Talk:Equivalence principle/Archive 2

Inertial motion
Can someone suggest what is inertial motion. —Preceding unsigned comment added by Superkan619 (talk • contribs) 13:26, 3 March 2008 (UTC)

Disputed tag
I removed the disputed tag (which I added). I am more-or-less happy with the present state of the article, and there doesn't seem to be much discussion about what to do on the talk page, so it seemed like the reasonable thing to do. Of course, the article could still be improved, but I think it is fairly accurate.

If anyone disagrees, please feel free to reinstate the tag. --Joke137 15:33, 3 May 2005 (UTC)


 * I suppose I should reinstate the disputed tag then.


 * In the physics literature the WEP is sometimes described as the newtonian assumption of the universality of free fall (UFF), and sometimes as a relativistic assumption. The current article is blurry on how the WEP is to be understood.


 * The current article skips explaining what the equivalence principle is about. Instead, it begins in three separate places to talk at length about highly specialistic matters: different flavors of the EP. The current article is, to my regret, very exclusive, it is written for readers who already have a thorough understanding of the equivalence principle.


 * I have put into the article history a version of the article as I envision it, and then I backed out. EMS has made it quite clear why he reverted: EMS wrote: "That is GR stuff, and the EP is not GR".


 * It appears that EMS chooses to understand the equivalence principle as a strictly newtonian principle, a choice that places the EP logically outside general relativity. It appears that in EMS' opinion the EEP and SEP are confined statements, about inertial frames of reference, making EEP and SEP logically separate from general relativity. (Separate from GR since the EEP and SEP formulations in the article do not describe which frames of reference are to be regarded as inertial frames of reference. Thus, EEP and SEP could equally well be meant to refer to, say, Nordstrøm's 1914 theory of gravity, that uses only special relativity.)


 * With no prospect of any agreement about the content of the EP, I backed out. --Cleon Teunissen | Talk 07:35, 4 May 2005 (UTC)


 * I think that the "bluriness" is a good complaint, and I have come to agree that both the equivalence of free-fall and inertial motion as well as the equivalence of accelerated motion and being on the surface of a planet are important. The issue is one of presentation, but that must await another rewrite.  (But please, Cleon, let me or Joke137 do it.  I myself hope to have time for this next month.  The EP is just a lot less trivial that I though it was, in part because my focus really is the WEP.)


 * I agree that the gist of the article is fine. I think that there is still some inconsistency in it that makes it less than clear what the EEP and SEP are.  I also have learned more about how the EEP arose that needs to be included. (EEP seems to be a 1915/16 concept as opposed to the 1907 concept, but the 1907 concept has the rule for determining by observation whether you are accelerated while the 1915 concept relies on geodesic motion to determine inertial motion.  In fact both are important since observation is needed to verify prediction.)


 * I also should make clear that my concern about EP vs. GR: IMO, the EP is a self-contained part of GR.  There is no need in describing the EP to bring up curvature and geodesic motion, and in fact I see the early introduction of those concepts as hiding instead of illuminating what the EP is about.  This is not to say that they should not be mentioned however.  Curvature is required due to a combination of the EP, inertia as geodesic motion, and the observation that (inertially) free-falling objects accelerate towards the center of the Earth.


 * Look at it this way, saying that "not all animals are men" does not mean that "men are not animals". The EP is not GR, but it is a part of GR.


 * --EMS 17:02, 4 May 2005 (UTC)


 * Part of the difference between you and me is that I claim that the 1907-paper-EP is different (deeper and wider in scope) from any of the versions of the WEP that I have encountered. Hence my claim that Einstein did not propose any of the WEP's. There is no logical (deductive) path from UFF to general relativity. Since GR is conceptually deeper and wider in scope there IS a logical (deductive) path from GR to UFF.


 * I claim that the 1907 paper goes full depth right away, and that the more shallow UFF merely serves as corroborating evidence in that paper. The only reason to mention the UFF at all is that it is a logical consequence of GR; a violation of UFF would constitute a severe problem for GR.


 * The article appears to state that some WEP was by itself sufficient to serve as the conceptual basis of the 1907 paper. I dispute that; all of the versions of the WEP that I have seen are narrower in scope than the conceptual basis of the 1907 paper. --Cleon Teunissen | Talk 18:38, 4 May 2005 (UTC)


 * WEP, at least as I see it, is embodied in the 1907 paper. The "physcial equivalence" of being on the surface of the Earth and being in a accelerated frame of reference.


 * You seem to have gotten a grip of why I say the the EP is not GR. WEP actually does lead to GR (or at least the concept of curved spacetime), but cannot and does not do so on its own.  You do seem to understand now the relationship between WEP and UFF.  I do like the UFF being in the article, but fundamentally it is explained and predicted by the EP.  UFF inspired the EP, but is not the EP itself.


 * --EMS 20:27, 4 May 2005 (UTC)

So here is my view on the problems. I realize that what I was arguing so strongly for, the inclusion of the WEP, EEP and SEP are important because they are how the term arises in modern scientific usage. They really arose with Dirac in 1937. However, the equivalence principle as a historical concept in the development of general relativity is also very important and something that I am pretty much totally unqualified to add to the article. The historical discussion should probably be the first thing in the article, as that is what visitors to the article are principally looking for. I'm going to make a few changes along these lines, but I hope someone else can insert more, and perhaps can say something direct about what Einstein actually thought the EP was. --Joke137 18:18, 4 May 2005 (UTC)

I would like to get my hands on the 1907 paper, but there doesn't even seem to be a reference to it in the article. This is perplexing. It may be true that there is a logical deductive path from the SEP to general relativity, but nobody really knows for sure, and the SEP is rather technical. It is true that the WEP/UFF does not lead to GR, but it is still an interesting and important observation about nature, and probably one of the best philisophical reasons to believe in metric theories of gravity, such as GR (as opposed to Newtonian mechanics, where there is no reason for inertial and gravitational mass to be identical). Again, I really only know about how people use the term now, and not so much about history. I think the article could really use a quote from Einstein. --Joke137 19:02, 4 May 2005 (UTC)


 * I will see what I can dig up in the way of reference, but it may be to the original source. Cleon may actually be able to help because he knows the web landscape for this stuff, and can refer this Wikipedia article to places where the 1907 article is discussed.  So while I do not want Cleon making changes to the content of this article, he welcome and encouraged by myself to help fix up the references.


 * --EMS 20:27, 4 May 2005 (UTC)


 * I like the accessability of the information from the history of science department of Kyoto university. The genesis of relativity, the 1907 paper --Cleon Teunissen | Talk 20:34, 4 May 2005 (UTC)

The UFF is is too narrow in scope for the 1907 paper on gravitation
The theory of special relativity had unified kinematics and wave-mechanics of the Maxwell equations. SR asserts that every frame of reference that is shown to be non-accelerating (by measurement with an accelerometer) is a frame in which the speed of light (as measured with for example an interferometer) is seen to be isotropic. The inertial frames of motion, and the isotropic frames of light propagation always coincide.

So when Einstein got his 1907 idea for a theory of gravitation, it was a concept that in itself automatically garantees the same correspondence of inertial motion and isotropy of the speed of light. There is no way that Einstein would be interested in anything less. The newtonian concept of Universality of Free Fall does not meet the requirement.

The only assumption that meets Einstein's requirement is the deeper assumption that in some form gravitational time-dilation is implicated. Einstein made that assumption from the start, any shallower assumption doesn't cut it.

In the 1907 paper and the 1911 paper the wave-mechanical propagation of light in a gravitational field is discussed.  --Cleon Teunissen | Talk 20:09, 4 May 2005 (UTC)


 * Be careful. Time dilation is the effect and not the cause.  Being in an accelerated frame of reference results is a gravitational time dilation being observer, not the other way around (as the author(s) of the gravity article seem to think).

--EMS 20:32, 4 May 2005 (UTC)

Reinstated the disputed tag
I decided to reinstate the disputed tag. In the current article it is stated:
 * From this principle, Einstein deduced that free-fall is actually inertial motion,

This means that the current article states that Einstein deduced the conceptual basis of the 1907 article from the newtonian principle of the Universality of Free Fall. I dispute that.

The way the historical introduction is set up is working up to the suggestion that there is a logical (deductive path) from the UFF to the equivalence conjecture of the 1907 paper. I dispute that line of thinking.

In the opening section the expression 'pseudo-force' is used, and it is suggested that gravitation should not be seen as force. That is a contradiction of general physics. Gravitation is one of the four fundamental interactions of nature. The theory of General Relativity unifies the description of gravitation and the description of inertia. There is nothing "pseudo" about gravitation. --Cleon Teunissen | Talk


 * From the general relativity page quoting from Einstein's 1907 article introducing the Equivalence Principle:


 * We shall therefore assume the complete physical equivalence of a gravitational field and the corresponding acceleration of the reference frame. This assumption extends the principle of relativity to the case of uniformly accelerated motion of the reference frame.


 * The business of the gedanken of a man falling off of his roof and Einstein realizing that his acceleration cancels the effects of gravity is the start of the development of the WEP, but this is where Einstein ended up, and the UFF is an integral part of it. The UFF exists in an accelerating rocketship in free space.  That is why Einstein made the statement quoted above.


 * Also, gravitation is not a force, but instead it is an interaction. A force causes an object to move along a non-inertial path, but gravitation is the tendency of object to move towards each other while they are traveling inertially.  The use of the term "psuedo-force" in the article is in the context of showing that gravity as it is perceived here on the Earth is such a psuedo-force, and it is.


 * You will either clarify your objections or I will remove that tag. I don't think that you have a leg to stand on here, but you have learned enough that you at least deserve that much of a chance.  Do note the Joke137 dug up a whole other side of the EP that I was barely aware of and had improperly discounted, which is why I left the decision to remove the his Disputed tag up to him.  In this case, you seem to be covering what in our discussions is old ground, and only demonstrating that you still lack a good understanding of what the EP is.


 * --EMS 16:49, 5 May 2005 (UTC)


 * P.S. I am amazed that you can refer us to excellent resources like The genesis of relativity, the 1907 paper but still miss such important points as how the UFF is an integral part of why Einstein devised the EP, which is documented in there very well.  It's quotings from the 1907 article includes:


 * As far as we know, the physical laws with respect to $$\Sigma_1$$ [(being in an accelerated frame of reference in free space)] do not differ from those with respect to $$\Sigma_2$$ [(being accelerated by a gravitational field)]; this is based on the fact that all bodies are equally accelerated in the gravitational field.


 * The boldened text states the UFF.


 * --EMS 17:59, 5 May 2005 (UTC)

Gravitation is an interaction.
That is the very point. Gravitational interaction causes acceleration towards the center of gravitation (more precise: acceleration towards the common center of mass).

In newtonian thinking it is assumed that in order to accelerate a force must overcome the resistence of inertia. In the transition from newtonian thinking to GR thinking, a decision needs to be made on how to define the term 'force' in the context of GR thinking. That is not a physics issue, that is only semantics. There are two options: (1) Retain the newtonian definition: it is a force if and only if there is resistence due to inertia.  (2) Redefine the term to fit in with GR. The generalized concept of force is: if it causes acceleration, it's a force.

To insist that gravitation is 'not a force' but instead 'an interaction' is not a physics statement, it's a semantic statement. In physics, a force and an interaction are one and the same thing; they cause acceleration with respect to the common center of mass.

An interaction is always between two participants. Usually the two participants are two lumps of matter: Suns, planets, moons, objects on the surface of a planet. In the case of, say, a rotating wheelshaped spacestation, then the artificial gravity elicited by the rotation does not involve interaction between two mutually attracting participants. When artificial gravity is elicited there is just the one lump of matter being accelerated.

The choice which the scientific community has made is option 1. Option 2 is equivalent if you say that it causes proper acceleration. As for the use of coordinate acceleration, you need to realize that one can always do a coordinate conversion to a system in which there is no coordinate acceleration no matter how the object is moving in another frame of reference. That is why the focus is on proper acceleration, which is an invariant.

This really is not semantics, for the reasons given previously. I know that people continue to think of gravity as a force and talk about it as one of the four forces of nature, but technically it really is not a force. That is why the term "interaction" is needed in this context.

--EMS 21:12, 5 May 2005 (UTC)

Didactics
In GR, it is necessary to always declare explicitly what size-scale the reader must take in mind. The expression 'gravitation' can refer to all of the gravitation around a planet, or it can refer to local manifestation in a volume of space the size of an elevator cabin. Of course, on elevator-cabin-scale the effects of gravitation or acceleration with respect to the local inertial frame of reference are fundamentally indistinguishable.

The current article mentions in the opening section. >>Albert Einstein's assertion that the gravitational "force" is actually the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.<< That is misleading, for it fails to tell the reader how to put that information into context.

No accelerated frame of reference can be devised that mimics at once the entire spherically symmetrical gravitational field surrounding a planet. That fundamental difference must not be glossed over, especially not in the very first sentences of the article. --Cleon Teunissen | Talk 19:02, 5 May 2005 (UTC)

OK. I will do an edit, and add "as experienced locally while standing on a massive body (such as the Earth)". Let me know what you think.

--EMS 21:12, 5 May 2005 (UTC)

Edit done
Done as promised above. I'm glad that I gave you the benefit of the doubt. You did have a good point, and I am glad to have responded to it.

Even so, the ball is now back in your court. You must now either show that more work is needed or remove the Disputed tag. Otherwise I will in a day or two remove that tag myself.

--EMS 21:28, 5 May 2005 (UTC)

Explanation of why the disputed tag was reinstated

 * I am amazed that you can refer us to excellent resources like The genesis of relativity, the 1907 paper but still miss such important points as how the UFF is an integral part of why Einstein devised the EP, which is documented in there very well. --EMS 17:59, 5 May 2005 (UTC)

What perplexes me is that you do not read what I write. I wrote: "I claim that the 1907 paper goes full depth right away, and that the more shallow UFF merely serves as corroborating evidence in that paper." I also wrote: "Since GR is conceptually deeper and wider in scope there IS a logical (deductive) path from GR to UFF."

I have stated that the 1907 paper EP implies the UFF, UFF is among the logical consequences of the 1907 paper EP.


 * The UFF was known before 1907. Galileo discovered it.  The WEP was devised to explain the UFF.  Because of that, the UFF had better be a consequence of the WEP.

Conjecture
About the UFF: The most solid, most simple hypothesis to account for the UFF is to go along with what had been hypothesized for centuries: that the force of gravity couples to all mass in proportion to the mass. That is a dull, drab, simple hypothesis, but in general it is good advice to hypothesize as frugal as possible; do not hypothesize beyond necessity!

Einstein's 1907 equivalence conjecture was a leap of imagination. (Of course, Einstein conjectured way beyond apparent necessity because he spotted the deeper necessity.)


 * I don't see anything to argue with here.

What constitutes a logical deductive path
The 1907 EP implies the UFF, but the reverse is not true, because the 1907 EP is more profound in content. The following statement is illogical:
 * From this principle, Einstein deduced that free-fall is actually inertial motion,

The 'free-fall is actually inertial motion' concept was a sheer conjecture. GR predictions have been confirmed and with that strong corroboration in the bag the equivalence conjecture has been elevated to the status of principle.

The correct statement is to write that the 1907 EP was a conjecture.


 * I am not going to play that kind of semantic game. As best I can tell, the Equivalence Principle was being called that back in 1907.  Yes, even as a "principle" it was also a conjecture.  But for Einstein's research it was as principle and was used as a principle, and that is what matters.

In the article it is stated:
 * The equivalence principle proper was introduced by Albert Einstein in 1907. An that time, he made the observation that the acceleration of bodies towards the center of the Earth at 1g (g=9.81 m/s2) is equivalent to the acceleration of inertially moving bodies that one would observe if one was on a rocket in free space being accelerated at a rate of 1g. From this principle, Einstein deduced that free-fall is actually inertial motion, while being at rest with respect to the Earth (while under the influence of its gravitational field) is an accelerated state of motion.

The above passage refers only to newtonian physics. I think that this gives the readers the impression that in 1907 Einstein employed a newtonian principle in writing his first article.

The article should state that in 1907 Einstein used a relativistic approach. (What I mean by a relativistic approach to the equivalence is what I described in the 'UFF is too small in scope' section.) --Cleon Teunissen | Talk 00:33, 6 May 2005 (UTC)


 * Yes. That passage refers only to Newtonian physics, and properly so.  The WEP is where Einstein breaks away from Newtonian mechanics in regards to the nature of gravity.  The achievement is therefore expressed in relation to the Newtonian concepts and related observations that drove it.


 * I am not impressed by what you wrote in the "UFF is too small in scope" section. The EP is not GR.  It can exist independently of relativity thoery.  However, GR cannot exist without the EP.  You may as well call a door a house.  The EP leads into GR, and that is what matters.  Those observations of the propagation of light are interesting, but they are not part of the deductive pathway that Einstein used to concoct either the WEP or GR.  In fact, one thing that Einstein noted even in the 1907 article is that the propagation of light is not isotropic in accelerated frames of reference due to the effects of time dilation.


 * Yet you are right on one count: The UFF/inertial frame relationship is too narrow to infer GR from.  And as you correctly note, even in the 1907 paper Einstein is going beyond the EP and dealing with issues of time dilation and the propagation of light.  However, the framework for those additional observations is the WEP in the narrow form that you so detest.


 * --EMS 04:22, 6 May 2005 (UTC)

It think I should add that I see in what way the 1907 paper does not represent the concepts of the 1915 general relativity theory. From 1907 to 1915 the equivalence conjecture remained the same in content, but a big change was that around 1912 Einstein recognized that the theory should not only account for gravitational time dilation, but also for gravitational space deformation. He recognized the necessity to use tensors. Minkowski had introduced the concept of a metric of space-time. --Cleon Teunissen | Talk 00:33, 6 May 2005 (UTC)


 * Yes. After 1912 Einstein is on a different path.  He is then looking for metrics and geodesics.  This where the WEP becomes almost superfluous and the EEP becomes more of a consideration.  BTW - It is Levi-Civita who is credited with getting Einstein to think in terms of tensors.  Minkowski introduced the concept of spacetime in 1908 and described the relationship that became the Minknowski metric, but it was not initially recognized as a metric (except by people like Levi-Civita).


 * You are not impressing me now. The full story of the development of GR is another subject in which the EP is only a player.


 * --EMS 04:22, 6 May 2005 (UTC)

How the EP relates to Special relativity

 * Yes. That passage refers only to Newtonian physics, and properly so.  The WEP is where Einstein breaks away from Newtonian mechanics in regards to the nature of gravity.  The achievement is therefore expressed in relation to the Newtonian concepts and related observations that drove it. --EMS 04:22, 6 May 2005 (UTC)


 * [...]The EP is not GR. It can exist independently of relativity theory.
 * [...], even in the 1907 paper Einstein is going beyond the EP and dealing with issues of time dilation and the propagation of light. --EMS 04:22, 6 May 2005 (UTC)

It appears that you believe that a theoretical physicist could also have attempted to formulate an alternative newtonian theory of gravity on the basis of the assumption that free-fall is inertial motion. It is hard to see how, given the newtonian assumption of space and time as fundamentally separate entities, such a theory could have been developed. (To my knowledge, Riemann explored physics theories that worked with non-euclidean space geometries, but to no avail.)

You sound like someone who claims that since historically darwinism broke away from creationism, darwinism should be explained/introduced in terms of creationistic assumptions.

It appears that you are oblivious of the core achievement of special relativity. Before SR, it was assumed that for kinematics the galilean transformations are the correct ones, and in electrodynamics the physicists found themselves ever more forced to use the Lorentz transformations in order to have a consistent theory at all. In special relativity there is only one set of transformations, the Lorentz transformations. SR unifies kinematics and the wave-mechanics of electrodynamics into a single framework of thought, that is the heart of SR. It appears that you fail to grasp that aspect of SR.

Your remarks give, finally, an insight into your thoughts and considerations. It is clearer now why there is no prospect of reaching any agreement. --Cleon Teunissen | Talk 07:36, 6 May 2005 (UTC)

I claim that from the start Einstein aimed for a theory of general relativity that would have special relativity as a limiting case. The 1907 paper is exclusively about the propagation of light. In the 1907 paper it is assumed (in order to maintain overall consistency) that the theory of general relativity that is to be developed will account for a gravitational time dilation. In 1907 Einstein cannot foresee how the future theory will account for gravitational time dilation, but by assuming a principle of physical equivalence of the propagation of light in an accelerated frame and in a gravitational field he gives an approximation of the magnitude of the gravitation-related time dilation. --Cleon Teunissen | Talk 08:13, 6 May 2005 (UTC)

The meaning of complete physical equivalence

 * Those observations of the propagation of light are interesting, but they are not part of the deductive pathway that Einstein used to concoct either the WEP or GR. In fact, one thing that Einstein noted even in the 1907 article is that the propagation of light is not isotropic in accelerated frames of reference due to the effects of time dilation. --EMS 04:22, 6 May 2005 (UTC)

From the way Einstein discusses the propagation of light in a gravitational field it can be seen that he employs a relativistic assumption, an assumption of physical equivalence of propagation of light in a gravitational field and the corresponding acceleration of the reference frame.

Einstein did not mention such an assumption separately. For Einstein it was sufficient to state the conjecture in the following, general form:
 * We shall therefore assume the complete physical equivalence of a gravitational field and the corresponding acceleration of the reference frame.

That general statement is obviously meant to cover both kinematics and propagation of light in one go, as the two had been unified by the theory of special relativity.

Of course Einstein notes that the speed of light is not isotropic in an accelerated frame of reference, that is an important clue for the direction in which a theory of gravitation must be sought. Gravitation must entail the same anisotropy in the speed of light, otherwise the physical equivalence would not be complete.

It is really weird that you insist that it must be assumed that the 1907 paper EP involves only a newtonian principle. Both the 1907 and the 1911 paper only discuss the propagation of light, using Maxwell's equations, yet you state that thinking about propagation of light was not part of the GR development effort. You are contradicting the evidence. --Cleon Teunissen | Talk 13:33, 6 May 2005 (UTC)

The EP is not GR. How many times do I have to write that?

Leave light out of this. Look again at the statement made in the 1907 paper: the complete physical equivalence of a gravitational field and the corresponding acceleration of the reference frame. That is the point of the WEP. That is where I stop in terms of expressing the WEP.

Of course light is involved in the development of GR. That and a lot of other things that you wished to cite in that "cancellation" version of this article. However, this article is not about the development of GR. It is about the Equivalence Principle. It can and should refer to GR, but as something that arose out of a set of principles that includes the EP.

You seem to think that the 1907 and 1911 articles are about the EP. They are not. Instead they are about the behavior of time, the propagation of light, and the attributes of massive objects in accelerated frames of reference. In each article, the EP is presented and then used to generalize the accelerated frame observations to the gravitational field. In the 1907 article, the "... physical equivalence ..." sentence is the end of the presentation of the Equivalence Principle. Everything written after that point is irrelevant to the EP, but is quite important to GR and its development.

Let me emphasize that the scope of this article is the EP and the EP only! When someone links to this article they are seeking an explanation of the EP itself and some explanation of how it hooks back into GR. They are not seeking a discussion of the development of GR. That is a legitimate topic for a Wikipedia article, and if you want to work on that be my guest -- Just don't do it here. Also, a person linking to this article does not need to have discussions of tensors, curvature, time dilation and/or the propagation of light. Those of GR topics. The EP is not involved with them, but instead is the bucket that Einstein used to "carry" those phenomena from an accelerated observer in free space to someone standing on a planet and perceiving gravity. The bucket is not the water. The door is not the house. The EP is not GR.

--EMS 15:53, 6 May 2005 (UTC)

P.S. I will move the "Disputed" tag to the bottom of the article. I do not see this as a major conflict.

Well, as you've probably noticed, I've been working on the article over the last few days and I hope it has been moving to the middle ground in this debate. In particular, although I'm no historian, I've tried to find out what Einstein actually said and add it to the article. I mentioned that he used the equivalence principle combined with special relativity to derive some of the consequences of a relativistic theory of gravity. However, I agree with EMS that the equivalence principle, as stated by Einstein in 1907 and 1911 is essentially the UFF (although Einstein takes it to apply to relativistic energy, not just matter as we usually think of it). I also agree that UFF$$\implies$$GR but not that GR$$\implies$$UFF. In all honesty, I'm a bit perplexed: I don't know what all the fuss is about.

As for whether gravity is a force or not, we've hashed through this before. It's semantics. It is clear to me, however, that in GR inertial frames are defined by the geodesic equation which is the GR equivalent of the Newtonian equation for acceleration in the absence of a force $$a=0$$. Thus, in this sense, gravity is definitely not a force, and this is what I was trying to get at. --Joke137 20:13, 6 May 2005 (UTC)


 * My only caution is that in my view the EP is not UFF but instead is an interpretation and explanation of UFF.


 * Beyond that, I like the editting that you have done. It has tightenned up the article and brought in a lot of the material that I myself was thinking about adding.  Perhaps in a few weeks, when I have more time to think about it, I will tweak it some more myself, but I no longer see a need to do a serious rewrite.


 * --EMS 01:48, 7 May 2005 (UTC)

The scope of the Equivalence principle article
In my opinion, a wikipedia article should be self-consistent, and reasonably self-contained.

The article states:
 * there is only one force acting on a person standing on the surface of a massive object, and that is the upward force of the surface on that person.

On the face of it that is a ridiculous statement: if there is only one force everything would be hurled to stars.

This means that there is a necessity to provide the readers with additional information that allows the readers to see themselves that postulating the equivalence principle makes sense in its own right, that when all is considered it is a self-consistent and indeed compelling concept.

That, in my opinion, should be the scope of the EP article: to provide just enough general context to see the promise of the EP. --Cleon Teunissen | Talk 23:11, 6 May 2005 (UTC)

It is not a ridiculous statement at all. It is a statement of the equivalence principle, and it takes some thought to understand. If the equivalence principle were utterly straightforward, it would not have been as profound a shift in thinking as it was. The article does not consist of that one statement you've quoted. If you don't think the article is pedagogical enough, that's fine, but I haven't seen any clear suggestion of what to do with it. --Joke137 00:14, 7 May 2005 (UTC)

I agree that it must be emphasized that the equivalence principle is a heuristic tool. It gives clues for the direction of efforts to find a relativistic theory of graviation and motion. I agree that the final GR theory relates to more profound concepts than the equivalence principle, so that in the end the EP is seen to be one of the logical consequences of GR, rather than a logical basis of GR.

The Einstein quotes added by Joke137 are, unfortunately, adding extra layers of confusion. Those quotes represent a line of thinking that has been abandoned.

The heuristic of the equivalence principle is helpful and necessary to find a relativistic theory of gravitation. The Einstein quotes that are added by Joke137 are written from a mindset of aiming to extend the principle of relativity of inertial motion to a principle of relativity of all kinds of motion, but seeking that sort of extension has turned out to be a dead end. --Cleon Teunissen | Talk 23:11, 6 May 2005 (UTC)

The whole point of the Einstein quotes is that the principle served as a powerful heuristic tool in the development of general relativity. That's why they're in the history section, and that's why you learn about the equivalence principle in school when they teach you about GR in hand-waving fashion. As I see it, there are only two reasons to care about the equivalence principle: These are the two issues I've tried to address in the article. Unfortunately, I'm not a historian of science, so my attempt at describing the history surely leaves something to be desired. --Joke137 00:14, 7 May 2005 (UTC)
 * because it was instrumental in going from special relativity to general relativity
 * because it provides a nice set of properties of general relativity that are experimentally testable, and thus useful to see if GR is the theory that describes the universe

Having taken a closer look at the article, I only have two comments:
 * 1) The assertion of EP = UFF made in the middle of the article is not in my view correct, for the reason noted above.
 * 2) The third Einstein quote does not add anything to the article:  We can make the point about the importance of the EP ourselves, and with much more clarity and brevity.  However, the other two quotes very much capture the essense of the WEP as Einstein formulated it, and make excellent additions to this article.

Cleon - You wrote that


 * The Einstein quotes that are added by Joke137 are written from a mindset of aiming to extend the principle of relativity of inertial motion to a principle of relativity of all kinds of motion, but seeking that sort of extension has turned out to be a dead end.

I assure the mindset that you cite was not a dead end. That goal was achieved, and the extension is called General relativity.

--EMS 02:40, 7 May 2005 (UTC)

Re: the comments. I agree that Einstein's formulation of the EP is different from the WEP/UFF (and even more different from the EEP!). But I think the WEP is the same as UFF. I agree that the quotes are too long, but the reason I added the third one is I wanted to make clear that Einstein's idea was not to point out simply that, gosh, in Newton's mechanics a uniform gravitational field looks the same as an accelerated frame of reference, but rather to elevate this observation to the status of a physical principle while developing a new theory of gravity. Just as he used the idea that the speed of light is fixed as the basis for special relativity, even while discarding the idea that clocks run the same for observers moving at different rates, he was forced to discard more cherished ideas (such as that clocks run at the same rate all over the universe) that are incompatible with the equivalence principle and special relativity. I'm still not sure how best to address this in the article. --Joke137 03:01, 7 May 2005 (UTC)

Maybe its just my orientation, but I see UFF as being the principle found by Galileo, and as such I see it as being independent of the WEP. I know that Clifford Will does interpret WEP as being the same as UFF, for testing puropses (which is Will's forté), that is very much the case. However, from a conceptual standpoint I cannot agree with that assessment. I suggest distinguishing between the two but to also note the existance of a school of thought that considers them to be the same.

You make a good point about the third quote, but I repeat that there may be a better way of achieving the goals of its inclusion. However, I have no suggestion to offer at this time, and no intention to touch it either. We can do better, but we can also easily do worse.

As for the clock issue: I would be cautious about including it. Time dilation is not a consequence of the EP, but instead the EP allows predictions made for an accelerated observer in free space to be placed in a more -uh- "down-to-Earth" context. IMO, things like that should be touched on, but only briefly and with an internal link to an article on the phenomenon itself.

--EMS 03:28, 7 May 2005 (UTC)

For the first point, I have nothing further to add. Does the terminology "weak equivalence principle" predate Will and his collaborators? In my recent edit, I tried to make it clear that what Einstein said, and what is mentioned in the first half of the article, is closely related to, but not the same as, what Will uses.

I agree with what you have to say about the third quote.

As for clocks, I am happy about what's in there now. Time dilation is a consequence of special relativity, clocks running at different rates in gravitational potentials is a consequence of special relativity and the equivalence principle, but I don't see any reason to confuse the issues further by adding this to the article.

By the way, I just took a stab at rewriting the fifth force article, which was pretty confused up till this point. Unfortunately, I made it probably too technical, but also reasonably accurate, I hope. I thought I'd mention it since it is closely related to the equivalence principle (really just another name for some bits and pieces of it). --Joke137 04:31, 7 May 2005 (UTC)

Stating that UFF=WEP is a very awkward choice
There is a fundamental difference between the UFF and even the weakest form of the equivalence conjecture.

Newtonian thinking is that during free-fall the force of gravity is still there, but under the assumption that the force of gravity couples to all mass equally, it is understandable that you don't feel it. Diamagnetism also has the property (with a few exeptions) of coupling to all atoms equally so a diamagnetically levitated accelerometer won't register acceleration.

The equivalence conjecture, even the weakest form, goes a fundamental level deeper than that. It is to conjecture that there is an analogy with presence or non-presence of a magnetic field of when considering electromagnetic induction. If you are stationary with respect to a wire carriying current there is not a magnetic field, if are moving with respect to that wire there IS a magnetic field. That apparent contradiction can be resolved only by a fundamental rethinking of space and time: shift to thinking in terms of unified space-time.

There are oodles of versions of the WEP around, but to state that WEP=UFF is a hopelessly awkward choice. Stating that WEP=UFF introduces a severe inconsistency. --Cleon Teunissen | Talk 07:02, 7 May 2005 (UTC)

I don't have access to any books right now, but here is what I could find on the internet. T. Damour and A. M. Polyakov, "The String dilaton and a least coupling principle," ''Nucl. Phys.'' B423, 532 (1994) [arXiv:hep-th/9401069], or at spires spires.
 * Indeed, present experimental data gives upper limits of order: [...] 10-11–10-12 on the universality of free fall (weak equivalence principle) (p.2)

See also Will, section 2.1:
 * One elementary equivalence principle is the kind Newton had in mind when he stated that the property of a body called "mass" is proportional to the "weight", and is known as the weak equivalence principle (WEP). An alternative statement of WEP is that the trajectory of a freely falling body (one not acted upon by such forces as electromagnetism and too small to be affected by tidal gravitational forces) is independent of its internal structure and composition. In the simplest case of dropping two different bodies in a gravitational field, WEP states that the bodies fall with the same acceleration (this is often termed the Universality of Free Fall, or UFF).

It's tough for me to see that any distinction is being made. --Joke137 14:28, 7 May 2005 (UTC)

I agree with you that Clifford Will does not make a distinction. However, whether Clifford Will recognizes the distinction or not, it is there. Experimentors can afford to be logically inconsistent in how they present the theory, their work does not depend on it. It is in the developmental stage of a theory that inconsistencies in thinking can be costly. Also encyclopedic articles must be self-consistent.

Before GR as well as after the introduction of GR experimentors were keen to test the UFF. In newtonian physics the UFF is a convenience (without UFF the newtonian force of gravity might couple differently to different planets, so a "stray" orbit might be explicable by way of violation of UFF) In the case of GR, UFF is a logical consequence of the suppositions of GR; a violation of UFF can show the way to theory that is deeper than GR.

A potential successor to GR must have GR as limiting case. Therefore string theories must either predict full compliance with EP (hence UFF), or predict such a minute violation of UFF that it has so far been undetectable. In the future, detecting deviation from UFF might help to narrow down among possible successors to GR. --Cleon Teunissen | Talk 15:27, 7 May 2005 (UTC)


 * What I did with my edits is to try and preserve the distinction between what Einstein said and the modern formulation used by Will and all the theorists and experimenters I have cited at the bottom. I don't know if the term weak equivalence principle exists outside this context. If this isn't clear enough from the article, one possibility is to add something to the History section describing how the statements by Einstein are conceptually broader than Galileo's demonstration of the universality of free fall. --Joke137 17:46, 7 May 2005 (UTC)

I have so far assumed that a newtonian UFF and a relativistic UFF are conceptually different; for a relativistic UFF you expect that both rest mass and energy contribute to gravitational mass. Nuclear binding energy contributes equally to mass and to gravitational mass, and it seems to me that the newtonian expectation pattern would be that the force of gravity only couples to mass. To what extend is this a technicality? Do 2 deuterium atoms have the same composition as one helium atom in the sense of the definition of the UFF? --Cleon Teunissen | Talk 16:30, 7 May 2005 (UTC)


 * Yes, that's true, in the relativistic UFF you expect rest mass and energy to contribute. This is important, because only about 1–2% of atoms are made up of rest mass, with most of the rest made up of QCD binding energy. The UFF is a macroscopic statement, though, so it is largely a technicality and the difference between the UFF in a Newtonian and relativistic context is slight. In the Lagrangian formulation of field theory, the UFF implies that all fields couple to the same metric which does not have to be the metric of gravity. The strong equivalence principle is what suggests that the metric whose geodesics matter follows is the same as the metric in the Einstein-Hilbert action. --Joke137 17:46, 7 May 2005 (UTC)

the UFF is a macroscopic statement

 * The UFF is a macroscopic statement, though, so it is largely a technicality and the difference between the UFF in a Newtonian and relativistic context is slight. --Joke137 17:46, 7 May 2005 (UTC)

Yes, the relativistic UFF is a macroscopic statement. By contrast, the 1907 paper EP is a general statement. The heuristic power of the 1907 pater EP is that it also covers the behavior of photons.

My thinking about relativistic physics is strongly influenced by the online book reflections on relativity. It is a rich source of ideas. From section 1.4 The dilemma of light:
 * (E2') The speed of light with respect to any system of inertial coordinates is isotropic and independent of the state of motion of the source.
 * This form emphasizes the fact that this principle can be regarded as an assertion of wave-particle duality for light, because if light behaves like material particles its speed ought to be isotropic with respect to any inertial frame, and if light consists of a wave in a material medium its speed ought to be independent of the state of motion of the source, whereas the above principle asserts that light exhibits both of these characteristics. In this sense, we see that the wave-particle duality commonly associated with quantum mechanics is actually at the core of special relativity as well. (It's not surprising that Einstein was occupied with a paper on light quanta at the same time that he was formulating his ideas about relativity.)

It is suggestive that there is a fundamental relationship between kinematic (macroscopic) inertia in Minkowski space-time, and the wave-mechanical properties of Minkowski space-time.

The wikipedia article about the Sagnac effect was co-written by EMS and me. (In particular, EMS took care of the formulas.) A sagnac interferometer measures its own angular velocity with respect to the local inertia frame of reference. It is the wave-mechanical counterpart of a gyroscope.

One of the aspects of a ring laser interferometer is that interpreted in terms of classical wave mechanics there is a pattern of standing waves in the ring laser interferometer that is stationary with respect to the local inertial frame of reference. The probability-distribution of a photon hitting a detector is stationary with respect to the local inertial frame of reference, at all angular velocities of the ring laser interferometer. External link Standing wave in a ring laser interferometer

There are also proposals for atom interferometers with a ring topology. Atom ring interferometers will be more sensitive than laser ring interferometers. Atom interferometry has been used in a test of the Principle of Equivalence.

Our theories describing kinematic behavior of atoms, and our theories describing the wave behavior of the corresponding atomic de Broglie waves yield the same outcomes. Acceleration can be measured kinematically or interferometrically and the two measurements always agree. It can hardly be a freak coincidence that relativistic physics has built-in compatibility with particle/wave duality. On the basis of current knowledge it is safe to say that the inertia of large objects must be the resultant of quantummechanical behavior of the individual atom waves.

The heuristic power of the 1907 paper EP was that it covered both propagation of light as a wave-mechanical phenomenon, and the photon-nature of light. So to me there is a vast difference between the macroscopic, kinematic UFF and the 1907 paper Equivalence Principle.

Of course, none of above ideas is suitable for the EP article. It just that it is so bizarre to suggest that the 1907 paper EP is rooted in newtonian concepts.

I shall once more try very hard to stay away from the Talk:equivalence_principle page. I insisted for a very long time. Only crackpots insist forever. --Cleon Teunissen | Talk 20:12, 7 May 2005 (UTC)

Concede that WEP=UFF
First of all, I wish to answer Cleon:


 * The heuristic power of the 1907 paper EP was that it covered both propagation of light as a wave-mechanical phenomenon, and the photon-nature of light. So to me there is a vast difference between the macroscopic, kinematic UFF and the 1907 paper Equivalence Principle.

That difference does not exist. Once the principle of geodesic motion was hit upon in 1912, all that was needed was for the metric to be locally Lorentz invariant. At that point, a failure to follow a geodesic path places a particle in an accelerated frame of reference and all of the predictions made in the 1907 and 1911 papers apply based on that fact and that fact alone. Indeed, the UFF applies to photons as well as massive objects, otherwise there would not be anything unversal about it.

As for the subject of this posting: It has finally occurred to me that the key is in the above paragraph:  After 1912, inertia was defined as geodesic motion, and so all that was left of the initial EP investigations was the UFF. The questions of "what is inertial motion?" now had an answer. Admitedly it would be a few more years before GR came to exist in its final mathmatical form, but that is where the transition occurred. Also, I am realizing the GR is something of a moving target: The EFE may be immutable, but how the field understands and uses GR tends to change over time. For example, it was over 40 years before it was proven that GR supports the existance of black holes. It seems that over the years the understanding to the EP has changed, and mine is mote a 1970s and before version of it. This makes sense, since that is when I studied physics as an undergraduate. However, I now see that UFF is the definition used by Wald in his landmark General Relativity book as well as by Will, while Ohanian cites MG = MI, which is effectively the same thing.

This also calls into question the propriety of my EP definition involving the identification of an accelerated frame of reference based on local observations. On one level the statement is correct, but it may more follow from the EP than be the EP. I wouldn't change how that quote is presented just yet, but I no longer think that it should be in the introduction. I'm just not yet sure what to do with it. --EMS 02:08, 8 May 2005 (UTC)

Allowing the reader to put information into context
The current article mentions in the opening section. >>Albert Einstein's assertion that the gravitational "force" is actually the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.<<  That is misleading, for it fails to tell the reader how to put that information into context. --Cleon Teunissen | Talk 19:02, 5 May 2005 (UTC)


 * OK. I will do an edit, and add "as experienced locally while standing on a massive body (such as the Earth)".  Let me know what you think. --EMS 21:12, 5 May 2005 (UTC)

In my opinion, the problem that it is an (unintentionally) misleading statement is still there. According to the theory of general relativity there is a spherically symmetrical curvature of space-time surrounding a planet. The point is that this planetery gravitational field is not a pseudo-field. The caveat 'locally' is in itself insufficient to help the reader put the statement into proper context. Why should a field that is real suddenly be a pseudo-field when the volume of space is that is examined is human sized? 

It is only after people have absorbed a lot of general relativity theory that the knowledge is there to appreciate that viewing just 'local' or viewing global can make a world of difference.

I think the expression 'pseudo-force' is chosen to fulfil a need, and I would like to describe that need as follows: should we or should we not view inertia as a force?<BR> Let's suppose that Minkowski space-time is like a fluid that exerts a drag on objects moving trough it. But drag is proportional to velocity: and inertia is proportional to the  time derivative of velocity. (So exit viewing Minkowski space-time as a fluid.)

(I like to think of iInertia as analogous to inductance; when there is a self-inducting coil in a current circuit, then there is no resistence to uniform current, only change of current strength is opposed.)

I believe that inertia should be viewed as an interaction of matter/energy with space-time geometry. That means that I follow the interpretation of GR that space-time geometry is a physical entity, an active participant in the physics taking place.

Inertia is the opposition to acceleration with respect to the local inertial frame of reference. Should this opposition be seen as a force or not? I believe a clear-cut decision must be made.

I believe that it is confusing to disguise inertia as something else. Do not call it halfheartedly a 'pseudo-force', call it by its proper name: inertia. <BR> --Cleon Teunissen | Talk 08:21, 8 May 2005 (UTC)

I think that you are somewhat right here in that the Equivalence Principle is dealing with the equivalence of inertial frames of reference. So I will agree with you that the introduction is misleading in that respect. However, I want to see something this zeros is more effectively on inertial side of the Equivalence Principle (instead of the acceleration side as I have done). I am not yet ready to do an edit (although I am thinking about it and I assume that Joke137 also is.) If you like, you can play with this in your sandbox. Just be warned that I am not about to accept anything of the ilk of your last major update of this page. "Cancellation" was a step backwards to a pre-1907 view, and "manifestation of inertia" is a buz-term that I continue to see as being inappropriate for this page.

The valid complaint that I see here is that free-fall as inertia is not being stated clearly and up front. However beyond that I see this article as being in much, much better shape that it was even a month ago. It still needs work, I do admit. The EP is just not the simple thing I treated it as being. --EMS 02:00, 9 May 2005 (UTC)

Bonk on the precis
My mind bonks on the precis
 * Whenever an observer detects the local presence of a force that acts on all objects in direct proportion to the inertial mass of the object, that observer is in an accelerated frame of reference.

There are problems in the references to
 * 1) a force
 * 2) all objects
 * 3) the inertial mass of the object

A. If the interpretation is that a force (#1) is acting on #3 then grammar dictates #3 should be the inertial masses of the objects

B. If the interpretation is that #1 is acting on #2 (all objects) then grammar dictates #3 should be the inertial mass of each object

Which sense is it? A or B? Ancheta Wis 10:36, 10 May 2005 (UTC)


 * B, thanks! --Joke137 19:26, 10 May 2005 (UTC)

The context of the UFF

 * Indeed, the UFF applies to photons as well as massive objects, otherwise there would not be anything universal about it. --EMS 02:08, 8 May 2005 (UTC)

The 'Universal' in UFF refers to the observation of independency of  where the falling takes place, and that free-fall trajectory is independent of the composition and internal structure of the macroscopic test masses.

In the physics community around 1900 there was the law of motion that in kinematics the Galilean transformations are the transformations of motion. The Lorentz Ether theory predicted that the frames of reference in which the speed of light is measured to be isotropic are related by Lorentz transformations. At the time this was not seen as violating the assertion of Galilean UFF, because prior to the acceptence of Special Relativity asserting UFF only applied to kinematics, not to the propagation of light.

Conclusion: when UFF is asserted as a principle of physics, it must be stated explicitly whether the assertion is made in conjunction with classical physics thinking, or in conjunction with relativistic physics thinking, because relativistic UFF includes the assertion that kinematic inertial motion and isotropy of the speed of light allways coincide.

You state that in the light of the 1912 progress, the principle that was stated in the 1907 paper was a relativistic UFF. In short:
 * WEP=relativistic UFF.

So we have the understanding that the 1907 paper EP is a thoroughbred relativistic concept. --Cleon Teunissen | Talk 11:29, 10 May 2005 (UTC)


 * You keep insisting on dragging the whole relativity edifice into this discussion. UFF is the independency of how an object falls with respect to when, where, composition, and structure.  I really don't see how relativity in any way, shape or form speaks to the equivalence principle.  On the other hand, the equivalence principle speaks very strongly to relativity.


 * You are correct in that the local isotropy of the propagation of light in inertial frames of reference [known as Local Lorentz Invariance (LLI)] very much drives how the UFF is used and what it indicates. However, I see LLI as being seperate from the UFF.


 * You want to start with the relativistic physics, while I want to start from the classical physics. The reason for my view is that I see the EP as being a bridge from the classical view of gravity to the relativistic view of gravitation.  The identification of free-fall as inertial motion demands the relativistic view that spacetime is curved.  You can't have a non-uniform acceleration field in a flat spacetime.  You can in a curved spacetime.  In addition to curvature, you also need spacetime to be a pseudo-Riemannian manifold, since otherwise you cannot get the sign of the gravitation of massive bodies to be correct.  (If spacetime was a Riemannian manifold, gravitation would be repulsive instead of attractive).  Also, if spacetime is pseudo-Riemannian, then the rules of SR automatically apply locally in inertial frames of reference.


 * In short, I see GR as flowing from the UFF and EP, not the other way around. --EMS | Talk 20:30, 10 May 2005 (UTC)

The transition of SR to GR

 * You keep insisting on dragging the whole relativity edifice into this discussion. --EMS | Talk 20:30, 10 May 2005 (UTC)

Wel, not the whole relativity edifice.<BR> I suspect what misunderstanding has been going on. What I insist on is recognition (in the article) of the importance of special relativity as a heuristic tool in the search for the theory of general relativity. It was not my intention to suggest that the finally achieved GR served all along as a guidance in finding the finally achieved GR.

I think I have underestimated the importance to explicitly treat SR and GR as separate theories. The conceptual shift from SR to GR is as deep as the conceptual shift from newtonian dynamics to SR. Special relativity was when it was developed a theory of its own, and only a few years later, in the road to GR, special relativity was a heuristic tool rather than a theory of its own.

The 1907 paper and the 1911 paper are very much transitional. Conceptually they go outside the boundaries of special relativity, and at the same time those papers rely for guidance on insights that came with special relativity. The logic of the 1907 paper already implies that in the still to be found theory of gravitation and motion the global Lorentz invariance of Minkowski space-time will have to be replaced by a concept of Local Lorentz invariance. The logic of the 1907 paper already implies that whatever the still to be found theory will look like, this theory's space-time will be different from SR space-time. (The technical term that I most encounter is that GR describes space-time as 'diffeomorphic space-time'.)


 * The identification of free-fall as inertial motion demands the relativistic view that spacetime is curved.  --EMS | Talk 20:30, 10 May 2005 (UTC)

Well, I do agree that the logical exposition of an idea does not necessarily have to follow the historical course of events; occasionally it is beneficial to deviate from the historical course of events. I suppose that with the full knowledge of GR available, it can be argued that if it is conjectured in a initially newtonian context that free-fall is inertial motion, then the only way to make that idea work in mathematically consistent form is to break away from the newtonian thinking, and come up with space-time continuum, and with curvature of this space-time continuum.

I encountered that in 1900 Schwarzschild had published an article with the title 'On the Permissible Scale of the Curvature of Space'. In this article Schwarzschild discusses light following geodesics of curved space. This illustrates that non-euclidean geometry and geodesics were explored prior to SR and GR, but the possibility of space-time continuum was not explored prior to SR and GR. (Max Planck institute for the history of science In the Shadow of the Relativity Revolution Download as PDF File (2,2 MB))

I think that in the case of the Equivalence principle it is disadvantagous to deviate from the historical course of events. Prior to the introduction of special relativity the idea of identifying free-fall with inertial motion was just a whacky thought, no discernable prospect to make that work mathematically.

Acceptence of special relativity confronted physicists with the necessity to rethink gravitation fundamentally, and at the same time SR opened a radically new perspective on how and where to look for a new theory of gravitation and motion.

I refer to the 1907 EP as a conceptually relativistic EP because although Einstein broke away from SR, he relied on SR insights to guide his explorations. --Cleon Teunissen | Talk 09:34, 12 May 2005 (UTC)


 * I think that you are finally starting to grasp the significance of the EP and where it fits into things. Do note that the 1907 paper was published a year before Minkowski gave his famous talk on "Space and Time" in which the metric structure of SR was described for the first time and the tensor nature of its operation described.  (BTW - Minkowski was professor for whom Einstein was a former student.  After Einstein heard about this speech, his response was "I should have paid more attention in his classes".)   So even historically, the EP was seperate from the development of the geometrical view of spacetime.


 * I would be careful about calling the spacetime of GR "diffeomorphic". Diffeomorphism is the same topology being expressed in different coordinate systems.  Technically, the symmetry group of GR is the diffeomorphism group, and the meaning of "diffeomorphic" is to indicate that two metrics are describing the same topology.


 * As for historicity: Be advised that even after the EP came about the idea of "making it work mathematically" still seemed to be a wacky thought.  Indeed, many scientists continued to think that it was wacky even after Eisntein published his field equations in 1915.  Just once again be advised that the EP is only part of the edifice that is GR. --EMS | Talk 14:44, 12 May 2005 (UTC)

Disputed tag
Is there any possibility of removing the tag in the near future? A lot of work has gone into this article, and I, for one, am reasonably happy with the form that has been established.–Joke137 18:59, 24 May 2005 (UTC)


 * I have let that business lapse for now. I am not totally happy with this article myself, but I need a block of time with which to tweak it and I do not know when I will get it.  I echo your feelings about the amount of work that we have put into this.  For me, what is galling about that "Disputed" tag is that it is Cleon's doing, and his objections are not anything that either of us care to entertain.


 * Since Cleon has agreed not to edit this page and has moved away from worrying about this, I think that it is time to remove it, and I will do so. If Cleon objects and restores it then we must consider an alternate course of action, but we can cross that bridge when we come to it. --EMS | Talk 19:59, 24 May 2005 (UTC)

Warning messages
I combined the warning messages to editors so the article isn't pushed down so far. -- Kjkolb 12:36, 25 November 2005 (UTC)

New section
The new section, added by an anonymous user, entitled "Proposal for an extended equivalence principle," doesn't make too much sense to me. While it is undoubtably a serious addition, my feeling is that something needs to be done to improve it if we are to keep it. Any opinions, EMS? –Joke137 15:41, 5 December 2005 (UTC)


 * The text includes the statement:
 * This suggests a generic mechanism for avoiding singularities.
 * That and the lack of a link or reference to a specific proposal leads me to the conclusion that we are dealing with original research here. Unless you object, I think that we should revert this section out.  (I agree that the author is serious about this proposal, but then again all such editors are.) --EMS | Talk 19:27, 5 December 2005 (UTC)

That sounds fine to me, unless the original author would care to come forward. The articles he links to are the sort of very formal relativity where I'm not too comfortable anymore, unfortunately. –Joke 20:48, 5 December 2005 (UTC)

"Proposal ..." has been removed
An anonymous editor added to the article


 * === Proposal for an extended equivalence principle ===
 * The equivalence principle remains an active research area, such as the proposed extension described below. The different versions of the equivalence principle discussed above are applied at a point, based upon the local flatness of spacetime at any point. In 1922 Fermi proved the existence of Fermi coordinates along any curve under which space is locally flat at each point along the curve (see for instance Nestorov ). Iliev observed that the equivalence principle can be applied in this larger locally flat space along the curve, rather than pointwise. Monroe  argues that time dilation of $$(1-v^2)^{-1/2}$$ observed in this locally flat space is indistinguishable locally (along the curve) from a relative velocity v. Therefore, the unbounded time dilation observed as an event horizon emerges at the center of a collapsing star implies that the center is in reality as well as appearance receding at a velocity approaching the speed of light, producing a bubble-like local inflation of the star's interior even as the star continues to collapse as seen from the outside (as conjectured by Shatskiy ). This suggests a generic mechanism for avoiding singularities. This extension of the equivalence principle has not been tested, although it is one of several proposed explanations for the Pioneer anomaly.

This is a higly technical and dense discussion. However, its most disturbing chatracteristed is that while it relies on other available work, it does not seen to reference a single document that describes the proposal itself. Perhaps I am mistaken about this, but unless the author can show otherwise, I must assume that this is original research, which is prohibited by Wikipedia policy. --EMS | Talk 02:12, 6 December 2005 (UTC)

additions
I haven't checked in on this page in a while. These tables have been added, but they weren't very carefully integrated into the text. The first table is a repeat of our table, except taken from a copyrighed source. The second, I can sort of fathom what it means, but it is not explained at all. –Joke 01:18, 30 January 2006 (UTC)

Ciufolini & Wheeler <I>Gravitation and Inertia</I> (Princeton University Press: Princeton, 1995) pp. 117-119<BR>

Beryllium-magnesium and beryllium-titanium respectively give 0.1919% and 0.2398% difference/average nuclear binding energies. These are among the largest net active mass composition tests possible. 400+ years of Equivalence Principle tests have given zero output within experimental error.

<SUP>a</SUP>(nuclear mass)/(atomic mass), corrected for isotopic abundance<BR> <SUP>b</SUP>globally aligned undecatiplet<BR> <SUP>c</SUP>iron core rather than homogeneous body

One unexamined Equivalence Principle test remains: Does a left hand fall identically to a right hand? Handedness arises from relative mass distribution. A set of atoms is handed if it does not exactly superpose upon its mirror image. Geometric parity is handedness in all directions simultaneously. Parity divergence gives net active mass 400-500 times as large as composition does for the same test mass loaded. Does a solid single crystal sphere of left-handed quartz fall identically to a chemically and visibly identical solid single crystal sphere of right-handed quartz?

Categorization
Mpatel - I must politely disagree with your decision to change the categorization of this page so that it appears at the beginning of category:general relativity. My concerns are: At the least, I would reserve that honor for "core" articles, such as general relativity and perhaps general relativity resources. The equivalence principle, while important, is not part of that core in which GR itself is explained in depth. So I will revert.
 * if you place too many articles up front, the impact of that placement is lost ("If everything is important, nothing is important."), and
 * I fear that people will tend to look for this article under "E" instead of at the start.

If you would like a suggestion, perhaps you can create a parallel category for the core concepts of GR, and add that category to the relevant articles. --EMS | Talk 04:17, 25 February 2006 (UTC)


 * Hi EMS. I think you're right. I may also take up that suggestion of a parallel category. MP  (talk) 08:03, 25 February 2006 (UTC)

Recent disagreement with the equivalence principle
An anonymous editor recently added:


 * All bodies at the same spacetime point in a given gravitational field will undergo the same acceleration. Which is incorrect! STOP RAPING!!!. Only when space is flat do its coordinates have direct meaning of distances or angles, and if one set of coordinates have direct meaning of distances or angles then the space must be flat. This is the famous Riemann theorem when he pineered the concept of curved space.
 * General relativists claimed curved spacetime. However, when testing GR with data, I see that all textbooks consider Schwarzschild coordinates to have direct meaning of distances or angles.
 * Therefore, I have strong feeling that they rape common human sense. If there is no scientific conspiracy here then there must be some relativist standing out to meet my challenge!! see here.''

You do not seem to understand curvature, metric tensors, or general relativity. Simply put, a metric tensor relates physical and coordinate distances, and in the process can also be used to determine the local angles between coordinate vectors. The relationship between coordinate changes and distances being arbitrary does not mean that distances (and angles) cease to exist. You are effectively telling me that you cannot define distances or directions on the surface of the Earth (since it is curved). Right.

If nothing else, these are your own personal opinions, but Wikipedia is not a soapbox. I refer you to WP:NPOV, part of the NPOV standard. The relevant part reads:
 * If a viewpoint is held by an extremely small (or vastly limited) minority, it doesn't belong in Wikipedia (except perhaps in some ancillary article) regardless of whether it's true or not; and regardless of whether you can prove it or not.

By placing your own personal opinions and original research here, you are vandalizing this encyclopedia. It is kindly but forcefully requested that you stop this NOW. --EMS | Talk 21:48, 28 August 2006 (UTC)

MISSING THE POINT
EMS apparently does not understand that "curvature - metric tensor - general relativity" are supposedly consequences of the application of the Equilalence Principle, and applied in ad hominem fashion, i.e. "well you don't understand these things so you must be stupid and that invalidates your argument", to the anynomous poster simply does not deal with the issue of what the Equivalence Principle is.

Contrary to EMS's objection that the validity of EP is an accepted "viewpoint", it is in fact a postulate, and subject to inquiry on a scientific and logical basis, particularly in an article which presumes to describe it. The simply stated version is that uniform [generated] accelleration is indistinguishable from the acceleration due to gravity. This is either true or it is not true. It is of no help to ignore obvious contradictions between this article and say the article on the N-Body problem, or facts gleaned elsewhere such that gravity acts towards the center of mass - inertia does not - the force of gravity increases as the distance between the bodies decreases - inertia does not, and other such information which is hardly in the realm of "personal opinion". Moreover it does not help to cloud the issue with so-called confirmation without providing a caveat and disclaimer regarding affirming the consequent, i.e. If EP is true all masses will "fall" at the same rate; we dropped some balls and could detect no difference in their rate of fall; therefore EP is true. This is a typical non sequitur.

Finally, "forcefully" requesting [demanding] no information contrary to the "popular viewpoint" is hardly germaine regarding scientific theory ... especially when such information is available in articles elsewhere in the Wikipedia.

DasV 15:34, 18 September 2006 (UTC)


 * I must admit to having some difficulty parsing a cohrent point out of this diatribe. First of all, it may be instructive for DasV to look at the edits of this anon:  and .  These are not thoughtful edits to an encyclopedia article, but instead are disruptive personal comments.  Beyond that, while historically the equivalence principle led to the "curvature - metric tensor" paradigm of general relativity, in general relativity itself the equivalence principle is a consequence of that paradigm:  Geodesic motion in curved spacetimes results in free fall being inertial motion, the point of the equivalence principle.
 * I won't disagree with the EP being treated as a "postulate" needing to be tested. (Even as a theorem of GR it's being tested in still valuable as that will also test the foundations of GR.)  However, so far all tests of GR and the equivalence principle have confirmed both the theory and the principle.  I also know of no coherent opposition to the equivalence principle at this time, but even if there was such a movement its existance cannot be announced by injecting POV statements that "this is incorrect" into an article. --EMS | Talk 00:47, 19 September 2006 (UTC)

Frankly I don’t see how the point could have been made more briefly or more coherently. As to “diatribe”, well that’s the point of discussion, and EMS’s comments to the anon were certainly sharp abusive denunciations. Nevertheless:

1. This "curvature - metric tensor - general relativity" are supposedly consequences of the application of the Equilalence Principle” and this, “the equivalence principle led to the "curvature - metric tensor" paradigm of general relativity” are simply two versions of saying the same thing. The point is that it is a fallacious argument to suggest that the anon must not be aware of these consequences, and thus his statements are proven wrong. Such reasoning detracts from the subject of the article … the equivalence principle.

2. EMS departs from the issue once again to inform the anon of the part of the NPOV standard regarding viewpoints. An axiom or postulate of a theory is not a “viewpoint” accepted or otherwise in the sense that a poll is taken on the street and whatever the majority thinks determines its inclusion or exclusion from an encyclopedic article. The postulate states and means something in and of itself whether everyone agrees or disagrees with its validity. The point of an article on such a postulate is to explain what it means, its derivation, how it used to build hypothesizes, how it is tested, possible variances, etc.

3. In the case of EP there is not only evidence to support it, but some evidence does not point to EP as original stated to the exclusion of all other explanations, and within other articles within the Wikipedia there exist articles that are at variance to the equivalence principle as originally stated, to wit the N-Body Problem, and articles on gravity and inertia which indicate that gravity acts towards the center of mass - inertia does not - the force of gravity increases as the distance between the bodies decreases - inertia does not. This is why by the way that other sources such as Britannica say that [EP] states that gravitational forces and inertial forces are of a SIMILAR nature and OFTEN indistinguishable.

4. This means you cannot have it both ways. Either [from the article] “gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is actually the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.”, or this is not the case. Either [from the article] “The origins of the equivalence principle begin with Galileo demonstrating in the late 16th century that all objects are accelerated towards the center of the Earth at the same rate.”, this is truly demonstrated or it is not. In other words if one’s “box” is five miles high and in uniform acceleration and a ball is dropped it will fall exactly, trajectory and all, as a ball dropped from five miles above the earth [excluding air friction], or it will not. And if you drop a 10 kilo ball from one mile above the moon it will fall to the surface in exactly the same amount of time as a ball with the mass of Jupiter will, or it won’t. There is no “viewpoint” involved here.

5. You cannot conclude the equivalence principle has been proven by citing experiments which support it. You can say it has wonderful predictive value, like epicycles, but citing reams of experiments without this caveat is not scientific.

6. Leading the reader of an article to believe a postulate of a scientific theory is a proven fact is not justified even if it is believed by every scientist on the planet.

Now as to EMS editing/removing the anon’s edits to the article, this was appropriate. Notwithstanding the anon’s feelings that the information was intrinsically wrong does not justify including material which simply confuses the issue even more and destroying the article. However, there is no reason the anon could not make his argument on the discussion page, and it stands or falls on its merits. This gives everyone an opportunity to see his objections and what he would propose to add or delete.

However the point is the article reads like a badly worded high school physics text, and is rife with obvious contradictions of information provided in other articles. The citations are verbose and unhelpful; the Britannica does it with two sentences, “All dynamic experiments yield the same results as obtained in an inertial state of uniform motion unaffected by gravity. This was confirmed to a high degree of precision by the Hungarian physicist Roland Eotovos.” Additionally, it ignores current investigation of the equivalence principle such as “Was Gallileo Wrong” http://www.nasa.gov/vision/universe/solarsystem/06may_lunarranging.html, “Evicting Einstein” http://science.nasa.gov/headlines/y2004/26mar_einstein.htm, and “Microgravity” http://www.nasa.gov/centers/glenn/research/microgex.html. I would hope EMS would not regard NASA as a “small or vastly limited minority”. Maybe EMS will take the time to observe some “coherent” questions regarding the postulate among scientists participating in the NASA missions.

I hope EMS finds this latest diatribe is more easily “parsed”. DasV 16:43, 19 September 2006 (UTC)

I will try to focus my comments on criticism of the article itself, since we're all in agreement that the anon's edits ought to have been reverted. I don't really see how the article reads like a "badly worded high school physics text." The equivalence principle, like Mach's principle, is not a single principle and not a simple part of modern physics. Many naïve statements of the principle have, in fact, been falsified. The sources cited in the article are many of the essential original forces used in developing the principle as it is now known, as well as current review articles on the subject. Before you suggest that the article "ignores current investigation of the equivalence principle" might I suggest you actually read it in its entirety?
 * 1) This doesn't seem relevant to the article.
 * 2) This doesn't seem relevant to the article.
 * 3) There is very little evidence suggesting that the equivalence principle is violated. There are some experiments that purport to measure a variation in the fine-structure constant, and the University of Washington Eöt-Wash group has some very tentative indications of strange gravitational behavior at very short distances. As for the examples you mention, you are fundamentally understanding what the principle means. The equivalence principle does not state anything about the way in which gravity acts – the force law and its direction –  it is merely a statement that gravitational motion is locally indistinguishable from inertial motion. These points have been discussed exhaustively in the talk page archives. If you think they are insufficiently clear in the article, feel free to propose clarifications.
 * 4) I fail to see your point here.
 * 5) I don't think the article says the equivalence principle has been proven. In fact, as you mention, there are several ongoing experiments designed to test the EP. These are mentioned in the article.
 * 6) Where do we do this?

Finally, let me say that I don't think your abrasive tone is particularly helpful. –Joke 17:25, 19 September 2006 (UTC)

Okay:

1 addresses the fallacious argument as detracting from the issue of why the edits were removed in the first place, as they apply to the article.

2 the NPOV is not appropriate since EP is not a point of view for the reasons stated. It is relevant to set forth the objective of the article.

3 the point here is the original stated version of EP, which I addressed originally, from which derivations have been made. I think you meant to say you think I am fundamentally “misunderstanding” what EP means. Perhaps, but if so then that’s the fault of the article isn’t it? Since after reading it I should have a fundamental understanding yes? The tag “locally” came after the formulation, and is not applied to such phenomena as Galileo’s falling balls. As to “locally” finding somewhere in the universe without gravity, you’ll pardon me if I’m unconvinced of that. Yes I agree they have been exhaustively discussed, nevertheless they will continue to be discussed because the statements are not sufficiently clear. That’s why it is commonly believed and taught that the mass of the balls will not effect their rate of fall. Yet Newton’s law of universal gravitation is exhibited in another article which states the contrary. That leads to 4.

4 says if you claim it is true that uniform accelerated motion and motion due to gravity are actually the same [EP] then the action of falling balls in either system will be exactly the same. Newton says that a ball dropped from 5 miles above the earth [assuming no air friction] will experience a change in the rate of acceleration – the force of attraction between the earth and the balls will be proportional to both masses – will fall towards the center of mass of the system, while the ball released to fall through the floor in the inertial frame of the elevator will continue in a straight line at a constant velocity as the “elevator” accelerates away from it. It cannot be both.

4 says if you claim that Galileo’s experiment demonstrated that balls fall towards the center of the earth regardless of the mass of the balls then any objects dropped in a gravity field will fall to the surface of the object creating the gravity at the same time. Newton says if you drop a 10 kilo ball from one mile above the surface of the moon and then later drop an object with the mass of Jupiter from one mile the time at which they each arrive at the surface will be drastically different due to the mutual attraction of the masses. It cannot be both. Are you saying that the original statements from Galileo and Einstein were naïve statements of the principle?

5 says that as presented in the beginning of the article and quoted in 4 the statements read like facts.

6 follows from 5

Yes I read the entire article, more than once. I entered the discussion on the points that EMS raised with the anon. I still say it reads like a high school physics text in that it does not adequately deal with the issues I cited. An encyclopedia article imho should address them, since your reader moves from your article on gravity to GR – SR, for which the equivalence principle is the bridge. There is nothing wrong with a high school physics text, except that it presents and manipulates concepts, without necessarily discussing impacts between them. The article is badly worded in that someone without first hand familiarity of many of the terms would be lost without looking up some of the theories presented. Much of it is over the head of most. This is a typical failing of “experts” in that what they perceive as obvious may only be obvious to themselves and God. I see no difficulty with adding such research as NASA is doing, and even discussing how it would change current understanding.

As to the abrasive tone, I think that is in the eye of the beholder, as is the “diatribe”. If you’d like to meet for a drink I’ll be quite sociable. DasV 19:16, 19 September 2006 (UTC)


 * DasV wrote above:
 * 4 says if you claim that Galileo’s experiment demonstrated that balls fall towards the center of the earth regardless of the mass of the balls then any objects dropped in a gravity field will fall to the surface of the object creating the gravity at the same time. Newton says if you drop a 10 kilo ball from one mile above the surface of the moon and then later drop an object with the mass of Jupiter from one mile the time at which they each arrive at the surface will be drastically different due to the mutual attraction of the masses. It cannot be both. Are you saying that the original statements from Galileo and Einstein were naïve statements of the principle?
 * Speak of missing the point! According to the equivalence principle, if you placed a ping-pong ball and a Jupiter-mass ball side-by-side 10 miles above the Earth, they both will fall towards the Earth at the same rate!  Admitedly both the Earth and the ping-pong ball will be very much affected by the Jupiter-mass (as they must both fall towards it too), but ignoring that you will find both accelerating together towards the Earth.  BTW - This is also the case in Newtonian mechanics.
 * More to the point is that the amount of "gravitational force" that each object reacts to is directly proportional to the mass of the object. Under D'Alembert's principle, the same applies to a ball dropped in an accelerating elevator in deep space if the observer in the elevator assumes that they are in an inertial state of motion and the ball is being acted on by a force.  (In other words, the pseudoforce on a ball dropped in a space elevator in proportional to the mass of the ball.)  That is what the equivalence that the equivalence principle is all about.  As for NASA:  It has long been known that tidal effects permit an observer to distinguish between being on the Earth or in an accelerating rocketship in deep space.  That does not contrdict the overall equivalence of the action in both cases being a manifestation of D'Alembert's principle.
 * I hope that this settles this issue. --EMS | Talk 03:48, 20 September 2006 (UTC)

Yes speaking of missing the point. You did not address the question, you changed the parameters. Calculate the times in the example given and say whether they are the same or not, i.e. time for a 10 kilo mass to fall to the surface of the moon and the time for a Jupiter size mass to fall to the surface of the moon both from 1 mile above the surface. Or use the surface of the earth if you like instead of the moon. Two separate incidents, not both at the same time. To be consistent with "Galileo demonstrating in the late 16th century that all objects are accelerated towards the center of the Earth at the same rate" the times must be equal. Any object regardless of mass must "fall" to the towards the center of the Earth [moon] at the same rate. If the times are equal in the example then it settles the issue ...simple.

And this, "Admitedly both the Earth and the ping-pong ball will be very much affected by the Jupiter-mass (as they must both fall towards it too), but ignoring that you will find both accelerating together towards the Earth." I assume is a 3-body problem; regardless of the fact that the mass of the ping pong ball is trivial all of the masses really do interact - according to Newton.

And yes it has been known that tidal effects [gravity] would permit an observer to distinguish between Earth and an accelerating elevator for all the reasons I set out above. Which means that the gravitational forces as experienced locally while standing on the Earth are ACTUALLY NOT the same as the pseudo gravitational "forces" experienced by an observer in a non-inertial (accelerated) frame of reference. It is curious that you do not understand this is inconsistent with "gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is ACTUALLY [emphasis added] the SAME [emphasis added] as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.” from the article. Nowhere in the article have you explained away the inconsistency or the necessary changes in the understanding of EP from inception to now.

BTW you can settle the issue by simply ignoring the inconsistencies. They are pretty much ignored in the mainstream. DasV 15:35, 20 September 2006 (UTC)


 * Feel free to invent "inconsistencies" if you like. However, the result is original research, and that cannot be presented here.  --EMS | Talk 03:51, 21 September 2006 (UTC)

Original research? So we go on.

From Wikipedia articles:

In physics, gravitation or gravity is the tendency of objects with mass to accelerate toward each other. Force equals G times mass of one object times the mass of the second object divided by the radius squared between them. On the Moon,the Moon's gravity: 2.4 km/s, on Earth,the Earth's gravity: 11.2 km/s, on Jupiter, Jupiter's gravity:59.5 km/s.

Under normal earth-bound conditions, when objects move owing to a constant gravitational force a set of kinematical and dynamical equations describe the resultant trajectories. For example, Newton’s law of gravitation simplifies to F = mg, where m is the mass of the body. This assumption is reasonable for objects falling to earth over the relatively short vertical distances of our everyday experience, but is very much untrue over larger distances, such as spacecraft trajectories, because the acceleration far from the surface of the Earth will not in general be g.

The earth itself experiences an equal and opposite force to that of the falling object, meaning that the earth also accelerates towards the object. However, because of the immense mass of the earth this acceleration is vanishingly small.

Every body perseveres in its state of being at rest or of moving uniformly straight ahead, except insofar as it is compelled to change its state by forces impressed. [Cohen & Whitman 1999 translation

EP … gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is actually the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference. The origins of the equivalence principle begin with Galileo demonstrating in the late 16th century that all objects are accelerated towards the center of the Earth at the same rate

Of course none of the above is “original research”. As is done in high school physics classes routinely, from the information above it is simple enough to derive:

1. Gravity acts towards the center of mass of the object, (system of objects), and the force of gravity is proportional to distance. Inertia ( object released in uniformly accelerated system) means an object will move uniformly in a straight line, and absent a force it will not change its rate of movement, (as an object in a gravity field will in accordance with the change of distance). (It may be trivially noted that the mass of the object and the mass of the rocket ship will interact – this is one of the interests of NASA’s Microgravity project.)

2. The rate at which an object with a mass of 10 kilos falls to the Earth is drastically different than the rate at which an object with a mass of Jupiter falls to the earth. (Understanding that the objects are in reality attracted to each other.) Had EMS taken the time to work the example problem he might have understood this. Or:

Quiz 1. Determine the rate of change in acceleration for an object falling from 5 miles above the Earth from its beginning position to its contact with the Earth. 2. Determine the rate of change in acceleration between a uniformly accelerating rocket ship and an object released from that rocket ship after a separation distance of 5 miles. 3. Is the rate of change in acceleration the same for both 1 & 2? 4. Determine the time for a object of 10 kilos to contact the Earth’s [moon’s] surface after released from 1 mile above the surface. 5. Determine the time for an object with the mass of Jupiter to contact the Earth’s [moon’s] surface after released from 1 mile above the surface. 6. Are the times in the two problems above the same?

Nothing “original” going on here; common everyday high school physics problems solved with information provided by Wikipedia articles.

Given the information provided in the other articles, the article on EP, (and EMS it appears), claims that:

Gravitational force = “pseudo-force” [inertia outside of a massive gravity field]

All objects are accelerated towards the center of the Earth at the same rate = objects are attracted in proportion to their mass

It should be obvious to anyone with the ability to pass the Quiz above that the answers are at a variance with the information provided in the EP article. This is what EMS claims are “invented inconsistencies”. Apparently the Britannica “invented” them long before I did, as they are careful in their articles not to make the above errors. To repeat the quote from Britannica, “[EP} states that gravitational forces and inertial forces are of a SIMILAR nature and OFTEN indistinguishable”. Nowhere do I find a statement in the Britannic claiming that all objects regardless of mass will fall to the Earth at the same rate.

As I said these inconsistencies are pretty much ignored in the mainstream, (popular articles, tv, high school classes, etc.), and therefore by the NPOV standard it is permissible to ignore them. However, it is unscientific and non-objective to claim they do not exist, were invented by this author, or constitute original research in any way. Such hyperbole in a reputable scientific journal would not be allowed. I hope this will not be followed by further alliteration. DasV 20:44, 21 September 2006 (UTC)

From the article:
 * The principle does not apply to large bodies, which might experience tidal forces, or heavy bodies, whose presence will substantially change the gravitational field around them.

Your example with Jupiter is caught by the latter exception, I believe. –Joke 13:19, 22 September 2006 (UTC)

I agree with the statement, but "large" is not quantified. Any mass of relatively significant size will produce a different falling time from that of a negligible mass. This is particularly demonstrable in the case of eliminating the "side by side at the same time" parameters, which produces a 3-body problem. Try it with Gaspra instead of Jupiter and see if you agree. The statement as it stands in the beginning of the article leads to many silly conclusions, i.e. the moon is “falling” towards the earth; replace the moon with EMS’s ping pong ball and see if the orbit is the same.I will not suggest that in reality any mass will react in proportion to its quantity, but that the behavior of negligible size masses in relation to Earth size masses poses significant problems in measurement [in light of the evidence so far] as I would be accused of “original research” … I will leave that task to NASA.

The exception(s) should be forthcoming in opening statements of the article. The Britannica example “[EP} states that gravitational forces and inertial forces are of a SIMILAR nature and OFTEN indistinguishable” is a more accurate statement than claiming gravitation and inertial forces are the “same”, and Galileo’s “demonstration” should be said to be limited to negligible masses WRT the Earth, or at the least as you have pointed out “except WRT significant masses which propagate substantial changes to the field. Few may read all the way to the statement you point out, and it should be clear in the introduction. DasV 15:29, 22 September 2006 (UTC)


 * I'll try one last tack here. First of all, we will assume GR and the strong equivalence principle (where even a Jupiter-mass object will accelerate towards the Earth or Moon as the same rate as a ping-pong ball.  Other theries (like Brans-Dicke theory lack this, but I am not discussing them.)  Now suppose that we attach hyper-powerful rockes to the Moon which produce enough thrust to counter the gravitational pull of the Jupiter-size mass.  In that case, a Juipter-size mass will reach the Moon from 5 miles above in the same time as a seperately dropped ping-pong ball.  That is the gist of the strong equivalence principle, and that has (so far) been confirmed by observation.
 * For tidal effects, I admit that there is a difference, and am willing to see a short and throughtful mention of it. OTOH, your Jupiter-mass-object example only demonstrates a lack of understanding of this principle. --EMS | Talk 16:37, 22 September 2006 (UTC)

You are reasoning fallaciously by assuming the consequent. In lay terms you have the cart before the horse. As Dr. Phillips and P. Barry at NASA correctly point out, “In particular, Einstein crafted his theory of gravity, i.e., the general theory of relativity, assuming the Equivalence Principle is true.” In other words the validity of GR, (and SEP), relies first on the validity of the EP from which it was derived. You cannot “assume” that GR is correct in order to prove that from which GR was derived. Nor can you claim observation has “confirmed” SEP, which is a consequence of GR, which is a consequence of EP, which proves EP.

Once again take a look at the Wikipedia on the subject: Using Newton's law of gravitation, proving Kepler's Laws for the case of a circular orbit is simple. Elliptical orbits involve more complex calculations. Using Lagrangian mechanics it is possible to develop a single polar coordinate equation that can be used to describe any orbit, even those that are parabolic and hyperbolic. This is useful for calculating the behavior of planets and comets and such. More recently, it has also become useful to calculate spacecraft trajectories.

Contrary to the above, the article claims that Newton’s law of gravitation does not enable the calculation of the behavior of planets and comets and such. The claim is the use of Newton’s law of gravitation to determine the time of “fall” of a Jupiter sized mass to the surface of the Earth [Moon] and the time of “fall” of a 10 kilo mass to the surface of the Earth [Moon] will not provide accurate times. According to EP they must be equal.

Your statement, “First of all, we will assume GR and the strong equivalence principle where even a Jupiter-mass object will accelerate towards the Earth or Moon as the same rate as a ping-pong ball”, “might” even be true, but it has not been demonstrated; especially not by reliance on another theory which relies on it having already been demonstrated it to begin with, which has never been done. It is simply circular reasoning. It will take a very well confirmed hypothesis, with high precision predictability, to tear up all the current books on celestial mechanics.

The article states as a fact that “The origins of the equivalence principle begin with Galileo demonstrating in the late 16th century that all objects are accelerated towards the center of the Earth at the same rate.”, which is categorically false, regardless of who believes otherwise. Galileo’s experiment demonstrated no such thing, otherwise all those researchers in your test table have been wasting their time on redundancy.

The statement needs to be revised to be accurate. Even, “The origins of the equivalence principle begin with Galileo’s experiments in the late 16th century which found there was no detectable difference in the rate of acceleration for objects of negligible mass, as compared to that of the Earth, moved by the force of gravity, regardless of the weight [mass] of the objects, as opposed to the Aristotelian concept that heavier objects fall faster than lighter ones.”, would be an improvement, though perhaps someone could phrase it better. That is what Galileo demonstrated. It is simple enough to be accurate in the article. That would settle this issue.

It may well be that I have a “lack of understanding” of what this principle is, but I clearly understand what it is not.

As to your suggestion on tidal effects, I agree that would be a good edit, except that a lay person will probably not understand that tidal means gravitational, however I leave that to your prerogative as a subject matter expert [SME]. DasV 20:39, 22 September 2006 (UTC)


 * The first and most important thing to deal with is your proposed edit. Overall, I like it.  It is factually and historically accurate, and does a much better job of relating Galileo's work to the equivalence principle.  I would edit it to
 * The origins of the equivalence principle begin with Galileo’s experiments in the late 16th and early 17th centuries. These experiments showed that the acceleration of test masses due to gravitation is independent of the amount of mass being accelerated.  This overturned the Aristotelian concept that heavier objects fall faster than lighter ones.
 * I use the term "test mass" as it is related to the relativity concept of a "test particle" and implies that the mass is small enough so that its own gravitational field does not affect the results. It also means that we will not have other editors cleaning up your text by removing the business of the masses being much smaller than that of the Earth due to its seeming trivial.   Let me know what you think of this, as your suggestion is useful.
 * As for your claiming that my noting that obsevation confirms that SEP is a circular argument: General relativity (GR) is a very specific theory.  It makes certain predictions.  In addition the SEP is a theorem of GR.  So observations that are in accord with the predictions of GR are confirmatory of both GR and SEP.  The original EP I am happy to step back from, but not too far.  It is true in the infinitessimal limit in both space and time in GR, but tides do refute it over extended space and time. --EMS | Talk 00:18, 24 September 2006 (UTC)

As I am editing from a remote location I will be brief. I believe your proposed edit captures the essence of the Galileo experiment, and the origin of EP. I agree that the WRT issue is dealt with adequately by the use of "test masses", and is a smoother statement. This coupled with the edit regarding the tidal forces will improve the accuracy of the article, the reader's understanding, while avoiding the "original research" and/or "mainstream understanding". I think this is good resolution, and I compliment you on it.

I will RTR to comment on the reasoning at a later time. DasV 17:34, 24 September 2006 (UTC)


 * The edit is done. If you wish to discuss my reasoning on how the EP is supported by observations, the feel free to e-mail me on it, as it is something of a distraction here.  OTOH, please feel free to make additional proposals for edits here.  Let's just say that I don't contest your facts, only the spin you give them. --EMS | Talk 00:24, 25 September 2006 (UTC)

Fair enough, although it may come up again and is germane to the "spin" [a good word for it I think]. It is exactly the "spin" or interpretation in the real world that is the basis of all controversy regarding scientific theory. I believe the edit(s) have improved the article; as it proceeds it travels areas which leaves less able travelers behind, but the introduction is sufficiently succinct so that a lay person can gain an accurate understanding of the principle sans mathematics. I don’t think I can add anything further which would not impart my own bias, which is not “mainstream”.

Allow me to close the subject with an old legend. Once upon a time, about 2,600 years ago a Pythagorean was walking to temple when he came across some workers who were laying out the foundation for a house. Observing their primitive tools for making the rectangular shape square, he stopped to enlighten them to an easier way to do it, which was of course the Pythagorean theorem, which he demonstrated with a 3-4-5 right triangle. The workers were amazed and thankful, except that when they tried the problem they simplified it even more by setting the lengths of the sides to 1 unit each. Arriving at the solution the square root of two, which was beyond their ability to calculate, they took the Pythagorean down to the river, drowned the troublemaker, and returned to their traditional methods.

This legend, which is probably not true [most good ones are not], illustrates the need for theories to correspond to reality and be practical. DasV 15:21, 25 September 2006 (UTC)


 * Correspondence to reality is the most practical concern in so far as scientists are concerned. There is a realm where Newton's theory of gravity is not as good as general relativity.  However, general relativity also shows that gravity gives you a more-than-adequate approximation for everyday situation here on the Earth.  So what is "practical" is to some extent a function of your needs.
 * As for any further edits, please feel free to make proposals here. Being aware of your biases is essential to doing good edits, and you seem to have that quality.  I am willing to see what points you wish to make and how well your concerns can be represented in this article.  I certainly am not going to accept any statement that the EP is junk, but if we can better describe its domain of applicability that will be useful. --EMS | Talk 16:52, 25 September 2006 (UTC)

I agree with you in the main here. EP being "junk" is not one of my [many] biases, and I think I would agree with your earlier statement that I do not understand what EP is, in the sense of all of the implications and derivations possible from its assumption, however I suggest that is likely not understood by scientists, since theories relying on it continue to evolve. Nevertheless after this exchange I think we both agree there is value in being as accurate as possible when dealing with language, which can be ambiguous and often imprecise. I hope my modest suggestions were helpful. I don’t think I can improve on the EP article without further scientific observations that would imply revision. Also I would not presume to alter an article without consensus.

Physicists build on principles such as Einstein’s formulation of EP, philosophers of science [natural scientists in the original sense] build on possibly the most important postulate he [and others] stated::

As far as the laws of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality. Albert Einstein, "Geometry and Experience", January 27, 1921

I look forward to going over the implications of this with you soon … in Time. :) DasV 15:23, 26 September 2006 (UTC)