Talk:Twin paradox/Archive 15

Archive 10

←

Archive 13

Archive 14

Archive 15

Archive 16

Article lead : Inaccurate summary of the references.

There has been much discussion of the use of "incorrect" and "naive" in the lead but it seems to me that a more significant problem has been missed. The current sentence is "This result appears puzzling because each twin sees the other twin as moving, and so, according to an incorrect[1][2][3] and naive[4][5] application of time dilation and the principle of relativity, each should paradoxically find the other to have aged more slowly.". I have emboldened the relevant phrase. My understanding is that it is not the rate of ageing that is paradoxical but the total amount by which each has aged. there are five references cited, the relevant extracts are:

[1] "When the travelling twin gets home, she has aged only a few years, while her sister is now old and gray."

[2] "To Speedo, the most significant change is that his brother Goslo has aged more than he and is now 60 years of age. Speedo, on the other hand, has only aged by 34.6 years. ... This leads to the paradox: Which twin will have developed the signs of excess ageing."

[3] "An (incorrect) application of relativistic time dilation yields the paradoxical conclusion that each twin would find the other to be older."

[4] "It follows that the twin B must be younger than the twin A when they meet again. ... it should be also possible to argue that A is younger than B."

[5] "Thus Terra has aged more than Stella. If we naively apply the time dilation formula in the reference frame of Stella, we would reach the opposite conclusion."

In each case, the paradox is not that one or the other "ages more slowly", it is that the total ageing is greater for the twin who moves inertially throughout. To correct this, I suggest the end of the sentence is rewritten as "... each should paradoxically find the other to have aged by a lesser amount."

George Dishman (talk) 19:32, 6 March 2017 (UTC)

Hi George, I don't agree. Time dilation means that A measures (with his t-clock) the time (dt) between two events on B's remote t'-clock as longer (dt = gamma dt', since dx'=0, as you can check with the Lorentz transformation). A says something like "what your clock calls a second, I call 4 seconds". So indeed A measures the aging of the remote clock as "more slowly than his own". If A remains inertial between B's departure and return events, then when they meet again and compare their elapsed times, indeed B's clock has aged more slowly (or just less), and B is indeed younger than A. That is the correct, informed application of time dilation. The converse is not true, simply because B bas not been inertial between the two events. The mistake of assuming that B remains inertial is the incorrect, naive application. Indeed A finds B to have aged more slowly, but B does not find A to have aged more slowly. - DVdm (talk) 20:05, 6 March 2017 (UTC)

Ha... I see what you mean: perhaps we should just remove source [3] because they have the typo: that should be: "... paradoxical conclusion that each twin would find the other to be younger." Good find! - DVdm (talk) 20:27, 6 March 2017 (UTC)

Hi DVdm, your disagreement is exactly why I put the view here rather than editing the text so thanks for replying.

You gave for instance "A says something like "what your clock calls a second, I call 4 seconds" but it'll be easier to use v=0.8c and gamma=1/0.6. Suppose each has a clock that ticks once per second. B departs and travels for 100 seconds at a constant 0.8c as measured by A before turning round and returning in another 100 seconds again at constant speed. Time dilation says that B's clock will measure 60s elapsed time during each leg of their trip giving a total of 120s compared to 200s measured by A. So far, so good. However, B can also apply time dilation to each leg. B says "my outward leg took 60s hence in that time A's clock should have ticked 36 times". Similarly for the return leg, B measures 60s and no acceleration so during that time A's clock should have advanced by another 36s. Thus when B returns, by applying time dilation to both legs, he expects A's clock to have advanced by a total of 72s. This application of the time dilation formula is entirely legitimate and correct for each leg which is why "ages more slowly" is not the nature of the paradox. The cause of the paradox is that using only time dilation means that B predicts that A's clock will have advanced less than his own overall when they are compared at the same location on B's return, but even when resolved, it is still true that each ticks "more slowly" than the other on both legs.

What is "naive" about this is to assume that only time dilation is relevant. When B turned round (assumed to be impulsive so take negligible time), he switched from the inertial frame in which he was at rest on the outward leg to that in which he is at rest on the return leg. That switch of inertial frame needs to be included via the "relativity of simultaneity". If we number the ticks for each clock starting at 0 when B departs, tick 60 on B's clock is simultaneous with tick 100 on A's clock as judged by A but with tick 36 on A's clock as judged by B. Similarly, in the return trip inertial frame, tick 60 on clock B is simultaneous with tick 164 on clock A as judged by B and tick 120 on B is simultaneous with tick 200 on A. Again, A has ticked "more slowly". The resolution of the paradox of course is that the "relativity of simultaneity" effect takes the A clock from 36 to 164 on the change of B's frame. This is illustrated in the diagram in the article and accompanying text. Adding that 128s to the 72s computed using time dilation alone resolves the paradox. The bulk of the article is perfectly correct, it is only the single sentence in the summary that, IMHO, is misleading and not representative of the sources.

You said at the end "but B does not find A to have aged more slowly" but that is incorrect, B does find that A aged more slowly both on the outward leg and on the return leg, there is no finite period when A wasn't ageing more slowly according to B, but on completion of the trip, B finds that A has aged by a greater amount overall, that is the paradox, hence "has aged more slowly" is not the same as "has aged less".

If you consider removing references due to a typo ("older" instead of "younger" which I should admit I hadn't spotted, I even had one like that in my initial submission), I would suggest removing reference [3] which IMHO is more seriously wrong, it states that "This asymmetry, which involves accelerated motion, cannot receive a complete analysis within special relativity since special relativity applies only to uniform motion. It requires the tools of general relativity which is introduced below.". Obviously including relativity of simultaneity in SR is sufficient to resolve the paradox without resorting to GR. It is also incorrect to say that SR only applies to uniform motion, see Rindler Coordinates for example. Reference [5] also uses GR by the way.

George Dishman (talk) 13:29, 8 March 2017 (UTC)

Ok, I have removed source [3] and replaced the somewhat awkward phrase "has aged more slowly" with "has aged less": [1]. That's clearly better. Thanks for having spotted this. - DVdm (talk) 16:33, 8 March 2017 (UTC)

Excellent, thank you. George Dishman (talk) 18:23, 8 March 2017 (UTC)

Parodying "Molière" in the "femmes savantes" , your discussion reminds me of two sligthly pedantic poets having an argument over two versions : "how beautiful is that blue bouquet ... " or "how blue is that beautiful bouquet ..." ! Once you have admitted that simultaneity is relative , that fact plays no further role. A and B have the same clocks , playing the same rhythm , unchanged on the inward trip and the outward trip. Indeed we can say that the switch between the simultaneity planes at the turnaround point has mostly a negligible numerical impact , and no theoretical importance for the resolutions of the "paradox". Cordially ; I apologise if necessary ...Chessfan (talk) 19:54, 10 March 2017 (UTC)

Perhaps I should explain the context, I've been trying discuss the thought experiment with someone who is a newcomer to SR, he understands the basics but has been arguing that the paradox can be eliminated by taking length contraction into account. However, his "solution" only resolved the difference in clock rates which he insisted was the paradox, it couldn't consider what would happen if the twins were reunited. The error on this page made it impossible to explain that the "Twins Paradox" requires one to turn round and return to the other. Changes the rate of ageing to the total amount resolves the problem. George Dishman (talk) 16:26, 17 March 2017 (UTC)

REQUEST FOR COMMENT: Should “naive” statement be deleted?

The following discussion is an archived record of a request for comment. Please do not modify it. No further edits should be made to this discussion.

A summary of the debate may be found at the bottom of the discussion.

---

ISSUE

The current version of the article (as of this time) states as follows in the first paragraph (with the citations links inactivated):

This result appears puzzling because each twin sees the other twin as moving, and so, according to an incorrect^[1][2] and naive^[3][4] application of time dilation and the principle of relativity, each should paradoxically find the other to have aged less.

As the host of this RFC, I propose that the “naive” be deleted as insulting and un-encyclopedic.

Let the community discuss this for a while (as long as editors are weighing in) to see if we can arrive at a consensus.

RFC Protocol:

• Wikipedians who weigh in should feel free to update their support/oppose (*vote*) statements as they see fit so long as no one has commented on it in the Rebuttal and discussion; merely update your autosignature with a fresh quadruple-tilde (~~~~) when finished.
• If someone has responded to your *vote* statement or referenced it with language such as “I agree with what so-n-so said,” do not modify your statement. If you have a change of mind, strike your statement (<s>TEXT TO BE STRUCK</s>) and post a new statement if you wish.
• In the Rebuttal and discussion, to begin a thread when responding to another editor, use an “@” reference, e.g. @User:So-n-so: You wrote “blah blah blah” but I have a different take.
• In the Rebuttal and discussion section, please abide by normal customs where you make no substantive edits to your posts after someone has responded; strike statements if you feel it appropriate.
• It is near impossible to discern who is taking what position when debate (protracted, indented replies and replies-to-replies) occurs in the support/oppose tally sections. Discussion belongs only in the Rebuttal and discussion section. The host reserves the right, as moderator, to transplant inter-editor back & forth discussion to the proper section in order to keep this RFC clean and free of visual chaos that makes it difficult for newcomers to understand the developing consensus.
• The duration of this RFC will remain flexible. The general objective will be to achieve a balance amongst three factors. One factor is to run this RFC long enough to give an opportunity to the widest segment of the community—uninvolved wikipedians and I.P. editors included—to weigh in. A second factor is whether—after providing opportunity for ample participation—a consensus one way or another has become apparent. And a third factor is whether there is a protracted period of participation having dropped off due to lack of interest.
• The procedure for calling this RFC will also be kept flexible. If the consensus is clear based upon a grin test, it's a simple call. If a consensus is anything short of crystal clear, we'll call in an uninvolved admin to bear judgement.

In advance, thanks to everyone for participating. Greg L (talk) 22:35, 30 March 2017 (UTC)

Support deleting “naive”

[Just single-paragraph posts here, numbered with "#"; discussion and replies below, not here]

1. Support deletion This isn't a huge deal. The issue is simple: Whether it is any way encyclopedic to state in the lede of an article that people are “naive” if they think the paradox exists. This wording has clearly been a source of controversy and much debate has occurred on this talk page on the subject; it is time to put this one to bed. As RFC proposer, it doesn't matter if certain cited experts believe it is naive to think a paradox exists; they are obviously referring to any other “expert” in the field to make the point that if one has a full understanding of relativity, one would perceive no paradox. Alas, Wikipedia is a general-interest encyclopedia directed to a non-expert readership. The simple fact is this article has 6000 words and lots of explanatory formulas, all of which also showed that the experts—including Einstein—discussed the apparent paradox at length and labored to explain it using various thought experiments and different points of view. For us to say it is “naive” for anyone to think there is paradox and come to Wikipedia to learn about it is insulting to our readership and is non-encyclopedic. Greg L (talk) 15:55, 31 March 2017 (UTC)

Oppose deleting “naive”

[Just single-paragraph posts here, numbered with "#"; discussion and replies to other editors in the below section, not here]

1. Oppose deletion. See all over the place on this talk page—including the previous RFC. There is nothing insulting about this well sourced qualifier. It holds no judgment about people and just accurately describes a mistaken (aka incorrect, as sourced) and uninformed (aka naive, as sourced) application of a theory. The qualifier is encyclopedic per the provided sources. The 6000 words of the article also solidly support both these qualifiers, so they are well-placed in the lead. - DVdm (talk) 20:57, 30 March 2017 (UTC)
2. Oppose deletion. See sources and previous RFC. --D.H (talk) 15:57, 31 March 2017 (UTC)
3. Oppose deletion It is a paradox, as it is easy to believe that they should age the same. Most people don't find special relativity easy to understand, but the twin paradox requires general relativity. Gah4 (talk) 19:06, 31 March 2017 (UTC)
4. Oppose deletion. It does not say that anyone is naive. It does not say that it is naive to think that the paradox exists. It is not insulting. This RFC does not make any sense. Roger (talk) 06:31, 1 April 2017 (UTC)

Section for rebuttal and discussion of other editors’ above position statements

[Unrestricted area for back & forth discussion]

The term non-relativistic is commonly used to describe physics that is correct in Newton's sense, but not in either special or general relativity. I don't know that there is a term for physics that is incorrect with special relativity, but correct with general relativity. The paradox comes when applying special relativity to a case that needs general relativity. Gah4 (talk) 19:08, 31 March 2017 (UTC)

Gah4: Are you clear what the issue is here? You wrote “The paradox comes when applying…” My issue is over how the article currently says you are naive for believing that. This RFC is whether it is appropriate to say that it is “naive” to suspect a paradox exists. That's rather rather insulting language for an encyclopedia directed to a general-interest readership trying to grasp the nuances of the Twin paradox. Greg L (talk) 19:14, 31 March 2017 (UTC)

Note that the article does not say that "you are naive for believing that", or that "it is naive to suspect a paradox exists". It says that an incorrect and naive application of some principles leads to a paradox. You make the same allusion on this other forum. Please don't misrepresent what the article says. And please have a look at WP:FORUMSHOP. - DVdm (talk) 19:27, 31 March 2017 (UTC)

Yes, but I didn't make it so obvious. Since general relativity isn't especially easy, it shouldn't be an insult to not understand it. Or, for the specific case, to not understand when it is needed and how to apply it. Telling someone that they don't completely understand general relativity should not be considered an insult. Gah4 (talk) 20:39, 31 March 2017 (UTC) There is a story, which I don't have a reference for, that the designers of GPS didn't know if the general relativity correction was needed. They included it with a switch to turn it on or off. Soon after it started working, it was found that it was needed. I suspect that they were all pretty smart. Gah4 (talk) 20:39, 31 March 2017 (UTC)

Oh… relax, DVdm. Too much drama—perhaps driven by some WP:OWN. There is a atomic-thin distinction between saying someone is naive and saying the perception that there is any paradox at all is founded upon a naive understanding of science. The text reads This result appears puzzling because each twin sees the other twin as moving, and so, according to an incorrect[1][2] and naive[3][4] application of time dilation and the principle of relativity, each should paradoxically find the other to have aged less. Greg L (talk) 20:19, 31 March 2017 (UTC)

The first time you alluded to intransigence and wp:OWNership was here. I chose to ignore that. Now this is the second time. Please stop resorting to personal comments. Thank you. - DVdm (talk) 21:10, 31 March 2017 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Error in the article?

the History section says, In his famous paper on special relativity in 1905, Albert Einstein deduced that when two clocks were brought together and synchronized, and then one was moved away and brought back, the clock which had undergone the traveling would be found to be lagging behind the clock which had stayed put.[A 4]

I in fact do not see that in Einstein's 1905 paper ?

It should be taken out of Wikipedia. — Preceding unsigned comment added by 47.201.179.7 (talk) 14:02, 17 April 2017 (UTC)

@47.201.179.7:

Can you provide source link to this paper. Then use the find and replace tool to search for the text. Once you have determined whether the information is valid, make an edit to this page, and include your source in your edit summary. If the page is semi-protected create an account. Good Luck, CopernicusAD ^(u) (t) :) 14:08, 17 April 2017 (UTC)

It's near the end of paragraph 4 of https://www.fourmilab.ch/etexts/einstein/specrel/www/

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.

- DVdm (talk) 14:56, 17 April 2017 (UTC)

That is not the twin paradox, that is only one way, with no return trip mentioned. It should be removed from Wikipedia, Einstein did NOT address the twin paradox in 1905.47.201.179.7 (talk) 15:20, 17 April 2017 (UTC)

Indeed, it is not the twin paradox. It is about time dilation, which plays an essential role in the twin paradox, just like the sentence form the article does, that you have quoted in your message above. The referenced journal supports it, so it should not be removed from the article, as it introduces the concept of time dilation in its historical context.

Oh, by the way, I now see that I had copy/pasted the wrong sentence. That should be:

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.

Sorry for the confusion. My bad. - DVdm (talk) 16:06, 17 April 2017 (UTC)

@DVdm: So are you saying the content should be removed from en Wikipedia? If so I will correctly source and remove the content. If it belongs let me know, or either way let me know too. From what I take away, that content should be removed. Good LuckCopernicusAD ^(u) (t) :) And PS, ping or leave me a talkback 17:53, 17 April 2017 (UTC)

Oh! Stupid me! Duh, keep content, case closed. — Preceding unsigned comment added by CopernicusAD (talk • contribs) 17:54, 17 April 2017 (UTC)

This is a false statement and must be removed from Wikipedia: In his famous paper on special relativity in 1905, Albert Einstein deduced that when two clocks were brought together... Einstein never said that. 47.201.179.7 (talk) 02:28, 18 April 2017 (UTC)

As you can see, the content is properly sourced and formally attributed in the article. - DVdm (talk) 07:19, 18 April 2017 (UTC)

The statement is FALSE. Einstein never talked about bringing the clocks together in 1905. It must be removed from Wikipedia. Einstein never said that in 1905. He said it years later, in 1911. So it must be removed from Wikipedia about 1905. He did NOT address the paradox in 1905. So remove it.47.201.179.7 (talk) 14:24, 18 April 2017 (UTC)

Indeed, as I said before, he did not address the paradox. He addressed time dilation, which is what is addressed here in the article too. I have put the relevant sentence in boldface. If that is not clear, perhaps the passage in the original language of the 1905 paper (http://onlinelibrary.wiley.com/doi/10.1002/andp.19053221004/pdf) is helpful:

Man sieht sofort, daB dies Resultat auch dann noch gilt, wenn die Uhr in einer beliebigen polygonalen Linie sich von A nach B bewegt, und zwar auch dann, wenn die Punkte A und B zusammenfallen. Nimmt man an, daB das für eine polygonale Linie bewiesene Resultat auch fur eine stetig gekrümmte Kurve gelte, so erhalt man den Satz: Befinden sich in A zwei synchron gehende Uhren und bewegt man die eine derselben auf einer geschlossenen Kurve mit konstanter Geschwindigkeit, bis sie wieder nach A zurückkommt, was $t$ Sek. dauern möge, so geht die letztere Uhr bei ihrer Ankunft in A gegenüber der unbewegt gebliebenen um ${\frac {1}{2}}tv^{2}/V^{2}$ Sek. nach.

I see no difference between this and the sentence in the article:

In his famous paper on special relativity in 1905, Albert Einstein deduced that when two clocks were brought together and synchronized, and then one was moved away and brought back, the clock which had undergone the traveling would be found to be lagging behind the clock which had stayed put.

Hope this helps. - DVdm (talk) 15:42, 18 April 2017 (UTC)

You are dead wrong. Time dilation is one thing, explaining the twin paradox is quite another. Nowhere in 1905 did he talk about bringing the clocks back together. This Wikipedia article is about the twin paradox, not time dilation. It must be removed about bringing the clocks back together, he NEVER said that in 1905 as Wikipedia incorrectly asserts. Remove it. 47.201.179.7 (talk) 16:44, 18 April 2017 (UTC)

I'm sure that most people will agree that the phrase "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" is precisely about keeping one clock at A, while taking another clock away from A and then bringing it back to A.

And yes, this article is about the twin paradox, which is based on an incorrect application of time dilation, which is introduced in the history section. No problem there. - DVdm (talk) 17:32, 18 April 2017 (UTC)

OK I agree, Einstein did have the twin paradox in his 1905 paper, not much, but it is there. 47.201.179.7 (talk) 19:18, 18 April 2017 (UTC)

All right everybody, were all done here. No more debating, I give to @DVdm:. Are we clear. Bye CopernicusAD ^(u) (t) :) 01:12, 19 April 2017 (UTC)

Incorrect, naive application?

I edited this to indicate that the application of time dilation referred to cannot be unequivocally ruled as 'incorrect naive', purely on the strength of two citations. The edit I made did not, as has been suggested, introduce incorrect information - on the contrary, it eliminated an expression which was not legitimate, for the reason just given. It's not acceptable for the article to be as dogmatic as it was. My edit simply indicated that the assessment of this application as 'incorrect naive' is a matter of the opinion of the authors of the cited references, or of the author of this article, on the strength of the content of those references. Please restore my edit. Ed Addis (talk) 14:33, 22 September 2016 (UTC)

Ha, sorry for having hit the wrong button - I should have hit the wp:NPOV in stead of the error-button. My mistake.

The content is properly sourced with two relevant sources, and I'm sure that we can find more sources. So there is nothing dogmatic about it. Your change ("... held by some to be incorrect and naive ...") used wp:weasel words, and more or less induced your personal opinion —doubts perhaps— about this. If sufficient relevant authors say that something is naive, then Wikipedia can (and should) say that it is naive. - DVdm (talk) 14:58, 22 September 2016 (UTC)

Nevertheless, the reality is that there are many, myself included, who do not agree with this interpretation. It's therefore not acceptable to just label it as 'incorrect'. It would be OK to say that there is a strong (or even a majority) body of opinion that favours a different interpretation, but you can't escape the accusation of dogmatic I'm afraid. I'd be happy to accept a different wording from the one I used, but it must allow for the existence of differing opinions on the key point of whether the paradox as simply stated here is valid. Or is this going to be another case of the WP police overriding valid objections?Ed Addis (talk) 17:06, 22 September 2016 (UTC)

Wikipedia reports on and reflects the scientific consensus. The overwhelming academic scientific consensus is that incorrect, naive application of the principles is right at the heart of the paradox. Pretending (by using wp:WEASEL words) that this is just a matter of different interpretations would be de-facto wrong, and, creating a false balance, would give undue weight to fringe theories. Please have a careful look at wp:UNDUE, wp:FALSEBALANCE and wp:PROFRINGE. - DVdm (talk) 17:32, 22 September 2016 (UTC)

It is always wrong to describe variant viewpoints as incorrect, when any differences of opinion exist. It is just academic imperialism, and sadly there's far too much of it on this platform. Unfortunately, it undermines the credibility of Wikipedia, and is contrary to its spirit. If you refuse to soften this dogmatic statement, I will take the matter to a higher authority.Ed Addis (talk) 19:20, 22 September 2016 (UTC)

Please indent talk page message as outlined in wp:THREAD and wp:INDENT. Thanks.

You will find that indeed Wikipedia is designed this way. Have you taken a look at the spirit of Wikipedia as it is described in wp:UNDUE, wp:FALSEBALANCE and wp:PROFRINGE? - DVdm (talk) 20:13, 22 September 2016 (UTC)

FWIW, I have added 3 more sources. There's a lot more, but that should be sufficient.- DVdm (talk) 22:11, 22 September 2016 (UTC)

@ DVdm. It doesn't matter if there are sources from experts who believe it is naive to believe there is a paradox and whether you've cited it to those experts. The simple fact is this article is full of 6000 words and lots of explanatory formulas, all of which also showed that the experts, including Einstein, wrestled with the paradox and labored to explain it in different ways. For an an encyclopedia directed to a general interest readership to say it is “naive” to believe there is any paradox is insulting, non-encyclopedic, and incorrect given the intended readership.

I deleted it yesterday and you put it back (∆ edit here) with the edit summary of This properly sourced qualifier was extensively discussed on talk page and there was no consensus to remove it.

Well, “well debated” it wasn't; it was just you and one other editor. Now I'm wading in and I feel precisely as user:Ed Addis does: your insulting declaration has no place. There is now a consensus to remove it so…

I’ve deleted it again. Please stop cybersquatting and reverting editors on this issue. It looks like you're more anxious to show off how insightful you are on the subject. We're not here to demonstrate that as wikipedians, we are terribly smart-smart by being insulting to our readership. Greg L (talk) 17:05, 30 March 2017 (UTC)

There was no consensus to make the change to delete it, so per our policy wp:NOCONSENSUS, we can keep it:

• In discussions of proposals to add, modify or remove material in articles, a lack of consensus commonly results in retaining the version of the article as it was prior to the proposal or bold edit. However, for contentious matters related to living people, a lack of consensus often results in the removal of the contentious matter, regardless of whether the proposal was to add, modify or remove it.

Read the remainder of this talk page. I have restored the properly sourced qualifier, as the text was there since a long time, and there was no consensus to remove it, so please don't try to turn this around as you did in this edit summary. You can always apply for other types of dispute resolution. DVdm (talk) 17:32, 30 March 2017 (UTC)

Well, I see that of the last 500 edits, 42 of them are nothing more than you pressing the “undo” button on other editors. I have better things to do in my life than mess around on Wikipedia with intransigent editors who think they own an article and quickly invite others to delve into “other types of dispute resolution.” Perhaps you thrive on all that drama. I don't. Greg L (talk) 19:48, 30 March 2017 (UTC)

I think there is a clear and (to me) obvious compromise here, and that is to just remove the naive. Being incorrect is a factual statement, and while it might not be appropriate to say that somebody is incorrect if they phrase the statement as a question i.e. "Wouldn't the time dilation from traveling at the speed of light violate the principals of relativity?", ultimately what the article is saying is that "Traveling at the speed of light violates the principals of relativity." is incorrect. However, adding that it was "naive" was redundant, opinionated and insulting. When I came to this article earlier today trying to understand what I saw as a clear violation, I honestly was a little insulted by how I was labeled naive just because I did not fully understand special relativity. Granted the article does not say "Incorrect, naive, stupid, foolish application that only a complete idiot could ever start to reason." However, the presence of "naive" slightly undermines the objectivity of the whole statement, and Wikipedia's standards are pretty close to perfection. As such I am going to just remove the one redundant word.50.53.86.168 (talk) 23:32, 29 October 2017 (UTC)

This is not the compromise here. There are two RFC's about this elsewhere on this talk page. There is no wp:consensus to remove the well-sourced term. On the contrary, there is a clear consensus to keep it. - DVdm (talk) 07:39, 30 October 2017 (UTC)

#oly crap you're right there are TWO other RFCs about this! And they are enormous. Forget it I am getting out of this before it's too late. I don't know why I ever decided to go to this talk page and then erase just two words. But now I see that they are cursed. The entire page should be erased before it consumes all of our lives. Just rewrite it from the ground up, then perhaps we can all move on. — Preceding unsigned comment added by 50.53.86.168 (talk • contribs) 09:00, 30 October 2017 (UTC)

Relax, there's no reason to take the word personally. I do at least two naive things each day—and that's even before breakfast . - DVdm (talk) 09:02, 30 October 2017 (UTC)

Take a stand on acceleration, and talk about length contraction more

The following discussion is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

---

This article seems to want to take an even hand in evaluating the role of acceleration and gravitational time dilation. This includes all four sources that note how the paradox arises from "naive" and "incorrect" applications of SR. They all invoke GR, and it's pretty well established that you don't need GR to explain it (e.g. Wheeler and Taylor, Introduction to Special Relativity Second Edition), so bringing it up again and again just clouds understanding. Anything that mentions the acceleration should be moved to a section called "Why Acceleration Doesn't Matter" with explanations why.

Also, as the section "Specific Example" shows, the concept that is necessary to clearly unravel the "paradox" is length contraction. Conceptually, the Earth observer is standing on the end of a 4 light year long ruler. The observer watching a ship whizzing over the ruler at 0.8c to the end and back observes the journey to take 5+5 = 10 years (even though the ship is length contracted). The traveling observer also sees the ruler whizzing by underneath at .8c, but it is length contracted to a mere 2.4 light years. The moving traveler therefore observes his journey to take 3+3 = 6 years. The key is simply that both observers are watching the same journey. That's it! There is no need to invoke different inbound/outbound frames, or talk about slow moving clocks, or draw planes of simultaneity to explain "missing time".

Bkennedy99 (talk) 02:27, 7 May 2018 (UTC)

There are many explanations possible, and the one you show here is one of them. Noting that the travelling twin needs to accelerate, whereas the home stayer needs not, is another way. Referring to the equivalence principle (and thus to GR) is yet another, which is mentioned at the end of the lead. Note that it's pretty well established (—see Taylor and Wheeler—) that invoking acceleration is not equivalent with invoking GR: it is well known that SR can perfectly handle acceleration. See also the third last paraphraph of the lead of article Special relativity. So the four sources don't all invoke GR. Note that the total elapsed time on Earth can be calculated as a function of (1) the elapsed time on the ship, combined with (2) the history of the proper accelaration only—see section Twin paradox#Difference in elapsed times: how to calculate it from the ship. - DVdm (talk) 09:02, 7 May 2018 (UTC)

The very first sentence is "the twin paradox is a thought experiment in special relativity". There are enough easy to understand, acceleration-free formulations of the "paradox" such that talking about acceleration is not necessary to understanding it in SR. The article should be about this simpler formulation. Otherwise, the risk is that the reader goes "oh, acceleration" and continues to not have even a basic grasp of length contraction, which is a core SR concept. Now if you could solve the Twin Paradox using only acceleration (SR or GR) and *not* invoke length contraction that would be interesting, but I have never seen that claim. It sounds dubious anyway as identical accelerations cab be invoked at ends of different differences, so what would the different observed time be proportional to? Bkennedy99 (talk) 14:08, 7 May 2018 (UTC)

Indeed, talking about acceleration is not necessary to understanding it in SR, which is what the subsection Twin paradox#Role of acceleration carefully explains to the reader (with 3 sources), even if 4 sources use acceleration. In the subsection Twin paradox#Relativity of simultaneity we have an explanation without acceleration and without length contraction, with 5 sources. The subsection Twin paradox#Travellers' perspective uses the length contraction explanation. So I think the article is fairly complete and balanced, and that balance is well reflected in the lead, and i.m.o. we don't need to worry about readers going "oh, acceleration". - DVdm (talk) 16:43, 7 May 2018 (UTC)

Length contraction is casually mentioned in the "Specific Example" section that describes the math of the distance, but its explicit role in gaining an intuitive understanding is not mentioned - namely, that the spaceship is simply traveling through less space. A reader should be able understand that a round trip from New York to Pittsburgh takes less time than a round trip from New York to Los Angeles. The only remaining conceptual component is that the traveler and the home observer are viewing the same ruler and same spaceship, but the apparent lengths are different - that is, plain-old length contraction. This completely accounts for the difference in observed times, and should be spelled out under "Resolution of the paradox in special relativity", with an example of a giant 4 light year long ruler. As it stands, the first paragraph under the section says things like "This acceleration, measurable with an accelerometer, makes his rest frame temporarily non-inertial. This reveals a crucial asymmetry between the twins's perspectives: although we can predict the aging difference from both perspectives, we need to use different methods to obtain correct results.". This, by the way, is before the section called "Role of acceleration". It's also just irrelevant (explained below) because even without this "crucial asymmetry", you still get the "paradox". It's not crucial at all, as it can be completely removed.

The next section, if there must be one, could be on the "Relativity of simultaneity". And while this doesn't use acceleration, it assumes length contraction, which *must be understood to make sense of this section*. This is while the red and blue lines are sloping. It's nice to be able to account for the "missing seconds" in some orderly way, but this doesn't resolve the paradox. You still need to explain why the spaceship got to the turnaround point in less proper time in their own frame.

Finally, I would advocate that anything mentioning acceleration just be thrown out - it's muddled thinking, and the sources that the articles link to are perpetuating it. In science, if we want to know whether experimental effect A (constant relative motion) or effect B (initial acceleration + turnaround acceleration) cause something (the apparent paradox), we simply run the experiment without one of the variables. And, lo and behold, we discover that removing acceleration through a slightly modified experiment gives the same result (the apparent paradox). If there is story that uses *only* acceleration and not length contraction, that would be very interesting - but it probably wouldn't be SR. Once the story includes length contraction, the role of acceleration should be dropped as unnecessary clutter as length contraction is sufficient.

I know you disagree, which is fine. But there is a ton of bad information out there on the Twin Paradox, and Wikipedia was not helpful in sorting through it. Maybe my thoughts on this talk page will be helpful to the next person who really wants to get their head around the issue. Honestly the whole article is all over the map, it should probably be rewritten by a single subject matter expert with a cohesive take on the issue. Bkennedy99 (talk) 18:56, 7 May 2018 (UTC)

Regarding "... anything mentioning acceleration just be thrown out ...": we tend not to throw out properly sourced stuff ^{(Ohanian, Harris, Rindler, Weidner)} in Wikipedia. DVdm (talk) 19:44, 7 May 2018 (UTC)

You can obtain correct results by fully exploring the Earth twin's perspective. This does not fully solve the paradox. As explained very effectively in this article, the apparent paradox arises from this doubt: "what about the traveling twin's perspective? Doesn't he observe space contraction and time dilation as well?"... In the Specific example section, for instance, this question is not answered, as only the Earth twin's perspective is fully explored. In that poerspective, the acceleration phase between the two legs of the trip plays no role. However, in the space twin's perspective, the acceleration phase explains everything (see Relativity of simultaneity), even if you imagine infinite acceleration. And by the principle of relativity, this perspective is as legitimate as the Earth twin's perspective.

Also, if you use the modified thought experiment in which the space twin is represented by two different inertial observers B and C (outgoing and ingoing), you need to explain why you cannot use the "opposite" or "reciprocal" experiment, in which the Earth twin is represented by B and C and the space twin is represented by only one, motionless, inertial observer. And you need "experienced acceleration" to explain that this is impossible ("experienced acceleration" makes the space twin non-inertial during the acceleration phase).

So, computing the correct results without considering acceleration is possible, but only if you start from the Earth twin's perspective (as in the Specific example section). However, in order to solve the above mentioned doubt, you need to fully explore both perspectives. Solving the paradox is not the same as computing correct results...

Paolo.dL (talk) 18:02, 8 May 2018 (UTC)

This is why acceleration needs to be removed from the "solutions". Imagine that at some point in the journey, the earthbound twin gets lonely decides he wants to catch up. So he gets in the same spaceship, accelerates to the same velocity, but changes his mind. He stops at the moon, and pulls the exact same U-Turn as his brother, experiences the exact same acceleration forces, returns to earth, and gets back to waiting. If acceleration alone explained age differences, both twins would have the same age because they had the same acceleration profile. But clearly they won't, due to... length contraction! *that explains the paradox*. If the star is 4 light years away in an inertial frame with earth, then the travelling twin is literally moving through less space than 8 light years of space to make the round trip. Thus the elapsed time is shorter. Full stop. The symmetry is that earth sees a squished ship, and the ship sees a squished distance between the stars. The "squishiness" is plain old constant velocity SR length contraction. Bkennedy99 (talk) 21:30, 8 May 2018 (UTC)

@Bkennedy99: they would definitely not have the same acceleration profile. For obvious reasons their maximum "halfway velocity" would be wildly different from the other traveller's. But this has become off-topic here. The article talk page is not the place where we discuss or explain the subject or aspects of it, in order to educate contributors. It is where we discuss the article — see wp:Talk page guidelines. Due to some serious misconceptions that you have about this, you proposed a change to the article, but that is not going to happen due to the proper sourcing of the content that you like to see removed. This is not the place where we can help you. If you like, you can go to our wp:reference desk/science, or read a good book, or find some online discussion board such as Usenet, but we have to cut this short here. I have put a little formal warning about this on your user talk page. Good luck. - DVdm (talk) 06:41, 9 May 2018 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Dennis Sciama

I had undone this edit by user Las1817 (talk · contribs) because it puts wp:UNDUE weight on an old Machian wp:FRINGE view from Dennis Sciama. It's also just a wp:primary source, and a rather old one: the book is from 1959. User Las1817 put it back. I think that this fringe view does not belong here. Comments welcome. - DVdm (talk) 09:01, 20 October 2018 (UTC)

Per wp:NOCONSENSUS and lack of comments, I have undone the edit again: "In discussions of proposals to add, modify or remove material in articles, a lack of consensus commonly results in retaining the version of the article as it was prior to the proposal or bold edit." Also note that the first ("Although...") part of the edit is even wp:unsourced and amounts to wp:original research. Without proper wp:secondary sources, this is just another wp:UNDUE opinion. Comments still welcome. - DVdm (talk) 09:58, 21 October 2018 (UTC)

Special Relativity alone can explain the paradox

General relativity is not necessary to explain the twin paradox; special relativity alone can explain the phenomenon. — Preceding unsigned comment added by 47.201.179.7 (talk) 14:10, 30 May 2017 (UTC)

References

Please sign all your talk page messages with four tildes (~~~~) — See Help:Using talk pages. Thanks.

Yes, but I made a little tweak, replacing that pop-sci blog with a proper textbook source: [2]. - DVdm (talk) 16:02, 30 May 2017 (UTC)

I'm not really getting which side everyone is on here, but I diagree on getting general relativity into this. The problem still works be it that the "stationary" twin stays on a planet or in a theoretical point in empty space free from the influence of any gravitational force. --uKER (talk) 15:35, 21 October 2018 (UTC)

Clarification/confirmation about jump discontinuity please

In the section "Relativity of simultaneity" I think it would be worth confirming the nature and direction of the discontinuity. Am I correct in thinking it means that the stationary twin "suddenly" seems older (rather than younger) from the point of view of the traveling twin? Am I also correct in thinking that the the stationary twin does not have any discontinuity in their view of the other (the traveling twin is viewed as smoothly continuing to age more slowly - apart from during any acceleration phase - and does not have any "sudden" age change)? FrankSier (talk) 09:50, 2 March 2019 (UTC)

The stationary twin does not suddenly seem older or younger form the point of view of the traveling twin. When the travelling twin calculates the "current age" of the remote stationary twin immediately before and immediately after the turnaround, they will get a different result. The stationary twin's calculated "current age" will suddenly increase. I think that is clearly explained in the section. The only thing the traveller would see, is a sudden blue shift of the image (of the stationary twin's clock) that was sent out a long time ago, and from then on, the remote clock at the stationary twin would seem to run faster than it seemed to be running during the first phase of the trip.

And indeed the stationary twin does not find any discontinuity in their calculation (or even view) of the travelling twin. - DVdm (talk) 11:39, 2 March 2019 (UTC)

Clarification of "the noted differences are not symmetrical, and the asymmetry grows incrementally" please

This is in the section "A non space-time approach". I think it is clear what is meant by "...the noted difference is symmetrical between the two parties" earlier on in the explanation (both see the other's clock as running the slow by the same proportion). But I do not think it is clear what is meant by the asymmetry at this second stage. Does it mean that both no longer see the other's as running slow (which does not seem right to me)? Or is it to do with the clocks not having been set to the same zero at any time? Or something else? FrankSier (talk) 09:36, 2 March 2019 (UTC)

See next section. - DVdm (talk) 11:41, 2 March 2019 (UTC)

Clarify Assumptions

It should be noted that this solution is only valid if one of the twins remains stationary in the original reference frame. It is presumably the Earth-bound twin who does so, but the more general case is that both twins go walkabout in their own spaceships. In that case, the elapsed time in the initial reference frame is t0 = gamma1 * t1 = gamma2 * t2 and their relative velocity is given by the velocity addition formula as +/-( v2 - v1 ) / ( 1 - v1 * v2 / c^2 ). The time transform is then t2 = gamma1 * t1 / gamma2. --Relativity Guy (talk) 22:09, 26 May 2019 (UTC)

Is there a WP:reliable source to back this up? Otherwise it would be wp:original research. - DVdm (talk) 08:21, 27 May 2019 (UTC)

I cannot find any documentation on this, but according to our local relativity expert (http://physics.usask.ca/~dick/251.htm), it is a well known fact in GR theory, which is consistently ignored in SR textbooks. It follows from conservation of momentum, since the reference frame in which the net 3-momentum is zero (throughout the experiment) has the maximum possible lapse of proper time. --Relativity Guy (talk) 01:51, 28 May 2019 (UTC)

Some professor's personal webpage is not sufficient. Wikipedia needs reliable wp:secondary sources for all challenged new content. See wp:RS and wp:BURDEN. - DVdm (talk) 08:06, 28 May 2019 (UTC)

Contradiction

The article says:

However, this scenario can be resolved within the standard framework of special relativity: the travelling twin's trajectory involves two different inertial frames, one for the outbound journey and one for the inbound journey, and so there is no symmetry between the spacetime paths of the twins. [...] General relativity is not necessary to explain the twin paradox; special relativity alone can explain the phenomenon.

Britannica, however, seems to disagree:[3]

The answer is that the paradox is only apparent, for the situation is not appropriately treated by special relativity. [...] A full treatment requires general relativity, which shows that there would be an asymmetrical change in time between the two sisters.

The role of acceleration is also unclear: SR-only explanations tend to say acceleration has nothing to do with the resolution of the paradox, but if this is true, what makes GR relevant? GregorB (talk) 20:28, 16 July 2019 (UTC)

Britannica, which would merely be a wp:tertiary source is wrong. - DVdm (talk) 20:53, 16 July 2019 (UTC)

It appears so, and that's remarkable. To me, the article itself, as well as all the explanations I've seen so far, are really frustrating because they appear to be circular in nature. Again, the article says:

However, this scenario can be resolved within the standard framework of special relativity: the travelling twin's trajectory involves two different inertial frames, one for the outbound journey and one for the inbound journey, and so there is no symmetry between the spacetime paths of the twins.

But, as stated in the formulation of the paradox, both twins may see themselves as stationary, and the other twin as moving, which is why the above doesn't really explain anything. Another example is Carlo Rovelli's explanation:[4]

‘In motion’ in relation to what? How can we determine which of the two objects moves, if the motion is only relative? This is an issue that has confused many. The correct answer (rarely given) is this: in motion relative to the only reference in which the point in space where the two clocks separate is the same point in space where they get back together.

So "the only reference in which the point in space where the two clocks separate is the same point in space where they get back together" is the rocket then?

IMO, a natural resolution of the paradox would be the following sentence: "The rocket-twin is wrong when he considers himself stationary and the Earth-twin moving because {insert reasons}." It is quite incredible that, to my knowledge, nobody has been able to come up with such a sentence. GregorB (talk) 22:26, 16 July 2019 (UTC)

That Rovelli sentence is of course wrong because there's an absolutely essential qualifier missing. And you just debunked it, because both the Earth (which is what he had in mind) and indeed the rocket (as you have in mind) are such "references". He should at least have said "the only inertial reference frame in which the point in ("its") space..." And even then, the phrase "a reference in which a point in space" is pretty poor.

Your proposal "...when he considers himself stationary..." is indeed correct, and it is of course fully compatible with the sentence in the lead, which is more general, and at that point does not reveal which twin "is wrong" yet. Note that that sentence is explitised further down in the article body, with proper references.

Note that there are tons of incomplete, poor, bad and wrong explations out there, but this is not the place to discuss them, unless they are mentioned in the article itself, and based on reliable sources treating them. Here we can only discuss the content and format of the current article, not the subject itself or how the subject is (mis)treated on the internet and in some sources. See wp:talk page guidelines. - DVdm (talk) 09:22, 17 July 2019 (UTC)

Thanks for the comment, this is finally illuminating.

My point here was not merely to comment on the poor quality of various explanations, but primarily on the difficulty of understanding the resolution of the paradox as currently presented by the article. I can't help thinking the article could do better in this respect, although frankly I can't suggest anything concrete right now. GregorB (talk) 10:13, 17 July 2019 (UTC)

The equivalence of biological aging and clock time -keeping.

I suggest to modify the last sentence as follows : "This means that biological aging would be slowed in the same manner as a clock, seen from some other inertial frame". --Chessfan (talk) 14:10, 8 September 2019 (UTC)

Indeed the source does not speak of "slowing biological aging".It says : "Total proper time, the aging along any given worldline, straight or curved, is an invariant: it has the same value as reckoned by observers in all overlapping free-float frames. This value correctly predicts elapsed time recorded directly on the wristwatch of the particle that travels this worldline. It correctly predicts the aging of a person or a mouse that travels this worldline."

That is totally coherent with the fact that wristwatch's are not sensible to displacement. --Chessfan (talk) 09:39, 9 September 2019 (UTC)

Acceleration

The following discussion is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

---

Since a constant speed (scalar) does not imply a constant velocity (vector) -think about circular motion-, the first should not be accounted as a proof of an acceleration-free description. Tommaso Righi (talk) 13:57, 26 May 2020 (UTC)

The "first", which is constant speed as you say, sounds like acceleration-free. But more to the point, as we can only discuss the article here —not the subject, see wp:Talk page guidelines—- where in the article does it say that constant speed is "accounted as a proof of an acceleration-free description"? DVdm (talk) 15:41, 26 May 2020 (UTC)

Just before the "History" paragraph: "However, it has been proven that general relativity,[9][10][11][12][13] or even acceleration, are not necessary to explain the effect, as it has been shown to apply even for a theoretical observer that can invert its direction of motion instantly, thus maintaining constant speed all through its trip.[14]"

Note that the uniform circolar motion has constant speed and it is not an acceleration-free motion. Tommaso Righi (talk) 16:46, 26 May 2020 (UTC)

Moreover, an "invertion of motion" is by definition a change in velocity, which is by definition provoked by acceleration. Tommaso Righi (talk) 16:50, 26 May 2020 (UTC)

Please indent talk pages messages as outlined in wp:THREAD and wp:INDENT — See Help:Using talk pages. Thanks.

Yes, we know that invertion of motion requires an infinite acceleration spike during an infinitesimal time (as is explained elsewhere in the article), but here in the lead, the cited source (https://www1.phys.vt.edu/~jhs/faq/twins.html) uses different physical observers on both phases of the trip. When the receeding observer (Prime) passes the "turnaround" point, their clock reading is taken over onto the approaching observer's clock (Doubleprime). This way the travelling observers don't have to accelerate. I clarified this in the article ([5] and [6]). This way we stay closer to the source as cited by user Uker. I think we have everything nicely covered now. - DVdm (talk) 17:32, 26 May 2020 (UTC)

In what i quoted (""However, it has been proven that general relativity,[9][10][11][12][13] or even acceleration, are not necessary to explain the effect, as it has been shown to apply even for a theoretical observer that can invert its direction of motion instantly, thus maintaining constant speed all through its trip.[14]") nor in the paradox statement there is no mention of a third observer substituting the travelling twin at some point (turnaround) of the journey. Tommaso Righi (talk) 17:55, 26 May 2020 (UTC)

Sorry, I did a wrong indentation again. Tommaso Righi (talk) 17:59, 26 May 2020 (UTC)

No problem, I fixed the indentation.

I already had made a change in the article to that part which you quoted here above. It's closer to the source now. - DVdm (talk) 18:54, 26 May 2020 (UTC)

Thank you. I would also specify that in the described scenario, while the two observers are inertial, so is not the transferred clock, which is instead accelerated during the transfer. Tommaso Righi (talk) 20:32, 26 May 2020 (UTC)

Ah, but it is not the clock that is transferred. Only the clock readings are compared, so only information is (locallly) transferred and nothing physical ever gets accelerated . - DVdm (talk) 20:48, 26 May 2020 (UTC)

My bad! Tommaso Righi (talk) 20:59, 26 May 2020 (UTC)

I think the change in the article does not solve the fact that calling an observer (one unique observer) "theoretical" (in the part I quoted) is not sufficient to perform a change in velocity ("can invert its direction of motion") with no acceleration. A change in velocity is straightforward the definition itself of acceleration. Tommaso Righi (talk) 10:00, 27 May 2020 (UTC)

Well, let's not forget that these are all thought experiments to explain the twin paradox puzzle, which itself is a thought experiment. The essental point is that with the sudden "theoretical turnaround" we have constant speeds during the coasting phases of the trip, and no accelerations are used or needed in the calculation and explanation. I have added this clarification as well: "... not necessary to explain the effect, as a thought experiment shows with a theoretical observer ...". The original formulation ("... it has been shown to apply for a theoretical observer ...") might indeed have given the impression that this was experimentally shown, which of course it wasn't. Waterproof now, I'd say. - DVdm (talk) 11:34, 27 May 2020 (UTC)

I think neither the fact that the experiment could be (not necessarily tho) "thought", nor the observer (or the motion) being called "theoretical" justify any change in velocity along a trajectory (or, in this case, a worldline) to happen without acceleration. Tommaso Righi (talk) 12:58, 27 May 2020 (UTC)

Moreover, it is said that this theoretical observer mantain constant speed: this is not true in general. In fact, if you stick to what you'd call "outbound frame", the speed before the turnaround is zero, while the speed after the turnaround is not. What is actually true is that the speed (but not the velocity!) measured in the earthbounded frame stay constant. Tommaso Righi (talk) 13:15, 27 May 2020 (UTC)

Besides, the purpose of a thought experiment is to reproduce reality, so it should not be different from a performed one, unless the theory used for the thought one is incorrect, which is not the case. Tommaso Righi (talk) 15:15, 27 May 2020 (UTC)

You're totally missing the point here. You're making it as if the whole reasoning is not valid if it's not reproducible physically, in real life. As it has been clarified to you, the whole twins paradox is just a thought experiment, a theoretical poser with no immediate viability for having any real physical macrosocopical entities participate in them, as are many other relativistic scenarios (Bell's spaceship paradox, Terrell rotation, ladder paradox, etc.). What you're doing is like telling a primary school teacher asking you how many watermelons you'd have if you bought 5 and then 5 more, that her postulate is not valid because no human being can possibly carry 10 watermelons. It is perfectly valid to pose a theoretical time-keeping entity that could switch speed directions with no deceleration, because in fact all the problem needs to consider is the existence of a frame of reference in which that stuff happens. Furthermore, in the cited example, and just for the sake of getting it even closer to something that has a semblance of physical viability, said theoretical entity capable of instant changes in motion is thought of as composed of two time-keeping devices, capable of transferring the time-keeping information on contact when passing by each other at what would otherwise be the turnaround point. --uKER (talk) 15:21, 27 May 2020 (UTC)

Hello. I don't think the things you're saying are true. In particular, you can't call "theoretical" or "thought" a thing because it does not follow physics' laws (like invert speed direction with no acceleration). I mean, you can, but this is not what we actually do in physics (take as source every physics text ever written, or thought, if you feel more comfortable with the framework). Tommaso Righi (talk) 15:41, 27 May 2020 (UTC)

As for the two-device scenario, this prove that the motion can be decomposed in two inertial regions, but it remains globally non-inertial: local properties of curves does not imply the same properties globally. Tommaso Righi (talk) 16:04, 27 May 2020 (UTC)

It's just a matter of disregarding irrelevant stuff. Will you want to consider the decreased acceleration due to the weight of the fuel necessary to make the trip too? Again, the reasoning theorizes about a frame of reference, not a spaceship, not even a person. --uKER (talk) 16:12, 27 May 2020 (UTC)

Note: let's make sure we do not start discussing the subject here, nor each other's unserstanding of it — see wp:Talk page guidelines. This article talk page is for discussion related to improving the article in specific ways, based on reliable sources — see, for instance, the standard user warning {{uw-chat2}}. As it is now, I think everything in the article is rather properly sourced. Discussions on some aspects of the subject will never stop, which is of course why the talk page guidelines are what they are. I propose we close the subject. - DVdm (talk) 16:23, 27 May 2020 (UTC)

Good call. --uKER (talk) 16:41, 27 May 2020 (UTC)

I disagree. There are some things against the scientific consensus. To begin with, edit the sentence where it is said that there exist a kind of observer capable of a change in velocity which is acceleration free, because it is not true. (In Latin, Isaac Newton, Philosophiae naturalis principia mathematica, volume 1 of a facsimile of a reprint (1833) of the 3rd (1726) edition, as annotated in 1740–42 by Thomas LeSeur & François Jacquier, with the assistance of J-L Calandrini]). Tommaso Righi (talk) 16:49, 27 May 2020 (UTC)

You're just refusing to acknowledge the validity of theoretical thinking. That's not a problem with the article, sorry. --uKER (talk) 17:37, 27 May 2020 (UTC)

I'm not: I'm just assuming that one of the hypothesis of the thought experiment here presented is that we assume physics' laws hold. If I'm wrong about this hypotesis, than I think that the article should specify that the hypothesis of the experiments are beyon the known physics. Still, I'm pretty sure I'm right, and I can search for references. Tommaso Righi (talk) 17:57, 27 May 2020 (UTC)

I notice that the "theoretical" bit was already circumvened in another way: [7]. To accomodate for the fact that the hypotheses of the experiments are beyond the physics known, we could just replace "theoretical" with "hypothetical". I.m.o. that should do the job too, and afaiac we could go back there and do it that way. - DVdm (talk) 18:24, 27 May 2020 (UTC)

You're also not acknowledging that the proposed thought experiment doesn't actually pose a body changing speeds instantly, but TWO bodies passing by each other and having an instant exchange of a small bit of information when they do so. You're also not acknowledging that in any case the experiment is sourced to a quite reputable source. Here's another source for the same explanation if you want, this time by a particle physicist from Fermilab, and here is a simplified one with no math involved. --uKER (talk) 18:54, 27 May 2020 (UTC)

@DVdm This is unacceptable: if you really think the scenario decribed in the paradox deliberately assume a framework violating both special relativity and classical physics, then you should declare it from the beginning of the article, since the contrary it's commonly believed. In this case, I would like to see a source or a reference which states the paradox explicitly declaring a framework violating both special relativity and classical physics. Otherwise, in special relativity (actually this is also true in classical physics) an observer can divert from a geodesic if and only if it undergoes an acceleration. A few references supporting my last statement:

• Einstein, Albert (1920). Relativity: The Special and General Theory.
• Einstein, Albert (1996). The Meaning of Relativity. Fine Communications. ISBN 1-56731-136-9
• Logunov, Anatoly A. (2005) Henri Poincaré and the Relativity Theory (transl. from Russian by G. Pontocorvo and V. O. Soloviev, edited by V. A. Petrov) Nauka, Moscow.
• Charles Misner, Kip Thorne, and John Archibald Wheeler (1971) Gravitation. W. H. Freeman & Co. ISBN 0-7167-0334-3
• Post, E.J., 1997 (1962) Formal Structure of Electromagnetics: General Covariance and Electromagnetics. Dover Publications.
• Wolfgang Rindler (1991). Introduction to Special Relativity (2nd ed.), Oxford University Press. ISBN 978-0-19-853952-0; ISBN 0-19-853952-5
• Harvey R. Brown (2005). Physical relativity: space–time structure from a dynamical perspective, Oxford University Press, ISBN 0-19-927583-1; ISBN 978-0-19-927583-0
• Qadir, Asghar (1989). Relativity: An Introduction to the Special Theory. Singapore: World Scientific Publications. p. 128. Bibcode:1989rist.book.....Q. ISBN 978-9971-5-0612-4.
• French, A. P. (1968). Special Relativity (M.I.T. Introductory Physics) (1st ed.). W. W. Norton & Company. ISBN 978-0393097931.
• Silberstein, Ludwik (1914) The Theory of Relativity.
• Lawrence Sklar (1977). Space, Time and Spacetime. University of California Press. ISBN 978-0-520-03174-6.
• Lawrence Sklar (1992). Philosophy of Physics. Westview Press. ISBN 978-0-8133-0625-4.
• Sergey Stepanov (2018). Relativistic World. De Gruyter. ISBN 9783110515879.
• Taylor, Edwin, and John Archibald Wheeler (1992) Spacetime Physics (2nd ed.). W.H. Freeman & Co. ISBN 0-7167-2327-1
• Tipler, Paul, and Llewellyn, Ralph (2002). Modern Physics (4th ed.). W. H. Freeman & Co. ISBN 0-7167-4345-0

Tommaso Righi (talk) 22:40, 27 May 2020 (UTC)

@Uker I think you don't understand my words. I said that in a physical framework (meaning assuming at least classical physics) one observer cannot be defined to be two or more observers, otherwise acceleration itself wouldn't exist in physics. Tommaso Righi (talk) 22:53, 27 May 2020 (UTC)

Divert from a geodesic? This whole experiment all takes place in a single line. What are you even talking about? --uKER (talk) 23:07, 27 May 2020 (UTC)

Wuold you add some sources for "This whole experiment all takes place in a single line", please? I can't see it. Tommaso Righi (talk) 00:07, 28 May 2020 (UTC)

P.S. By "line" I assume you mean a timelike trajectory (or curve) in Minkowsky spacetime, right? Tommaso Righi (talk) 08:49, 28 May 2020 (UTC)

Note: I think that everything (in the article) is properly sourced by reliable sources, and whatever we think about the subject is irrelevant here on this talk page—again, see wp:Talk page guidelines. Here we can only discuss things (with their sources) to add to or modify in the article. On top of this page it says, in bold: This is the talk page for discussing improvements to the Twin paradox article. This is not a forum for general discussion of the article's subject. Not adhering to that is talk page disruption, so this discussion should be closed now. If there is a particlar, specific thing to add or modify and you have a source to directly support it (without wp:SYNTHESIS), then we can open a new section for it here below. - DVdm (talk) 09:09, 28 May 2020 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Acceleration being posed as a possible explanation

There's numerous explanations like this one that show that the effect doesn't require the involvement of any acceleration. --uKER (talk) 16:14, 25 May 2020 (UTC)

Sure, but there's also plenty of really good, established sources that do use acceleration. There are many explanations, as we know, and I think the article can mention them all. As long as properly sourced of course . - DVdm (talk) 16:33, 25 May 2020 (UTC)

I'm not sure I can agree with that. There's already an explanation that explains it just with SR. If tommorow I come up with an explanation that takes into account special relativity plus the price of petrol, doesn't it probably indicate there was no point including the price of petrol in the first place? I'm not saying we remove mention of the ones that include acceleration (I hadn't done so), but they should at least be mentioned as probably being incorrect. --uKER (talk) 17:26, 25 May 2020 (UTC)

Ah, perhaps you think that acceleration needs general relativity. But see, for instance, in article special relativity: "Special relativity, contrary to some historical descriptions, does accommodate accelerations as well as accelerating frames of reference."

References

- DVdm (talk) 17:42, 25 May 2020 (UTC)

The explanations I have seen using acceleration cite it for for GR effects (read, gravitational time dilation). The lead itself seems to say that explainations are either simultaneity being affected by the frame change (SR) or gravitational time dilation due to the acceleration (GR). The affirmation that it can be explained by acceleration here in the lead isn't sourced, so I don't know what it's suppopsed to mean exactly. In any case, I always thought non-inertial observers fell outside of the scope of SR and can't figure out how else it could be explained. Are there any explanations other than these two possibilities, that somehow fit acceleration into SR? Can you cite any? BTW, I meant SR in my previous message. Edited. --uKER (talk) 18:25, 25 May 2020 (UTC)

Yes, through the equivalence principle GR can also give an explanation — also. That was the old view. But it does not need to. And non-inertial observers can fall well inside of the scope of SR. The modern view is that GR is only really indispensable where gravitation comes into play, but of course it can also be used where none does, as after all, SR is a proper subset of GR. The acceleration bit in SR is extensively explained in the section Twin paradox#Resolution of the paradox in special relativity and subsections, with sources galore . - DVdm (talk) 19:14, 25 May 2020 (UTC)

Alright, right now I can't get into those, but I'll take your word for it. What I will do, however, is specify that acceleration is not needed for the explanation either, as there's explanations that don't need it. --uKER (talk) 19:33, 25 May 2020 (UTC)

I got severely confused by this.

This article doesn't cite a source for the claim that acceleration doesn't matter and proper acceleration is the only thing that breaks the symmetry between the twins.

To say that the question can be resolved without reference to acceleration is nonsense.

In his video Don makes the mistake of begging the question: Why does he replace one and not the other twin with two space ships? It can only be, because one is accelerating. SimonR94 (talk) 00:52, 20 July 2023 (UTC)

This is completely wrong and begs the question.

Why does he replace one twin rather than the other with two spaceships?

The answer is that one is accelerating. An absolute acceleration is the only thing that breaks the symmetry. Therefore both the video and the article are nonsense. 141.70.6.133 (talk) 00:34, 20 July 2023 (UTC)

Unsatisfactory "Specific example" section

In the "Specific example" section, a reasonable argument is made why both the earth-bound and space-ship crew will agree that the space-traveller twin will be 6 years old on return to earth. It is also clear why the earth-bound crew will expect the earth-bound twin to be 10 years old at that time. However, there is no explanation of why the space-ship crew will expect the earth-bound twin to be 10 years old. It is merely asserted. It seems to me that the very heart of the "paradox" has been avoided. Zero^talk 10:09, 29 July 2020 (UTC)

The fist line of the travellers' perspective section says: "The ship's crew members also calculate the particulars of their trip from their perspective" (where I emphasised "also"). These travellers are assumed to be suffiencty smart to also being able to calculate the results form the Earth perspective, namely that "the round trip will take t = 2d/v = 10 years in Earth time." See the sources. Hope this helps. If not, you might try the wp:Reference desk/Science. )- DVdm (talk) 10:33, 29 July 2020 (UTC)

Thanks, but the "results from the Earth perspective" is not what the travelers have to calculate. What they have to calculate is the age that the earth-bound twin will have at the end of the experiment. They should be able to do it from their own perspective, using their clock and the rate at which they see the Earth receding and then approaching. It is alleged in this section that both calculations give the same results, but the space-ship calculation is only half given. Zero^talk 11:53, 29 July 2020 (UTC)

They know "the age that the earth-bound twin will have at the end of the experiment" because they know how far they will travel and at which speed in the Earth's perspective, so they know how old the Earth twins will be. The calculation is trivial. - DVdm (talk) 12:13, 29 July 2020 (UTC)

You are doing both the earth calculation and the space-ship calculation using the earth's frame of reference, so of course you will get the same answer. The whole point of the "paradox" is to explain why calculations in two different frames of reference will get the same answer. I don't believe that this problem exercised the minds of many very clever people if it was as trivial as you say. Should the entire rest of this long article be deleted? Zero^talk 12:56, 29 July 2020 (UTC)

The content is properly sourced. If you have a problem with sourced content in an article, please try the wp:Reference desk/Science, where someone might be prepared to explain. Per the wp:talk page guidelines we can't do that here. - DVdm (talk) 16:44, 29 July 2020 (UTC)

I'm a professional mathematician with university qualifications (albeit rusty) in special relativity. I'm also an administrator so I know that discussing the quality of an article is exactly what talk pages are for. You say that the content is properly sourced, but did you check? The first source (Minguzzi) is cited for "Dirac delta distribution acceleration". Two sources are given for the calculations. The first one (Jain) gives the calculation from the earth perspective and finishes with "The surprising result of this Example...is known as as the twin paradox" but doesn't suggest how the paradox is to be resolved. (Also, I don't think that science articles should rely on first-year undergraduate textbooks, which are necessarily dumbed-down.) The second one (Sardesai) is the real source, and here we find the part of the argument that this section of our article is missing: "It cannot be argued that because of reciprocity [earth-bound] Geeta should be younger than [space-ship] Seeta by 4 years. Whereas Geeta is in an inertial frame throughout the trip, the same is not true of Seeta. During initial acceleration to speed 0.8 c, reversal of speed at the station and the period of deceleration prior to reaching back on earth, Seeta is essentially in a non-inertial frame." So, unlike this section of our article, the source acknowledges the problem and gives a reason why it is not really a paradox. We should do that too. It seems to me (without reading Minguzzi in detail) that the "Dirac delta distribution acceleration" part is supposed to indicate that acceleration/deceleration is not a problem whereas the main source (Sardesai) says it is the problem. In total, this section of the article is not properly sourced and not satisfactory. Personally I find the explanation in the "Doppler shift" sections to be the most satisfying but too long, and I'm wondering if it can be summarised in a short section to put near the start. Zero^talk 04:31, 30 July 2020 (UTC)

It's probably a good idea to add some clarification from the second source to make it complete. But do note that "paradox" in essence has the meaning of a "seemingly contradictory" statement, so phrases like "it's not really a paradox" when appearing in some sources, should be treated with great care, as some authors tend to take a paradox as a real contradiction. - DVdm (talk) 10:17, 30 July 2020 (UTC)

The meaning of paradox is given with a sentence that has two parts. You consider the paradox as in the second part while many others consider it as in the first part: "a logically self-contradictory statement". So while we wait for other sources to appear to be cited that "solve" it as a self-contradictory statement I think the proper route to follow in this case is:

report the thought experiment possibly reporting some of the variants with citations, and report some of the possible "solutions" with the necessary citations making it clear that it can be seen also as a self-contradictory statement that cannot be solved as it is. I know that it appears as if there are no sources that see it as a "real" contradiction but even though it should be left as a possibility. 95.251.245.221 (talk) 14:39, 30 April 2023 (UTC)

"Specific example" wrong logic

The following discussion is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

---

In "Travellers' perspective" crew members calculate time for their (ship) frame from earth's perspective (ship is moving, earth isn't moving). But from perspective of ship's crew they are actually not moving and earth is moving. According to principle of relativity, crew must count that round trip will take t = 2d/v = 10 years (in "Travellers' perspective") and do calculations for "Earth perspective" as εd = 0.6 × 4 = 2.4 light years and εd / v = 2.4 / 0.8 = 3 years. Thus on arriving crew will be older (10 years for them against 6 years for earth) from their perspective. It obviously contradicts what you wrote in the example. 178.140.161.126 (talk) 10:03, 30 January 2021 (UTC)

If, from the ship's perspective, the Earth is moving, then the star is also moving, and in the traveller's frame, the distance between Earth and star is not 2d but 2εd. Also, the content is properly sourced, and discussed at length here above and in the searchable talk page archives. - DVdm (talk) 17:39, 30 January 2021 (UTC)

The distance between initial point of the star and initial point of ship in frame of reference of ship doesn't change. Star/earth just travel to ship and away this distance. All changes in time happen on earth/star and distance between star and earth changes in the earth frame. Do you understand what is invariance of inertial frames of reference?178.140.161.126 (talk) 11:51, 31 January 2021 (UTC)

As I said, see here above and in the searchable talk page archives. The content is propely sourced. We cannot discuss the subject here per wp:Talk page guidelines. Try our wp:Reference desk/Science. Additional warning on your user talk page. - DVdm (talk) 12:13, 31 January 2021 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Aging rate

I would like to mention the effect of aging rate in the twin paradox because the biological age of a person is not a simple count of elapse time, but the product of the elapsed time and aging rate. In special relativity, the time of the moving frame becomes shorter than the time of the stationary frame, but the aging rate of the moving frame becomes faster than the aging rate of the stationary frame. The relativistic effects of the time and aging rate cancel each other in the product so that the biological age of the traveling twin is always the same as that of the twin staying on the earth. Relativistic effect won't make traveling twin younger. Is there any error in my reasoning? 209.141.152.151 (talk) 14:24, 26 August 2021 (UTC)

Please put new talk page messages at the bottom of talk pages — See Help:Using talk pages. Thanks.

We can't add anyting that is not directly backed and supported by wp:reliable sources, and we can't discuss the content or aspects of the subject here. So we can't discuss your reasoning. We can only discuss the article itself, and we should do that based on reliable sources as well. See wp:Talk page guidelines.

Consider going to some public forum with your question. Usenet sci.physics.relativity is a good place to start. Google will bring you there. - DVdm (talk) 14:57, 26 August 2021 (UTC)

"Specific example" is incorrect

I have a degree in physics and a phd in maths. The author had clearly made a logical mistake in the "Specific example" section. I find it really sad that the author has dismissed all criticism that in my opinion are valid, without reflecting on the substance of the critical arguments, but rather deflecting the conversation by pointing at Wikipedia "policies" and "properly sourced" references. This deflecting strategy is a strong sign that the section is in fact incorrect, otherwise the author would be addressing the substance of the critical arguments. I understand that the author might consider themselves an expert in the field and therefore they perceive any criticism as a fundamental threat to their self-image, but like any decent scientist, they should still make an effort to find errors in their reasoning, instead of desperately trying to silence critical voices.

Here is my attempt to convince the author about the incorrectness of the argument in the "Specific example section": consider the same example with the only modification that not the spaceship but the earth is experiencing the Dirac-delta accelerations in exactly the same manner as the spaceship in the original example. Because none of the section's arguments made use of acceleration, they still apply in their original form, arrivig to the conclusion that the earthbound twin aged 10 years while the twin on the spaceship aged only 6. But this is clearly a contradiction, since in the modified example the Earth took the spaceship's role therefore the conclusion should have been the opposite. This contradiction shows that the arguments put forward in the "Specific example" section must be incorrect.

I am begging the moderators to take up this issue, not let the author dismiss the valid criticism put forward by myself and others, and finally enable the community to correct the article. 2A02:C7D:F01D:900:31B2:1B7F:6582:3065 (talk) 04:57, 23 November 2021 (UTC)

The "Dirac-delta accelerations" model an "infinitely accelerated jump" from one inertial frame to another. If the Earth undergoes that, and the spaceship remains inertial, then it results in the earthbound twin aging 6 and spaceship twin aging 10 at the end of the "Earth trip".

The current content is properly sourced and thus, for Wikipedia, considered "correct", so unless you have a relevant source that supports your claim, we cannot discuss this here per the wp:Talk page guidelines. - DVdm (talk) 09:36, 23 November 2021 (UTC)

Thank you for reflecting on the core of my argument, I agree with your conclusion in the modified example when Earth undergoes acceleration.

I would like to improve the "Specific example" section by having your conclusion added as a clarification: "Note that the reason why the twin on the spaceship ages slower is that the spaceship undergoes acceleration and jumps from one inertial frame to another when it turns back. In case the spaceship would stay stationary in an inertial frame of reference and the Earth would undergo acceleration when turning back and jumping from one inertial frame to another, then it would result in the earthbound twin aging 6 and the twin on the spaceship aging 10 years at the end of the 'Earth trip'."

Given that we agree on the correctness of this conclusion, I assume we also agree that adding this important clarification would improve the article? Peter Div (talk) 10:27, 23 November 2021 (UTC)

Please indent your talk page messages as outlined in wp:THREAD and wp:INDENT — See Help:Using talk pages. Thanks.

No problem, provided we can point to relevant reliable source. I'm afraid that, even if correct, per our policy regarding wp:original research, without a source we can't add that conclusion. - DVdm (talk) 10:39, 23 November 2021 (UTC)

As pointed out by Zero, the second source (Sardesai) has the clarification that our section is missing. Given that we not only have a primary source supporting the clarification, but 3 editors agree on the correctness with none disagreeing, I assume it would be against our policies to not improve the article by adding this clarification? Peter Div (talk) 11:05, 23 November 2021 (UTC)

On which page does Sardesai explicitly say that:

In case the spaceship would stay stationary in an inertial frame of reference and the Earth would undergo acceleration when turning back and jumping from one inertial frame to another, then it would result in the earthbound twin aging 6 and the twin on the spaceship aging 10 years at the end of the 'Earth trip'."

I don't see it in example 7 on pages 27, 28.

I don't think we should even add something like

The reason why the twin on the spaceship ages slower is that the spaceship undergoes acceleration and jumps from one inertial frame to another when it turns back.

because that is also just an interpretation of what appears in Sardesai's example. Besides, such a statement is already present elsewhere in the article, so I don't see a reason to repeat it in the specific example section. - DVdm (talk) 11:16, 23 November 2021 (UTC)

Sardeasi page 27-28 example 7:

Note that the motion of twins is not symmetrical. It cannot be argued that because of reciprocity Geeta should be younger than Seeta by 4 years. Whereas Geeta is in an inertial frame throughout the trip, the same is not true of Seeta. During initial acceleration to speed 0.8 c, reversal of speed at the station and the period of deceleration prior to reaching back on earth, Seeta is essentially in a non-inertial frame. Calculations done by taking into account the periods of accelerations and decelerations though beyond the scope of this book, give the same result as obtained above by cutting the problem down to our size!.

The reason Sardeasi added this note to the example is that without this the whole example is misleading and is not addressing the core of the paradox. This is what me, Zero and others have point out several times, this is the core issue with our section.

reflecting on your two arguments for not adding the note:

"because that is also just an interpretation of what appears in Sardesai's example" -> if it was "just" an interpretation then Sardeasi would not have found it important to add it. But it is not "just" an interpretation, as pointed out by myself, Zero and others, without this note the whole example is misrepresented.

"such a statement is already present elsewhere in the article" -> the section in question is misleading and is a misrepresentation of the original source without this note, regardless of such a statement being present somewhere else in the article.

Here is a summary of why we have no choice but to add the clarification to the section:

(1) the primary source does have this note which is an essential clarification, vital to the consistency of the example. Without the clarification the example is out of context and falls apart as pointed out by multiple editors. Not adding this note to our section would be a misrepresentation of the original source, therefore I am afraid we don't really have a choice here.

(2) several editors, independently of each other have pointed out that the whole section is "Wrong", "Unsatisfactory" and in general misleading without such a clarification. This includes yourself when you said "It's probably a good idea to add some clarification from the second source to make it complete."

In order to improve the article by resolving the issue of misrepresentation of the original source, this is what I am proposing to add to the end of the Specific example section:

"Note that the motion of twins is not symmetrical. It cannot be argued that because of reciprocity the earthbound twin should be younger than the twin on the spaceship by 4 years. Whereas the earthbound twin is in an inertial frame throughout the trip, the same is not true of the twin on the spaceship. During initial acceleration to speed 0.8 c, reversal of speed and the period of deceleration prior to reaching back on earth, the twin on the spaceship is essentially in a non-inertial frame." Peter Div (talk) 12:38, 23 November 2021 (UTC)

I still see no reason to repeat the core of the paradox in this mere numeric example. The core is abundantly mentioned elsewhere in the article. - DVdm (talk) 12:48, 23 November 2021 (UTC)

This note has nothing to do with the repetition of the core of the paradox. Without this note the example section is not only logically incorrect, wrong and misleading, but is also a misrepresentation of a primary source, as has been pointed out by multiple editors. In order to be able to decide whether this discussion has come to a standstill and therefore ready for the next step in the dispute resolution process, would you be able to confirm if your decision is final? Peter Div (talk) 13:09, 23 November 2021 (UTC)

I don't agree that the example section is logically incorrect, on the contrary. And it is well sourced. Also first note that, in articles, we don't put comments that start with "Note that..." — see MOS:EDITORIAL (which also refers to wp:NOR) and WP:NOTED. Second, please be aware of the meaning of wp:primary sources in Wikipedia. I don't think that you have that in mind.

I'd be glad to hear comments from other (more regular) contributors to the article, not just from passersby. - DVdm (talk) 14:12, 23 November 2021 (UTC)

Neither in the Earth's perspective subsection, nor in the spaceship's perspective subsection is it assumed that either of those frames are non-inertial. Therefore regardless of which of the two frames is non-inertial, the arguments of the subsections apparently still apply and should result in the same conclusion in both cases. However, as we have already agreed, in case the Earth's frame is non-inertial, the correct result is the opposite. This is a contradiction, a logical error pointed out by multiple other editors that you for some reason refuse to see. The root cause of the logical error is the misrepresentation of the argument from the original source: an important note has been omitted, the purpose of which is exactly to clarify this logical error which is why the author of the original source found it essential to include and which is why it's essential to include it in our section.

You keep repeating that the section is well sourced but it's not, it is missing the most important half from the original source, the note in question, without which the example makes no sense.

Thank you for the stylistic advice MOS:EDITORIAL and wp:primary sources, it's unfortunate that you refuse to engage with the core issue and instead bring up such secondary topics. Portraying me as a passerby is an ad-hominem attack.

During our discussion I have shown flexibility and willingness to find a compromise, to the point where all I was suggesting was to include the vital note in it's original form. You on the other hand have constantly refused to negotiate and dismissed every single suggestion not only from me, but also from other editors (even though you have agreed at some point that a clarification would be necessary, not sure what changed since then). You are preventing multiple people from correcting a section by adding an important note from the original source.

Yes, I would also be happy to hear other peoples opinions who have relevant background in the field. I have posted this for a Third Opinion Peter Div (talk) 14:57, 23 November 2021 (UTC)

Response to third opinion request:

I am responding to a third opinion request for this page. I have made no previous edits on Twin paradox and have no known association with the editors involved in this discussion. The third opinion process is informal and I have no special powers or authority apart from being a fresh pair of eyes.

I think the current content of the example section is fine. In reading the whole article, it appears to be laying out the basics of the situation for later analysis and explanation. I don't see a good reason to briefly note the explanation in the example section when it's adequately explained later (and already summarized in the lead). Firefangledfeathers 03:32, 1 December 2021 (UTC)

I Don't Think This is the Twin Paradox

I don't think the twin paradox was about why incorrect solutions to the twin problem contradict themselves. Generally in a paradox, you solve the problem correctly and the result conflicts with your senses or intuition. But now we seem to be saturated with "why does my ignorance of relativity contradict itself" as examples of the paradox. Indeed, the first paragraph in this article states that.

In his 1905 paper on SR, Einstein presented the original version of this problem involving two clocks, one stationary, and one making a round trip. He noted that it was "peculiar" that the traveling clock is now behind the stationary clock. While he didn't say "paradox", this real example of time dilation was paradoxical to the average person (and physicist). This has been referred to as the "Clock Paradox".

https://www.fourmilab.ch/etexts/einstein/specrel/specrel.pdf

In 1911, Langevin published a paper (The Evolution of Space and Time) and presented the same problem but replaced the traveling clock with an actual person and noted the fact that when the person returned they would be younger than those who stayed behind. This of course was even more interesting and paradoxical because now we are talking about the effect on living organisms. Though Langevin didn't use twins, this is the origin of the "Twin Paradox".

https://en.wikisource.org/wiki/Translation:The_Evolution_of_Space_and_Time

Neither Einstein nor Langevin questioned the validity of the solution or suggested absurd alternate solutions (such as supposing that the earth and universe made the round trip). The correct solution of the problem was itself paradoxical to our sense of time. Especially then.

Some suggestions...

1. The title of this article be changed to "Twin Problem" with "Twin Paradox" redirecting to it.
2. The opening paragraph be reworded to reflect the paradoxical nature of the correct result referencing the originators of the scenario (Einstein and Langevin) and time frame (1905 - 1911).
3. A simple walkthrough of the correct solution (the ship jumping to one moving frame, to another moving frame, then back to the stationary frame.
4. Examples of verifications (experiments) and actual use (GPS).
5. A section devoted to the common technical difficulties naive people have when they attempt to solve or follow the solution.

Examples of (5)...

1. Why can't we assume that it was the earth that made the trip and the space craft stayed stationary?
2. If the reason against (1) is that it was the ship that accelerated, then is the dilation due to acceleration?
3. Did the time difference happen when the ship turned?
4. Etc.

Identifying the "paradox" with all of these bad arguments has mangled it beyond comprehension. Two weeks ago, I was looking for a decent example for my son, and now I am here typing this. Note: After dozens of youtubes, I did find one (https://www.youtube.com/watch?v=1xb-z8ZyvI4&t=6s).

Just to be clear, when I first encountered this problem in high school, I fell into the "why does my ignorance contradict itself" paradox group. Rsccore (talk) 07:00, 7 August 2022 (UTC)

Regarding your suggestion to rename the article, we can't do that as the literature clearly prefers the term twin paradox:

Google Scholar	Hits
"twin paradox" relativity	4430
"twin problem" relativity	138

After all, a paradox is usually seen, not as a real contradiction, but as a seeming one. See the article Paradox and, for instance, the first meaning at https://www.dictionary.com/browse/paradox - DVdm (talk) 14:02, 7 August 2022 (UTC)

Agreed. That makes sense that this scenario retain its commonly known name (Twin Paradox). And the result that one twin ages less is paradoxical. My suggestion to rename was overkill. I do hope though that the rest of my suggestion to move the discussion of common mistakes and misconceptions to its own section, after the description and clean solution of the scenario, is considered. It seems that introducing and misidentifying incorrect solutions as "the paradox" has greatly affected the coherence of not just this article, but discussions/videos everywhere (not suggesting that we can fix those). It has spurred a crusade to solve "the paradox", that to people familiar with the principles and solution, doesn't even exist. And that crusade has spawned many misinterpretations (i.e. the difference happens all-at-once when the ship turns around) that only multiply. I just hope that the article can be restructured to begin its focus on the twin scenario, its solution, the paradoxical nature of the result, and then go on to discuss common misconceptions, which is certainly important. Obviously the most common being the "why can't we say that the whole universe is moving and the ship is stationary?". This would probably help us clean up the misconception discussion as well. Currently, misconceptions and explanations are too disorganized to see easily which goes with which. For example, the ship must accelerate to change inertial frames, but acceleration doesn't matter? I realize that these are two separate contexts, but the article doesn't make this clear. In the context that it is the ship that jumps from one inertial frame to another, that can only be done with acceleration. But in the other context, the acceleration doesn't need to be factored into the dilation calculations. 107.145.107.169 (talk) 21:16, 7 August 2022 (UTC)

That reply was from me, I thought I was logged in. Rsccore (talk) 21:18, 7 August 2022 (UTC)

Proposed Rough Draft

The following is a rough draft of what I propose the article to look like. I know this is bold, and that is why I am posting it here. Unless there is some sort of consensus, this draft is as far as this idea goes. Essentially, the only thing different about this version and the current article is point (2) below.

1. There isn't a paradox in the solution.
2. The reason for (1) is that the experiment is symmetric.

I think all relativists believe (1). The issue is (2). When the suspicious student asks, “Why can’t we use the principle of relativity and say that the spacecraft is stationary, and the Earth and star do all the moving…?” We have been conditioned to believe that a mythical paradox awaits us if we go down that path, so we reply “We can’t, because the experiment is asymmetric, and the spaceship is accelerating, and {insert 1,000 reasons }…” But every such explanation comes up an inch short of satisfying the student (or even us). So we create even more explanations of why the experiment is asymmetric and bring in even more advanced concepts and add stuff to the problem (i.e. acceleration) that wasn’t even there. But as hard as we try to explain that the experiment is asymmetric, we come up an inch short, because it isn’t.

By changing (2) from the conditioned version “The reason for (1) is that the experiment is asymmetric.” to new version “The reason for (1) is that the experiment is symmetric.”, we don’t have to explain anything. When the suspicious student asks why doesn't the principle of relativity apply, we reply "It does" and show them that we get the same result. Huge win for the sake of coherence. I address the student’s question in the discussion section after the solution.

Note: in my solution for the case of a stationary spaceship, yes, I am accelerating the earth and star back and forth. Instantaneously no doubt, just like with the ship. Justification? It is a thought experiment. We have plenty of leeway. We aren't saying that acceleration is relative, and the experiment has nothing to do with acceleration anyways.

<< beginning of draft >>

The twin paradox is a thought experiment in the study of special relativity involving two twins and space travel. The experiment begins with two twins on Earth. One twin (the traveler) boards a spaceship and travels to a distant star and back at a substantial fraction of the speed of light, while the other twin (the Earth twin) remains on Earth the whole time. When the traveler returns and reunites with the Earth twin, they find that the traveler has aged less than the Earth twin. This thought experiment was first introduced in 1905 by Einstein in his paper on special relativity using two clocks. Later, in 1911, Langevin introduced a similar thought experiment using people in place of the clocks, which became the basis of the current version using twins. The result was very controversial when it was introduced, as time was generally thought to be absolute and independent. Later, over several decades, as experimental results supporting it mounted, it became accepted as true.

The Solution

Let us assume that the star is 3 light years from Earth and that the spaceship travels at 0.6 times the speed of light. Also, for simplicity, we will assume that the spaceship can accelerate and decelerate instantaneously. According to the principle of relativity, we can examine this scenario from either the Earth’s frame of reference (the spaceship is moving) or the spaceship’s frame of reference (the Earth and star are moving). First, we will examine it from Earth’s frame of reference.

Analyzed in Earth’s Frame of Reference (the spaceship is moving)

In this analysis, we consider that the Earth and star are stationary, and the spaceship moves from the earth to the star and then back to the earth.

From the Earth twin’s perspective, the traveler travels at 0.6c to a star 3 light years away. The trip takes 5 years. Likewise, the return trip will also take 5 years, for a total of 10 years.

From traveler’s perspective, the distance to the star is only 2.4 light years due to length contraction, and the trip only takes 4 years. Likewise, the return trip will also take 4 years for a total of 8 years.

Analyzed in the Spaceship’s Frame of Reference (the Earth and star are moving)

In this analysis, we consider that the spaceship is stationary and that the Earth and star are moving (together). We start with the Earth at the same position of the spaceship, then the Earth and star move until the star is now at the position of the spaceship, and finally the Earth and star move in the opposite direction until the Earth is back at the position of the spaceship.

From the Earth twin’s perspective, even though the Earth and star are moving, the distance between them is still 3 light years, and it takes 5 years till the star gets to the position of the spaceship. For the return trip, the same thing occurs, for a total of 10 years.

From the traveler’s perspective, since the Earth and star are moving towards him, the distance is contracted to 2.4 light years (as in the other scenario), and it takes 4 years for the star to reach him. It takes another 4 years for the reverse, and the total is 8 years.

<< Add animation for both scenarios and perspectives >>

Discussion

As can be seen, whether we analyze this experiment in Earth’s frame of reference (the spaceship is moving) or in the spaceship’s frame of reference (the spaceship is stationary), we get the same result, the traveler ages less. And this is expected according to the principle of relativity. But this result conflicts with the naive belief that each observer should see the other observer’s clock running slow, and thus, from the spaceship’s frame of reference, the Earth twin should have aged less. But this reasoning is flawed. Particularly, the belief that each observer should see the other observer’s clock running slow.

Imagine two observers, A and B, traveling past each other.

If A pulls out a ruler and times how long B takes to traverse it, end to end, and B also times how long it took to traverse A’s ruler, end to end, then A will record a longer time than B, and B a shorter time than A.

But if B pulls out a ruler and times how long A takes to traverse it, end to end, and A also times how long it took to traverse B’s ruler, end to end, then B will record a longer time than A, and A a shorter time than B.

<< add animation for both cases >>

So, it is true that either observer can see the other observer’s clock running slow, but they can’t both see this at the same time. To compare clocks, you must use the same ruler. If we choose A’s ruler, then A and B will agree that A’s clock is running faster. If we choose B’s ruler, then A and B will agree that B’s clock is running faster. This lies at the heart of the relativity of simultaneity.

In the twin paradox, the ruler chosen is the distance between Earth and the star, which is in Earth’s frame of reference. Since we are using Earth’s ruler, the twins will always agree that the Earth twin’s clock is running faster and the traveler’s clock is running slower, regardless of whether we analyze the experiment in Earth’s frame of reference or the traveler’s frame of reference.

History of the Experiment

Most of this is present in the current article.

• History of the origination of the thought experiment.
• History of criticism of the result (there was plenty originally)
• History of experimental corroboration
• History of application (particle physics, astrophysics, GPS)

Common Misconceptions

Not much left here, since we no longer must prove or explain that the experiment is asymmetric. We covered the primary misconception above in the discussion of the solution. Most of the other misconceptions seem to stem from unneeded explanations of how the problem is asymmetric.

More Discussion

There is plenty of extended discussion in the current article.

Looks very much like wp:original research and wp:SYNTH. Remember that everything needs solid wp:Reliable sources. Also note that Wikipedia is not a textbook (WP:NOTTEXTBOOK) and we don't use the MOS:WE style. The article currently seems to be nicely inline with the relevant policies and guidelines, so I don't think much needs to be changed to it. - DVdm (talk) 08:57, 11 August 2022 (UTC)

Other than the "original research" claim, the other claims I think are non issues. This is a draft, but sources exist for all of it. It is elementary relativity with clocks and rulers. With regard to "We", I thought in the analysis of a physics solution, that was allowed? Textbook? I will have to look at that more, but I felt I was writing this in the same spirit as many other physics articles here. It is draft, I am sure it needs tweaking.

Original Research

This is a tough one. One important part of what I am doing here is just trying to restructure the article from the current "Twin Paradox Controversy" to "Twin Paradox". For example, this article about the Ladder Paradox is about the Ladder Paradox, not the Ladder Paradox Controversy.

https://en.wikipedia.org/wiki/Ladder_paradox

You can review several other articles about thought experiments on wikipedia, and see the same thing. This article is so different. It is not a source I would send anyone to to read about the twin paradox. I know the twin paradox has history of controversy, but why not focus on the thought experiment first, and the history of controversy second?

But you do have me on the second part (which I only realized while attempting the first part). My statement that the problem is perfectly symmetrical and we never had to explain its asymmetry to begin with. Again, textbook relativity with clocks and rulers, and plenty of sources. But the myth of asymmetry has grown so large, in modern times, it may be too late. Unless I can find enough references to back up that view, I may just have to publish this as a paper and not a wikipedia article.

Maybe the Twin Paradox article will never be as clear and simple as all those other paradox articles. The twin paradox entered the mainstream and was tarred and feathered, while those other paradoxes stayed safe in physics textbooks.

Maybe look at some of the other paradox articles here, and then at the current article here. Hopefully, some improvement can be done. Even if we stick with the myth of asymmetry, must it be debated in the beginning, rather than later on? When I first read this article, before I even got to the actual example, I was unsure if I even could understand the twin paradox. Rsccore (talk) 13:56, 11 August 2022 (UTC)

Velocity is relative, acceleration is not. If you have an inertial system (1) and another system that the inertial system sees as accelerating (2), system 2 will feel the effects of acceleration (e.g. being pushed back in their seat), system 1 will feel nothing. That is asymmetric. The twin that goes out and comes back will feel the acceleration at the turnaround point, the twin on Earth will not. That is the assymetry. PAR (talk) 07:42, 13 October 2022 (UTC)

1 is inertial also if it has a constant velocity that is uniform with respect to 2, so if 1 changes its velocity and then keeps it constant it then sees a different acceleration of 2 hence acceleration is relative. What 1 and 2 "feel" on their own or of the other is irrelevant also because is relative.

As a reply to the proposed new article: Sob! "As can be seen, whether we analyze this experiment [...] we get the same result, the traveler ages less." in fact this is the "real" contradiction because we can not know who is the traveler. 95.251.245.221 (talk) 14:33, 30 April 2023 (UTC)

No. What they feel is not relative. The traveling twin can make experiments to find out whether he is in an inertial frame or not. SimonR94 (talk) 01:28, 20 July 2023 (UTC)

Instantaneous turnaround implies acceleration

I removed the phrase "as the effect still applies to a theoretical observer that can invert the direction of motion instantly, maintaining constant speed all through the two phases of the trip " and added the two-observer scenario instead. Acceleration is the change in velocity per unit time, not the change in speed per unit time. An observer inverting the direction of motion instantaneously is experiencing infinite acceleration at that point. PAR (talk) 07:25, 13 October 2022 (UTC)