfectly or approximately collapse the wavefunction?   --Michael C Price 14:12, 19 May 2006 (UTC)
 * Knowledge and potential knowledge can be defined as well in terms of some information theoretic notion as is done in quantum information theory.  See for instance, Chris Fuchs article on quantum mechanics as information and a little more.--CSTAR 14:16, 19 May 2006 (UTC)


 * Knowledge OK -- but wavefunction collapse not just the acquisition of knowledge, otherwise we would just have some new classical physics and no quantum paradoxes (e.g. particle interference fringes). What about defining observers, and what about imperfect and approximate measurements?--Michael C Price 14:32, 19 May 2006 (UTC)
 * That's a very good point, and here I think real divergence between the various theories/interpretations appear.  These however in my view are ontological questions (which I think are important) rather than questions about mathematical formalisms.--CSTAR 14:37, 19 May 2006 (UTC)
 * I agree -- it's good to see us on the same wavelength and communicating clearly; these are issues of physics not of formalism. One further point, Cramer's  transactional interpretation suffers from the same defects as the CI, it's just that "wavefunction collapse" is called "transaction formation".  And Bohm's hidden variables contains many-worlds since Bohm never collapses the wavefunction.  So really we are left with just MWI and hence the many-worlds theory. --Michael C Price 14:57, 19 May 2006 (UTC)


 * Information theory doesn't suffice. Take the neutron scattering example of the Feynman Lectures on Physics where a spin flip makes the scattering amplitude incoherent. --DenisDiderot 14:44, 19 May 2006 (UTC)
 * You got me there, I don't see this. In particular, "Information theory doesn't suffice" ... to do what? In addition quantum information measures are much different from classical information measures. An n-qubit space has 2n+1 - 2 real dimensions. An n bit space is discrete, although it can be considered as a vector space over Z mod 2 of dimension 2n.--CSTAR 14:54, 19 May 2006 (UTC)
 * Feynman's neutron scattering example shows that decoherence doesn't have anything to do with "our knowledge" and thus with the information theoretical construct you mentioned. Moreover, you didn't show the connection between that construct and the physical collapse of the wavefunction. Mentioning some branches of mathematics at random doesn't solve the problem at hand. To be definite, let's first define what a measurement is in CI. In MWI it's just a cascade of entanglements. Nothing more, nothing less. --DenisDiderot 15:51, 19 May 2006 (UTC)

Everett offered an elegant definition: a measurement-like interaction is an interaction between any two systems that induces correlations between their states. We may designate one system the object and one the observer, it really doesn't matter. Nowadays we called such coupled correlations entanglement. --Michael C Price 16:35, 19 May 2006 (UTC)

Max Tegmark's reply to my email.
Professor Max Tegmark, Ph.D. Dept. of Physics, MIT

70 Vassar Street Rm. 37-626B

Cambridge, MA 02139

'''Subject: Re: Greetings Earthling! Prof. Max Tegmark

'''Date: 5/19/2006 9:54:51 A.M. EDT

'''From: tegmark@MIT.EDU

'''To: MichaelDWolok@aol.com

CC: tegmark@mit.edu

Hi Michael,

''' Thanks for your message. I'm too swamped by various '''deadlines right now to respond in detail to your MWI questions. . .''' > I claimed Hugh Everett's Many Worlds Interpretation

> offers the following advantages over the Copenhagen

> Interpretation.

> 1. It more simply and more naturally resolves the

>     paradox of wave-particle duality.

I agree. > 2. It justifies the anthropic principle.

I agree, but only partially, since Level III adds nothing new over Level II - see http://space.mit.edu/home/tegmark/multiverse.html

> 4. It automatically generates Feynman's sum-over-histories.

>     The Copenhagen interpretation does not generate

>     Feynman's sum-over-histories.

> 5. In other interpretations Feynman's sum-over-histories is

>     a mere mathematical quirk, because in these interpretations

>     every path is not really taken.

I don't quite agree here, since many of these histories are far from semiclassical "parallel universes". > 6. It simply explains Schrodinger's Cat paradox.

Certainly. > 7. It returns Quantum Mechanics to a deterministic theory.

>     God does not play dice. By doing this, it makes

>     Quantum Mechanics more compatible with relativity

>     which along with all other scientific theories are a

>     deterministic theory.

Agreed. > 8. It eliminates the problem of trying defining what exactly

>     constitutes "measurement."

Agreed. > 9. It eliminates Von Neumann's boundary problem: where

>     to draw the line between the micro world where Quantum

>     Mechanics works, and the Macro world where it doesn't. Agreed. > 10. It eliminates the special place for an observer and

>      human consciousness.

Agreed. > 11. It restores objective reality to the universe between

>      measurements.

Yes. > It seems Einstein's main objection with Quantum mechanics

> had to do with the Copenhagen Interpretation and not the

> theory itself.

I agree.

'''Best wishes,

''';-)

'''--

'''Prof. Max Tegmark

'''Dept. of Physics, MIT

'''70 Vassar Street Rm. 37-626B

Cambridge, MA 02139

Wave-Particle Duality Paradox
The paradox has not disappeared. The paradox remains a paradox. It may not be a true contradiction. In order for it not to be a contradiction you have to accept the principle of "complementarity" which many physicists find problamtical or you have to accept the Many Worlds Interpretation of QM. Just because ***YOU*** don't find the principle of "complementarity" problematical does not mean wave-particle duality is not a paradox. Heisenberg himself called Wave-Particle Duality a paradox.

"People in the past who called it paradoxes probably thought that it actually seems self-contradictory to be wavelike and particlelike."

What an ignorant statement. Richard Feynman thought it really wasn't understandable. Nothing has changed that would have made Wave-Particle dualiy more understandable to Richard Feynman. Sometimes light acts like wave!!! It doesn't act wave-like. Sometimes it acts EXACTLY like a wave. Sometimes it acts exactly like a particle, not particle-like!!! Light NEVER acts wavelike or particlelike. How it acts depends upon how you choose to observe it. According to the CI interpretation it doesn't even exist between measurements as anything real. GET REAL! Between measurements light is neither a particle nor a wave, it is not wave-like, nor is it particle-like. This is at the very least paradoxical.

The most respected and authoritative dictionaries of American English are The American Heritage Dictionary, the latest Merriam-Websters Collegiate Dictionary, and Merriam-Webster's Third International Dictionary. They define the word paradox as follows:

From the latest Merriam-Webster Collegiate dictionary:

Main Entry: par·a·dox  Function: noun  Pronunciation: 'par-&-"däks Etymology: from Greek paradoxon CONTRARY TO EXPECTATION 1 : a tenet contrary to received opinion 2 a : a statement that is seemingly contradictory or opposed to common sense and yet is perhaps true b : a self-contradictory statement that at first seems true c : an argument that apparently derives self-contradictory conclusions by valid deduction from acceptable premises 3 : one that possesses SEEMINGLY contradictory qualities or phases

From the American Heritage Dictionary

par·a·dox   ( P )  Pronunciation Key  (pr-dks)  n. A SEEMINGLY contradictory statement that may nonetheless be true: the paradox that standing is more tiring than walking. One exhibiting inexplicable or contradictory aspects: “The silence of midnight, to speak truly, though apparently a paradox, rung in my ears” (Mary Shelley). An assertion that is essentially self-contradictory, though based on a valid deduction from acceptable premises. A statement contrary to received opinion.

[Greek paradoxon CONFLICTING WITH EXPECTATION]

Not all dictionaries are created equal. "Believe It or Not," some publishers print dictionaries just to make a buck. They hire dozens of graduate students with degrees in any subject to concoct definitions as fast as possible. It is amazing to me that any intelligent person would rely on a definition from such a dictionary.

There are only two dictionaries of American English with full-time lexigraphers. They are Merriam-Webster and the American Heritage Dictionary. Both have 100+ luminaries reknown for their command of the English language that oversee and participate in their frequent redactions.

It is necessary to know the difference between "paradox" and "contradiction" to answer correctly one of the questions on many SAT exams.

Nothing has changed in the last half-century that makes wave-particle duality any less a paradox.

In the words of Albert Einstein, Bohr and Heisenberg have brainwashed a generation of physicists not to see any difficulty with Quantum Mechanics.

Roger Penrose, Stephen Hawking's thesis advisor and a world reknown quantum physicist" points to the paradoxes of Quantum Mechanics and claims that it is quite possible that Quantum Mechanics is just plain wrong and will have to be replaced by a better theory.

See Max Tegmark's repsonse to my addition that Lethe disputed.

>> I claimed Hugh Everett's Many Worlds Interpretation

>> offers the following advantages over the Copenhagen Interpretation.

>> 1. It more simply and more naturally resolves the

>> paradox of wave-particle duality.

>

> I agree.

In presenting a theory you have to present the POV of its adherents.
In presenting a theory you have to present the POV of its adherents. If you don't your presentation is not complete.

It is one thing is make sure an article has a neutral point of view. It is quite another to prevent any editor from giving any reason why anyone would accept the theory. Suppose we did not know if the Earth was flat or round. Further suppose in an imaginary Wikipeida article on "The Theory of a Round Earth, some editors try to list the reasons of those who think the Earth is round for favoring this theory. But other editors refuse to allow these reasons in the article because they claim it violates Wikipedia's NPOV policy, and because these reasons are just plain wrong.  Whether these reasons are correct or not, they should be in the article.


 * A NPOV article should state that the views of respectable people in the area, and state that it is an opinion. In this case, it is also happens not to be a scientific view, as it is untestable. But it is a valid, albeit minority, interpretation of the theory. Stephen B Streater 06:30, 21 May 2006 (UTC)


 * Since no other interpretation is scientifically coherent (see points raised about the flaws about the CI and Cramer's and Bohm's interpretations raised a couple of days ago) then every test that validates wave-particle duality tests many worlds. -- MCPrice


 * I must have missed the bit where you proved that no other interpretation is scientifically coherent. I thought you only referred to currently known interpretations. And you may have missed my point below questioning the need for "interpretations", when the equations are the consistent, non paradoxical interpretations. Stephen B Streater 15:15, 21 May 2006 (UTC)


 * Yes, of course I only referred to the currently available interpretations. Obviously any theory may be overturned by new empirical or theoretical developments; that didn't seem worth pointing out.  To be pedantly exact: I am claiming that many-worlds is the only currently credible scientific quantum theory.  As for your point below questioning the need for "interpretations", when the equations are consistent, well the CI has inconsistent equations (collapse vs unitary evolution with ill-defined domains).  Same criticism applies to all collapse theories/interpretations (such a Cramer's).  Bohm's hidden variables postulates the same unitary dynamics for the wavefunction as Everett, but since the wavefunction never collapses it contains Everett-worlds anyway. -- Michael C Price 17:06, 21 May 2006 (UTC)

Those who favor Hugh Everett's theory see a paradox in wave-particle duality. They believe Many-Worlds does a better job explaining the paradox than the does CI. If you want to add that those who follow the CI don't see any paradox at all in wave-particle duality, I have no problem with that statement. The princiiple of complimentarity is a creation of the CI, without it, there is an apparant contradiction, unless you are able to offer some other explanation. Michael D. Wolok 23:20, 20 May 2006 (UTC)


 * This clearly presents a major flaw in the "adherents" view. Reality is described by the equations. All experimental results in the domain of QM are accurately predicted. There is no need for another "explanation". Hence no need for a paradox. Any paradox arises entirely out of arbirary and innaccurate infliction of POVs about what reality "should" be like on interpretation of the meaning of the equations. Do the paradox people claim GR is a paradox because it does away with the concept of well ordering of events in time, or because as a deterministic theory it does away withh free will, which "clearly" people have? Stephen B Streater 06:30, 21 May 2006 (UTC)


 * Reality is not completely described in CI's equations, since it states only in nebulous words when certain events like the collapse are supposed to happen. MWI does away with such ambiguities and clearly shows mathematically when and why decoherence is observed. --DenisDiderot 23:02, 21 May 2006 (UTC)

OK. My main problem now is the claim that there is no wave function collapse in MWI. Do you have a cite for this from a reliable source which I can see online? Stephen B Streater 14:05, 22 May 2006 (UTC)
 * This is really a serious problem. Obviously you haven't understood the topic you're editing, otherwise you would know the answer to your question. Read Everett's original article first. --DenisDiderot 14:40, 22 May 2006 (UTC)
 * Now you know why I'm not editing the article - I'm leaving that to others. So there is no problem. I studied standard QM where obviously collapse of the wave function is important. Also I've read half a dozen popular science books which cover MWI. My impression from these was as I've stated above: the wave function still collapses, but the world splits at the point when it collapses with one branch per possible outcome. This is different from what you are claiming. Do you have an on-line copy of your source which is apparently Everett's original article so I can verify your claim and straighten out any misunderstanding? Stephen B Streater 15:09, 22 May 2006 (UTC)
 * OK. Someone here's just explained it. I might read what the article says these days and add his explanation for discussion if it is clearer. Stephen B Streater 15:18, 22 May 2006 (UTC)
 * The whole point of MWI is to avoid the problems of the collapse -- when, why and how it's supposed to happen. In MWI, measurements are cascades of entanglements and thus just a special case of the standard unitary time evolution. Everything else, including the concept of parallel worlds, is just a necessary consequence of this single idea and the standard superposition principle of quantum mechanics. There is no collapse at all. Check out the section "An illustrative example" in the article for an explanation. --DenisDiderot 16:07, 22 May 2006 (UTC)

One lesson to draw from this is that the MWI article isn't clear enough on the no-collapse <==> many worlds identity. We all need to make it clearer. Michael C Price 16:42, 22 May 2006 (UTC)
 * The lesson might also be to read the article more carefully before posting. --DenisDiderot 16:47, 22 May 2006 (UTC)
 * This is certainly not the lesson. I am going to take an independent look at the article. That is why I have been discussing MWI with you on the talk pages. Once I know what's what, I'll check the article. Stephen B Streater 17:04, 22 May 2006 (UTC)
 * Re: Now you know why I'm not editing the article - I'm leaving that to others. So there is no problem.
 * Stephen, you did edit the article several times. Why don't you tell the truth? This is a problem! --DenisDiderot 16:47, 22 May 2006 (UTC)
 * I did edit it but as I said I am not editing it now. Which edit do you currently have a problem with? My only edit this month seems to be this one: - enforcing WP:OR policy. Do you have a problem with enforcing WP:OR? My April edit still stands, apart form a spelling mistake. I'd also prefer it if you didn't edit my posts to the talk pages. Please revert your last change to my comments. And if you continue to break WP:NPA and WP:AGF it will reflect badly on you. Stephen B Streater 18:38, 22 May 2006 (UTC)

Speculative Fiction
This section currently reads:

From the point of view of quantum mechanics, these stories however are deficient for at least two reasons:

''There is nothing inherently quantum mechanical about branching descriptions of historical events. In fact, this kind of case-based analysis is a common planning technique and it can be analysed quantitatively by classical probability.  The use of historical events complicates matters by introduction of an issue which is generally believed to be completely extraneous to quantum theory, namely the question of the nature of individual choice''

Surely both these deficiencies are incorrect? All historical events are intrinsically quantum mechanical, since all events are quantum mechanical. By the same token the question of free-will is not outside the domain of quantum mechanics, although I wonder why it is mentioned here.

I suggest that this section is substantially rewritten, which I'm happy to do if no one else volunteers or objects.

--Michael C Price 21:10, 23 May 2006 (UTC)
 * Well it's my fault. In my defense, what was there before was a hodge-podge of stuff from science fiction stories etc., which are now (mostly) in the parallel universe page. Although I still would argue that some phenomena can be accounted for well without QM (for example, planning using an event tree or branching in stochastic processes) and some of the science fiction "examples" would fall into this class or involve modal realism. But please feel free to change it. If someone has a disagreement with what you write, it will surely be brought up and talked about here.--CSTAR 21:19, 23 May 2006 (UTC)


 * OK, replaced above text with According to many worlds most of the historical speculations entertained within the alternative history genre are realised in parallel universes.--Michael C Price 05:36, 24 May 2006 (UTC)


 * Looks good to me.--CSTAR 05:43, 24 May 2006 (UTC)

PS. I'm puzzled why Jimbo Wales dropped by on the talk page. Though there is the occasional spat here in the talk page, as far as the article is concerned, it's NPOV as far as I can tell. AM I missing something? --CSTAR 13:36, 24 May 2006 (UTC)


 * I believe that Wolok must have emailed Tegmark and Wales in an attempt to get support for his text. I haven't been able to read Wolok's extensive and badly formatted posts here, so I haven't engaged in a discussion about his text. -lethe talk [ +] 13:46, 24 May 2006 (UTC)


 * Perhaps this is just where the action is on WP at the moment ;-) Stephen B Streater 13:56, 24 May 2006 (UTC)

Zeno's Paradoxes are still Paradoxes
Zeno's paradoxes would still be paradoxes today even if they were all answered. But as a matter of fact, many mathmematicians today believe Zeno's paradoxes still have no completely satisfactory explanation. See the Wikipedia entry on "Zeno's Paradox." I can also cite many article written on Zeno's Paradox by leading mathematicians. Michael D. Wolok 23:20, 24 May 2006 (UTC)


 * Well, maybe, but is this really an issue of major disgreement in this article? Is there something specific in the article you object to? Talk pages are mostly for discussions about writing the article. It doesn't seem to me there is major disagreement on anything currently written in the article.--CSTAR 23:26, 24 May 2006 (UTC)
 * PS I don't consider myself an MWI proponent but I think the article is NPOV.--CSTAR 23:28, 24 May 2006 (UTC)

Atten: Supporters of NPOV.
I have added the following text to the article:

Reasons Given By Those Who Favor Many Worlds

Those who favor Hugh Everett's Many Worlds Interpretation claim Hugh Everett's theory offers the following advantages over the Copenhagen Interpretation:

1. It more simply and more naturally resolves the paradox of wave-particle duality.

2. It simply explains Schrodinger's Cat paradox.

3. It returns Quantum Mechanics to a deterministic theory. God does not play dice. By doing this, it makes Quantum Mechanics more compatible with relativity which along with all other scientific theories are completely deterministic.

4. It eliminates the problem of trying defining what exactly constitutes "measurement."

5. It eliminates Von Neumann's "boundary problem": where to draw the line between the micro world where Quantum Mechanics works, and the Macro world where it doesn't.

6. It eliminates the special place for an observer and human consciousness.

7. It restores objective reality to the universe between measurements.


 * I think there is a lot of overlap between the above reasons. It could be summarised as something like, MWI removes the observer-dependent role in the quantum measurement process, see for example Schrodinger's Cat paradox.  There are already good explanations of MWI on the other WP pages; I know, I've been adding them :-).  All we need to do is link to them.  We must try to avoid verbose repetition of other pages; that defeats the whole point of hypertexting.  It also makes the MWI page harder to read.--Michael C Price 08:01, 25 May 2006 (UTC)

It seems Einstein's main objections with Quantum Mechanics had more to do with the Copenhagen Interpretation, than with the theory itself. While Hugh-Everett's Many-Worlds does not quite generate the kinds of worlds necessary to justify the anthropic principle, it is a step on the way to Stephen Hawking's No Boundary Proposal and Max Tegmark's All Universe Hypothesis which do justify the anthropic principle.

If Lethe or some other editor removes this text, I ask you to put it back. Apparently, this is the way Wikipeida works. My addition has the explicit support of Max Tegmark. And Jimbo agreed with me on the meaning of the word "PARADOX". Please note that the word "PARADOX" is synonymous with the word "riddle." If anybody finds something "puzzling" it is a "paradox." Anything that at first glance is hard to explain is a "paradox." Just because something has a valid explanation does not meant it is not a paradox. I will get Mr. Wales more involved in this article if necessary. Lethe, your continuing gratuitous ad hominem attacks are uncalled for. If you keep attacking me personally, I will report your conduct to Mr. Wales. Michael D. Wolok 23:55, 24 May 2006 (UTC)


 * I think you've misjudged what Jimbo's role in this project is. Reporting my conduct to Mr. Wales won't get you very far.  He doesn't resolve content disputes, nor does he resolve policy disputes.  I thought we had a content dispute at this article, but now you've accused me of personal attacks, a violation of policy.    I do apologize if anything I've said seemed to be a personal attack.  It was not intended as such.  Maybe you're referring to my comment that your posts are badly formatted?  Well, I don't think that's really a personal attack.


 * So if you want to report me for making personal attacks, you should visit the personal attack intervention noticeboard. You can also file a request for comment, a request for mediation, and finally a request for arbitration against me, in that order.  The last one is kind of the supreme court of wikipedia.  If you think I've made personal attacks, that is your recourse, not bothering Jimbo.


 * Finally, Jimbo's opinion (or mine, for that matter) about what "paradox" means is pretty irrelevant. -lethe talk [ +] 01:02, 25 May 2006 (UTC)


 * I think the point of Jimbo's intervention was to tell us how to resolve disputes - find reliable sources and cite what other people have said, not make it all up ourselves. Stephen B Streater 08:48, 25 May 2006 (UTC)

Citations?
Einstein made numerous objections to Quantum Mechanics. Each one of these objections completely evaporates under Hugh Everett's Many-Worlds Interpretaion. For example, Einstein said, "God does not play dice with the universe." Under Hugh Everett's theory the universe is deterministic and not random. Einstein had a major problem with objective reality disappearing between measurements. This problem only arises in the CI. It does not arise in the Many-Worlds Interpretation. I think Michael Price can help me out here.
 * Noted. But the citation is for the claim that "EInstein objected only to Copenhagen". This needs an outside source as per WP policy.
 * My understanding is that Einstein merely disliked CI's indeterminism, but that his fundamental objection was to having the role of observation hard-wired into the physics, instead of emerging from the physics. In a letter to Schrodinger I believe, although I'd have to check.  But Einstein died before Everett published so how he would have reacted to MWI is conjectural.--Michael C Price 09:30, 25 May 2006 (UTC)

As to Hugh Everett's Many Worlds leading to a theory that does justify the anthropic principle, I cite the following article: http://space.mit.edu/home/tegmark/multiverse.html
 * Well the please put it in the refernces. We need to set up footnote format.

Hawking and Hartle's No Boundary Proposal is exactly analagous to Hugh Everett's Many-Worlds. I don't see how it is possible to dispute this. I can't see how it needs a citation.


 * I dispute it, but rather than explain why, the point is that other people dispute it, and therefore it needs a citation. WP is about consensus, not your or my viewpoint. Purely from a pragmatic POV, uncited opinions will be removed by someone else with a different opinion; it's a waste of time adding them.  The only statements that will persist are neutral or very well-cited ones.  Remember WP is not the place for original research, no matter how obvious and elementary. --Michael C Price 08:08, 25 May 2006 (UTC)

Hugh Everett's theory posits "Matter-Energy" taking every possible quantum path available leading to countless universes. Hawking and Hartle's No Boundary Proposal posits "Space-Time" taking every possible quantum path available leading to countless universes. Is it really hard to see the connection between the two theories?

"On the other hand, Feynman's imaginary time is meant to explain quantum mechanics, which by many is said to predict all other equally possible worlds, which is known as the Many Worlds Theory." everythingforever.com/hawking.htm

"One reason why many cosmologists now take the anthropic principle seriously is that the "many worlds" interpretation of quantum mechanics seems to be the only sensible context in which to discuss "quantum cosmology"


 * I think this is the wrong way around. Those that take the anthropic principle seriously do so because of the evidence for fine tuning.  That may lead them to take MWI seriously.  --Michael C Price 08:29, 25 May 2006 (UTC)

- the branch of physics that tries to describe what happened near the big bang. As emphasized by Jim Hartle, quantum theory allows many mutually incompatible histories. However, it only makes sense to consider the initial conditions that led to the classical behaviour that we observe today. (With complete ignorance of the initial conditions, the quantum fluctuations could be arbitrarily large and the emergence of a classical world would not be possible.) Within this restriction, quantum cosmology allows many different worlds or "branches", all with different values of the constants, and this validates the strong anthropic principle."

physicsweb.org/article/world/14/10/3

Michael D. Wolok 00:32, 25 May 2006 (UTC)
 * Cite at least a reference to Hawking and Hartle's No Boundary Proposal.

--CSTAR 00:38, 25 May 2006 (UTC)

See above.

Perhaps, Michael Price could help me out here. I will continue to look for other cites.

"Purported" carries a negative connotation
The word "purported" carries a negative connotation. As such it runs contrary to Wikipedia's NPOV policy. 64.12.116.67 04:34, 25 May 2006 (UTC) Michael D. Wolok 05:29, 25 May 2006 (UTC)


 * I'm afraid you're imagining things here.--CSTAR 04:41, 25 May 2006 (UTC)

Pros must proceed Cons
Before you cite problems with the MWI, you should give the reasons why people favor it in the first place!!! 64.12.116.67 04:36, 25 May 2006 (UTC) Michael D. Wolok 05:28, 25 May 2006 (UTC)

I am happy with "Advantages of MW" but . ..
I am happy with Advantages of MW but. . . that seems to favor the MWI. Some physicists may disagree with one or more of the items I enumerated saying they are not true. A more objective heading is: "Reasons given by those who favor the theory." I think all the reasons listed are valid reasons for favoring MW, but I make allowances for others who have an opposing viewpoint. Michael D. Wolok 05:28, 25 May 2006 (UTC) 64.12.116.67 04:54, 25 May 2006 (UTC)


 * The text below the heading makes this very clear.--CSTAR 04:56, 25 May 2006 (UTC)

Michael Price
You should be at the forefront making changes to this article. —The preceding unsigned comment was added by 64.12.116.67 (talk • contribs).


 * I appreciate the sentiment, but comments like this, directed to a specific user and of no interest to the wider public, should be directed at my talk page, rather than clutter up the MWI discussion page. Click on anyone's signature to get their user and thence talk page.  BTW Michael (Wolok), do you read your own talk page?  They're invalauble for everybody, including yourself.  Find "my talk" at the top of your WP page; go there and I can offer suggestions more easily there. When I hear from you I'll delete this section from the discussion page. --Michael C Price 07:48, 25 May 2006 (UTC)


 * Micheal C Price, if you want to access Wolok's talk page, it is at User talk:Michael D. Wolok. It was in fact created quite some time ago.  You can't get there directly, since he doesn't include a link to it in his signature.  Instead, you have to click the redlink for his user page, then click the discusssion tab at the top.  But then, we have to do the same thing to access your own talk page since your signature also links to your userpage, not to your user talk page.  So remember: user talk page, if you want to talk.  -lethe talk [ +] 08:23, 25 May 2006 (UTC)
 * Thanks for the correction. I've amended my previous text to reflect this --Michael C Price 08:38, 25 May 2006 (UTC)

CSTAR quit this.
"Quantum mechanics becomes a deterministic theory making it more compatible with the theory of relativity and all deterministic theories."

I wish to make clear in this point that all other theories in physics are completely 100% deterministic, and contain no randomness.
 * Yes, but doesn't mean that MW is more compatible with "other theories" such as relativity.--Michael C Price 07:24, 25 May 2006 (UTC)

CSTAR keeps changing my point. CSTAR if you want to make revisions in my addition, discuss them here first.
 * Well, you added the section and posted it here afterwards. Isn't C* just following the same MO?  --Michael C Price 07:24, 25 May 2006 (UTC)

According to MWs:

"Quantum mechanics becomes a completely deterministic theory making it more compatible with the theory of relativity and all other theories in physics which are deterministic and not random." By introducing randomness and indeterminism, the CI makes quantum mechanics depart from EVERY orther scientific theory to date.


 * This is not true AND far to partisan for WP. First deterministic and indeterministic theories are not incompatible.  Second, other indeterministic theories had been proposed before (Newton proposed one such for light; I doubt he was the only one).  Third, given that indeterministic results appear in the lab, the idea that an indeterministic theory is required is quite natural.--Michael C Price 07:24, 25 May 2006 (UTC)

This is a major reason not to accept the CI, and to accept MW. By removing these fact, this reason becomes less of a reason than it is. I also believe Einstien's famous quote is relevant here: "God does not play dice with the universe." This quote clearly evinces Einstein's displeasure with the CI, and it reveals how Hugh Everett's MWI causes Einstein's objection to QM to evaporate. Michael D. Wolok 05:27, 25 May 2006 (UTC)

"God does not play dice with the universe."
Quantum mechanics becomes a deterministic theory making it more compatible
 * I don't believe the compatibility argument. At the very least it needs a cite.--Michael C Price 09:17, 25 May 2006 (UTC)

with the theory of relativity and all other physics theory to date which are all deterministic. The Coppenhagen Interpretation introduced indeterminacy and randomness into science. Aside from the Coppenhagen Interpretation of Quantum Mechanics there is no scientific theory that includes indeterminacy or randomness. Einstein particularly objected to this aspect of the Coppenhagen Interpretation. In response to it, he said, "God does not play dice with the universe."

I would be happy if anyone could reword this point without losing content. Michael D. Wolok 05:50, 25 May 2006 (UTC)


 * But most of the remaining substantive points are already made in the article's opening sentence, viz: The many-worlds interpretation (or MWI) is an interpretation of quantum mechanics that rejects the non-deterministic and irreversible wavefunction collapse associated with just measurement in the Copenhagen interpretation; instead in MWI the conventional deterministic and CPT-reversible laws of quantum physics that govern the dynamics of unobserved systems describe measurements or observations as well.
 * And
 * MWI proponents ... argue that all other, non-MWI interpretations, are actually inconsistent, illogical or unscientific in their handling of measurements.


 * There is really no need for the repetition of these points within the article; it actually makes it less readable. --Michael C Price 09:16, 25 May 2006 (UTC)

This Modern World
Here is absolute proof of time travel! Here is proof some people alive today are actually living in the past:

dancris wrote: "The modern explanation of this PARADOX—the wave/particle duality—is that both light and particles have BOTH a wave nature and a particle nature."

Gee, and all this time, I thought this was Neils Bohr's "Principle of Complimentarity" which Richard Feynman found impossible to comprehend. Could you possibly explain how the modern explanation differs from Neils Bohr explanation? Or maybe it is as Hitler said, if you repeat a big lie often enough people will without questioning it accept its truth.

Richard Feynman said in his famous lectures: "You had to know which experiment you were analyzing in order to tell if light was waves or particles. This state of confusion was called the 'wave-particle duality' of light, and it was jokingly said by someone that light was waves on Mondays, Wednesdays, and Fridays; it was particles on Tuesdays, Thursdays, and Saturdays, and on Sundays, we think about it! . . . 'I think I can safely say that no-one understands quantum mechanics… Do not keep asking yourself, if you can possibly avoid it, 'but how can it be like that?'… Nobody knows how it can be like that."

By the way, dancris, no one who knows anything about physics says particles have a wave nature. Roger Penrose and many others explain that this is an incorrect way at looking at things.

[User:Michael D. Wolok|Michael D. Wolok]] 06:06, 25 May 2006 (UTC)


 * Citing Roger Penrose is not a good idea. Despite his contributions to physics many of his more philosophical pronouncements (especially on the mind and QM) are widely regarded as cranky.--Michael C Price 07:30, 25 May 2006 (UTC)


 * Here is a more humorous example. Stephen B Streater 08:32, 25 May 2006 (UTC)
 * "Penrose must err to believe this sentence." :-) --Michael C Price 10:03, 25 May 2006 (UTC)

MWI first rejected
Hugh Everett left the field of physics because of the almost universal negative reaction his theory received at the time. At first Hugh Everett's theory was rejected by most physicists. In fact, for the most part, few physicists even heard of his theory. His thesis became forgotten and buried.
 * Exactly. It was ignored, not rejected.  Very different things. --Michael C Price 07:38, 25 May 2006 (UTC)

Only with the failure of super-gravity and other attempts to fully explain quantum mechanics did his theory slowly gain popularity. It really didn't catch on till the advent of quantum cosmology which is virtually built on this theory.
 * This is conjecture and I doubt it is true. The pros and cons of MWI are evident even at the non-relativistic level of Schrodinger's equation; they have nothing to do with either super-gravity or quantum cosmology, except insofar as such researchers tend to be very familar with quantum theory.  MWI's reasons for acceptance and rejection by physicists go much deeper.


 * Also remember Planck's law -- "A new scientific truth does not establish itself by its enemies being convinced and expressing their change of opinion, but rather by its enemies gradually dying out and the younger generation being taught the truth from the beginning".  --Michael C Price 07:38, 25 May 2006 (UTC)

How can any article on Hugh Everett's Many Worlds fail to tell the story of its early rejection and its growing acceptance. Of course, those who oppose the theory don't want this information out because it shows the trend, which is greater and greater acceptence from utter rejection.
 * That sounds like paranoia. Be careful. Those who oppose MWI think the polls are merely irrelevant.--Michael C Price 07:38, 25 May 2006 (UTC)

Michael D. Wolok 06:37, 25 May 2006 (UTC)

Eisntein seems to come up a lot. Was Einstein aware of MWI, or is this just conjecture that he would have liked it? SR does not propose zillions of universes after all. "I'm sure Einstein would have supported ME" needs some proof. Stephen B Streater 09:01, 25 May 2006 (UTC)


 * Einstein died in 55, Everett published in 57. Even if there had been overlap, I understand that Einstein became quite disconnected from developments in quantum theory later in life. -lethe talk [ +] 13:45, 25 May 2006 (UTC)