Talk:Twin paradox/Archive 12

Proposed Update 2
Obviously, we would like to see the enhanced history section restored - with changes as discussed in our response above.

The only other major change to the article would be to insert the "Proposed Solutions and Associated Questions" section (see the "To: Wikipedia Twin Paradox Entry Administrators" entry above) after the current "Specific Example" section and before the current "Resolution of the paradox in special relativity" section.TwPx (talk) 04:08, 14 December 2007 (UTC)


 * To me it obvious that TwPx is trying to sneak an originally researched essay into Wikipedia. The central thesis of the essay is that the twin paradox is an ongoing symmetric debate with no clear outcome, between those who think that special relativity is a valid theory and those who don't. As I already stated, this is false. There is no debate. There is a group of lay people failing to understand the basics of a somewhat advanced part of physics. Misguided by the trivial mathematics which is used in this part, they feel they should be able to understand this, but they just don't. Putting Herbert Dingle central in this (see removed sections "Dingle" and "Post Dingle") is putting heavy undue weight to a person who made an eternal fool of himself in his -sorry- pathetic "Science at the Crossroads". Clearly TwPx thinks that Dingle had a case. Dingle did not have a case. If TwPx thinks otherwise, then TwPx clearly is a member of the aforementioned group of lay people. Taking into account the fact that TwPx "studied the Twin Paradox in depth for around 40 years" and fails to see how, for example, Dingle's central theme is trivially refuted, and thus still fails to understand the basics of special relativity, my well-meant advice is that he should seek another hobby. Wikipedia is not a publishing service for essays.
 * There is no open debate with no clear outcome among biologists about whether religious fanatics have a point pushing creationism. They don't have a point.
 * There is no open debate with no clear outcome about whether some people have a point thinking that the 1-2-3-4-5-6-7 lottery combination is far less likely than any other combination. They don't have a point.
 * DVdm (talk) 12:13, 14 December 2007 (UTC)


 * Once again, DVdm's argument is based on perjorative words and unsupported statements, e.g., "sneak", "this is false", "There is no debate", "group of lay people failing to understand the basics", "Misguided by the trivial mathematics", "they should be able to understand this, but they just don't", "fails to see", "is trivially refuted", "fails to understand the basics of special relativity", "he should seek another hobby". I also don't see the last two paragraphs adding logic that supports DVDm's assertions.


 * I think that I've been clear in my comments about what I wanted to do and in the actual proposed sections. DVdm has not said why the proposed section should be considered as something inherently objectionable (e.g., "an essay") as opposed to what's currently written for this article or any other Wikipedia article. DVdm seems to have misread the proposed sections. I'm simply accurately recording what was said about the Twin Paradox. I wrote about what the literature contains as represented in summary form by the references given above.


 * I nowhere say that Dingle was correct. I mentioned his views including the fact that those views changed and that when he was asked for an alternative to special relativity, he had none. I also quoted what was the most recent indepth review of Dingle Twin Paradox debate . If DVdm thinks Prof. Hasok Chang "fails to understand the basics of special relativity" and "should seek another hobby", then DVdm should give a specific quote from the referenced paper and give the detailed logic that refutes what Chang has written. The Chang paper was one of 3 Notes already in the article when I first read the article, I did not add it. I think Chang is very intelligent, professional and objective who thoroughly analyzed all written material and wrote a well reasoned paper.


 * Personally, I'm not a Dingle-ite. I've always thought special relativity was a valid theory. I have not made any assertions that special relativity isn't valid. I have accurately stated the ideas in the literature whether or not those ideas were put forward by people whose opinions agreed with mine.


 * Twice DVdm has mockingly quoted that TwPx "studied the Twin Paradox in depth for around 40 years". Actually, that forty year in depth study of the Twin Paradox literature is relevant. From reading DVdm's objections (e.g., what he erroneously referred to as speculations), it seems that many of DVdm's objections stem from not having the same broad exposure to the literature and misinterpreting my review of the literature as my expressing my personal views.


 * (For the 2nd time above, I asked DVdm to give supporting logic or retract his claim that TwPx “fail[s] to understand the definitions of (and difference between) proper time and coordinate time”. There's still no answer from DVdm.)


 * The alternative was entered into the Discussion on 10/30/07 (I since suggested it be broken into two sections and added (the first section as an enhanced History) to the current document.) You can read DVdm's comments. Everyone, including wwoods and Gscshoyru, should make his views known below as to whether they agree or disagree with DVdm's (and Tim Shuba's) content and style. (I see by DVdm's insightful comment following Tin Shuba's remarks that he's in agreement with Shuba.) If DVdm/Shuba don't represent the consensus view, then we can move on. If DVdm/Shuba do indeed represent the consensus view, it looks like we will go to Arbitration and we would like to be efficient and just go once so all who agree with DVdm/Shuba can be dealt with collectively. ThanksTwPx (talk) 05:45, 17 December 2007 (UTC)


 * Above, in this section, I proposed an addition. DVdm objected. I noted that DVdm's assertions could not be supported and then asked if anyone agreed with DVdm. No support has been given for DVdm's position. I will therefore add the section.TwPx (talk) 20:38, 3 January 2008 (UTC)


 * No one agreed with your additions. No support has been given for your position. There is no consensus. The fact that no-one replied to your proposal does not imply that "DVdm/Shuba don't represent the consensus view".
 * Reverted back for reasons already stated. Wikipedia is not a soapbox, nor a publishing service for originally researched essays. Try a private publisher or a blog on your personal website. Good luck, DVdm (talk) 21:27, 3 January 2008 (UTC)


 * The reasons DVdm gave above were clearly rebutted. No reply was given. DVdm continues to make unsubstantiated claims. DVdm has routinely deleted additions made in good faith by others. The deletion is not consistent with Wikepedia guidelines - see WP:Revert.TwPx (talk) 22:27, 3 January 2008 (UTC)


 * Repeat: No one agreed with your additions. No support has been given for your position. There is no consensus. The fact that no-one replied to your proposal does not imply that "DVdm/Shuba don't represent the consensus view". DVdm (talk) 22:38, 3 January 2008 (UTC)


 * DVdm continues to avoid the points raised and gives no substance. The reasons DVdm gave above were clearly rebutted. No reply was given. DVdm continues to make unsubstantiated claims. DVdm has routinely deleted additions made in good faith by others. The deletion is not consistent with Wikepedia guidelines - see WP:Revert.TwPx (talk) 01:52, 4 January 2008 (UTC)


 * Two applicable Wikipedia policies in this situation are the prohibitions against original research and constructing novel narratives from available sources. In addition, many of your statements are simply mis-representations of the alleged sources. For example, Einstein's 1905 paper does not say what you say it says. Your entire proposed section consists of original research, novel narrative, and misrepresented sources. Granted, the fact that your original research is idiotic drivel and your novel narrative is pure crackpotism may prejudice some editors against your proposal, but those facts are irrelevant. Simply by virture of being original research and novel narratives and misrepresentation, your proposal must be rejected. There's no need for (and no point in) any detailed rebuttal of your research and narrative.Lumpy27 (talk) 08:16, 4 January 2008 (UTC)


 * Lumpy27 follows the standard mode of DVdm of citing this or that Wikipedia policy as the reason for deleting contributions from others - in this case, "Original Research". At the same time, you and DVdm have consistently shown total disregard for following Wikipedia policy. For example, being rude and insulting (e.g., "idiotic drivel", "pure crackpotism"). Regarding the "Original Research" charge, I've already rebutted that above. Again, if you were familiar with the literature, you'd be aware that it's an accurate portrayal of the Twin Paradox Debate. You conclude, "There's no need for (and no point in) any detailed rebuttal of your research and narrative." That's a very convenient position since you unable to point out specific problems.


 * The one specific charge was "Einstein's 1905 paper does not say what you say it says." Please give the details. What did I write that was "wrong" and what specific statements in the 1905 paper do you think it contradicts - no generalities, be specific. ThanksTwPx (talk) 17:19, 4 January 2008 (UTC)


 * It's rather odd that you object to people basing their rejection of your edits on Wikipedia policy, rather than by giving explanations of the specific errors in your proposed material. Wikipedia policy is supposed to be the basis for evaluating edits. Editors are explicitly prohibited from basing their edits on anything else. For example, the "no original research" policy was formulated explicitly to deal with physics cranks, because the founders of wikipedia realized that it is impossible to ever convince a physics crank of the erroneousness of his beliefs. That's more or less the definition of a crank. So, rather than allowing wikipedia to lapse into a giant forum for endless arguments with physics cranks over content, the "no original research" and "no novel narrative" policies were created. Here is some relevant excerpts from official wikipedia policy explanations:


 * Just been reading the link to your page "A trivial refutation of one of Dingle's Fumbles". Wasn't clock A synced (using Einstein's method for clocks stationary to each other) to clock H, therefore making rate of A identical to rate of H? Ditto clocks B and N? I think Dingle tried to explain this on page 229.
 * --Jimbo2x (talk) 00:10, 9 January 2008 (UTC)

"Wikipedia's founder, Jimbo Wales, has described original research as follows: The phrase "original research" originated primarily as a practical means to deal with physics cranks, of which of course there are a number on the Web. The basic concept is as follows: It can be quite difficult for us to make any valid judgment as to whether a particular thing is true or not. It isn't appropriate for us to try to determine whether someone's novel theory of physics is valid; we aren't really equipped to do that. But what we can do is check whether or not it actually has been published in reputable journals or by reputable publishers. So it's quite convenient to avoid judging the credibility of things by simply sticking to things that have been judged credible by people much better equipped to decide. The exact same principle will hold true for history" (WikiEN-l, December 3, 2004).

The phrase "original research" in this context refers to untested theories; data, statements, concepts and ideas that have not been published in a reputable publication; or any new interpretation, analysis, or synthesis of published data, statements, concepts or ideas that, in the words of Wikipedia's founder Jimbo Wales, would amount to a "novel narrative or historical interpretation".

If a viewpoint is held by an extremely small (or vastly limited) minority, it doesn't belong in Wikipedia ... regardless of whether it's true or not; and regardless of whether you can prove it or not.

What counts as a reputable publication?

Reputable publications include peer-reviewed journals, books published by a known academic publishing house or university press, and divisions of a general publisher which have a good reputation for scholarly publications."

So, you see, the question is not whether your proposed research and narrative is "true", the question is whether it reflects the point of view of more than just a very small minority of individuals, and this question is to be answered by citing reputable sources, and those sources must be presented in proportionate fashion. In other words, if 1000 reputable sources say X, and 3 reputable sources say Y, then Y deserves only about 3/1000 as much attention in the article as X, and since most articles are of a length that 3/1000 would be virtually nothing, there is an effective lower bound on viewpoints held by extremely small minorities. Such viewpoints do not belong in wikipedia, per the editing guidelines. You are trying to insert into this wikipedia article material that is explicitly prohibited by wikipedia policy. You should stop trying to do that.Lumpy27 (talk) 18:29, 4 January 2008 (UTC)


 * I understand the statements in bold and agree with them completely. However, as you should know, it's not relevant to what I wrote. I'm not proposing anything new. I'm simply, as you should know, recounting what was written about in the Twin Paradox Debate. I've gone into this, including the rationale, in great detail above.


 * If I tried to discuss Lorentz Aether Theory (LAT) in the Special Relativity article, then what you wrote would be relevant. However, this topic is different as discussed in detail above. I'm glad you brought up the point about reputable sources as virtually all the literature on the Twin Paradox discusses the "Proposed Solutions and Associated Questions" I discuss. Few if any have the main topic that there never was a Twin Paradox debate.TwPx (talk) 22:22, 4 January 2008 (UTC)


 * Your proposed addition is entirely original research and novel narrative. Just to give one simple example so you have some idea what I'm talking about, we need go no further than the second sentence in your essay. (The first sentence is rather poor as well, but might be passable.) Your second sentence is

"In that paper, he derived the (special relativity) time dilation equation that said that all inertial (i.e., non-accelerating) observers would observe all other inertial observers’ clocks to be running slow by the reciprocal of the Lorentz factor. So the greater the relative velocity between two inertial observers the slower they would observe each other’s clock to be running."

Presumably your source for this sentence is Einstein's 1905 paper, but you have not quoted the paper, you have instead offered your paraphrase, which unfortunately embodies the very ambiguities and misconceptions that underlie the fallacies in all crackpot "reasoning" about special relativity. The paper nowhere says "inertial observers would observe all other inertial observers’ clocks to be running slow". That's a meaningless assertion as it stands, because it doesn't define what it means for an observer to observe a moving clock. The paper refers, instead, to the rate of a clock when viewed from (i.e., in terms of) "the stationary SYSTEM" or from any "SYSTEM in uniform motion". The crucial word there is 'system'. This word has been defined previously and very carefully in the earlier section of the paper. Anyone who grasps the difference between what Einstein wrote in the cited paper and how you paraphrased it will also understand all the fallacies and non sequiturs in the remainder of the proposed alternative version of this article. And this is just the second sentence. Each of the subsequent sentences is crammed full of just as much (or more) misrepresentation and original research and novel narrative.

As a constructive suggestion, I recommend that you withdraw your current proposal, and re-write it, this time without paraphrasing. When you want to tell the reader what Einstein said, tell the reader what Einstein said, verbatim. I also suggest that you focus on presenting the history of the subject as it is presented in any of the numerous reputable sources that are widely available. Do not go back and dig up letters to the editors of Nature in 1956 and string them together with your own interpretations, and then claim that you are not engaging in original research or novel narrative. If you base your summary on the 99.99% consensus views expressed in reputable sources, rather than your own interpretation of the 0.01% views of a couple of retired engineers and armchair philosophers, I think the result will be more likely to be accepted here in Wikipedia.Lumpy27 (talk) 01:01, 5 January 2008 (UTC)

Here is a quote from Einstein's 1905 paper, for reference:


 * Further, we imagine one of the clocks which are qualified to mark the time t when at rest relatively to the stationary system, and the time $$\tau$$ when at rest relatively to the moving system, to be located at the origin of the co-ordinates of k, and so adjusted that it marks the time $$\tau$$. What is the rate of this clock, when viewed from the stationary system?


 * Between the quantities x, t, and $$\tau$$, which refer to the position of the clock, we have, evidently, x=vt and
 * $$\tau=\frac{1}{\sqrt{1-v^2/c^2}}(t-vx/c^2).$$


 * Therefore,
 * $$\tau=t\sqrt{1-v^2/c^2}=t-(1-\sqrt{1-v^2/c^2})t$$


 * whence it follows that the time marked by the clock (viewed in the stationary system) is slow by $$1-\sqrt{1-v^2/c^2}$$ seconds per second, or--neglecting magnitudes of fourth and higher order--by $$\frac{1}{2}v^2/c^2$$.


 * From this there ensues the following peculiar consequence. If at the points A and B of K there are stationary clocks which, viewed in the stationary system, are synchronous; and if the clock at A is moved with the velocity v along the line AB to B, then on its arrival at B the two clocks no longer synchronize, but the clock moved from A to B lags behind the other which has remained at B by $$\frac{1}{2}tv^2/c^2$$(up to magnitudes of fourth and higher order), t being the time occupied in the journey from A to B.


 * It is at once apparent that this result still holds good if the clock moves from A to B in any polygonal line, and also when the points A and B coincide.


 * If we assume that the result proved for a polygonal line is also valid for a continuously curved line, we arrive at this result: If one of two synchronous clocks at A is moved in a closed curve with constant velocity until it returns to A, the journey lasting t seconds, then by the clock which has remained at rest the travelled clock on its arrival at A will be $$\frac{1}{2}tv^2/c^2$$ second slow. Thence we conclude that a balance-clock7 at the equator must go more slowly, by a very small amount, than a precisely similar clock situated at one of the poles under otherwise identical conditions.

--Jimbo2x (talk) 22:30, 8 January 2008 (UTC)

Stationary in relation to which frame?
In section 3 (Resolution of the paradox in special relativity), it is stated that "his position is constant in space, moving only in time" (that is the position of the first, Earth bound twin).

Can the frame of reference to which twin one is not moving here be explicitly mentioned? Is the the Earth frame of reference? Or the "absolute" frame of reference? Surely, it cannot be the frame of reference of the ship. —Preceding unsigned comment added by Jimbo2x (talk • contribs) 02:12, 2 January 2008 (UTC)

Agreed. I have edited the sentence to clarify. Timb66 (talk) 10:40, 2 January 2008 (UTC)

Thanks for that. Would it be possible for someone that is familiar with Minkowski's diagrams to prepare a set of diagrams for the twin in the space ship (i.e. for all x' coordinates). It would make the "no symmetry" point clearer. --Jimbo2x (talk) 22:11, 2 January 2008 (UTC)


 * Not sure what you have in mind there. Minkowski diagrams are not "for someone". They show events as points, and objects and observers as wordlines. One cannot draw a diagram "for the twin in the spaceship". One can draw a diagram that shows both twins. On such a diagram, the stay-at-home twin will live on a single straight line, and the travelling twin will either live on a curve (if the situation is modelled with accelerations), or on three different straight line segments (if we model it with 'frame jumping'):
 * before departure and after return, on the line of the stay-at-home twin,
 * while receding, on a tilted line (outbound),
 * after turnaround, while approaching, on another tilted line (inbound),
 * Now you can choose one of those 3 lines (or yet another one) as the "vertical" line (and consequently orthogonal with the spatial axis) - see here. Is this what you have in mind?
 * Or are you thinking about a fancier version of a drawing like this? DVdm (talk) 12:05, 3 January 2008 (UTC)


 * I meant the first set of diagrams you pointed to (GIF) --Jimbo2x (talk) 18:45, 3 January 2008 (UTC)


 * Ok, but don't you think that all four 'views' show the asymmetry of the situation equally well as the little diagram in the bottom right of the section? After all, the stationary twin clearly lives on the straight line, whereas the travelling twin lives on the broken line,
 * [[Image:Twin_paradox_Minkowski_diagram.png]]
 * so what kind of additional diagram would you like to see? DVdm (talk) 19:35, 3 January 2008 (UTC)
 * I think you're right, actually. One drawing seems sufficient. --Jimbo2x (talk) 20:19, 3 January 2008 (UTC)


 * Nobody picked up on it, but I proposed a diagram like this a few months ago, showing the events in all three reference frames:
 * [[Image:Twin 5.png|400px|border]]
 * v = c/√3  (γ = √1.5)


 * O is the point at which the traveling twin leaves the stay-at-home.
 * E is the point at which the traveling twin reverses course.
 * D is the point at which the traveling twin returns home.


 * A is the point simultaneous with turnover, in the outbound twin's rest frame.
 * B is the point simultaneous with turnover, in the stay-at-home twin's rest frame.
 * C is the point simultaneous with turnover, in the returning twin's rest frame.
 * The three frames of reference are the rest frames of the stay-at-home twin, the outbound twin , and the returning twin . The third is translated so that the traveling twin has the same coordinates after turnover as before. The arrows are the twins' worldlines, the thin lines are their lines of simultaneity at turnover. The dashed diagonal lines show the light cone from the start.
 * —WWoods (talk) 21:51, 4 January 2008 (UTC)


 * Looks excellent, but I'm afraid it's going to require a lot of supporting text and explanation, don't you think? And don't forget to include a 4th view where none of the frames coincides with the (- not too confused yet? -) lay reader :-) DVdm (talk) 22:03, 4 January 2008 (UTC)


 * Oh sure, the purpose of the picture would be to illuminate the text.
 * Does it really need a view from a none-of-the-above frame? This article doesn't have to explain the general concepts of inertial reference frame, the Lorentz transformation, or the relativity of simultaneity, though it needs enough to explain why the twin paradox is counterintuitive but not self-contradictory.
 * —WWoods (talk) 02:09, 5 January 2008 (UTC)


 * Doesn't really need that 4th view indeed, but while you're at it, I don't think it would be harmful. I recall a long Usenet exchange which only came to an end when that 4th view was shown. It sort of gives an attractive aerial view, at least i.m.o.
 * Feel free to download the "new" version here. Cheers, DVdm (talk) 11:37, 5 January 2008 (UTC)


 * I think it'd be better with the x and t axes for that frame, and the event coordinates need to be recalculated. What velocity relative to the others would be best? Halfway between those of the stay-at-home and the outgoing twins? Put the origin at the start as in the first two, or offset as in the third?
 * —WWoods (talk) 15:29, 5 January 2008 (UTC)


 * I wouldn't take halfway. Rather something like 2/3 vs 1/3, something that looks not too symmetric, and the origin as in the first two.
 * I also would -definitely- replace "point at which" with "event where" or "event when" or (ultimately) "event when/where", and -perhaps- "turnover" with "turnaround", but the latter is a matter of personal taste. DVdm (talk) 15:49, 5 January 2008 (UTC)

What To Do About Crackpot Edits
It appears that TwPx is reverting every attempt to remove his POV essay from the article, and it flagrantly violating the 3rr rule. Multiple editors have explained why his essay is not suitable for this Wikipedia article, since it consists of original research and a novel narrative (along with numerous flat-out misrepresentations of alleged sources). I tried to make a constructive suggestion (see above) for how he might try to make his essay conform to Wikipedia policy, but he doesn't seem interested in making any changes to his crackpot essay. Maybe we should just place a banner at the top of the article stating that it has been taken over by a crackpot and should be ignored until further notice. Is there such a banner? If there isn't, there ought to be. This sort of situation will (I'm afraid) crop up more and more often. There needs to be some more efficient way of coping with determined physics cranks.Lumpy27 (talk) 04:09, 5 January 2008 (UTC)


 * I don't think that any special action is called for at this moment. TwPx seems to have been blocked for a while and the article has been reverted to its original state, so it certainly is not taken over. Regarding such a banner... perhaps... but then of course, insertion and removal should be restricted to admins, and they already have everything at their disposal to deal with this kind of disruption, provided they can base their judgment on properly documented reverts and 'sec' user page warnings. Cheers, DVdm (talk) 16:06, 5 January 2008 (UTC)

A more precise setup?
My impression is, that many discussions suffer from disputable experimental setups. A clock could be embodied by a counter, which counts the frequency of a certain emission mof a simple atom, e.g. hydrogen. Two clocks in relative movement compare their own radiation to the other ones. This is a undisputable setup, as I suppose. Undoubtly, with speed zero, the ratio observed/own frequency will be one. But what, if there is speed <> 0. Will the ratio be <1, >1, and will it be the same for both observers? ErNa (talk) 18:13, 10 January 2008 (UTC)


 * When clocks are considered in this article, they are always taken to be identical and as ideal as engineeringly possible.
 * Altough this is not really the place for the other question, for the ratio, take a look at the time dilation overview. Whether they are the same for both observers, you should be able to decide from the carefully explained meanings of the variables in the equation. DVdm (talk) 18:43, 10 January 2008 (UTC)

In the twin paradox, the persons measure time by aging and twins stands for ideal engineered. But to be carefull with identical: if there are two identical clocks, there has to be an observer to state this identity. In the twin paradox, there is no observer, but the twins observe themselves. That is, there has to be a first comparing mechanism as an integral part of the twin, which can compare a inner property (the own age) to an outer property (the twins age) and a second mechanism, that can compares the first results. The outcome of this second comparison tells us, whatever we are, which twin (clock) passes more time in between a period, determined by two events. We should or can debate the TP only after reaching an agreement about such an experimental setup. Anything else is just shouting to the wind. ErNa (talk) 06:47, 11 January 2008 (UTC)


 * Yes, but don't forget that one also cannot really start to debate the TP after reaching an agreement about the size and the colour of their underwear, and, by extension, about the gender of their guardian angels. And of course, don't forget that there has to be an additional observer to state and verify the latter. DVdm (talk) 09:25, 11 January 2008 (UTC)

Ok, but please don't devaluate my efforts to be more precise by making jokes, even good ones. To make the scenario most simple and unambiguous, we define, which abilities the "twins" have. A twin has a clock, creating ticks. A measure for the amount of time, that passes for this twin, is embodied by a counter, incremented with every tick. A possible mechanism for the clock is a resonator, tuned to the radiation of certain arrangement of elementary particles :-), lets say, hydrogen atom. This can be seen as a laser, and by tuning the resonator, the frequency and the wavelength can be determined. Now we have to presumtions: proton and electron will have certain energy levels and the speed of light is invariant. These presumptions give us scales to measure time and space. True for every single twin. Now there is need for an instrument to compare the radiation, generated by to occurances of such twins. This can be reached by broadening the capabilities: it take a grating and a detector to determine the deflection of an light beam (and a beam former too). To twins are situated in a distance with relative velocity 0 (We suppose, we can agree, who this is defined). The twin can measure the deflection of his own beam and of the beam, received from the other one and he can calculate the ratio of deflection of these two beams. Is this setup a mutual basis for exchanging arguments? ErNa (talk) 11:05, 11 January 2008 (UTC)


 * Ah. Since you didn't use a smiley with your previous reply, I didn't use one either, and I had assumed that we would understand each other. So, in case you are not joking around, feel free to read my previous reply as if I was not either. Cheers, DVdm (talk) 11:51, 11 January 2008 (UTC)

When I try to follow discussions on the field of relativity, I have the impression, that words are most inappropriate to guide a conversation. So, why should it be different here. True: a clock should be as ideal as engineeringly possible. But that says nothing about an engineers skills. And not, what a clock is! Therefore, the discussion goes round and round for years and ages. ;-) Einstein discovered the theory of (special) relativity. Since that time, space and time are no longer separate items, but entangled. One presumption was: the speed of light is invariant, therefore time and space can no longer be absolute. And this fact confuses many people. And very early some fought against this theory by creating a paradox from what Einstein said to be "eigentümliche Konsequenz". And we should help to make this paradox and the misuse of the SRT more obvious and easier to understand.ErNa (talk) 14:48, 11 January 2008 (UTC)


 * Are you saying the article needs a more rigorous definition of clocks and/or the biological aging process? It seems to me that the notion of a clock and an aging biological organism is fairly unambiguous in these circumtances. We're obviously not talking about sun-dials or hour-glasses. Take average wrist watches and average human beings. Is there some ambiguity here that you think has a significant effect on the description of the twins paradox?


 * As you may know, special relativity takes clocks and measuring rods as primitive entities. Einstein pointed out that this was not entirely satisfactory, since ideally the phenomena that we identify as clocks and rulers should emerge from the theory, rather than being the basis of the theory. But he also believed that physics was far from being able to give a fundamental theory of the most primitive aspects of nature, so in order to make any progress it is necessary to adopt some provisional concepts, and the ideas clocks and rulers are fairly basic. The fact that co-located atomic clocks seem to keep time consistenly with biological aging and wrist watches, etc., is not too surprising to most people, so I'm not convinced the article needs to dwell on this. I haven't checked, but there are probably Wikipedia articles on the more general subjects of time (and clocks) and space (and measuring rods), so those articles might be more suitable places for a philosophical consideration of the intelligibility of those concepts.130.76.32.182 (talk) 23:21, 11 January 2008 (UTC)

As I know, special relativity is a framework, that leaves physical laws unchanged when switching between different inertal frames of reference. Clocks or rods are just used to describe physical properties. Using the word clock we imply, what a clock does, but not, what it is and how it is done. Therefore I'm looking to find a setup, that has only the features, that are needed and, on the other hand, is physically existing and correctly described. So: two (flat) mirrors form a kind of resonator. Somehow a electromagnetic wave is exited in between these mirrors.... This description is incomplete, as long as we do not really know, what every object "is". The fact that co-located atomic clocks seem to keep time consistenly with biological aging and wrist watches, etc., is not too surprising to most people. That is true. But when discussion the twin paradox, then we HAVE TO aggree, that atomic clocks measure time. AND: two co-located atomic clocks measure time to the same amount. So, time is a quality and a quantity. And that is not clear to everybody. How can You determine, that every (lasting?/running?passing?)) second of an inertial moving clock (in a world without gravity, to make it more simple) represents the same amount of time? This is not as clear, as it seems! But it is true by definition! and coinsides with my private observation (only of statistical relevance ;-) ). That is the reason, why I think, we have to have this very elementary and undisputed setup. First: co-located elementary particles cannot be discriminated by properties. They are equal, wherever they came from and however they were "created". Second: For example, a proton and an electron, brought together, alway for a atom of hydrogen. Atoms of H can be exitated and emit electromagnetic radiation. Third: This radiation can create a standing em-wave between to mirrors if and only if the distance of this mirrors has certain values. And again, it is not obvious, how we can measure this distance! All these aspects have to be defined and aggreed on, before! we can discuss, what the twin paradox is and why! If not, it leads to endless discussions, frustration, people called crackpots, and, and, and. Suppose, I know a truth. And I am not able, to convince other people, that this will always be my fault, for I am not able, to express myself in a way, that other people can understand. The same is true for everyone. And we all know, even twins can be differently talented teachers. ;-] ErNa (talk) 13:52, 12 January 2008 (UTC)


 * Again, the "twins paradox" concerns certain consequences of a particular theory, namely, special relativity. As mentioned above, special relativity takes clocks and measuring rods as primitive entities, so your comments about the underlying nature of time, space, and the inexplicable intelligibility of the world are not really relevant to this article. If you wish to add to Wikipedia some published ideas (from reputable sources) on the general philosophical topic of why the world is as comprehensible as it (apparently) is, it would be more suitable for an article dedicated to that subject. Again, the twins paradox concerns consequences of special relativity, which takes clocks and measuring rods as primitive elements, so your comments are not relevant to this article.


 * If I may add one other comment, I notice that you haven't cited any reputable sources for your philosophical ideas. If you have no sources, and are simply expressing your own ideas, then I'm afraid there is no place in Wikipedia for them. Any original ideas you have on the subject that you're talking about are explicitly excluded from Wikipedia, per the prohibition against original research. So I want to stress that you should only be trying to convey ideas that are already published and accepted. If you cite the reputable references whose ideas you are trying to convey, we might be able to suggest suitable articles to include them.63.24.100.132 (talk) 19:44, 12 January 2008 (UTC)

no original research
There is no need for a source, when I show, that it is not a priori clear, that every second takes the same amount of time. That is a integral part of SR and follows from invariance of speed of light. Seconds measured -or, more precise, counted periods of an oscillator- by a single, inertal clock, are equal by definition. That is, how time is defined. And the setup, I try to describe, should use a physical clock, embodied by a system, counting the oscillation of a very simple arrangement. Whilst clock and rod are philosophical items, not physically defined. A big advantage of this arrangement is, that the oscillator automatically generates a signal (a light), that can interchange information with a second, identical osci. Again, I do no OR, but just show, that it is very difficult to express something exactly. My only OR is, that I found, that all the misunderstanding of SR comes from implied settings, when using "normal" words. ErNa (talk) 22:38, 12 January 2008 (UTC)


 * What you've written above strongly confirms that you are, in fact, advocating "original research" and a "novel narrative", both of which are excluded by Wikipedia policy. Remember, the policy against original research says it doesn't matter whether what you say is true, or even whether you can prove it to be true. The only acceptable material for Wikipedia articles are views that have been previously published in reputable sources. It doesn't matter at all whether what you are saying is true, even if you can PROVE it to be true. All that matters is that you can cite a reputable source that conveys whatever it is you are trying to say. All your comments (and actions) imply that the point of view you are trying to express is not (as far as you know) expressed in any reputable published source. Therefore, the point of view you are trying to express is prohibited from Wikipedia.63.24.43.71 (talk) 23:38, 12 January 2008 (UTC)

Capitalisation
A quick check in a couple of well known text books ('Introducing Einstein's Relativity' by Ray d'Inverno and 'Special Relativity and its Experimental Foundations' by Yuan Zhong Zhang) confirmed that 'special relativity is not normally capitalised. Martin Hogbin (talk) 10:09, 31 December 2008 (UTC)


 * Indeed, Downwards' edit was on the mark, Eeekster's revert was off. DVdm (talk) 12:18, 31 December 2008 (UTC)

Article needs massive rewrite
The twin's paradox is poorly stated here. The issue of acceleration is a red herring. The real paradox is this: If Twin T travels in an inertial frame near the speed of light, he doesn't age according to Twin S in the stationary frame. This is an experimental fact: See muon lifetime. Therefore, Twin T outlives Twin S, according to Twin S. All inertial frames are equivalent. Therefore, according to Twin T, it is Twin S that is moving near the speed of light. Therefore, according to Twin T, Twin S dies first.

The paradox is that we have two apparently contradictory histories: in one history Twin T outlives Twin S, and in the other history things are reversed. Paradox occurs because we believe that only one of these two histories can be true.

I'd guess that the resolution of this paradox is not in any acceleration, because we need not speculate upon a return trip to establish matters. The resolution may be that simultaneity of events is not universally agreed upon. So various observers in various frames all will have varying views of which twin died first.

What say you? Brews ohare (talk) 22:09, 1 January 2009 (UTC)


 * Muon lifetime is not directly related to the twin paradox. The essence of the TP is that both twins are together at one event, get separated, and then reunite in another event, arranged in such a way that, between those two events, one twin remains in one inertial frame, whereas the other twin does not. The latter either undergoes accelerations, or jumps from one inertial frame to another. Failing to understand this difference is the origin of the paradox. You seem to confuse the TP with so-called mutual time dilation. Check The Twin Paradox in the Usenet Physics FAQ. DVdm (talk) 22:25, 1 January 2009 (UTC)


 * I was just about to say much the same. Martin Hogbin (talk) 22:28, 1 January 2009 (UTC)

Maybe you can help me out here. I feel that the usual explanation of the twin paradox provided in the article based upon acceleration really is an easy cop-out that amounts to saying: "Hey, there is no paradox because you broke the rules; you have to stay in an inertial frame." It's true that the rules were broken, but "no fair" is neither a very interesting nor a very persuasive explanation. However, to me there is a more interesting paradox, which does not break the rules. You refer to it as the mutual time dilation paradox, but there seems to be no discussion of same on Wikipedia. Is that so?

This paradox is that, with no tricks about acceleration, only different inertial frames, Twin T says that he aged more than Twin S, but Twin T says the opposite.

As an example, suppose Twin S and Twin T move toward each other at speed c/(2 + ε) according to observer O. They both die on his doorstep at the same time. All observers agree on simultaneity of events occurring at the same location. Observer O says that Twin S and Twin T aged the same amount.

Now let O move toward Twin T at constant speed. Then Twin T appears to move rapidly toward rendezvous with a slow clock, while Twin S moves slowly toward rendezvous with a fast clock. We arrange that O arrives at rendezvous exactly as Twins S & T meet and expire. For things to work out, we require that Twin S travel a short distance and Twin T a long distance so both have the same number of clicks on their clocks upon meeting. In other words the rendezvous according to O occurs just enough nearer Twin S's house, and just enough farther from Twin T's house. I suppose this what relativity predicts?

I find this version more interesting because it draws upon relativity of simultaneity, and not a cop-out. Brews ohare (talk) 23:10, 1 January 2009 (UTC)


 * I do not quite follow your example, but it is not what is generally known as the twins paradox, see the history section of the article. Martin Hogbin (talk) 23:53, 1 January 2009 (UTC)


 * There are plenty of paradoxes in SR based on the relativity of simultaneity, for example the Ladder paradox. Martin Hogbin (talk) 23:57, 1 January 2009 (UTC)


 * 'Brews ohare', you're talking about time dilation or the relativity of simultaneity, more than the twin paradox, which is a way of using those to show the counterintuitive nature of relativity. You're right that acceleration isn't required for time dilation; you can even do a variation of the twin paradox without acceleration, by having three ships resetting their clocks as they coast past each other.
 * I think saying someone is 'in an inertial frame' is problematic. A frame of reference is a set of coordinates overlaid on spacetime — all events of interest can be located in any frame. It's just that some frames are convenient to use, because some events occur at the same time or in the same place.
 * You might be interested in the diagram in http://en.wikipedia.org/wiki/Talk:Twin_paradox/Archive_12#Stationary_in_relation_to_which_frame.3F showing the events of the twin paradox in the three rest frames, side by side.
 * —WWoods (talk) 23:10, 2 January 2009 (UTC)

I had not looked at the archives; thanks. Brews ohare (talk) 06:11, 3 January 2009 (UTC)

Rotational motion
Most particle accelerators use rings, and particles move in circles. Circular motion is non-inertial. Thus, it would seem that muon lifetime experiments based upon collider experiments are outside special relativity. See Muons in accelerators & Muons in flight. Is it just fortuitous that lifetime dilation in circles is the same as in straight paths? Do you know of some discussion of same?

If the twins were placed on contrary rotating concentric carousels, they could compare lifetimes periodically when they happened to ::meet. If we sit in a stationary frame, and the carousels rotate in opposite directions at the same rate, the twins age the same way, and have the same age each time they meet. Suppose for example, the two twins die simultaneously on their third meeting. Then any observer must agree that this event occurred on the third meeting, because everybody agrees on simultaneity of events occurring at the same location.

If, on the other hand, we rotate with angular rate Ω, one twin ages faster than the other, according to us. That means one will die earlier, and just how much earlier depends on how fast we rotate, and not upon the twins at all!! Nonetheless, on their third meeting, both must be observed by us to die, regardless of our Ω.

Perhaps the circumference of one carousel shrinks and the other enlarges so the twins always meet at the same number of clicks on their differently paced clocks?? That would require the radii of the carousels to shrink or enlarge, even though the radii are everywhere perpendicular to the rotational velocity, and would lead to increased radial separation of the perimeters of the carousels as our Ω increases.

So a more likely explanation seems to be that our notion of the angular rate of the carousels changes with our Ω in a way that reconciles the clocks of the twins? The faster clock must travel less far than the slower clock before rendezvous, so rendezvous occurs at the same number of clicks on each clock? Brews ohare (talk) 01:23, 2 January 2009 (UTC)


 * Well, exploration of the literature shows that this subject is complicated by gravitational time dilation. I haven't found a thorough discussion, though I added some "Further Reading" to the article. Brews ohare (talk) 04:40, 2 January 2009 (UTC)


 * I removed the totally ridiculous vanity-press book you added, but I question the "further reading" section entirely. There are already (way too many) external sources in the special relativity article. This section adds nothing to the article and just invites people to add or spam their personal favorites, or use internet searches to book-mine potentially crappy sources they have never read or do not understand. Tim Shuba (talk) 06:45, 2 January 2009 (UTC)


 * Brews, referring to your "...particles move in circles. Circular motion is non-inertial. Thus, it would seem that muon lifetime experiments based upon collider experiments are outside special relativity.". Please note that circular motion and even non-inertial motion is completely and 100% within the realm of special relativity. The only thing that is outside SR, is gravitation. DVdm (talk) 16:58, 2 January 2009 (UTC)


 * DVdm: It appears that to the stationary observer the rotating clock is slowed by SR but for a rotating observer the clock is slowed by the gravitational time dilation due to centrifugal force. So the effect on clocks seems to be outside SR for the rotating observer (e.g. the muon). Do you agree? See Grøn Øyvind Brews ohare (talk) 20:50, 2 January 2009 (UTC)


 * Brews, I had not (nor have now) read your above comments beyond your first paragraph that I quoted and commented upon. Whatever you had in mind, I had to cut it short early. That's also the reason why I had reverted (without paying sufficient attention,) your section on the rotational version. DVdm (talk) 21:32, 2 January 2009 (UTC)


 * Bear in mind that, if we are going to delve onto this subject, we need to make the distinction between an object that is held in circular motion by gravity (for example the moon) which is in inertial motion, and one that is held in circular motion by other forces (for example a particle in a collider) which is not in inertial motion. I would suggest most of this discussion is outside the scope of this article.Martin Hogbin (talk) 11:18, 4 January 2009 (UTC)


 * Yes, by all means. That's why I still think we don't need that little section. I vote for removal. DVdm (talk) 11:21, 4 January 2009 (UTC)


 * I have made a correction. I suspected something was wrong, so I checked the reference. See exercise 9.25 in book on page 227. Since R > 3 mu, we have Delta(Tau_B)/Delta(Tau_A) > 1, so Alice is younger than Bob. She must be, as can be easily seen from an inertial frame attached to the center of the planet. Alice orbits and Bob does not, so her proper time integral is smaller than his. Afaiac, the section can stay now. I have added the page and a direct scholar search link to the reference. DVdm (talk) 13:07, 4 January 2009 (UTC)


 * Martin: an object that is held in circular motion by gravity (for example the moon) which is in inertial motion, and one that is held in circular motion by other forces (for example a particle in a collider) which is not in inertial motion. I believe this statement to be an error from the viewpoint of SR, which shares with Newtonian mechanics the view that any accelerated frame, whether the acceleration originates in gravity or a Lorentz force, is a non-inertial motion. Brews ohare (talk) 16:00, 4 January 2009 (UTC)


 * Yes but if we are concerned with gravity we need to use GR as SR cannot deal with gravitation. Martin Hogbin (talk) 16:48, 4 January 2009 (UTC)


 * Indeed, but in this particular case we don't really need gravity. After all, Alice's orbital speed can be expressed as a function of the R and mu (which are used in the expression of the proper times ratio). So in this particular case we can ignore gravity, and use her speed w.r.t the inertial frame attached to the center of the planet to calculate her proper time integral. Same conclusion. DVdm (talk) 17:04, 4 January 2009 (UTC)

"Rotational version" irrelevant
How does this section appear irrelevant? It is a twin paradox like that outlined in the intro, with the difference that the paths are circular. In fact, the author of the cited reference draws the parallel several times in the book. Brews ohare (talk) 20:53, 2 January 2009 (UTC)


 * Oops, sorry, you are right, I was confused. I thought you had gravitational time dilation in mind, where the concept of "mutual time dilation" is not present, and therefore cannot be the cause of wrongly claiming symmetry and thus generating the paradox. I slightly changed the text to make it silently implie that the twins remain at the same distance from the center of the gravitating body, for if they don't, the result can depend on whether Bob travels towards or away from the center and back. DVdm (talk) 21:23, 2 January 2009 (UTC)

Terracentric universe, redux???
I'm a bit confused.

Surely the homebody is subject to several sources of rotational accelaration: the Earth's axial rotation, the Earth's orbit around the sun and the sun's orbit around the galactic core, as well as whatever gravitation forces are affecting the Milky Way's celestial transit.

There's one bit that may address this, but I've no way of knowing whether it actually does or not: ''The standard textbook approach treats the twin paradox as a straightforward application of special relativity. Here the Earth and the ship are not in a symmetrical relationship: the ship has a "turnaround" in which it undergoes non-inertial motion, while the Earth has no such turnaround.'' (my emphasis)

So does this "non-inertial" bit mean that relativity doesn't apply to gravitational acceleration? Or is it that the acceleration experienced by the Earth is negligible in relativistic terms (the speed of the Earth's rotation being a sixth of a millionth of c, and orbit a ten thousandth)?

I think we should be told!

Prof Wrong (talk) 14:16, 4 July 2008 (UTC)


 * The twin paradox is a Gedankenexperiment, and as such assumes a simplified set-up. In this context "Earth" is meant to represent an inertial observer, one that doesn't undergo accelerations. To avoid confusing readers like Prof Wrong, the article should use something else, like another spaceship, or at least point out that the of effect of accelerations on Earth-based observers is negligible compared to what the traveler suffers. Paradoctor (talk) 00:24, 1 May 2009 (UTC)

History
Based on the description of the well known historian A.I. Miller (ref. 1), I rewrote the intro and the history section to include the contributions of Max von Laue. Already in 1913 he alluded to the fact, that one twin uses two frames for his journey, while the other remains in one, and this accounts for the different aging. Laue also was the first to illustrate those connections by using Minkowskian spacetime. German (p. 58): Von allen Weltlinien, welche zwei gegebene Weltpunkte 1 und 2 verbinden, hat die gerade Verbindung die längste Eigenzeit. (My translation: ''From all world lines, which connect two given world points 1 and 2, the straight connection has the maximal proper time.") --D.H (talk) 15:42, 4 January 2009 (UTC)
 * That is interesting history. The attribution to Lord Halsbury in several papers and texts goes back to a reference supposedly in Discovery in 1955, which appears in fact to be non-existent, as does Lord Halsbury himself as a contributor to relativity. Brews ohare (talk) 16:03, 4 January 2009 (UTC)
 * Discovery was a British science magazine published 1920-1966. They carried part of the second Dingle controversy, consisting of letters from Fisher, Mccrea, Dingle, Halsbury and a couple of others, and an article by Bondi. Also commenting on the debate is a rather amusing paper by Goodhart on biological time featuring cold-blooded physicists and astronautical frogs. If somebody wants my scans, drop a line on my talk page. Paradoctor (talk) 22:29, 30 April 2009 (UTC)

Apples and Oranges
Is it valid to say that, in the "Specific example" given in the article, the travelling twin effectively moved at 1.73c? I understand the length contraction but for practical purposes the aim of his trip was to travel 8.9 light years (from the home reference frame) and he accomplished it in 5.14 years (from his own reference frame). 83.146.14.12 (talk) 11:34, 14 April 2009 (UTC)


 * Speed is defined as distance divided by time. Normally one does not divide distance as measured by one person by time as measured by another. DVdm (talk) 16:26, 14 April 2009 (UTC)

Accelerated rocket calculation
IMO the "Accelerated rocket calculation" doesn't add anything valuable to the article, and certainly doesn't add enough to justify its length.--76.93.42.50 (talk) 15:36, 17 March 2008 (UTC)
 * Seconded. Oddity- (talk) 09:14, 7 May 2008 (UTC)
 * What does it add, in your opinion? BTW, you might consider getting a named account, all the cool editors have one. ;) Paradoctor (talk) 19:52, 6 May 2009 (UTC)

Making the article easier to understand
The twin paradox is much easier to understand if one imagines a long line of stationary, evenly spaced, and synchronized clocks extending from the stationary twin to the point where the other twin turns around. Imagine that as these clocks pass the moving twins window a strobe flashes so he can read off the elapsed time. Even though the non-moving twins clock seems to the moving twin to be ticking at half the rate of his own, the elapsed time, as told by the clock immediately outside his window, is passing at twice the rate of his own. More importantly, just before he stops, in order to turn around, the line of clocks are, from his perspective, out of synch but the moment he stops the line of clocks will be perfectly synchronized again which means that the nonmoving twins clock now reads the same as the clock he is next to. That means that his calculation of what the nonmoving twins clock said jumps suddenly while he decelerates (which leads to general relativity). Of course, this doesnt add anything to the article that wasnt there before but it does make it easier to understand. Em3ryguy (talk) 08:17, 20 May 2008 (UTC)


 * There are language and reference [frame] problems in your essay. Also sounds like WP:OR because most of the time dilation issues in the article are discussion points on old/accepted papers and books on relativity, and you quote none. Jok2000 (talk) 02:55, 21 May 2008 (UTC)


 * First, its a suggestion not as essay. Second, it can be rewritten by anyone to any form they like. Third, since it doest change anything or add anything to the article that wasn't there before and moreover since every statement made is trivial I dont see why a citation would be necessary. I am simply suggesting it as a way of illustrating what is being talked about. Em3ryguy (talk) 03:57, 21 May 2008 (UTC)


 * Where do you draw the line between stating the obvious and publishing original research? If one can do nothing but repeat what has already been published and one cant even make a trivial observation then I have to question the usefulness of WP. People can read the published articles themselves. They look to wikipedia to illustrate and explain these complex ideas in simple terms. At least, I do. Em3ryguy (talk) 08:45, 24 May 2008 (UTC)


 * What is obvious to you may very well not be obvious to others. I have different ideas about what happens. More importantly, this is an argument you came up with. As long as you don't get it published and peer-reviewed, or at least create a public uprorar about it, it is neither notable nor reliable, but simply original research. Though the 'original' is a bit ironic, seeing as your argument has been brought up more than once. If I know my DVdm, he can pull a suitable link for that out of his left nostril, if you ask nicely. :P Paradoctor (talk) 20:03, 6 May 2009 (UTC)

deleted note explaining acceleration
Someone reading about special relativity can be expected to either know the physicist's definition, or at least be able to follow a link/use a search box. If not, they need presumably need more exposition than a single encyclopedia article can provide. Paradoctor (talk) 19:18, 5 May 2009 (UTC)

Archive review
Just finished my quick'n'dirty review of the talkpage archive, permalink. If you think that I overlooked salvageable material, drop a section on my talk page with "review . " as title. Please note: the specific ratings are rather loose, the important thing to look out for is either an OK rating or some comments by me, anything without could be just as well deleted IMHO. Oh, and the comments sometimes contain material only of interest to me. ;) Paradoctor (talk) 19:27, 5 May 2009 (UTC)

Added review of this version of the main talk page Paradoctor (talk) 21:18, 6 May 2009 (UTC)

The symmetrical clock paradox
Some people get lost in the twin paradox presented here (with accelerations and all), so maybe a simpler, symmetrical paradox is in order. So, here is that "gedanken" experiment:

Let's have two identical rockets, carrying two identical high precision (in today's technology that would be atomic) clocks, with digital displays. The displays show the number of ticks (cycles) that each clock recorded (this could be for instance number of nanoseconds - a simple integer) since last reset. The rockets and clocks include a mechanism that fires the thrusters on detection of blue light and resets the clocks on detection of red light.

These two rockets are placed in space, away from any gravitational fields, so that they are pointed to one another, on a straight line. The rockets are far away from each other and we shall call the left one A and the right one B. Midway between rockets A and B we place a source of light we shall call S, capable of emitting blue or red light, each simultaneously to left and right, toward rockets A and B.

Rockets A and B, together with S form a single inertial frame of reference S for now (in other words, they are all stationary to one another).

Now we send blue light simultaneously from our source S toward rockets A and B. This starts the thrusters and the rockets burn their fuel, therefore being accelerated toward one another, on a collision course. The rockets burn all their fuel and each reaches constant velocity v in relation to our stationary frame of reference S, but in opposite direction. The speed at which the rockets are travelling is "normal" speed (i.e. not nearly close to the speed of light, but something a normal rocket could do).

After the burn is completed, we send red light simultaneously from our source S in the direction of rockets, therefore resetting the counters of our clocks. Now we have a system of three inertial reference frames, A - for rocket A, B - for rocket B and S - for our source of light S, in linear motion in relation to one another. And we have our clocks on rockets A and B synchronised, as observed from S.

Just before the collision occurs (i.e. when rockets A and B are in immediate vicinity of light source S) and if the trip of rockets is sufficiently long, we shall have the following situation in relation to clocks, as observed from different frames of reference, due to time dilation as per special theory of relativity:


 * clock A displays N as observed from reference frame A
 * clock A displays M (different from N) as observed from reference frame B
 * clock A displays L (different from N and M) as observed from reference frame S

And due to symmetry of motion and identical construction of clocks:


 * clock B displays N as observed from reference frame B
 * clock B displays M (different from N) as observed from reference frame A
 * clock B displays L (different from N and M) as observed from reference frame S

In comparison, such a system would show N on both clocks as observed from any frame of reference using only Newton's mechanics. --Jimbo2x (talk) 20:19, 3 January 2008 (UTC)


 * Since this is entirely original research we can't of course put it in an article, and it doesn't really belong on this talk page either. You might try Usenet. As a matter of fact it just so happens that someone (or you?) just asked exactly the same question on Usenet in sci.physics.relativity.
 * Anyway, I'll give you two hints here. (1) You write: "... in linear motion in relation to one another. And we have our clocks on rockets A and B synchronised." When they are in relative motion, they are not synchronized. They both see the other one's clock "run slower". (2) You created a symmetric situation, so you get symmetric results. Good luck on Usenet :-) - DVdm (talk) 21:44, 3 January 2008 (UTC)


 * Point taken about original research. I thought someone must have done this before (it seemed unlikely that this would be original). And it wasn't me on Usenet - so obviously not all that original ;-)


 * In terms of "synchronised" - they are synced in the sense that they are both running equally different as observed from S. About symmetric - yeah, that was my point. I think it would be useful to show that observers in different reference frames would read the same clock differently in immediate vicinity of each other according to SR.
 * --Jimbo2x (talk) 21:58, 3 January 2008 (UTC)


 * The idea of symmetry is of course not new, but I had never seen this kind of setup with the central light.
 * Little note on "... synchronised in the sense that...". Sure, but that's another kind of synchronization. From the three clocks in the setup, no pair of clocks is synchronized in any way. You could perhaps say that the clocks A and B are sort of equally desynchronized according to S, but as soon as you forget S, A and B are just two systems in relative motion - nothing special about them. In fact, one can construct a "central S" for every pair of systems in relative motion. That can't possibly make the pair physically special.
 * Do try the Usenet thread - it might become interesting :-)
 * Cheers, DVdm (talk) 22:25, 3 January 2008 (UTC)


 * There are only two clocks in the setup - A and B. If two equidistant stationary clocks can be synced by passing a beam of light simultaneously from the midway to them to "reset" them, so can the same two equidistant clocks in uniform opposing motion be synced as observed from S, unless the beam of light somehow travels to the left at different speed than to the right :-) Sure, from A, B is observed to be different (due to relative motion) and vice versa, but for an observer in S, they will be in sync (i.e. explicitly, both clocks display L when rockets meet, as observed from S). I never thought nor do I think that S would make A and B special. S is just another inertial frame of reference and an observer, just like you said.
 * --Jimbo2x (talk) 23:07, 3 January 2008 (UTC)


 * Well, you really do have a third clock in there. The minute you mention "reference frame S", you have it. No reference frame without a clock :-)
 * Do try Usenet for this - it's better suited for this kind of thing. This page is for discussions about the format and content of the article, not the subject. Cheers, DVdm (talk) 08:59, 4 January 2008 (UTC)


 * No worries. --Jimbo2x (talk) 09:41, 4 January 2008 (UTC)


 * Actually, all observers in the above example would see one and the same value on both clocks. That is to say, my example does not reach correct conclusions. --Jimbo2x (talk) 23:19, 14 February 2008 (UTC)

The problem is: whenever somebody tries to make a setup, which clearifies, what one is talking about, this "problem" is solved with the axe original research. The twin paradox has to be a mystery. There are so many ways to misunderstand what is argued. Simple example: what is a clock. It could be so simple: Two identical laboratories are connected by a loaded spring. This mechanism is released and according to conservation of momentum both labs gain the same speed, relative to each other. Now the have the following instruments: a source of light, that is a resonator, amplifying a certain spectral line of a certain gas of atoms and a prisma or better, grating. This light is directed to a semi reflecting mirror, twisted 45°, so 50% of the light is directed to the grating, 50% is send in direction of the second "twin" lab. So, the grating deflects two different beams: one from the local source and one from the distant source. It should be possible, by elementary calculation to determine the ratio of the detected deflection angles. And it should be possible to have a shutter which modulates the outgoing beam with this determined ratio on and of. And again it should be possible to compare the on/off ratio of the received and send signals. I have to point out: this is no OR, it is just a try to find a common language of what is a clock, how to measure a frequency..., how to measure distance and time in an environment, where nothing is absolute, except "c". ErNa (talk) 06:32, 9 May 2008 (UTC)


 * The common language as regards editing the article is in the literature. Check (=read) the references, and you'll find definitions of "clock", "distance", "time" and lots of other terms. If you go and invent your own definitions, chances are extremely high that you will either get it wrong and somebody has it done before, or you get it right and somebody has it done before. If you're really lucky, you might get it wrong in a spectacular new way. If you want to learn about relativity, you might consider enrolling in Wikiversity. ;) Paradoctor (talk) 00:52, 1 May 2009 (UTC)

Wooops! Misread the article, everything is fine —Preceding unsigned comment added by 85.223.172.189 (talk) 09:45, 19 May 2009 (UTC)

Separate page for discussion of Twin scenario itself
I have set up a separate page. Please post your reply there, as this is really the place for discussions on how to improve the article. Martin Hogbin (talk) 10:04, 17 January 2009 (UTC)


 * Quote from the boilerplate on top of this page: "This is not a forum for general discussion about the article's subject.", and "This" means all of Wikipedia. Please self-delete the page, or at least move it to your user space. The latter would be against Wikipedia policy, too, but my interest is limited to keeping the talk page on topic. Paradoctor (talk) 00:08, 1 May 2009 (UTC)


 * On what basis do you claim that 'this' refers to all of Wikipedia.Martin Hogbin (talk) 16:52, 1 May 2009 (UTC)


 * What Wikipedia is not: "Wikipedians have their own user pages, but they may be used only to present information relevant to working on the encyclopedia." (my emphasis)
 * Talk page guidelines: "Talk pages are for discussing the article, not for general conversation about the article's subject (much less other subjects). Keep discussions on the topic of how to improve the associated article. Irrelevant discussions are subject to removal." Paradoctor (talk) 21:00, 1 May 2009 (UTC)


 * I set up a separate page to keep the discussion of the twins paradox itself off the article talk page, something I am sure that you will agree with.Martin Hogbin (talk) 16:52, 1 May 2009 (UTC)


 * ?!? I explicitly asked you to delete or move that page, what on Earth gives you the idea that this constitutes agreement, rather than the exact opposite? And moving to a subpage is not moving it off the talk page, it is just a reorganization of the talk page. Paradoctor (talk) 21:00, 1 May 2009 (UTC)


 * If my page were removed there would be little either of us could do to prevent off-topic discussions from taking place on the talk page.Martin Hogbin (talk) 16:52, 1 May 2009 (UTC)


 * So you propose to keep off-topic matter off the talk page by putting your own off-topic discussions there? Paradoctor (talk) 21:00, 1 May 2009 (UTC)


 * There is also the matter of exactly where a subject becomes off-topic. Martin Hogbin (talk) 16:52, 1 May 2009 (UTC)


 * In your own words: "Please post your reply there, as this is really the place for discussions on how to improve the article.". Paradoctor (talk) 21:00, 1 May 2009 (UTC)


 * What about someone who wants to change the article because they believe that it is incorrect? Where do they state their case?Martin Hogbin (talk) 16:52, 1 May 2009 (UTC)


 * Here of course. But, the subpage contains almost no references to the article, as per your design! For the few spots that do, and in the spirit of pragmatism that's an integral part of the Wikipedia community, I have offered you the option of moving the off-topic discussion to your user space. You may think it's no biggie, but we disagree on that. This talk page has already accumulated a dozen archive pages, totalling more than a megabyte of text, and much of it irrelevant to work on the article. Now imagine someone comes along, trying to find out if a particular question has been addressed before. You see the problem? Now imagine how much fun it will be for me to reorganize this huge mess into a usable toolbox for the main article.
 * Please do not take my request as a judgment on the discussions, I am simply trying to keep a sane working environment. If you want a less "restrictive" environment to debate in, why not make your own wiki? You can have your cake and eat it! ;) Paradoctor (talk) 21:00, 1 May 2009 (UTC)

I must agree with Paradoctor: this talk page is not supposed to be a place where the subject can be discussed or explained. On the other hand, ever since Martin created the sub-page, It looks like the noise has significantly gone down here. So I guess Martin's initiative was not without merit. Anyway, I do think that Paradoctor makes a very strong point, and moving the sub-page to User:Martin Hogbin/Twin Paradox explanations looks like a very good idea. DVdm (talk) 10:49, 2 May 2009 (UTC)
 * That "noise" remark sooo reminds me of my old usenet days. ^_^ Anyway, I didn't mean to imply his idea was bad, I just just think he didn't go far enough. Paradoctor (talk) 13:43, 2 May 2009 (UTC)


 * In that case I suggest that we just delete the page. I have no strong attachment to it. Martin Hogbin (talk) 20:00, 2 May 2009 (UTC)


 * Like DVdm I find I must agree with Paradoctor. The material on the subpage is about the twin scenario itself, in that sense that subpage does not serve wikipedia purposes, at least not directly. --Cleonis | Talk 21:07, 2 May 2009 (UTC)

Since Martin prefers deletion, I'll wait a couple of days in case somebody else wants to give the subpage a new home before MfDing it. Paradoctor (talk) 20:03, 3 May 2009 (UTC)

Ok, page is up for deletion at WP:MfD (permalink). Paradoctor (talk) 15:16, 8 May 2009 (UTC)

Someone near a library?
I've been looking for the clock paradox bibliography by Mildred Benton, WorldCat WorldCat (apparently duplicate entry). According to Marder, this bibliography contains about 250 entries, and it's the only sizable bibliography on the twin paradox I've not yet obtained. I could have scans sent to Berlin, but that costs at least 35 US$ and would take at least three months, I've been told. Commercial delivery services are totally out of my so-called budget.

So, I wonder if someone among you living near one of the libraries would feel motivated to spend a few hours and a few bucks copying the book (47 pages) and uploading scans? I think I can confidently predict a barnstar for such a kind soul ... ;) —Preceding unsigned comment added by Paradoctor (talk • contribs) 16:07, 4 May 2009 (UTC)

Have it. Paradoctor (talk) 18:24, 26 June 2009 (UTC)

In Twin Paradox Twist, the Accelerated Twin is Older
Just when you thought you were beginning to understand the twin paradox (maybe), scientists have found something new to ponder. In the original version of the famous thought experiment on time dilation, one twin stays on Earth while the other twin takes a rocket at nearly light speed into space, and returns to find that he is younger than his twin on Earth. But a new version of the story now shows that the twin who experiences an acceleration can be older than the twin who doesn’t accelerate, under slightly different conditions.

In the new scenario, both twins are in circular orbit at different velocities around a large body, with the velocities measured by observers rotating with zero angular momentum with respect to the sky. Abramowicz and Bajtlik considered what happens when twin A stops moving, and so has a velocity of zero, and therefore a non-zero acceleration. Twin B continues to orbit at a set velocity corresponding to Keplerian free orbit and therefore has zero acceleration. Twin A is the accelerated twin, and twin B is not accelerated. As the scientists calculate, contrary to the classical version of the paradox, twin B is younger.

Quoted from In Twin Paradox Twist, the Accelerated Twin is Older —Preceding unsigned comment added by Softvision (talk • contribs) 19:27, 14 July 2009 (UTC)

Softvision (talk) 19:28, 14 July 2009 (UTC)


 * The report quoted above is about arxiv:0905.2428v1. In this context, The twin paradox on the photon sphere (DOI:10.1103/PhysRevA.75.044101, arXiv preprint) should be mentioned. Paradoctor (talk) 21:03, 14 July 2009 (UTC)

Unneeded advertising and trivia section
I think the trivia section should indeed be removed and that Auspex1729's edit was right on the mark. Apart from being unneeded and irrelevant, I think it is even more so ridiculous. And wikipedia is not an advertising bureau for coffeeshops and 'Pocket PC connectivity Companies". Let's get rid of it? DVdm (talk) 18:09, 4 August 2009 (UTC)


 * In the absense of response, I boldly removed the trivia. DVdm (talk) 11:47, 6 August 2009 (UTC)


 * I concur. Plastikspork (talk) 16:49, 6 August 2009 (UTC)

Travelling without extreme accelleration
I think the article is missing a section explaining the situation when the travelling twin is not experiencing extreme accelleration, e. g. by travelling with an accelleration of just 1g. This he could do by either returning his thrust after travelling half the way etc. or by travelling in a circle, experiencing the 1g to keep on the circular track.

Why is there no such paragraph explaining the TP in this situation?

I see explanations using the STR with unaccelerated ISes and I see explanations using the GTR with accellerated ISes, but this does not explain what will happen if the travelling twin will experience the same accelleration as the non-travelling twin (who stays on earth, i. e. also in a 1g-environment). Rotational issues should be considered (and explained) as well.

--Alfe (talk) 23:34, 14 September 2009 (UTC)


 * Look at the section Difference in elapsed time as a result of differences in twins' spacetime paths and consider the trivial case a = g and coasting time T = 0.
 * Also note that the non-travelling twin, in his permanent 1g-environment is not subject to kinematic time dilation, but to Gravitational time dilation.
 * For the rotational version, see section A rotational version with explantion (including equations) its reference and footnote.
 * DVdm (talk) 06:36, 15 September 2009 (UTC)


 * Thank you for your quick answer. So I think you tell me that in the case that both twins experience the same acceleration throughout the time of the experiment, both will have aged the same when they reunite?  That's what I think as well; a confirmation on that fact would be very welcome.
 * But I think to point at the equations and say "compute it yourself!" is not the best solution for the WP which should explain facts on a level to be understood by laymen. Furthermore, It does not really explain what the twins experience when they watch each other's clocks (while considering the time the light needs for the travel).  As far as I understand this, even if the traveling twin accelerates with just 1g, he eventually will reach a large speed which should make his time appear to progress slower to the earth-bound twin.  So the earth-bound twin should see a previous time on his twin's clock all the time, until and including the time the twin comes back.  Likewise the traveling twin should see the earth and his brother move fast away, so the earth-bound clock should appear to move slower from his view.  By preventing extreme accelerations (turnaround of the travel shall happen with also just 1g), he should never see a massive speed-up in the other one's clock as he definitely would in the simpler examples on the main page.  So, I think the page could really use some explanation on what the twins will experience in this experiment.
 * The rotational version you point to does not apply constant acceleration but speaks about a stopping and re-getting of rotational speed, so it does not fit my second example which all the time applies constant acceleration of 1g to keep a rotational track (of large diameter). --Alfe (talk) 11:14, 15 September 2009 (UTC)


 * Remarks:
 * "... in the case that both twins experience the same acceleration ..." => Yes, that is precisely explained in the section "Difference in elapsed times: how to calculate it from the ship" in which the coordinate time is given in terms of the proper acceleration.
 * "... what the twins experience when they watch each other's clocks ..." => In the case of "instantaneous" acceleration this is explained in the section What it looks like: the relativistic Doppler shift. To show or explain this with realistic accelerated motion is complicated. See if you can find a source and whether it can be included "on a level to be understood by laymen", as you say. I doubt it.
 * "The rotational version you point to does not apply constant acceleration ..." => Indeed, I misunderstood. But note that in your "second example which all the time applies constant acceleration of 1g to keep a rotational track (of large diameter)", it is the tangential orbital speed that determines the time dilation, not the centripetal acceleration. I should have made this remark in my first reply, sorry.
 * DVdm (talk) 11:52, 15 September 2009 (UTC)
 * Alfe, I am not sure that what you are asking for is possible in WP. There are a great many possible questions that could be asked concerning the aging of twins who follow different paths through spacetime, twins on different orbits round the same body for example.  You can probably find many more possible questions in the problems in a GR text book but you cannot expect that WP will provide easy solutions for them all. The twins paradox is similar to asking why two people who both go from  A to B by different routes do not travel the same distance as one another. Not really much of a paradox, just a question of knowing how to do the appropriate calculation. Martin Hogbin (talk) 17:05, 15 September 2009 (UTC)
 * Martin, thanks for your consideration. Let me put it this way:  I'm an (interested) layman, a friend of mine is, too, and we both, independently, had the same train of thoughts, so this might be rather common.  It goes like this:
 * Special relativity states that unaccelerated frames of reference experience different flows of time. This leads directly to the seemingly paradox of the twin.  This paradox is solved here for two cases: (a) if the traveling twin is experiencing acceleration in order to make the trip (special relativity does not apply to these cases, and general relativity explains time dilation due to gravitational/accelerational effects) and (b) if the frame of reference is switched.  So the explanation is either large accelerations or switching of the frame of reference.  It doesn't take a genius to then think of a trip which avoids both, i. e. a trip without large accelerations and without switching the frame of reference, and this is the scenario I propose to explain here.
 * I do not expect this page to explain the computations in all detail, but one physicist should sit down for awhile, compute this scenario through and then explain briefly what the twins will experience, i. e. what they will see and how the paradox is solved in this case. I would come up with this if I could but I think others are better equipped to do so ;-)  --Alfe (talk) 08:39, 22 September 2009 (UTC)


 * Alfe, perhaps you missed my first reply to you in this section. Someone already sat down for awhile and computed this scenario on behalf of you and your friend. You can find the computation in the section Difference in elapsed time as a result of differences in twins' spacetime paths. In these equations you can take any values that you like for the parameters a, A and T and feed them into your calculator. When you take a "small" value for a, you have a "small acceleration".
 * Note of course that in this model the travelling twin lives, while he feels the acceleration, in a non-inertial frame of reference, so your requirements ("trip without large accelerations and without switching the frame of reference") are satisfied. However, in a way this means that he is in fact continuously switching between inertial frames of reference. This is precisely the reason why special relativity is perfectly capable of handling this, without the need for general relativity, which is only called for in situations where gravity is involved. Now you might say that gravity is indeed precisely what the non-travelling twin experiences. That is correct, but this effect is so small that it can be (and is) neglected in the context of this article, even for small values of the travelling twin's acceleration. DVdm (talk) 11:35, 22 September 2009 (UTC)


 * DVdm, I'm sorry I did not answer completely before; I understood and considered your reference to the section in the article, but as I stated before: pointing at equations and and saying "compute it yourself" is not what I expect of a WP article (as it may be too hard for laymen, the typical readers of WP, to do so), at least not for the questions which come up automatically. I tried to point out why I think that the 1g-travel-version of the thought experiment comes to mind automatically (I hope this can be followed).  What I expect of the WP is to give answers to the then-arising questions (a) Why does SR-related time dilation due to large relative speeds do not have an effect in the end?  and (b) What will the twins see on each others clocks when?
 * I think a paragraph like this could be what I'm looking for:
 * If both twins experience the same acceleration (e. g. one due to Earth's gravitation and one due to the thrust of the rocket), actually no different aging applies eventually; they both will have exactly the same age when the traveling twin returns. By accelerating a rocket with "just" 1g it takes months to reach relativistic speeds, but once reached, time dilation according to SR will be observable for both twins symmetrically, i. e. both see each other age slowly, no matter whether the traveling twin is flying away or already on his journey back.
 * The reason why this still is no real paradox lies in the relativity of the acceleration applied. What the traveling twin will measure as 1g will be measured by the Earth-bound twin as a smaller acceleration (due to length contraction and time dilation); in smaller accelerations, clocks run faster according to the GR, annihilating the slower-clock-effect of the SR due to the high relative speeds.  Vice versa the traveling twin will measure the acceleration of Earth's gravitation as smaller (again due to length contraction and time dilation), so from his point of view, the acceleration of the Earth-bound twin is smaller, so the clocks there are running faster again.
 * I think this explains quick and rather "unmathematical" the effects taking place on a qualitative level and without formulas. But I'm not sure I'm right with my explanation and I still don't see how the small GR effect of different accelerations can annihilate the (I assume) much larger effect of the time dilation due to SR and large speeds.  Maybe someone can brush up my text with something about this.  --Alfe (talk) 12:21, 24 September 2009 (UTC)
 * Ah, I forgot: My proposal draft above is still missing some sections: (a) what the twins see at each moment on each other's clocks (is it constantly the same as the observer's clock (after accounting for the time the light needed for the distance)?), (b) the relativity of the turn-around point of the traveling twin and how each it is observed by the two twins, (c) how and why the effect due to SR (with growing relative speed tends to actually stop the clock of the brother) can be annihilated by the small effect of different accelerations which are not large (1g agains 0g is very small, as we can observe with astronauts today).  --Alfe (talk) 12:50, 24 September 2009 (UTC)


 * Remarks:
 * "... pointing at equations and and saying "compute it yourself" is not what I expect of a WP article (as it may be too hard for laymen, the typical readers of WP, to do so)" => Many subjects are very difficult and complicated for laymen. I guess that's a fact of life. Wikipedia articles about such subjects tend to explain in the lead, i.e. before the first section, in broad terms what the subject is about, taking into account the layman's limited knowledge of the subject, and using (more or less) lay language. The remainder of the article usually assumes some basic knowledge (and sometimes advanced,) from the reader's part. This seems to be a difficult subject indeed.
 * "a) Why does SR-related time dilation due to large relative speeds do not have an effect in the end?" => It has an effect.
 * "b) What will the twins see on each others clocks when?" => That is briefly explained in the section What it looks like: the relativistic Doppler shift and further in the article Relativistic Doppler effect. This article (twin paradox) is not about what they see on each others' (remote) clocks during the trip of the traveller. It is about what they see when they are reunited and compare their clocks locally, at the same place.
 * "If both twins experience the same acceleration (e. g. one due to Earth's gravitation and one due to the thrust of the rocket), actually no different aging applies eventually; they both will have exactly the same age when the traveling twin returns." => But different aging does apply eventually. I just explained this in my last sentence. Assuming you haven't seen that sentence, I will repeat it here, with some emphasis added: "Now you might say that gravity is indeed precisely what the non-travelling twin experiences. That is correct, but this effect is so small that it can be (and is) neglected in the context of this article, even for small values of the travelling twin's acceleration".
 * "The reason why this still is no real paradox lies in ... ... so the clocks there are running faster again." => I'm sorry, but this entire paragraph is... (no offence intended) nonsense. I wouldn't know where to begin to comment on this.
 * "... I don't see how the small GR effect of different accelerations can annihilate the (I assume) much larger effect of the time dilation due to SR and large speeds." => It does not annihilate the effect.
 * "Maybe someone can brush up my text with something about this." => I'm sorry, but your text is completely wrong. A brush is not what is needed.
 * Alfe, I really think that this is one of those articles of which you should refrain from going beyond the lead. Don't let that put you off though. There are thousands of articles like that for me as well, even in my own field. I doubt, but nevertheless hope this helps. DVdm (talk) 16:29, 24 September 2009 (UTC)

(I'm dropping some indentions to ease reading.)

I think you contradicted yourself (how fitting for this article about a paradox). I asked (way up) whether the two twins have aged the same if they experienced the same acceleration for the whole time of the experiment and you said Yes, that is precisely explained in the section "Difference in elapsed times: how to calculate it from the ship" in which the coordinate time is given in terms of the proper acceleration. Now you say that they do not have aged the same: But different aging does apply eventually.  How can both be true? Which is true?

I think you are wrong when you say that the gravitational effect the non-traveling twin experiences can be neglected due to it being too small because AFAIK gravitational time dilation is the same as "accelerational" time dilation (see also the lemma about the gravitational time dilation, stating exactly this: ''It can also be manifested by any other kind of accelerated reference frame such as an accelerating dragster or space shuttle. Spinning objects such as merry-go-rounds and ferris wheels are subjected to gravitational time dilation as an effect of their angular spin. This is supported by the general theory of relativity due to the equivalence principle that states that all accelerated reference frames possess a gravitational field. According to general relativity, inertial mass and gravitational mass are the same. Not all gravitational fields are "curved" or "spherical"; some are flat as in the case of an accelerating dragster or space shuttle. Any kind of g-load contributes to gravitational time dilation.'')

But even with believing that there is a difference between the two kinds of time dilation (gravitational and "accelerational"), we can cut out the gravitational stuff by altering the experiment a little: Both twins start in free space (at 0g), the one twin starts a long 1g trip (to and fro), his brother does lots of much shorter ones, say, 10 meters to and fro, so he "swings" back and forth all the time. Both then experience 1g all the time without any mass-related gravitation and should age the same (if I understand you right). Or shouldn't they? Or how can you explain that one twin aged more than his brother in this case?

BTW, I don't feel offended; the main goal of my proposal was to show what kind of paragraph I expect from the WP article (i. e. without reference to formulas etc.). I was well aware of its ... inadequacy. If there really is a different aging, then of course such a paragraph does not make sense as it just states a special case without different qualities (then don't put it in, but still say clearly that in this case still a different aging applies and why). But if there is no different aging (what I still believe and, BTW, what several sources on the net support), then a paragraph like mine (just cleared off all wrong stuff and fit with the correct description) should be part of the article.

Concerning your opinion about the parts beyond the lead of an article: Yes, if it is not possible to explain something without the formulas, then you are right. I see this situation given e. g. in the article about Bell's Theorem. However, in the article about the TP, I think the case lies differently; here just the explanation can be bettered beyond the point of just pointing at the equations (also no offense intended; the equations are nice, but not the whole world). --Alfe (talk) 12:12, 25 September 2009 (UTC)


 * Remarks:
 * "I think you contradicted yourself ... ... Which is true?" => There is no differential aging when both twins experience the same acceleration when no gravity is involved, i.o.w. when they both go on some kind of round trip which is symmetrical as seen in some third inertial reference frame like .e.g. the Earth. That is what I assumed you hand in mind (way up) the first time. The second time you asked, you explicitly said "(e. g. one due to Earth's gravitation and one due to the thrust of the rocket)...". In that case there is differential aging.
 * "... because AFAIK gravitational time dilation is the same as "accelerational" time dilation..." => it is not the same. I already gave a link to Gravitational time dilation.
 * "... a paragraph like mine (just cleared off all wrong stuff...)" => I'm sorry, but that paragraph would be empty. I've no other way of putting this.
 * DVdm (talk) 13:26, 25 September 2009 (UTC)


 * Hi, DVdm, sorry to be obviously bugging you like this. Obviously I understood your first comment wrong and you always meant that even given the same acceleration for every participant, still a different time dilation can occur; just because I relied on what I understood there, I wrote the rest of my statements.  But still things are a little strange here.  For one thing you gave a pointer to a source which contradicts your statements:  You say that accelerational time dilation differs in at least one way from gravitational time dilation and point at the lemma gravitational time dilation which in turn states exactly the opposite, as I quoted above.  And in my example thought experiment I gave above (one twin going back and forth all the time while his brother makes the large trip), both experience the same acceleration all the time, so "mass-induced" gravitation does not come into play at all.  Still I now think that a time dilation occurs which somehow is due to the changes in direction of the acceleration.  And I still think this should be reasoned in the article which&mdash;as it stands now&mdash;suggests that the acceleration can be used to reason why a time dilation after the return will occur.  --Alfe (talk) 09:38, 28 September 2009 (UTC)


 * This is the 5th time you make remarks about statements that were never made. Please stop doing this. I left |a warning on your talk page. Thank you. - DVdm (talk) 11:38, 28 September 2009 (UTC)


 * Your obviously emotional response prevents an objective discussion, so I'm sorry I will never find out what you meant when you said that gravitational time dilation is different from accelerational time dilation (quote: it is not the same) which I think is in contradiction to the lemma gravitational time dilation you refer to yourself (I won't quote the part again). If you really think I refer to statements that were never made (5 times!), I'm sorry but I don't know where you think I did this.  I on my part have the feeling you did not really take the time to understand my remarks.  I did my best by quoting what I find contradictory.  But as I cannot force you to see what I mean, maybe some other reader of this discussion can give a more objective opinion about my concerns now.  If other users also have the feeling that I'm just trolling around, I'd be happy to at least know this.  --Alfe (talk) 12:37, 28 September 2009 (UTC)
 * In your scenario it is true that both twins would experience the equivalent acceleration force, but only the one in the rocket is increasing his velocity relative to the twin on earth whose gravitational acceleration doesn't increase his velocity because the acceleration just presses him against the earth. —Preceding unsigned comment added by 206.53.153.59 (talk) 23:10, 20 November 2009 (UTC)