Talk:Twin paradox/Archive 05

From Wikipedia, the free encyclopedia

This is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.

Archives
/Archive 01: 2004 to December 2005 /Archive 02: December 2005 to April 2006 /Archive 03: April 2006 to September 2006 /Archive 04: September 2006 to October 2006 /Archive 05: November 2006 to December 2006

Contents 1 Please don't remove peer reviewed references that are cited from article 2 unclear addition by Moroder 3 Twisting the Twins 4 GR section removal 5 Origin of the Paradox section - rewrite? remove? 6 Why is neutrality of article being challenged? 7 What is this discussion about 8 Content dispute and structure of this article 9 EMS 9.1 Note on "kinematics"

Please don't remove peer reviewed references that are cited from article

One or two users remove reference to a paper that is cited in the article, thus removing reference contrary to WP:V. Note that the paper by Unnikrishnan also expresses a current notable recent opinion, but it may not be necessary to emphasize that.

To turn this into something positive, we may use this occasion to start making this article conform to Wikipedia:Scientific_citation_guidelines. Then we can make that reference simply a footnote for verification, and limit the reference section to a more limited number of selected publications.

Harald88 21:42, 25 November 2006 (UTC)

No comment? OK then I start that now. Harald88 22:45, 25 November 2006 (UTC)

[More comments, copied from my (Harald88 23:40, 25 November 2006 (UTC)) Talk page:]

I must admit, I'm having trouble with this reference Harald. The article makes some very good points, but the conclusions basically reintroduces an absolute reference frame as a physically significant one:

"The failure of the accepted views and resolutions can be traced to the fact that the special relativity principle formulated originally for physics in empty space is not valid in the matter-filled universe. Planck’s assertion2 that there is no physical method of measurement of the velocity of motion through space is made void by the various markers available in cosmology, especially the dipole anisotropy of the CMBR."

In other words the author denies the relativity principle. Maybe this is some subtle argument from general relativity? It just seems a bit odd to me. It needs to be put into context.WolfKeeper 23:11, 25 November 2006 (UTC)

He seems to refer to GRT, with his own interpretation (with which I don't agree, if I understand him well). But I reduced it to a footnote, as the reference is simply about his agreement with Builder on his criticism of Einstein's "real" gravitational fields due to acceleration that nowadays are called "pseudo fields", in disagreement with Einstein's 1918 POV -- see [1]. Harald88 23:37, 25 November 2006 (UTC)

Correction: actually also tht phrase sounds rather compatible with Einstein and hJansen , see below. Harald88 02:29, 26 November 2006 (UTC)

Oh Harald88! By all means you know, that there are zillions of scientific papers published every year. We are advised to use standard textbooks and review papers if available, and use editorial judgement (with the help of the citataion indexes, despite all voiced doubts about their relevance) whether and which research papers to include. --Pjacobi 23:22, 25 November 2006 (UTC)

Hi Pjacobi, see above, I wanted to ask you to watch people here who delete the note that general relativity nowadays is regarded as a theory of gravitation. If you know another recent paper that criticizes Einstein's 1918 Twin paradox paper please add it, thanks. Harald88 23:40, 25 November 2006 (UTC)

Actually Einstein's "solution" of 1918 may simply be regarded as a mistake (and obviously is regarded so by most), for not only nowadays we regard acceleration as "absolute" (that is, relative to space) but so also did Einstein clearly in 1920 as Einstein explained with much elaboration:

Newton might no less well have called his absolute space "Ether"; what is essential is merely that besides observable objects, another thing, which is not perceptible, must be looked upon as real, to enable acceleration or rotation to be looked upon as something real. It is true that Mach tried to avoid having to accept as real something which is not observable by endeavouring to substitute in mechanics a mean acceleration with reference to the totality of the masses in the universe in place of an acceleration with reference to absolute space. But inertial resistance opposed to relative acceleration of distant masses presupposes action at a distance; and as the modern physicist does not believe that he may accept this action at a distance, he comes back once inore, if he follows Mach, to the ether, which has to serve as medium for the effects of inertia.

and according to Jansen such was already the case during development of GRT; as Jansen puts it,

the rotation of the water and bucket with respect to the shell and the earth is not relative in the “Entwurf” theory or in general relativity and the rotating frame has resulting unphysical degenerate values of the metric at infinity and General relativity thus retains vestiges of absolute motion and Einstein thus accepted that the shape of the water surface in Newton’s bucket and the bulging out of one of the globes in his own thought experiment is caused by the rotation of the water and the globe with respect to the metric field and that the metric field cannot be reduced to matter. Even in general relativity these effects are the result of acceleration with respect to space(-time) just as in Newtonian theory and in special relativity.

That Einstein was a little confused in 1918 is also held by Jansen as he states that

Einstein’s correspondence with Hans Thirring in 1917 shows that this misunderstanding persisted for at least another year and a half - http://www.tc.umn.edu/~janss011/pdf%20files/annalen.pdf

I like that paper too but I don't know if it corresponds to a peer-reviewed paper. Harald88 02:29, 26 November 2006 (UTC)

Yesterday an anon (probably Moroder) deleted the whole last section (insread of improving it) because it could be misunderstood to mean that GRT can't deal with acceleration. Indeed the phrasing was clumsy, I now fixed that. Harald88 12:43, 26 November 2006 (UTC)

I also assert that the article will be improved with the removal of the Unnikrishan reference. Worse yet is the Builder reference. Geoffrey Builder is not notable as relativity author. He was a radio engineer who published a handful of papers relating to special relativity shortly before his death. Despite being around for just short of five decades, none of these papers seem to have more than an extremely minimal citation record. I am glad to see what hopefully will be a clear consensus to dump the Unnikrishan reference from this article, and certainly hope the Builder reference goes as well. Unfortunately, even with their removal, this article will remain quite poor. In particular, the section Origin of the Paradox reads as little more than an essay promoting an extreme fringe point-of-view. Tim Shuba 14:18, 26 November 2006 (UTC)

The journal in which he published is notable as is his POV (apparently he was the first to include accelerated frames into SRT and to make the Twin paradox a non-GRT issue, both impressive achievements), and for science articles it's the peer review process that matters for Wikipedia. OTOH I fully agree that reference to less known papers belongs in the footnote section and not in the references section - we should reduce that to the "best" general publications. Harald88 21:15, 27 November 2006 (UTC)

Hi. I wrote some, maybe most of the Origin of the Paradox section. I will gladly edit it if you would explain in some detail exactly where I have introduced "an extreme fringe point-of-view". What I wrote is what I learned in courses and textbooks. green 12.30.216.138 16:21, 27 November 2006 (UTC)

I am including a new section below. Tim Shuba 18:22, 28 November 2006 (UTC)

Agreed on mostly all points.

The "Origins" section has one use: It is common misconception, that the paradoxical of this paradox would be different age of the twins (i.w. time dilation per se), not the non-symmetry. Even our competetion gets it wrong [2]. --Pjacobi 15:12, 26 November 2006 (UTC)

I thought I made it clear in the Origins section that the paradox is NOT that the twins age differentially. Maybe this is what your meant by its value. green 12.30.216.138 17:41, 27 November 2006 (UTC)

Peter, the non-symmetry is expressed by the different aging of the twins; thus why do you claim that that lexicon "gets it wrong"? In fact different people see the paradox differently and have different opinions; this is one of the most contentious subjects in physics and Wikipedia is here to discusses all notable opinions (and it is not for us to claim that an opinion is "wrong"!).

BTW does your lack of reply mean that you also don't know if that article by Jansen has been published? Or would you know another similar paper? It's rather important for this article, as the Twin paradox took off with misunderstanding Einstein's claim that all motion is relative. Harald88 19:09, 27 November 2006 (UTC)

unclear addition by Moroder

I move the following addition by Moroder here for discussion:

There was never any confusion on this subject. For example in his 1934 edition of "Relativity, Thermodynamics and Cosmology", prof. R.C. Tolman was noticing (page 27) that while "a constant velocity in system S' implies a constant velocity in system S, it is interesting to note that a constant acceleration wrt system S' , would not in general imply a constant acceleration in system S". In other words, there was never any confusion relative to the absolute nature of acceleration.

That there was confusion on that subject is well established by a number of sources, one of which is referenced in the article and another well known expert in this field claims the same, as cited above. Leaving that aside, the Tolman citation plus accompanying remark does not clarify much and the "in other words" does not seem to match the cited contents; this risks to instead confuse the readers. Harald88 12:57, 26 November 2006 (UTC)

What article, harry? The crank paper of Unnikrishnan that you keep plastering everywhere, whether it fits or not? Do you understand the mistakes in the "paper" that you keep inserting? Have you read it? Can you follow it? Moroder 16:18, 26 November 2006 (UTC)

Twisting the Twins

[I move this from my Talk page. Note: I asked Moroder to put discussions on the corresponding Talk pages; I also informed him/her that I won't respond to "harry" and instead will delete wrongly addressed comments -- here I make a one-time exception, and move it to this page instead of just deleting it.] Harald88 18:50, 27 November 2006 (UTC)

harry, the article is about the Twins Paradox. Trying to hijack it into becoming the criticism of a more obscure, didactic paper (Einstein 1918) is not a good thing. Trying to further hijack it in order to promote the Unnikrishnan ideas of "universal preferential frame" and "Einstein was wrong" is downright wrong. Could you please stop plastering the Unnikrishnan paper on multiple subjects, whether it fits or not? Thank you Moroder 16:47, 26 November 2006 (UTC)

GR section removal

The section the resolution of the paradox in GR has been removed as its stated "resolution" is totally and completely erroneous. There is no gravitational time dilation at work in this case. Instead the issue is how one's view of spacetime changes due to acceleration in an SR universe. See an old sci.physics.relativity posting of mine for the details on how the traveling observer views the situation. (GR is needed to explain the case of the traveling twin achieving turnaround by passing close enough to neutron star or black hole. However, the overall geometric change in how the spacetime is viewed once away from the enabling object is still explained using SR instead of GR.) --EMS | Talk 19:29, 27 November 2006 (UTC)

EMS you are mistaken: I fully agree with you that the GRT solution is incorrect and that is also indicated as being the mainstream opinion, so that your edit doesn't make sense, really. Nevertheless it is a notable (Einstein's!) opinion, it is still held by a number of teachers (I happen to know one) and it is still discussed in the peer-reviewed literature; it is also (although less accurately) discussed in the physics FAQ (The "General Relativity" Explanation, [3]; it has also been established that it is strongly connected to the origin of the paradox (see Dingle etc.). In short, it is necessary to include it. But to address your sensitiveness I'll follow their example and put in quotes.

We discussed this topic in the past, and I then suggested to make it a separate page, linked from the main article that limits itself to modern popular textbook versions.

However, at that time there were no reactions [4]. Maybe now we can get agreement on this? Harald88 20:28, 27 November 2006 (UTC)

Einstein was not error-free by a long shot, and his "1918 opinion" may be just one more example of this. Personally, I need to see examples of this being covered in the peer-reviewed literature (and that excludes Unnikrishnan's Current Science article. Kindly be aware that Current Science is not only not peer-reviewed, but is meant for middle and high school students. I have no doubt that the editors of Current Science failed to understand what Unnikrishnan was saying and/or the errors in his article). I would also like to know if this was a view that Einstein held for many years or if he was quickly disabused of this notion (as I suspect was the case). In the later case, the 1918 opinion is just a transient phase and not significant. I would like to make it very clear that I would prefer not to cover erroneous opinions unless notability can be proven. That goes double in an article like this, where there are already too many views being put forward. --EMS | Talk 21:31, 27 November 2006 (UTC)

I am surprised to hear that that journal is not peer reviewed as I checked its website [5] and [6] that claim otherwise; and that paper is certainly too compicated for high school students. In any case that's not the issue, as you could see in the discussion above. That the "GRT" solution is notable is beyond doubt, as it is included in the FAQ, apparently was the standard explanation until Builder tried to shoot it down, and it has been taught as "correct" at least until recently at my university. I can try to find out what book that teaching was based on, possibly Moller who, if I remember well, advocated it.

As you apparently didn't read my link, I'll here repeat my suggestion:

• I have thought of splitting the article up in two distinct articles, the question is how to call them and how to link them. For sure Einstein's 1918 paradox is less "history" than the 1905 SRT text book exercise. Maybe the following is a good idea, in line with E4mmacro's suggestion: we can have an article called "Twin paradox", that starts with the two meanings, for example "as commonly discussed in textbooks" and "as originally discussed by Einstein", and then link to "Einstein's twin paradox" (Or "Einstein's clock paradox"). The "Twin paradox" page can then limit itself to the text book exercise. I also found guidelines for splitting: Wikipedia:NPOV_tutorial#Article_splitting Harald88 22:02, 27 November 2006 (UTC)

  Harald - It seems that you have latched onto an Indian journal, whose name is the same as the magazine I thought that you meant (and found in a web search). Even so, peer review in the third world journals is not what it is in western journals, and so my objection to Unnikrishnan's article stands.

  In so far as the "GR" view is concerned, it has hit me that this is a mechanism by which the simultaneity shift is determined to have occurred, and so may be more appropriate than I had given it credit for. There is still the issue of how to "spin" it though. Let's just say that the twin paradox has generated a lot of confusion over the years, and this article's state seems to be a demonstration of that.

  As for splitting the article: I strongly advise against that. Either the GR explanation is approrpiate or it is not. If it is approrpiate, then it should be included here. If it is not, then being off on its own won't change that. --EMS | Talk 22:52, 27 November 2006 (UTC)

Clarification: as this topic is very complex and lengthy (it's not yet fully covered), the proposal is to split this article up as is the rule for such cases. There are typically two kinds of readers. Many people just want to know how it is commonly and most simply calculated, they can't be bothered with the deeper issues. Other people who want to dig deeper and understand its origins should have that information as well. There is no need, nor is it beneficial to the readers, to put everything about a complex topic on one single page.

From Wikipedia:Content_forking:

Sometimes, when an article gets long, a section of the article is made into its own article, and the handling of the subject in the main article is condensed to a brief summary. This is completely normal Wikipedia procedure; the new article is sometimes called a "spinout" or "spinoff" of the main article, see for example wikipedia:summary style, which explains the technique. Even if the subject of the new article is controversial, this does not automatically make the new article a POV fork. However, the moved material must be replaced with an NPOV summary of that material.

Harald88 00:09, 28 November 2006 (UTC)

PS I see that you again removed notable information; if some piece of text is according to you not according to the sources (something about accelerating observers?), please move 'that to the Talk page for discussion or demand an exact citation, thanks in advance! Harald88 00:24, 28 November 2006 (UTC)

What I have done this time is to add a note equating the effects of gravitational time dilation with the effects of the relativity of simultaneity. Without that connection, the GR explanation appears ridiculous. With it, I have a working tie-in to SR.

I once again counsel you not to split this article. It is a difficult article because it is poorly written and laid out. I see no advantage being gained by making people go all over the place to find the same difficult material. The article first needs to be cleaned up and the physics better explained. Only afterwards can splitting it be considered.

I will look the article over some more first, but I think I can dispense with the "accuracy" tag for now given a context for accepting the "GR" explanation. --EMS | Talk 03:05, 28 November 2006 (UTC)

Imo it's a major mistake to delete the section on GR. The Twin Paradox exists in GR as well as SR, and some attempt should be made to explain the state of its resolution in GR. We should retain what was in the article previously and edit where appropriate. Also, imo we should ignore appeals to split the article up. Harald has never seen an article he hasn't wanted to split up. green 12.30.216.138 04:41, 28 November 2006 (UTC)

Glad to see the section on GR has been re-introduced in latest iteration. I removed the quotes from the phrase "General Relativity" in section header. green 12.30.216.138 05:11, 28 November 2006 (UTC)

I now have a better sense of what the GR view is about, how it works, and even how the objection of the "field" being universal and turned on and off is mistaken. None the less, I also find it to be a very esoteric exercise and wonder how useful it is as part of an encyclopedia. Then again, this whole article is a bunch of over-wordy hand-waving which takes too long to get to the point, and needs a solid rewrite. --EMS | Talk 15:35, 28 November 2006 (UTC)

I see you removed the section on GR again. Why? green 12.30.216.138 20:21, 28 November 2006 (UTC)

Look at the article. It's still there. I just cut out the last half of it, which I came to realize was somewhat POV and based on references to articles in obscure journals. The first three paragraphs, which are the "meat" of that section, remain. (A rewrite is needed, especially of the last paragraph, however). --EMS | Talk 22:26, 28 November 2006 (UTC)

OK, but I now see many more than three paragraphs, eight paragraphs plus an indented one if one counts one sentence paragraphs. Can you reproduce here what you cut out? It looks as if you retained all that was present previously. If I have time, I'll try to edit the Origin section along with what comes earlier in the article, and perhaps restructure everything in those sections. When I edited the Origin section, I retained some previously existing paragraphs at the beginning and end. This may have contributed to the lack of clarity in the overall presentation. 12.30.216.138 00:21, 29 November 2006 (UTC) green

Green here follows the deleted part; the purpose of putting the GRT solution at the end was to be considerate to the readers (they should be our main concern). Of course, if we put the whole history on a separate page for those who want to have more insight, that problem won't arise in the first place. And there is so much interesting and notable information about the complicated history that we should write, that in the long run there is bound to be a separate article on its history anyway. Harald88 00:47, 29 November 2006 (UTC)

It is sometimes claimed that the twin paradox cannot be resolved without the use of general relativity, by which it might be meant that age difference cannot be calculated by the travelling twin without general relativity, something we have tried to show can be done. On the other hand, the claim that general relativity is necessary, may be a claim that someone who doesn't believe the argument which rejects the first (erroneous) calculation by the space-ship crew is strong enough to convince the crew that they should perform the second correct calculation instead. The general relativity explanation says: if you are going to claim your reference frame is good, and deny the implications of changing reference frames, you will need to consider the inertial forces as equivalent to gravity forces and then account for the physical effect of gravity.

This explanation was popular among a number of physicists (Møller 1952) and continues to find adherents today. However, that calculation and its related interpretation have met with serious criticism. For example, after remarking that the general relativity calculation only corresponds to perceived reality for the traveler, According to Builder (1957)^[1], Einstein's solution of the twin paradox is invalid because the induced field must appear everywhere at the same time:

The concept of such a field is completely incompatible with the limited value c for all velocities [...], so that the specified field would have to be created simultaneously at all points in S' and be destroyed simultaneously at all points in S₀. Thus the principle of equivalence can contribute nothing of physical significance to the analysis.

Similar opinions have been expressed more recently by others^[2].

Thus in later years, physicists have increasingly treated general relativity as a theory of gravitation, while including acceleration with special relativity; consequently, acceleration is commonly regarded as "absolute" after all.

Thanks. Oddly, my link to the article doesn't always bring up the latest version. Since the TP occurs in both SR and GR, my inclination is to include the sections on GR that were deleted, perhaps editing them, but not starting another article. green 12.30.216.138 01:50, 29 November 2006 (UTC)

See below in the next header; from your comments there it appears that you now realise how difficult it is to the history justice in only a few lines. Of course the history should be summarized and approriately linked from this article while the (modern) "GRT solution" still should be mentioned in this article as that view is not dead. Harald88 10:53, 2 December 2006 (UTC)

Origin of the Paradox section - rewrite? remove?

First, there is a linguistic ambiguity in the word paradox that I'd like to mention. Different meanings are possible.

• paradox (sense 1) - A seemingly contradictory situation, which upon careful and logical examination is not.
• paradox (sense 2) - An unresolvable situation.

I suspect there is little serious disagreement that the twin paradox is a paradox in sense 1, but not in sense 2.

Now, on to the section itself. This essay advances a rather bizarre and unverified point of view that:

1. in 1905, the situation was "peculiar" but not paradoxical (why not? when does peculiarity rise to the level of paradoxical (sense 1)?)
2. in 1911, although the scenario was explicity presented using the personification of twins, the unsubstanstiated opinion of Wikipedia editors is that there is nothing paradoxical about it. Strange, as this is precisely why the twin paradox has generated interest in the general lay community who may know little else about relativity.
3. years later, sometime after the introduction of general relativity, this situation became paradoxical. Again according to Wikipedia editors, with no verification given.

Another problem with the section is the repeated use of the term "dynamically symmetric". While I understand what is meant, it is like that an average reader will just become confused. The content is already sufficiently addressed in the short top section, and even has a picture to show that the situation is not symmetric. Furthermore, in the context of general relativity, it is completely unclear (e.g. twins flying out and back "symmetrically" to two different bodies of widely differing masses would not be expected to have aged the same).

This section is an essay about the application of general relativity to the twin paradox, which even if properly sourced (which it is not) mostly ignores the main topic of the article. The twin paradox is stated in the first two sentences of the article. The fact the this "Origins" section takes nearly the space of three useful sections (the top, "Specific Example", and "Resolution of the Paradox in Special Relativity") is indicative of a serious POV problem (cf. "undue weight").

Good points, and simple to improve. Indeed the history of the paradox is complex, and this is just a compact summary.

Point 1: if you consult the dictionary, you may understand the difference between the possible meanings of "paradoxical" (which you presented) and "peculiar" (which you didn't present). Simply put: Einstein suggested that it was surprising, but he didn't suggest that he saw a problem with it.

Point 2: this was not the opinion of the editors but of the cited author, Langevin. It may be good to rephrase that into "For Langevin there was nothing paradoxical about it" - I'll do that now.

Point 3: No, it was before GRT, when Einstein was developing GRT that the paradox arose in the literature, partly due to his "general principle of relativity". Einstein explains that rather clearly in his 1918 paper. If that is not sufficiently clear in the article, it should be improved. Harald88 01:01, 29 November 2006 (UTC)

You state above that the Origin section is about GR, but it is not. I concur that the phrase "dynamic symmetry" should be explained. However, I don't think the Origin section is excessively long if one wants to clearly point out the error at the root of the alleged paradox, which the earlier sections fail to do adequately imo. Nonetheless, time permitting I will try to shorten it and move some of the historical-type paragraphs to the introductory section. 12.30.216.138 00:34, 29 November 2006 (UTC) green

The simple version of the twin paradox as outlined in the first two sentences of the article is a situation that is entirely resolvable in special relativity. The fact that the physical and mathematical relationships were unclear in the nascent days of relativity deserves no more than one sentence, if that. Today, our current best verifiable knowledge (isn't that what Wikipedia is allegedly about?) is that special relativity is the flat spacetime limit of general relativity, valid in the tangent spaces of the manifold. It would be silly to add that statement to this article, but at least it would be better than couching what appears to me to be fringe opposition to the concept as an "Origin" section. Tim Shuba 18:22, 28 November 2006 (UTC)

Please explain what, if anything, you consider a "fringe" position in the present version of the Origin section. You alleged this recently in another section of this Talkpage. What I wrote was hardly controversial about the error underlying the so-called paradox. green12.30.216.138

I don't see opposition in that section as much as I see wordiness and near-confusion. That section more covers the history of the paradox than anything else, and so should be moved to the bottom of the article and renamed. It also should be condensed so that it is 1/3 to 1/2 its current size. Even then it needs to be properly sourced. Otherwise it should be removed. --EMS | Talk 18:35, 28 November 2006 (UTC)

I don't mind a smooth rephrasing, but do mind complete removal of notable criticism that some editors here dislike - empoverishing Wikipedia is not required, as outlined just above. Therefore I now added the NPOV banner.

About history, the original Twin paradox had little to do with the textbook exercise of today, and readers who want to have a deeper insight in the cause of old debates need to be informed about the history. In fact there is much more to be added, but it would make this article too long. Thus I mainstain my recommandation to move the whole history to its own page, just as has been done with for example SRT and GRT. In other words, the more the history is summarized in this article, the more it becomes necessary to write a full article about it. Harald88 00:35, 29 November 2006 (UTC)

Absolutely not. The section has been killed and needs to stay dead. This nothing but yet another attempt to bring back the inept Unnikrishnan paper which in turn is the Trojan Horse for your pet theory, the "preferential reference frame". Do you think we are THAT stupid as not to see what you are trying to do? Moroder 01:48, 29 November 2006 (UTC)

Additional note: I see your remark elsewhere that you have "killed the last half of the offending section, including that silly reference". May I remind you that our personal judgements are irrelevant, and that your taking offense with opinions is not a valid excuse for deleting them. Also, Builder's paper was real progress since he included accelerating frames into SRT; thus an abbreviated version of his paper, "Resolution of the Clock Paradox", was also published in the American Journal of physics which is for example cited in [7] Harald88 01:39, 29 November 2006 (UTC)

So how about you did something useful: take the equations in the AmJPhys paper and write up a short section entitled "Resolution of the Twins Paradox for Accelerated Rockets". I just started doing this on my own, it is much more useful for the wiki readers than the metaphysical garbage that has been mercifully taken out by User:Ems57fcva Moroder 01:48, 29 November 2006 (UTC)

Harald - We each bring our own personal opinions and viewpoints into Wikipedia. Often, they get reflecting the articles. Sometimes that is good, and sometimes that is bad. One good thing about Wikipedia is that the bad personal opinions get challenged. Something about that Unnikrishnan paper seems to really fascinate you. I know that it is well written and published in a peer-reviewed journal, but what I am learning is that the journal matters more than the peer review. That Inidian journal is obscure and I am left to wonder what the credentials of the reviewer or reviewers are. What I can tell you as a trained physicist and relativist is that Unnikrishnan's views are junk! Under the NPOV undue weight guidelines, Unnikrishnan's views are one of those extremely small minorities such that it is not appropriate to present their viewpoint.

As I wrote on a user's talk page earlier today, you are an excellent bibliographer, able to find all sorts of interesting references. However, you need to realize that you do not yet have the training and experience needed to sort the wheat from the chaff. Sometimes odd viewpoints are just that, and do not merit coverage in Wikipedia.

As for the material I deleted from the GR section: As I got more oriented I found that it was adding an extra burden of detail to that section that was not needed. Part of why I hate that section is because it is bringing some very advanced concepts into an already confusing subject. By removing the extra commentary, I managed to restrict that section to the issue at hand, namely how a GR-based view provides a resolution. You also need to realize that less is sometimes better, as well as to realize the bad or inapproprate refereces are worse than none at all. --EMS | Talk 05:17, 29 November 2006 (UTC)

I did a major reorganization and edits of the Origin and the Introduction sections. Please check it out. I do believe I've cleared up a lot of confusion as to the origin of the "paradox" -- what it actually is -- as opposed to what it is not; namely, the strange result of asymmetical aging. green 12.30.216.138 05:21, 29 November 2006 (UTC)

The edits to the lead are good. As for the "Origins" section: It still says a lot of things that are repeated in the "Resolution ..." sections. I would rather see the "Origins" section removed and replaced by a "History" section at the end of the article. --EMS | Talk 05:31, 29 November 2006 (UTC)

I think the Origin section should be retained, since it gives a good general explanation of the cause of the paradox. The other sections might explain it in bits and pieces, and some repetition is not necessarily a bad thing. I could try to shorten it somewhat. In the meantime, please don't remove it. green 12.30.216.138 06:04, 29 November 2006 (UTC)

LOL! I'm not going to remove it. The material is good, but that section is doing too much, as it is covering both some history and some basic background on the physical basis of this paradox. What disturbs me is that the underlying lack of symmetry between the observers as described in the "Origins" section, and then again in the "Resolutions" section. It seems to be that if the "Origins" section had done its job, then the "Resolution" section would not need that text. (Most likely, the "Origins" section is a later addition, but just removing it deletes useful material such that I prefer to cautious about any such change.)

The biggest problem is that the "Resolution" section in the "meat" of this article. While some set-up is needed before the reader gets into that material, the amount of set-up currently provided is way too long. --EMS | Talk 16:06, 29 November 2006 (UTC)

Actually, in my previous cursory look at the material in some of the "Resolution" sections, I noticed some redundancies as well as obscure comments related to the cause of the paradox that are inconsistent with what I wrote in the Origin section and perhaps even misleading. I was intending to fix this, but so far have not. What I'd like to do is shorten the Origin section, and see if I can do some fixes to the other sections to create consistency and less redundancy. My main reason for not wanting to delete the Origin section is that the root of the paradox -- thinking it's OK to consider the twins in symmetric situations by making false inferences from some of Einstein's comments -- has a subtlety that should be fully explained. Btw, as you correctly surmise, much of the Origin section -- the part I wrote -- was a late addition to the article. green 12.30.216.138 16:26, 29 November 2006 (UTC)

In that case, we are more or less on the same page. Feel free to take your time and do a good job. I will let you know if I see any serious problems, but at this time I feel that it is more important to chase down any remining references to Unnikrishnan than to worry about details here. I do need to tweak the GR section a bit and even rewrite its last paragraph. (What that paragraph is trying to say is good, but the semantics are horrible. There is no such thing as an "inertial force", for instance.)

BTW - It would be nice if you got an account. --EMS | Talk 17:06, 29 November 2006 (UTC)

For the section on GR, I hope you are able to show using the assumption of asymmetric frames for the twins -- hence, not the "paradox" per se -- that one can derive the same result as SR. If not, it is important to point this out since the implications are significant imo. To do this you might need to revisit, and edit, the paragraphs you deleted. I have not been closely following the discussion about Unnikrishnan so I have no enlightening comments. Btw, I have noticed something flakey about Wiki software. When I link to the article I often get previous versions even though the edit file shows a newer version. Are they aware of this problem and doing something about it? Regards, green 12.30.216.138 22:57, 29 November 2006 (UTC)

First the version problem: I have not noticed that. You would need to show me an example of this. Do note that there is a way of linking to prior versions, but it requires the use of an explicit web link (delimited with single square brackets) instead of the usual internal link (delimited with double square brackets). Most likely this is a bug in the latest edition of MediaWiki (the open source software that "drives" Wikipedia) and will be resolved soon.

On the GR side of things: I have no doubt that the result is the same as for SR. I realized some time ago that gravitational time dilation is an affectation of the relativity of simultaneity: It is not so much that the clock at a lower potential runs slower as it is that after an elapsed time "x" for the upper clock, the simultaneous time for the lower clock is for an elapsed time of less than "x". So the effect of the "gravitational field" that exists while accelerating is to treat the stay-at-home clocks as having sped up a lot while the acceleration was ongoing. Even so, the real "physical" effect is the change in what event of the stay-at-home twin is simultaneous with the traveling twin in the traveling twin's frame-of-reference.

I am not going to do the calculations, but assume that they are sound as the USENET Relativity FAQ claims so. My question about that section is whether it adds anything to the understanding of this exercise. I can explain it, and I can justify the explanation. That does not mean that it belongs here, however. --EMS | Talk 05:12, 30 November 2006 (UTC)

EMS, first you came here removing the GRT solution with the claim that it is "wrong" and next you claimed that it is "right" so that you reinserted it while trying to block mention of peer reviewed criticisms about the inappropriateness of the "GRT solution".

Thus, please don't add confusion to this article on a subject that continues to confuse so many people including yourself but clarify your WP:Original research claim that "Unnikrishnan's views [on the point where they agree with Builder's] are junk", in view of your additional comment that you are "not going to do Einstein' calculations, but assume that they are sound", eventhough the usenet FAQ disagrees with Einstein calling that solution "GRT" and the fields "real" and while that FAQ fails to mention the physical impossibility (according to GRT) of such universe encompassing instantaneous "real" fields.

Harald88 10:47, 2 December 2006 (UTC)

I don't follow your GR explanation of the differential aging. There's no need to do the full calculation in GR. However, it is necessary to show using, say, a plausible physical argument -- which I think you claim to have -- that in GR one gets the same result (traveling twin ages less than stay-at-home) from each twin's pov. This is the criterion for solving the twin problem (not the "paradox" per se) that is stated in the Origin section -- that one gets consistent results using the pov of each twin. For completeness, one must do this for SR and GR since the problem arises in both theories. I would really like to see it. The previous explanation in the article was very unclear imo. green 12.30.216.138 06:20, 30 November 2006 (UTC)

The explanation is the article is a bunch of confused hand-waving. The gist is correct: An accelerating observer perceives the Earth as being subject to gravitational "time dilation", which ironically speeds clocks at higher potentials up. Because the ship is accelerating towards the Earth, Earth is at a higher potential, and because of the distnac to the Earth that is a much, much higher potential. Hence the needed "speed-up" being perceived for the traveling twin during turn-around. The equivalnce principle does explain why gravitational time dialtion exists in that case. However, the field being "physical" and the acceleration of intertially moving objects being accelerated due to "inertial forces" is total and absolute junk.

BTW - The issue is NOT differential aging, but rather what the accelerating observer perceives at turn-around. Remember that the traveling twin must find the "home" clocks time dilated on each leg of the trip. So for a 10-year trip in which the traveling twin ages only 5 years, he can only have the home clocks advance a total of 2.5 years for both legs of the trip. This exercise is about accounting for the "missing" 7.5 years of home time.

Oh well, enough ranting. I will try to edit that section over the weekend. --EMS | Talk 17:14, 30 November 2006 (UTC)

EMS I see that you are on the same page as Builder and Unnikrishan without realising it, as you call Einstein's explanation "total and absolute junk". However, that is not up to you to say, but it is up to us to cite the scientific literature about it.

Thus please don't go editing your own ideas into the article but cite the peer reviewed criticisms. I will take a backseat for a moment, and look what you make of it. Harald88 10:47, 2 December 2006 (UTC)

I'll be looking forward to reading it. Please explain in the article which twin is at a higher potential and why, and if there is an issue about the alleged instantanous propagation of the gravity field if one uses the equivalence principle as I believe was claimed in the previous version of the section. Also, imo, there should be an indication as to whether there is a consensus that the twin problem can be solved in GR, and that the solution presented is the consensus solution. I plan to re-edit the Origin section by next weekend as well. green 12.30.216.138 19:49, 30 November 2006 (UTC)

Please don't ask for too much. This is an encyclopendia, not a research journal. Kindly remember that I originally wanted to delete that stuff! The reason is that GR is not needed to solve the twin paradox. The "GR" solution is valid, but it is a very esoteric exercise. BTW - The "field" propagaes instantaneously because it is based on the observer's perception of spacetime and is not inate to the spacetime itself! Once again I emphasize that this fundamentally is another way of describing the simultaneity shift of SR. --EMS | Talk 20:02, 30 November 2006 (UTC)

(Reset indent). One of the issues that interest me here is that the asymmetical aging in SR, or if you prefer the time dilation of the clocks being compared, can be calculated using relative velocity, but in GR I think it depends on the magnitude of the acceleration. On its face this is odd if one assumes in the GR solution that the Earth has no gravity field (as I think we do and must to make the formulation of the problem in GR the same as in SR). green 12.30.216.138 23:49, 30 November 2006 (UTC)

Good questions. Basically you need to see equivalence principle. Accelerated observers see inertially moving objects as being accelerated with respect to themselves, and if they so choose may ascribe this effect to a "gravitational field". Look at it this way: Under GR, free fall is inertial motion, and the only force on us as we stand on the Earth is the surface of the Earth accelerating us "upwards" and away from a free-fall trajectory. This is in contrast to Newton's view in which a force of gravity is being countered by the force from the surface.

The "key" to this business is that all accelerated observers perceive the existence of gravitational time dilation. As I told you before, this is a manifestation of the change of simultaneity as one is accelerated. Now as a practical matter, Einstein derived the gravitational time dilation effect by considering the view of an accelerated observer in a Minkowski spacetime. In other words, this is an effect of SR and not of GR to begin with, but it does carry over into GR. So the name of the section itself is something of a misnomer. --EMS | Talk 02:50, 1 December 2006 (UTC)

That it is an SRT effect because, as you put it, Einstein derived the gravitational time dilation effect by considering the view of an accelerated observer in a Minkowski spacetime, is indeed what the FAQ claims, putting "GRT". Builder's articles in both AJP's were apparently the first to provide that argument as well, on top of providing the equations for accelerated frames. If you know of a earlier article that forwards that argument and includes accelerated frames inside SRT, please tell us. Note that an anon (12.30.216.138) changed the title back from The solution in "General relativity" to 'The solution in General relativity. Harald88 10:47, 2 December 2006 (UTC)

Why put quotes around 'General Relativity'? Btw, the quotes recently reappeared, so I deleted them. Either you have a solution in GR or you don't. No need for quotes unless you believe the solution being presented is bogus. In such case the quotes should be around 'solution'. green 12.30.216.138 00:09, 3 December 2006 (UTC)

I missed that remark of you. I used the quotes following the cited physics FAQ reference; it is a matter of taste if the quotes should be around GRT or around solution. But the mainstream view appears to be unfavourable to it, and that's what the quotes are meant to warn the readers about. Harald88 11:35, 9 December 2006 (UTC)

Would you agree that in GR the traveler's clock slows due to the equivalence principle compared to the stay-at-home twin, who is really assumed to be in a zero gravity situation (to model the problem in a manner completely comparable to SR)? If so, in this view there is no need to bring in the issue of how fast the induced gravity field propagates to the stay-at-home twin --- indeed there is no physical propagation -- making Builder's analysis irrelevant. Btw, I don't see how the alleged change in simultaneity is involved. It doesn't seem necessary. green 12.30.216.138 04:32, 1 December 2006 (UTC)

The GR explanation is only active at turnaround, and during turnaround the traveling twin IS NOT in a zero-g situation. Note that the "gravitational field" that exists during turn-around is perceptual, so Builder's analysis is indeed irrelevant and even misleading. As for simultaneity: That is essential. Otherwise you cannot account for the missing 7.5 years. --EMS | Talk 04:44, 1 December 2006 (UTC)

I wrote (and meant to convey) that the stay-at-home is in a zero-g field always in the way we are modeling the problem, and agree that the traveling twin is not in a zero-g gravity field at turnaround (and only at turnaround). Builder is certainly irrelevant and the deleted paragraphs are misleading in this respect. I still don't understand the simultaneity issue. green 12.30.216.138 05:08, 1 December 2006 (UTC)

Green, how can you claim that an opinion about an issue is irrelevant while at the same time you admit that you don't fully understand the issue?! Harald88 10:47, 2 December 2006 (UTC)

Ah - Now I see your confusion. For the stay-at-home twin, you are indeed correct that that the "GR" solution is irrelevant, and that there is no simultaniety issue. The stay-at-home twin is moving inetially in a gravitation-free environment such that GR is identical to SR. The volumes of simultaneity for the stay-at-home twin are therefore always parallel and evenly spaced, and the time dilation experienced by the traveling twin is directly accounted for.

Becuase the stay-at-home twin's view of things is so simple, I couldn't care less about it. Now for the traveling twin, there is a simultaneity issue: Before turnaround, the stay-at-home twin at 2.5 year after launch (stay-st-home time for a 10-year [stay-at-home measured] trip by the traveling twin at 0.866c) is simultaneous with the traveling twin in the traveling twin's frame of reference. After turn-around, the stay-st-home twin at 7.5 years after launch is simultaneous with the traveling twin in the traveling twin's frame-of-reference. In a nutshell, the volumes of simultaneity from before the turnaround are not parallel with the volumes fo simultaneity after the turnaround for the traveling twin. That is shown in an illustation in the article. (You can also see this old sci.physics.relativity posting of mine for another view of the situation.) --EMS | Talk 15:58, 1 December 2006 (UTC)

I think you misunderstood what I was meant to convey -- it was simply that whether we use SR or GR, we are, or should be imo, formulating the problem as twins in empty space, with one accelerating away and returning. The problem with saying that the stay-at-home is on Earth, is that it introduces in the GR solution (only, I think) the irrelevant issue of how many g's the stay-at-home is pulling, and whether that is being factored into the issue of which twin is at higher or lower potential. That said, and cmiiaw, but with the twins in empty space, solving the problem in SR or GR amounts, I think, to the decision of whether we apply the Lorentz Transformations or the Equivalence Principle. As to role of simultaneity in all this, I haven't clue. I'll try to read your reference this weekend to see if it helps. Finally, I just re-edited the Origin section. You will probably say it is decent, maybe even "good", but too long. I find it difficult to do justice to the subtlety of the issue with fewer words. green 12.30.216.138 06:48, 2 December 2006 (UTC)

Green, that's what I have been trying to convey all along: because of the subtleties involved as well as the great number of involved key publications, to do the history justice a whole article is required. That is also the appropriate place for a detailed discussion of Einstein's 1918 solution as it was a much disputed view (e.g. Dingle) but nowadays it's rarely mentioned. Harald88 10:47, 2 December 2006 (UTC)

I am not opposed to making the article longer to include some discussion about issues in contention. In fact, it seems appropriate in order not to leave the reader with the false impression that everything has been neatly resolved. However, I see no advantage in splitting the current article into several articles. Even if we decide to go into these issues in some detail, I see no gain in requiring the reader to link to another article. green 12.30.216.138 21:44, 2 December 2006 (UTC)

As far as Dingle goes, I still haven't read the stuff I got from you. However, I am progressively coming to the conclusion that SR, and maybe SR combined with GR in the example that follows, is an inconsistent theory. Why not have the twins start in empty space, and separate and return in a completely symmetric way? Using GR and the EP, during the symmetric periods of acceleration both clocks will slow by the same rate (and one doesn't need to worry about Builder's clain of the need to posit an instantaneous propagation of the fields since, iiuc, this is irrelevant). Otoh, applying SR during the periods of constant velocity, each observer will conclude that the other twin's clock is going slower. In sum, when the twins are reunited, it seems as if one gets a paradoxical result -- that each twin sees the other as younger. I think people claim that issues of simultaneity resolve this paradox, but I am far from convinced. green 12.30.216.138 17:05, 2 December 2006 (UTC)

To start with an symmetrical situation is a very interesting idea. Both will do, feel and see the same, there is no obviously difference. Now, the asymmetrical situation. Only one does and feels, but both still see the same. No difference?

If you're referring to what I wrote immediately above for the symmetric problem where both twins undergo accelerations and return, then I conjectured that each twin sees the other as younger upon return (since SR can be used on the constant velocity legs) and SR will have been falsified! The asymmetric problem is the actual twin problem -- that is, where only one twin accelerates. Here, they both see the same thing, but only one feels forces. If you are going to limit the analysis to what the twins "see", you are making the mistake of assuming that kinematics alone can solve the problem. green 12.30.216.138 00:52, 3 December 2006 (UTC)

Please read again the guideline on Article spinouts: [8] In agreement with the guidline I did not suggest to split this article into several articles but to reduce the history in this article to a compact summary and to describe the history in full in a separate article that is referred to, as we habitually do, see for example History of general relativity Harald88 11:23, 9 December 2006 (UTC)

Why is neutrality of article being challenged?

—Preceding unsigned comment added by 12.30.216.138 (talk • contribs) 14:42, November 30, 2006

See this diff. In a nutshell, Harald88 is upset over my removal of the last half of the "Resolution in general relativity" section, including the removal of the Unnikrishnan reference. I will therefore propose that it be removed if noone other then Harald objects to that. (Let's give this a few days and see if anyone has anything to say about it.) --EMS | Talk 03:04, 1 December 2006 (UTC)

While the "GRT solution" is notable, also notable criticism must be mentioned with some of the best examples. Einstein's "GRT" solution was never published in the English literature except in discussions and criticisms; nowadays that solution is rarely advocated. Those facts must be mentioned and explained, citing peer reviewed sources following WP:V. This is both required by WP:NPOV and out of respect for the readers who should be properly informed. Similarly, the different notable explanations must be cited in stead of suppressed; I notice now that also Langevin's explanation has been so much truncated that it is effectively suppressed, while also the reference has been deleted against WP:V (see Wikipedia:NPOV_tutorial#Information_suppression). Harald88 14:56, 2 December 2006 (UTC)

I've reproduced (in italics) the deleted paragraphs below for reference. The first paragraph is not clear and therefore difficult to evaluate as to content. Personally I am not totally convinced by the alleged solutions using SR. Although I have yet to carefully read the SR solutions in the article, from my previous studies and presumption as to the article's content, I am not sure about their validity. E.g., when using the frame jumping method, is the behavior of the traveler's clock adequately analyzed at the instantaneous turnaround, or is this issue swept under the rug? ... Concerning the critique below of Einstein's GR solution, I am not sure I understand the claim that using the equivalence principle requires the assumption of an instantaneous propagation of the gravity field to the stationary twin. Whether one uses SR or GR, one always calculates based on local observations, and then applying the principles and transformation equations of the theory being used. Otoh, for there to be a real difference in clock readings, we must be dealing with bonafide differences in gravity fields. In this view, Builder's critique would seem to have merit. ... In sum, I think this entire issue needs to be thought through with much care before we leave the GR section trucated. green 12.30.216.138 04:20, 1 December 2006 (UTC)

It is sometimes claimed that the twin paradox cannot be resolved without the use of general relativity, by which it might be meant that age difference cannot be calculated by the travelling twin without general relativity, something we have tried to show can be done. On the other hand, the claim that general relativity is necessary, may be a claim that someone who doesn't believe the argument which rejects the first (erroneous) calculation by the space-ship crew is strong enough to convince the crew that they should perform the second correct calculation instead. The general relativity explanation says: if you are going to claim your reference frame is good, and deny the implications of changing reference frames, you will need to consider the inertial forces as equivalent to gravity forces and then account for the physical effect of gravity.

This explanation was popular among a number of physicists (Møller 1952) and continues to find adherents today. However, that calculation and its related interpretation have met with serious criticism. For example, after remarking that the general relativity calculation only corresponds to perceived reality for the traveler, According to Builder (1957)[1], Einstein's solution of the twin paradox is invalid because the induced field must appear everywhere at the same time:

The concept of such a field is completely incompatible with the limited value c for all velocities [...], so that the specified field would have to be created simultaneously at all points in S' and be destroyed simultaneously at all points in S0. Thus the principle of equivalence can contribute nothing of physical significance to the analysis.

Similar opinions have been expressed more recently by others[2].

Thus in later years, physicists have increasingly treated general relativity as a theory of gravitation, while including acceleration with special relativity; consequently, acceleration is commonly regarded as "absolute" after all.

I see that the neutrality issues have not been corrected but the banner has been removed. Please don't remove the banner without addressing these issues, thanks! Harald88 22:41, 5 December 2006 (UTC)

What is this discussion about

The twin paradox has to faces: 1.Twins are no longer of same age. Today, this is no longer a problem, we generally accept, that time is not an absolute physical item. 2.The misunderstanding of symmetry: Movement is relative. Two object independently determine the change of their distance over time and determine the same velocity. It is not evident that both measure (compute the relation of changing in space distance and the time distance) the same value for this velocity! But when exchanging this value (for example, with a light signal), they both can independently see, that they measured the same value of speed for the relativ motion. To now, they don't know, if their "aging" is different. The paradox arises from the "fact", that according to special relativity both can "see" the other one moving and "know", that the other ones time is dilated. When they meet, one is older, one is younger. So, what happens, if both can not only exchange information about speed of relative moving, but also speed of their own time? How do we measure time? In principle by counting events, that are obviously periodical. For example by generating electromagnetic radiation with help of certain atoms. We believe, that the behaviour of atoms defines time. So, what happens, if the twins agree before they start, to exchange the value of relative velocity by sending a pulse width modulated signal of a certain light atomic source, where the pulse width reflects the relation of relative velocity to independently in place determined velocity of light. When they receive the signals, they receive two informations: 1. PWM represents relative speed. 2. The relation of frequency (or wavelength) of the received message and the sended message. This later is a measure for the "time" speed. The question is: will they both get the same value or not. ErNa 09:08, 1 December 2006 (UTC)

Content dispute and structure of this article

Related to the above POV dispute we also have a content issue: Builder's criticisms in that one paper seem to best convey the modern point of view (physical acceleration is considered to be "absolute"; neglecting gravitation, GRT is artificial for the twin paradox and there are simultaneity problems that are incompatible with a physical field); also his paper appears to have been the first to point that out, making it particularly notable. As Builder's paper dates from half a century ago, the addition of the reference to Unnikrishnan's article was meant to be a first of a number of additional references to illustrate the current status of discussion of the topic that bears on how the "GRT solution" is viewed nowadays by different scientists in peer reviewed publications. I don't insist on citing an Indian journal but we must be against prejudice; see also [9] (countered by [10]) for Unnikrishnan's understanding of GRT and this kind of problems. Harald88 11:54, 2 December 2006 (UTC)

Moreover, the article was incomplete and poorly structured and it appears to be getting worse. After providing the basic SRT calculation exercise (which could easily fill up one article by itself), the most notable explanations should be discussed: it correctly started with Langevin's (Lorentz-based) absolute space explanation and went on to Einstein's induced gravitational field explanation, but still lacking (although requested) was a discussion of Minkowski's physical spacetime explanation, which appears to be the most popular one nowadays. Harald88 14:56, 2 December 2006 (UTC)

I think the current introduction is far superior to what existed previously. It's now chronologically sound and hence more comprehensible, plus the previous discussion therein wrt defining the "paradox" was somewhat misleading imo. Concerning the Origin section, I see no grounds in claiming it has gotten worse. I tend to agree that overall the article might need some reorganization. green 12.30.216.138 21:50, 2 December 2006 (UTC)

I don't see this article as being helped much by the recent edits. The "Origin of the paradox" section is now a highly philosophical discussion that covers the same ground as the rest of the article, but in a much less comprenhensible fashion. The article is now also devoid of a clear statement of what the paradox is!

If this is your opinion, then obviously you need a crash course in reading comprehension. I won't be participating in this activity any further. green 12.30.216.138 05:34, 3 December 2006 (UTC)

The more I think about it, the more ridiculous your critique becomes. The paradox is caused by a purely kinematic treatment of the problem, which implies frame symmetry and the prediction that each twin sees the other as younger when reunited. This is stated in the concluding paragraph of the Introduction and explained in some detail in the Origin section. Why anyone would think it's OK to use pure kinematics to solve the problem goes to the manner in which relativity is misinterpreted. Unfortunately, in the intellectually decadent culture in which we reside (Sound bytes longer than 20 seconds need not apply!), anyone trying to explain the error of a purely kinematic analysis in terms of Einstein's seminal comments, suffers the criticism of being too "philosophical". What you want to do, apparently, is give a shallow, defacto pseudo-explanation that cannot be construed as indulging in a no-no -- "philosophy". Be my guest. green 12.30.216.138 23:53, 3 December 2006 (UTC)

This IMO is a severe defect, and should be in the introduction itself!

It is! Learn to read. I'm done. green 12.30.216.138 05:38, 3 December 2006 (UTC)

As it currently stands, this article would be much better served by the removal of "Origins of the paradox". I also would still prefer to see the "general relavity" section removed. The twin paradox is not a GR problem, and I see that section as a distraction. Finally, much of the history should be removed to a section on that topic at the end of the article. (Perhaps that may be the place to discuss Einstein's "GR solution".)

As for the loss of the references opposing the GR solution: I would be more sympathetic to Harald's position if they were not so blatantly mistaken. I am especially disturbed by Unnikrishnan's article which is so blatantly anti-relativistic that it obviously is not an acceptable source for this article. I don't see any sign of an ongoing debate on this topic in the field. As for the references that Harald is promoting above: The first is in regard to gravitational shielding and does not address that twin paradox. The second is even worse, being a response to the first one published in a geological journal instead of a theoretical physics journal! I think it speaks to the extent of the "conflict" that Harald is claiming that he is unable to document it in articles published in premier theoretical physics journals. --EMS | Talk 05:31, 3 December 2006 (UTC)

EMS, I did not promote those references for this article, sorry if you don't understand my English. And higher up on this page I already expounded that I don't care too much which articles are selected, as long as key opinions are justly represented. I know of no earlier modern mainstream opinion than the 1957 article by Builder according to which GRT "brings nothing", so that to me it looks most appropriate to at include at least that one. It's irrelevant if you think that Builder, Baez, Jansen etc. are mistaken and that their opinion should not be mentioned, their opinion is very notable to say the least. Similar with Einstein's "general POR" opinion without which this whole paradox wouldn't have taken off, and with which they disagree. Harald88 22:40, 5 December 2006 (UTC)

EMS

As per your edit preferences, I tried to delete Origin and GR sections, as well as the present introduction to remove what offends you but it was considered vandalism. Please remove them ASAP, and henceforth edit the article to your heart's content. I won't be doing any further Wiki editing. I have better uses for my time. green 12.30.216.138 10:51, 3 December 2006 (UTC)

Anon - I have restored your version (but then also restored the lead. I wanted that expanded if anything). I have also chided the editor who called your edit vandalism, as you did it in response to my requests. Hopefully you will get an apology out of her. (Perhaps your removing the lead was the issue, but until and unlessCharoletteWeb makes her thoughts known I cannot be sure of that.) I kindly ask you to settle down and consider continuing to be an editor. You are not the first person to suffer "wiki-shock" here. There is a guideline called "Please do not bite the newcomers", which was unintentionally violated in your case. So you are on firmer ground than you think. BTW - I still advise you to get an account. As you have just found out, anons are often suspected of being potential vandals, and to be honest with you most anonymous edits are vandalism. --EMS | Talk 22:42, 3 December 2006 (UTC)

I made some minor edits to my long comment immediately above, so you might want to read it again. As I stated therein, it is easy to succinctly state what the paradox is, and its cause; namely, the erroneous assumption that kinematics alone can be applied to the problem; which implies frame symmetry; which implies that each twin predicts seeing the other as younger at reunion. Of course, for any thinking person this explanation is woefully insufficient. It begs the question as to why it is an error to rely solely on kinematics. Why can't we simply ignore the dynamical difference between the twins? This is the issue I was trying to address in the Origin section. If you can explain the error more simply than I did, or more concisely, please do so. But to ignore a fundamental issue of such significance is absolutely unacceptable imo. Fwiw, I feel strongly that the twin problem needs to be solved in GR as well SR. For anyone relying on kinematics alone, the same paradox presents itself in GR. Further, it is worthwhile to provide the solution GR, if only to show that SR and GR produce consistent results (not a trivial issue imo). Btw, I have edited Wiki articles before, a few months ago. I might get an account someday. My motivation for participating is to teach and learn, and I don't need an need an apology from Charlotte. green 12.30.216.138 00:12, 4 December 2006 (UTC)

I see you have not fully restored my version. You deleted the Origin section. I wonder if you have any idea what really needs to be addressed in the article. I tried to explain it again, immediately above. Einstein wrote that only relative motion (i.e., kinematics) matters. If so, we have a paradox. Was Einstein wrong? Were his words misconstrued? If so, exactly how? A thinking reader will want to know! Also, since Harald and I agree that GR solution should be included in the article, we still have a POV issue. So the tag should be restored. green 12.30.216.138 02:05, 4 December 2006 (UTC)

Anon - THIS IS A WIKI, and I am not its boss or owner. While I know relativity quite well and have my own opinions, I don't have much extra time or energy for editing this article. At this time, you are more "in charge" of this article than anyone else IMO! So you do what you think is right and proper. I am perfectly capable of deleting content I dislike enough. You are under no obligation to delete it for me. So if you don't want it gone, then you can (and probably should) restore it.

I might add more to the Introduction instead of totally restoring the Origin section. Btw, I am not totally anonomous. I have a name-identifier. green 12.30.216.138 04:45, 4 December 2006 (UTC)

On the issue the "kinematics", I agree with you wholeheatedly, but not with the use of that word.

Not sure what you mean. I could be mistaken, but imo "kinematics" refers solely to an analysis of motion, or relative motion, whereas "dynamics" includes an analysis of the forces in play. In the twin problem, when we include the effects of acceleration, we are including dynamics in the analysis. green 12.30.216.138 04:45, 4 December 2006 (UTC)

I find it to be a bad sign when I have to start thinking about what someone means, as it means that the average reader has little chance of comprehending it. That is not to say that I want this article writen for the lowest common denominator, as relativity is just not a simple subject. All that same, Wikipedia is not a research journal either. I have not found words that I like, but a statement such like "The issue is that from the standpoint of the traveling twin, it is the stay-at-home twin that is time dilated throughout the trip. This creates the impression that when the twins rejoin, each should be younger than the other." is needed in the introduction of the article.

The last statement in the Introduction is somewhat like that. The problem with what you have is that it doesn't emphasize that each twin sees the other's clock as time-dilated and hence younger (via the false assumption of symmetry). Each twin is entitled to do the analysis, and a good resolution analysis includes calculating the situation as seen from the pov of each twin and that they are consistent (give the same result). Maybe I'll edit what we have to make the paradox more explicit. green 12.30.216.138 04:45, 4 December 2006 (UTC)

Also IMO, the twin paradox was not fully solved until the simultaneity analyses of the 1950s were published, but I am not sure if this view is widely held amongst those familiar with the issue. --EMS | Talk 03:56, 4 December 2006 (UTC)

I'd like this included, perhaps in the section on GR if you have time, or at least in the context of SR. green 12.30.216.138 04:45, 4 December 2006 (UTC)

So many words written during this discussion. Hard to identify the points of view. Everything goes always forward and backward. What about logic? What we say is: we dont have any preassumptions. A body radiates monochromatic light. Now this body is devided into two part of equal mass. These two parts exchange momentum and start relative movement. Both parts compare their own radiation with the others radiation an determine the same relation. There should be no disaggreement about this. And now we change this experiment a litte and confusion starts: One mass is a bit (incrementel) smaller then the other. One party says: there will be no difference in the above comparison, the other: there will be a difference. Both parties are right. This is the paradox. Isn't it? ErNa 08:36, 4 December 2006 (UTC)

Note on "kinematics"

Green - "Kinematics" is the right word. My concern is editorial as opposed to content. If that word is to be used, then it needs to be explained. That is a not a word that I hear often, and I assume that many readers will be completely unfamiliar with it. My concern with this article is that is just dives into intellectually "deep waters", and assumes that the reader can keep up. While it is fair to presume a basic understanding of relativity, you should not need to be a college graduate to make sense of this. The twin paradox is subtle and the details do get quite technical. Even so, the kinematics can be described without using too advanced a vocabulary. --EMS | Talk 13:43, 4 December 2006 (UTC)

I am a fairly experienced writer. For this reason I defined "kinematics" in the last version of the Origin section as well as restating the "paradox".

"That is, although it is undeniable that twins are in symmetric kinematic situations -- which means that each twin observes the other leave and return within a set of velocites and accelerations that are indistinguishable except for direction -- using this data alone and ignoring the fact that external forces act on the traveling twin only, results in a clearly impossible conclusion; namely, that each twin sees the other as younger when they are united."

I can accept criticism. The section can be improved. E.g., I should have made "kinematics" a link and noted that including an analysis of forces is called "dynamics", with a link to its definition. But it is unacceptable to trash the section without reading it with minimal care. green 12.30.216.138 15:36, 4 December 2006 (UTC)

Green - If you want to go back to a version with those sections included, then please revert the page to that. I am not going to stand in your way. I am not going to make major changes without a very good idea of where I want this page to go. I can name parameters, but doing the edits is another matter. I thought that you agreed about removing those sections, but if you don't then let's go back to the status quo ante and figure out how to make that material work well. All that I will say is that doing the revert is your job, but I will support it if you choose to make that change. I will not do an edit war with you. --EMS | Talk 17:11, 4 December 2006 (UTC)

I'm not sure what I want to do, except to enhance the introduction. Do you think Einstein believed that physical phenomena can be completely described in terms of kinematics, that is, relative motion, or have his words been misconstrued? Did he ever clarify this issue in his writings? green 12.30.216.138 19:08, 4 December 2006 (UTC)

I'm a student of physics and relativity. I am not a student of Einstein and his writings. Perhaps Harald can shed some light on this question. Certainly SR is a kinematic theory. Yet in his 1916 article of GR, Einstein points out the issue of how one is to define the inertial reference frames of SR in that rotation and acceleration do seem to be absolutes with respect to such reference frames. Even so GR is another kinematic theory. Now with that said, I do not believe that Einstein believed that all physical phenomena could be understood through kinematics alone, but instead I perceive that Einstein wanted to get the kinematics right so that the dynamics could be better studied. --EMS | Talk 19:48, 4 December 2006 (UTC)

If SR is restricted to inertial frames -- which it is essentially by definition -- it can be nothing other than a purely kinematic theory. But this raises the question whether it is legitimate to try to solve the TP within SR, given the acceleration of the traveler. Therefore, strictly speaking, a solution completely within SR must be viewed with some degree of skepticism. Iirc, the SR solutions I have seen indulge handwaving when it comes to calculating the time dilation during an assumed instantaneous turnaround. I presume our article does the same, and if so, is another defect that needs to be addressed. These issues strongly suggest the need to present solutions using GR.

If Einstein didn't believe that kinematics alone is sufficient to describe physical phenomena, he surely made statements that can be construed to mean just that! I'd really like to know what his position was in his later years. Perhaps Harald can shed some light. In any event, imo the TP shows the impossibility of such a dream. Kinematics alone leads to a paradox in SR. Otoh, if one analyzes the TP using GR, one still needs to distinguish frames to solve the problem. So I think a purely kinematic solution to the TP, in either SR or GR, simply cannot work. I therefore disagree with your claim that GR is a kinematic theory. How can it be? green 12.30.216.138 23:05, 4 December 2006 (UTC)

I reread the introduction. It is very clearly stated: from SR followes a "peculiar" result. And that's it. The confusion only arises, because some people say: for there is no absolute space, two observers, watching each other, can not determine, which one was accelerated. But this is wrong. Two objects in relative motion, sending a normalised light signal, for example 257 nm Hg line, may see different wavelength and by that determine their state of movement. The principle of relativity only says, the laws of physics are not depending on absolute motion! ErNa 17:30, 4 December 2006 (UTC)

Agreed. This discussion is about how best to impart that knowledge to the readers. Also do be advised the doppler shifts are only one way of viewing this problem. --EMS | Talk 18:02, 4 December 2006 (UTC)

How do you define "absolute motion"? Without a clear definition, I don't know what it means for anything to depend on it? Btw, wrt Doppler shifts, if both twins were to send the same frequency, wouldn't they receive the same but different frequency depending on their relative velocities? That is, wouldn't there be a difference in the received frequency only if the accelerating twin sent a signal during a period of acceleration? green 12.30.216.138 19:17, 4 December 2006 (UTC)

I try to explain. Like most students, I learned to pass the examinations. I was not interested in heory for 25 year. Than I tried to understand "action". Searching Wikipedia I found these disputes on relativity and was very astonished. Ok, here I am. Physics can be devided in two parts: reversible and irreverible events. Irreversibility give time a direction. But that is not our item here. Here, what goes forward, has to go backward in time. Whenever I join an electron and a proton, I get the same hydrogen. Independent of the history of the particles. Even when they are materialised during a collider experiment. To me this means: I cannot change a particles properties by any kind of acceleration. Since SR we know, there is no "absolute time". But: there is a "now". Let be c a constant. All events have a spacial distance to me. (let us say:) All events generate "photons", for electromagnetic force come from exchange of photons. The light of all events, that take place "now" can be seen "here" after a time (spacial distance /c). SR only knows a closed system. That means, momentum of the whole system (the univers) never changes. Therefore there is a special frame of reference for the whole system with momentum = zero. Suppose, all the matter of universe is in the centre of mass at rest (this is not possible, but, suppose!) (like said in kinematics: "without considering the factors that cause ") Then I transfer kinetic energy to all the particles in the universe to produce exactly what is "now". It takes a certain amount of energy. If I would have chosen a different starting point, that is, momentum a start was not zero, than I would need more kinetic energy (I think this is easy to proof). Therefore: The inertial frame of absolute rest is the frame connected to the centre of mass. Einstein say: (as I understand it): By watching a closed system, I cannot determine my velocity relative to the centre of mass. He didn't say: to object in relative motion can not determine which one moves faster. In Fact: It is possible to determine the absolute speed by experiment. At least, a thought experiment, for no one will pay for it. It will be very expensive! ErNa 20:29, 4 December 2006 (UTC)

Treating the whole universe as a system is certainly interesting, and there is a "preferred" frame in the cosmological solutions which is the Cosmic Microwave Background frame in which the proper time from the Big Bang is maximized. However, that is not a matter of there being absolute motion, and in relativity the laws of physics are independent of one's state of motion. The issue here is how the paradox works out given that indendence. --EMS | Talk 21:49, 4 December 2006 (UTC)

Why cannot the CMBR frame be considered the frame defining absolute motion? green 12.30.216.138 23:20, 4 December 2006 (UTC)

In a Robertson-Walker universe, coordinate spatial motion is motion with reepect to the local CMB reference frame. However, defining that coordinate motion as being "absolute motion" is a convention instead of a physical reality. I agree with what you wrote below that it is convenient, but in relativity it is also unnecessary. --EMS | Talk 05:28, 5 December 2006 (UTC)