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The current explanation of the twins paradox type of experiment does not seem complete. I suggest the following new initial section:-
The twins paradox experiment is a specific example of a general test of time dilation/contraction. The essence of the general test involves the measure of the time duration between two time separated events by two identical and accurate clocks. The clocks are synchronized when stationary next to each other. The first event is their separation and the second event their re-union. At re-union the clocks read each other and find that they are no longer synchronized. As the two clocks differ then one clock reads the other to be slow and, naturally, that clock reads the first clock to be fast.
The Hafele and Keating experiment is one example of a real experiment which proved this effect. In the experiment one clock remained stationary while the other was flown right round the Earth in a jet plane to eventually return to the stationary clock.
The only theory available to the physics establishment remotely capable of predicting this effect is special relativity which predicts time dilation as a consequence of relative velocity. However the postulates of special relativity specify that the two clocks must be inertial; yet one clock at least must accelerate during the experiment.
The twins paradox version of the general test severely restricts the paths that the clocks may make during their separation (the twins act as identical clocks in human form). The object of the restriction is to adapt the test specifically such that the suitability of special relativity is maximized. To this end one clock is made inertial throughout while the necessary accelerations of the other are confined to three periods of duration short relative to two long periods of inertiality. The application of special relativity to the general test with unrestricted paths, inferring constant acceleration, does not seem possible. END
My source is Einstein's special relativity. Plus Hafele and Keating. Your comments are not very specific.Are you unhappy with any particular part of my proposal and if so what and why? My point is that the Wiki explanation of the twins paradox is deceitful.The current explanation concentrates on one very specific example of the general experiment in order to put special relativity in the best light. It is not Wiki's job to support any particular scientific theory. Surely it is Wiki's job to explain to the public the difficulties that exist in the explanation of this particular phenomenon. i2.96.245.214 (talk) 09:10, 9 March 2013 (UTC)
It certainly is not Wikipedia's job to explain to the public the difficulties that exist in the explanation of some phenomenon. This is an encyclopedia — not a forum, not a textbook, not a publisher of original thought. For an overview of what Wikipedia is not, see wp:NOT.
Have a look at how reliable sources are actually used in our articles. The policy on wp:original research does not allow us to draw conclusions from a source, or give a new twist to it. Also, the policy on wp:synthesis forbids drawing conclusions form combined sources. Your analysis and your conclusions are wrong, and my being not specific is due to the fact an article talk place is not the place to discuss someone's personal views about the subject — see wp:talk page guidelines. If you are looking for comments, I can recommend sci.physics.relavity on Usenet, or a personal blog, or some open forum. Wikipedia is not the place for this. Good luck. - DVdm (talk) 10:14, 9 March 2013 (UTC)
I see the point as this. The twins paradox experiment is very well known. Hence wiki has a page on it. The reason that it is well known is because the desynchronisation of two clocks, in itself a simple thing to appreciate, cannot be explained by modern science. People expect modern science to be able to explain these things and when it can't it becomes an issue of great interest. I agree that it is not down to wiki to attempt an explanation but I believe that wiki should fully explain the circumstances of the experiment and also why modern science fails in this case. The current wiki explanation of the experiment is deceitful in that it does not explain the experiment properly and fully but distorts it so as to put the theory of special relativity in the best possible light. Wiki is not there in order to support particular theories, no matter how eminent the author. I repeat, the point of interest in this wiki article is that the observations cant be explained.2.96.245.214 (talk) 12:34, 9 March 2013 (UTC)
I don't think you see the point yet. It is not Wikipedia's task to explain "why modern science fails in this case", even if it would indeed fail. Wikipedia reports and shows how it is explained in the mainstream scientific literature. Whether that explanation is deceitful or unsatisfactory for you, is a matter of opinion, and this is not the place to vent opinions. - DVdm (talk) 14:12, 9 March 2013 (UTC)
I take it, therefore, that you wish to keep secret from the readers of this article that the twins paradox journey path is only one of an infinite number of possibilities, and that that one path has been specially chosen to suit a particular theory. Please confirm if that is your position or not.RFNo (talk) 11:44, 10 March 2013 (UTC)
If we can find a few high quality, mainstream scientific, reliable sources that state that "the twins paradox journey path is only one of an infinite number of possibilities, and that that one path has been specially chosen to suit a particular theory," then perhaps we can put something along that line in the article. Otherwise we can't. That is Wikipedia"s position. - DVdm (talk) 16:45, 10 March 2013 (UTC)
One does not need an eminent physicist to qualify the obvious. The Hafele and Keating experiment (which I have quoted)is an example of a different journey between separation and reunion of the two clocks to the Twins Paradox. In that case the journey is complex for both clocks. The object of a Wiki article is surely to explain something to the reader without prejudice, distortion and deceit. The current Wiki article is arranged merely as an advertisement for special relativity (and a poor one at that)at the cost of the truth. No wonder readers have difficulty in understanding it. I ask you to reconsider your position.RFNo (talk) 10:17, 11 March 2013 (UTC)
I cannot see what you are complaining about. All of my comments and suggestions are based on the most impeccable scientific source of all, ie. Einstein and his theory, Special Relativity. The postulates of special relativity specify inertial reference frames. Hence the theory only operates between inertial bodies. Are you denying that Einstein is a qualifying source or are you denying the postulates of SR? RFNo (talk) 13:10, 11 March 2013 (UTC)
Wikipedia is not interested in comments and suggestions that are based on a source. It is interested in what is written in the sources.
It looks like you haven't understood some basics about special relativity. The first postulate demanding that laws be the same in inertial frames, does not imply that the resulting theory is only valid for inertial frames. Quite on the contrary. Special relativity can perfectly deal with accelerated frames and clocks. See, for instance,
Note that Wikipedia is not the place to go to have it explained. If someone does not agree with the mainstream sources (which clearly say that special relativity can deal with accelerated frames and clocks), then one must go elsewhere. I already gave some suggestions above. - DVdm (talk) 14:07, 11 March 2013 (UTC)
I too am only concerned with what is written in the source, the theory of special relativity. The theory of special relativity is based upon observations between inertial frames. It is you who are mis-interpreting this ( mis-interpreting is hardly the word, perhaps over-turning is more correct)to also apply to non-inertial frames. I do not care how many other sources that you quote to back you up for NO-ONE may mis-interpret SR in this way. My source is primary, your sources are secondary. Where secondary sources disagree with the primary source they should be discarded. The Wiki article should dispense with quotations and calculations from such secondary sources. Why do you think that Einstein specified inertial reference frames in his theory if he did not mean INERTIAL.RFNo (talk) 14:50, 11 March 2013 (UTC)
Again, the first postulate demanding that laws be the same in inertial frames, does not imply that the resulting theory is only valid for inertial frames. Quite on the contrary.
And even if your primary source would explicitly say that special relativity cannot handle acceleration, even then does Wikipedia prefer wp:secondary sources over primary ones. It's policy:
"Wikipedia articles should be based on reliable, published secondary sources and, to a lesser extent, on tertiary sources."
I do not deny that the laws of physics are either identical or nearly so in accelerating frames as in inertial ones. What I do say is that this cannot be predicted from the basis of a theory which relies on inertial frames to make its conclusions. Any secondary or tertiary theory which proclaims to have done so is talking nonsense. It is like a man sitting on the branch of a tree that he is sawing off.The fact remains that readers cannot understand the twins paradox article. The article most definitely needs to state that the twins paradox path is only one of an infinite number which has been specially chosen to maximize the employment of special relativity.If this is not done the article is deceitful.RFNo (talk) 10:49, 12 March 2013 (UTC)
If we can find a few high quality, mainstream scientific, reliable secondary sources that state that "that the twins paradox path is only one of an infinite number which has been specially chosen to maximize the employment of special relativity," then perhaps we can put something along that line in the article. Otherwise, no, we can't. - DVdm (talk) 11:20, 12 March 2013 (UTC)
It is obvious to anyone that the twins paradox path is only one of an infinite number of possible paths. You are using bureaucracy to stifle improvement. Wiki wants an improvement but you dont.RFNo (talk) 09:44, 14 March 2013 (UTC)
Wikipedia does not allow improvement by original research, even it would be valid. Write a book or an article in a scientific journal, wait a few years until it gets quoted in other mainstream articles, journals and textbooks, and then Wikipedia can take some of it on board. Wikipedia wants high quality reliable secondary sources. If you cannot produce them, we can —and must— close this discussion. - DVdm (talk) 11:44, 14 March 2013 (UTC)
not proper acceleration as defined?
In the rocket example, it is stated that the rocket undergoes proper acceleration, but fails to define this. Typically proper acceleration is defined as where u is the proper velocity . The only thing is, working backwards from the stated result implies that the thing that they are keeping constant is instead . I don't believe there is anything wrong with this physically, it is just not what people call the proper acceleration typically. — Preceding unsigned comment added by 63.131.30.172 (talk • contribs) 17:06, 5 May 2013 (UTC)
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Yes, of course v(t) is the coordinate velocity, not the proper one, and indeed is constant here. I have added a wikilink to the the article on proper acceleration where the thing is properly(NPI) defined Cheers - DVdm (talk) 17:36, 5 May 2013 (UTC)
"Travailing"!
It is bullshit.
If some experiments may suggest it all right, it must rather refer to something else than SPEED - it can be gravity or whatever...
Because there can't be any such ABSOLUTE difference in time measurements (in this case pertaining to SPEED) -- because, as the same exact science states that there's no evidence of any ABSOLUTE CENTRE OF THE UNIVERSE (hence the ULTIMATE FRAME for all speed measurements), there CAN'T EXIST a SCIENTIFIC HYPOTHESIS insisting that a certain absolute determination depends on such RELATIONAL rubbish as SPEED. 213.87.132.224 (talk) —Preceding undated comment added 15:54, 4 September 2013 (UTC)
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The first sentence of this article states:In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more.
This is slightly inaccurate. On the traveler's return, the twins find that the traveler's clock is running behind that of his/her stay-at-home twin. You may *infer* that the traveler is younger, but that inference is *not* logically obligatory, and has some rather odd consequences. The reason why that inference is *not* logically obligatory is because the words, 'age', 'younger', 'older', are uniquely defined only in a world of absolute simultaneity. A world of absolute simultaneity, (our normal reference frame, GMT), is essentially a one-clock world. The world of special relativity is a multi-clock world, thousands of essentially different clocks all doing their own thing. Suppose the twins die the moment the traveler returns. Absolutely every observer left alive believes that the twins were born at the same moment, and died at the same moment, and so died at the same age. They will differ, of course, as to what that age actually was, but none of them will be disposed to believe that they lived different amounts of time, unless you can convince them that their clocks are irrelevant and the twins clocks are the only ones that count.
(Unauthorized Persons Inc. (talk) 04:04, 18 November 2013 (UTC))
Welcome to Wikipedia. "Age", in this context, means "elapsed proper time" since birth. So, yes, for determining the twins' ages, their clocks are the ones that count. BTW, twins are rarely born simultaneously. Ouch. ;) Paradoctor (talk) 04:52, 18 November 2013 (UTC)
Please allow me to point out that neither the first sentence of this article, nor Einstein's original article, (On the Electrodynamics of Moving Bodies), mentions or defines "proper time" which is a moderately arcane mathematical notion. As far as I know this notion wasn't invented in 1905. I don't even know whether it is possible to invent a clock which measures it. If this notion obviates the natural ambiguities in Einstein's original suggestion, well and good, but then the article should *teach* the notion, not merely keep it in reserve to handle potential challenges. (Unauthorized Persons Inc. (talk) 23:08, 18 November 2013 (UTC))
"I don't even know whether it is possible to invent a clock which measures it." Of course it is possible. No complicated calculations are involved. Each twin's clock measures proper time. What is so "arcane" about that notion? — Preceding unsigned comment added by 173.57.52.219 (talk) 05:52, 19 November 2013 (UTC)
We are arguing about a definitional preference. (Such arguments can go on for ever, this one had better not.) Most prefer to say that an observer's age is an interval between his/her birth and a hypothetical present reading of his/her clock. I prefer to say that a process has as many ages as there are intelligent clock-users observing the process from the beginning. There are two advantages. This disinfects the notion of age from Newtonian presuppositions. It also allows discussion of the relation between public and private clocks.
Consider the following scenario: Twins are born (at roughly the same moment) on Earth. Immediately after birth one twin heads for a distant fixed point at 0.9c. On arrival, he does the usual instantaneous about turn and returns to Earth. On arrival at Earth, he does another instantaneous about turn and returns to the distant fixed point. He visits it five times in all, and on his fifth return disembarks on Earth and he and his twin both die. If you consult their private clocks, the travelling twin is younger. But suppose you cannot do that. For some observers, the twin's motion *is* a clock. By that clock the twins die at the same age. By me that result is just as true as the conventional reading. (Unauthorized Persons Inc. (talk) 07:33, 27 November 2013 (UTC)]
(Off-topic subject discussion) He and his twin both die, one with a long beard and brittle bones, the other without a beard and in perfect shape. That should tell these observers for whom the twin's motion *is* a clock, that they are making a very bad choice of clock. The length of their beards and the brittleness of their bones would have been a far better choice. The essence is that the proper times of the twins are compared. Your observers for whom the twin's motion *is* a clock, don't do that, they use one and the same coordinate time for both, which is bound to produce a faulty result. (We should stop discussing this—see wp:talk page guidelines. Otherwise you can go to our wp:Reference desk/science)
(On-topic article discussion) The thing that you prefer to call age already has a name: proper age, and, yes, Indeed, "most prefer to say that an observer's age is an interval between his/her birth and a hypothetical present reading of his/her clock", so that is what we are supposed to have in this article, per wp:DUE. - DVdm (talk) 07:57, 27 November 2013 (UTC)
"What you prefer is of interest to Wikipedia only if it is published in reliable sources." Point taken. My last post on this one.
"He and his twin both die, one with a long beard and brittle bones, the other without a beard and in perfect shape." The beard and bones stuff is an *inference*, not an experimentally proven fact. If twins conducted the standard experiment they would still be twins after it was over.
"That should tell these observers for whom the twin's motion *is* a clock, that they are making a very bad choice of clock." This may be the only clock they have. They may not be close enough to know whether the twins possess clocks, or to verify the beard and bones stuff.
Re "The beard and bones stuff is an *inference*, not an experimentally proven fact.": every act of reading a clock—or any instrument—is an inference. In physics, time is defined as a clock reading. Counting your heart beats to time a process is an inference too, and it produces the experimental fact that the process lasted a specific number of your heart beats. Measuring the twin's beard lengths or the strenght of their bones can be quantified and so they produce facts.
Re "This may be the only clock they have": it is essential that each observer uses his own locally worn (or carried along) clock, by definition of time, in physics.
From this text it appears that only a travelling twin brother who turns around and comes home would have aged less. But that is not true. Also if he travels to a distant star and lands on an earth like planet, he will have aged less. In fact, he will age less under any condition, also if the rocket continues to fly and never comes back. — Preceding unsigned comment added by 94.226.95.29 (talk) 06:07, 5 January 2014 (UTC)
La lecture du véritable texte de de Langevin permettra aux lecteurs curieux de passer outre les milliers de présentations tronquées et fausses du véritable problème originel :
Page:Scientia - Vol. X.djvu/58
extraits du texte originel de Paul Langevin :
"Il suffirait pour cela que notre voyageur consente à s’enfermer dans un projectile que la Terre lancerait avec une vitesse suffisamment voisine de celle de la lumière, quoique inférieure, ce qui est physiquement possible, en s’arrangeant pour qu’une rencontre, avec une étoile par exemple, se produise au bout d’une année de la vie du voyageur et le renvoie vers la Terre avec la même vitesse. Revenu à la Terre ayant vieilli de deux ans, il sortira de son arche et trouvera notre globe vieilli de deux cents ans si sa vitesse est restée dans l’intervalle inférieure d’un vingt-millième seulement à la vitesse de la lumière. Les faits expérimentaux les plus sûrement établis de la physique nous permettent d’affirmer qu’il en serait bien ainsi. (...)
Le calcul montre ainsi que chacun d’eux verra vivre l’autre deux cents fois plus lentement qu’à l’ordinaire. Pendant l’année que durera pour lui ce mouvement d’éloignement, l’explorateur ne recevra de la Terre des nouvelles des deux premiers jours après son départ ; pendant cette année il aura vu la Terre accomplir les gestes de deux jours. (...)
Pendant le retour les conditions seront inversées : chacun d’eux verra vivre l’autre d’une vie singulièrement accélérée, deux cents fois plus rapide qu’à l’ordinaire, et pendant l’année que durera pour lui le retour, l’explorateur verra la Terre accomplir les gestes de deux siècles : on conçoit ainsi qu’il la trouve au retour vieillie de deux cents ans. Il la verra d’ailleurs pendant cette période par l’intermédiaire d’ondes qui pour lui seront lumineuses mais qui pour elle appartiendront à l’extrême infra-rouge (,..) Pour qu’il continue à recevoir de la Terre des signaux hertziens, celle-ci devra, après les deux premiers jours et pendant les deux siècles qui suivront, employer une antenne de transmission deux cents fois plus longue que celle du voyageur, quarante mille fois plus longue que celle employée pendant les deux premiers jours.
Pour comprendre la dissymétrie, il faut remarquer que la Terre mettra deux siècles à recevoir les signaux envoyés par l’explorateur pendant son mouvement d’éloignement qui pour lui dure un an : elle le verra vivre pendant ce temps dans son arche d’une vie deux cents fois ralentie ; elle lui verra accomplir les gestes d’un an. Pendant les deux siècles au cours desquels la Terre le verra ainsi s’éloigner, elle devra, pour recevoir les signaux hertziens émis par lui, employer une antenne deux cents fois plus longue que la sienne. À la fin de ces deux siècles parviendra à la Terre la nouvelle de la rencontre du boulet avec l’étoile qui marque le commencement du voyage de retour. L’arrivée du voyageur se produira deux jours après pendant lesquels la Terre le verra vivre deux cents fois plus vite qu’à l’ordinaire, lui verra accomplir les gestes d’une autre année pour le trouver au retour vieilli seulement de deux ans. Pendant ces deux dernières journées, pour recevoir des nouvelles de lui elle devra employer une antenne de réception deux cents fois plus courte que l’antenne du voyageur.
Ainsi la dissymétrie tenant à ce que le voyageur seul a subi, au milieu de son voyage, une accélération qui change le sens de sa vitesse et le ramène au point de départ sur la Terre, se traduit par ce fait que le voyageur voit la Terre s’éloigner et se rapprocher de lui pendant des temps égaux chacun pour lui à un an, tandis que la Terre, prévenue de cette accélération seulement par l’arrivée d’ondes lumineuses, voit le voyageur s’éloigner d’elle pendant deux siècles et revenir pendant deux jours, pendant un temps quarante mille fois plus court.
(Ont été laissés en citation tous les passages concernant l'écoulement du temps et omis ceux concernant les appareillages techniques nécessaires - antennes, énergie etc...L'entièreté du texte est lisible sur Wikisource))
/* Présentation */ simple ajout des extraits principaux du texte originel pour en finir avec toutes les fausses citations interprétations qui empêche le véritable débat d'avoir lieu.
Inventant de fausses anecdotes est intolérable !
The problem consists in the use by Langevin of the gamma function of Einstein equal to 1 over (1 minus root v ² / c ²). At the beginning with a value 100
(v / c = 19999/20, 000 according to the quotation)
But Paul Langevin don't keep that value.
Then Langevin uses a new value gamma = 200.
A new value that needs a new speed (v=.997497 c)
With no explanation.
In the 2nd part of his presentation, this value gamma=200 becomes gamma power 2 = 40000.
Langevin transforms 2 days in 1 year. Then transforms 1 year in 2 centuries.
A strange calculation completely absent in Einstein 's writings about special relativity.
An error calculation based on the absurd belief of an identical Doppler for the fast body and the slow body.
Langevin did not understood that the Terran will see the Voyager 2 days before his return ... one year old .
Langevin did not understood that the Terran ( in two days of earth time ) will see that the Voyager has aged a year in two days.
There is no factor 40000 in this problem . Except in the Langevin's absurd delusions — Preceding unsigned comment added by Yanick Toutain (talk • contribs) 22:37, 5 February 2014 (UTC)
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The following conversation took place more than four years ago:
"...
I agree with the concern of Frack99 about the current sentence in the article "So switching frames is the cause of the difference, not acceleration per se." because as he correctly
argued: ""Switching frames" cannot be done without acceleration, and acceleration causes the accelerated object to switch frames". (Even though in his following argumentation was incorrect). My reasoning: What happened, when the time reading of the outbound clock is transferred to the inbound clock? Photons (in the form of e.g. a radio signal) have been sent from the outbound ship to the inbound ship. When the photons, which obviously have a certain impulse, arrive at the inbound ship they very slightly slow this ship down while they are themselves radically accelerated (direction earth). At least this scenario is not devoid of acceleration since transfer of information (the clock reading), too, means acceleration of at least photons. This should really be looked into even though Max von Laue may have said what is now written in the article.
Felix Tritschler (talk) 17:49, 12 December 2009 (UTC)
It can be arranged that the information photons are sent immediately after the ships have passed each other. That way the inbound ship is slightly sped up.
So, in order not to be bothered by such quibbles, we imagine that the photons are sent exactly while the ships pass each other. This way they hit the ship orthogonally to the direction of relative motion and there is no effect on the inbound component of the inbound ship's velocity. DVdm (talk) 19:29, 12 December 2009 (UTC)
...
"
Back then I was stunned by your answer and did not reply accurately. So here are my thoughts, more than four years later (not that it took me four years):
The direction of acceleration of the incoming ship by the photons is irrelevant because it (the ship) never left earth.
What, however, did leave earth and returned to it is the mass that was lost from the outbound ship when it emitted the signals and that was gained by the incoming ship when it was receiving those signals. And this mass was always accelerated towards earth and always yielded exactly the delta v of the ships, no matter if the signals were sent before, during or after the rendezvous.
There is no twin paradox without acceleration, period.
Please let me know your thoughts, DVdm, I am curious.
This is Wikipedia and not the DMV, right? I hoped to get a short comment from you, DVdm, even if it does not lead to a change in the article. I am quite sure that it will be interesting for other readers. I know this does not comply with the Wikipedia policy so feel free to remove it. --Felix Tritschler (talk) 19:45, 18 April 2014 (UTC)
The first paragraph of the article says "one twin is accelerated more than the other. Therefore the Twin paradox is not a paradox in the sense of a logical contradiction." This idea, which is repeated in various places in the article, is not correct, because, as stated later in the article (under "Transfer of clock reading in a twin paradox trip") acceleration has nothing to do with the effect. The essential point is the proper-time length of the traveller's trajectory compared to that of the twin who stays at home. Unless I hear any objections, I will make the appropriate edits in the next few days. Dark Formal (talk) 20:00, 11 May 2014 (UTC)
But "the proper-time length of the traveller's trajectory compared to that of the twin who stays at home" can be attributed to the fact that "one twin is accelerated more than the other". That is also explained in the article, and everything is properly sourced. See also the section Twin paradox#Difference in elapsed times: how to calculate it from the ship, where the proper-time length of the traveller's trajectory is a given, whereas the coordinate time of the trip is calculated from the proper acceleration of the traveller. Furthermore, note that the article should be accessible for people who understand the concept of acceleration, but perhaps not (yet) that of proper-time, so mentioning acceleration first might be more appropriate. I don't think this article needs to be modified along the line you have in mind. - DVdm (talk) 20:25, 11 May 2014 (UTC)
Dark Formal: I just want to bring the article in to line with the standard consensus, which is given in Ref [4] http://dx.doi.org/10.1119/1.18252, which says that the acceleration is not the important difference. Right now there are incorrect statements, such as the caption of the first figure which says that the inequivalence of the twins is "the ship experiences additional acceleration due to changes in the direction of travel". This is misleading, and needs to be corrected. I agree that we don't want to fill up the article with jargon about proper time. I just mentioned that for the experts who inhabit the Talk page. Dark Formal (talk) 21:22, 12 May 2014 (UTC). The fact that physical acceleration plays no explanatory role is made clear by the diagram below By adding an arbitrarily small journey (B,C,D) to the stay-at-home twin's trajectory, we can give him exactly the same accelerations as those felt by the travelling twin (at A,T,E)., versions of which can be found in the literature on the subject (e.g. Fig 11 of "Philosophy of Physics: Space and Time" by Tim Maudlin ISBN-13: 978-0691143095). I plan to add a diagram like this to the article. Dark Formal (talk) 00:07, 13 May 2014 (UTC)
"p. 390: Finally, the conventionality approach to the twins’ differential aging can also be used to illustrate that discussions which try to pin the age difference to the direction-reversing acceleration are misconceived. Recently Boughn has stressed this point by considering the case of two identically accelerated twins who nevertheless age differently as a result. [...] p. 391: It remains true, of course, that without acceleration, then in Minkowski space-time at any rate, it is impossible to have twice intersecting trajectories so as to formulate the twin paradox with the twins starting and finishing in spatial coincidence. So, in this sense, acceleration is an essential ingredient in understanding the twin paradox. It may be noted however that even this role for acceleration can be eliminated in formulations of the twin paradox in curved spacetime, where the twins can fall freely along space-time geodesics between successive meetings."
So we have to be careful in what we call "twin paradox": It's true that the amount of time difference only depends on the length of the worldlines, not on acceleration per se (as seen in the three-brother-scenario or in your image on the right). However, regarding the standard scenario where one twin returns and the other one stays at home, the chosen acceleration profile clearly plays a fundamental role in explaining why the twins are travelling on different paths in spacetime. --D.H (talk) 09:57, 13 May 2014 (UTC)
Also note that in the figure on the right, in the two cases the accelerations as functions of proper time are not identical. The "outer" path of the traveller has acceleration "peaks" more or less like this:
| A T E
| _|________________________________________________|_____
| |
| |
whereas the "inner" path has of the modified stay-at-home twin has something like this:
| B C D
| ___________________|___________|________________________
| |
| |
So it is certainly not true that "by adding an arbitrarily small journey to the stay-at-home twin's trajectory, we can give him exactly the same accelerations as those felt by the travelling twin." These are pretty different acceleration functions. As the velocity function completely determines the elapsed time, and as the acceleration function completely determines the velocity function, it follows that the acceleration function completely determines the elapsed time. - DVdm (talk) 21:56, 13 May 2014 (UTC)
Dark Formal: You are exactly right when you say "the amount of time difference only depends on the length of the worldlines, not on acceleration per se".
I just want to modify the article to make this clear.
For example, the statement that the resolution of the paradox is "the ship experiences additional acceleration due to changes in the direction of travel" is misleading, because it talks about the experience of acceleration, which we agree is irrelevant.
As your quote from the Debs and Redhead article says, "discussions which try to pin the age difference to the direction-reversing acceleration are misconceived". If you are OK with it, I will make modest changes to the article in this direction. Dark Formal (talk) 00:31, 14 May 2014 (UTC)
The experience of acceleration is tangential. The "three brothers" example makes that clear. It is straight forward to design a thought experiment in which both twins experience an identical magnitude of acceleration, but in which their final proper times when they meet again are very different. Acceleration, without saying what about it does the trick, is not good at creating intuition about why it introduces this particular asymmetric effect. Use of inertial frames, and in particular how simultaneity differs between them, makes a lot more sense. Differences in velocities associated with differences in worldline locations, rather than acceleration, is far more directly connected to the matter than acceleration itself. —Quondum06:12, 14 May 2014 (UTC)
But during a trip, over which the total time is integrated, the "magnitude of acceleration" is a function of time. Can you design a thought experiment in which both twins experience an identical magnitude of acceleration as a function of time, but with different final proper times? In the above example they clearly do not share the same experience, even if the magnitudes are identical during a few phases of their existence. - DVdm (talk) 07:24, 14 May 2014 (UTC)
I was thinking of a situation in which each is accelerated in a circle, only with different radii. Obviously the vector of acceleration differs as a function of time. Clearly acceleration, as a function of time, determines velocity, which determines position (with suitable boundary conditions). Similarly, jerk, or any of a number of quantities from which position and velocity can be recovered, would work. Considered as a function of time, velocity is a far more direct determiner of the proper time difference than is acceleration: one does not need ever to differentiate velocity in the calculation, one only needs to integrate an expression involving it. Infinite acceleration fits into the formula by not featuring. The only intuitive value of acceleration is in pointing out an asymmetry in the specific case of when one twin experiences no acceleration in flat space. When the endpoints of the comparison are neither in the same place nor at the same velocity, comparison of proper time is meaningless (up to a margin), again showing that it is position and velocity which are the primary determining factors. —Quondum13:12, 14 May 2014 (UTC)
Sure, but let's not include that misleading figure with its caption in the article. Whatever we change here, let's make sure it is properly sourced, and preferably in more than one secondary source. - DVdm (talk) 13:37, 14 May 2014 (UTC)
Oh, I'd missed the intention to include it, and thought it was simply part of the discussion. I agree that the diagram above should not be included; while it illustrates a point, it is not a point we need in the article particularly. However, de-emphasizing the role of acceleration in the article may be reasonable. —Quondum21:50, 14 May 2014 (UTC)
Dark Formal: We already agree that the crucial thing is the length of the spacetime trajectories. As the diagram I showed makes clear, the amount of acceleration experienced is not the essential difference. What is relevant is the amount of space-time trajectory between accelerations. Right now there are several sentences in the article that obscure that point and make it sound as if the presence or absence of acceleration is significent. That, as we all agree, is misleading. I'm planning to correct it. I hope that's OK. Dark Formal (talk) 22:03, 14 May 2014 (UTC)
Do not forget to include reliable sources supporting this. If I know my literature on this topic, there are quite a few sources that state that acceleration is essential, and you can rely on people challenging the result found here sooner or later. Whatever we may think of that, we report what the sources say, not what we deem correct. Paradoctor (talk) 22:24, 14 May 2014 (UTC)
Indeed, there is a massive amount of literature that says that the presence or absence of acceleration is significant. And of course we do not all agree that this is misleading, so your plans might not be not OK. - DVdm (talk) 05:59, 15 May 2014 (UTC)
Dark Formal: Well, DVdm, you're the one who said correctly that the crucial thing is the length of the spacetime trajectories. Do you then agree that that what is relevant is the amount of space-time trajectory between accelerations rather than the magnitudes and directions of the accelerations themselves (which as the diagram showed, can be the same for both twins)? That's the point I want to article to reflect. Concerning Paradoctor's point about sources: there are sources that analyze the "acceleration is crucial" claim and refute it, and as far as I know there aren't any of the acceleration-believers who have come back with a response to the critique. Mostly they just seem to be ignorant of it. If you can find an example of a counter-refutation, that could change the way we present the issue. Dark Formal (talk) 21:29, 15 May 2014 (UTC)
If you can paraphrase a reliable source that gives a clear analysis of this, it would be great. Simply synthesizing something, no matter how sensible, is clearly not going to fly. So here perhaps discuss the content of specific sources that present the discussion in a way that we all feel fits into the WP guidelines? —Quondum22:19, 15 May 2014 (UTC)
Dark Formal, all I'm saying is that, whatever is being said, it will be contentious, and it will need to be cited. You can state that there are authors who say acceleration is not essential if you cite them, and you can say that their argument has not been challenged if you can find a reliable source for this claim. Sources, sources, sources. Paradoctor (talk) 05:02, 16 May 2014 (UTC)
But of course the crucial thing is the length of the spacetime trajectories, because that "length" is exactly the same thing as the elapsed time on the clocks who live along these trajectories. So saying something along the lines of "the crucial thing that causes the elapsed times to be different, is the fact that the lengths of the spacetime trajectories are different", is saying that "the crucial thing that causes the elapsed times to be different, is the fact that the elapsed times are different." The crucial thing is the difference in the acceleration histories of the twins, and as far as I can see that is covered in the article. If it isn't, perhaps we can just add a word or two here or there to fix that. - DVdm (talk) 09:44, 16 May 2014 (UTC)
Dark Formal: OK, I will have a go at it. I will try to make it non-contentious and of course I will cite sources. Actually, two of the existing references already make this point, so it won't be hard. Dark Formal (talk) 14:07, 16 May 2014 (UTC)
Dark Formal: I have made the simplest revisions. The next issue is that the sections "Resolution of the paradox in special relativity" and "Transfer of clock reading in a twin paradox trip" contradict each other about the role of acceleration. It would be nice if we could combine them into a single section that gives a coherent (well sourced) explanation. I'll do that if there are no objections. Dark Formal (talk) 15:02, 16 May 2014 (UTC)
Looks good so far. As far as combining goes, I'll have some tea. As a word of advice: A coherent explanation would be nice. The problem is that coherence is not a hallmark of the literature when it comes to paradoxes, and the twin paradox is certainly not an exception. Your safest bet is to find a solid source (e. g. a textbook on relativity) containing such a coherent explanation, and then to summarize it here. Otherwise you run the risk of producing a synthesis. That's an awfully easy mistake to make. Paradoctor (talk) 17:58, 16 May 2014 (UTC)
I noticed this addition. Looks good, but the Redhead wp:primary source is referenced four times now and probably gets serious wp:undue weight. As far as I can see, it's seldomly referenced by others, and then only by philosophers. I don't have access to it, but I'm skeptic. Can we avoid it altogether and have something more solid, and less recent? - DVdm (talk) 20:43, 19 May 2014 (UTC)
Dark Formal: Two of the citations to Debs and Redhead are just because they had a nice pithy quote I could use. One is for relativistic Doppler which could easily be changed to cite someone else. I'm not sure if Debs and Redhead is a primary source: American Journal of Physics is not a research journal, it's a review journal aimed at college and university physics teachers/students. I think that makes it a secondary source. Dark Formal (talk) 00:40, 20 May 2014 (UTC) By the way, how do I wikify a reference to a different section of the same article? Dark Formal (talk) 00:48, 20 May 2014 (UTC)
AJP is not a review journal: "The Journal is particularly interested in manuscripts that can be used to bring contemporary research in physics and related fields into the classroom. Such manuscripts should not be review articles, but rather self-contained articles that describe a particular piece of research in such a way that it is accessible to as many physicists as possible." [1]
Dark Formal: Ha! Thanks for pointing that out. It raises interesting questions about what counts as a review. They claim not to be a review journal, and then immediately give an article description that sounds much like certain types of review article. Compare the self-description of the Colloquia section of Reviews of Modern Physics, which says "RMP Colloquia describe recent research of interest to a broad audience of physicists. Their aim is to... communicate to one's colleagues, including junior graduate students, research results at the frontier and offer new insights into concepts that link many different subfields of physics" [[2]]. Dark Formal (talk) 01:06, 21 May 2014 (UTC)
The paper presents original research: "This paper attempts to provide an approach to the twin paradox, suggested by one of us" [...] "Having discussed a new approach" (my emphases). Whether it is to be considered a primary or secondary source depends on what you use it for. For the Doppler formula it would be a secondary source.
Dark Formal: Yes. Luckily we are not citing them for their "new approach" (which shows up as a footnote in the Stanford Encyclopedia article on Conventionality of Simultaneity). I think the things we cite them for are items that they are just reviewing from the wider literature. If I find anything that is more secondary than D&R then I will cite it. Dark Formal (talk) 01:06, 21 May 2014 (UTC)
I have read the paper now. Indeed, we probably shouldn't use this source to mention—in the context of this article—"conventionality of simultaneity". I agree that the source's usage is appropriate for the other refs, so I don't think there's a need for more secondaries. - DVdm (talk) 06:49, 21 May 2014 (UTC)
Linking to a section is done exactly as in HTML: [[#Acceleration_is_not_the_explanation]]. To find out the section's target name, hover your pointer over the section title in the "Contents" box, it should be displayed somewhere. If not, just click, and you can see it in the page address field. Most of the time, replacing spaces with underscores is all that's needed. More at Help:Link. Paradoctor (talk) 18:56, 20 May 2014 (UTC)
Specific example assumes Lorentz contraction of space
I thought lorentz contraction is applicable only for particles. Is it applicable to distance/space too?
"The ship's crew members also calculate the particulars of their trip from their perspective. They know that the distant star system and the Earth are moving relative to the ship at speed v during the trip. In their rest frame the distance between the Earth and the star system is εd = 0.6d = 2.4 light years (length contraction), for both the outward and return journeys. Each half of the journey takes 2.4/v = 3 years, and the round trip takes 2 × 3 = 6 years. Their calculations show that they will arrive home having aged 6 years. The travelers' final calculation is in complete agreement with the calculations of those on Earth, though they experience the trip quite differently from those who stay at home." — Preceding unsigned comment added by Ajaynarewade (talk • contribs) 09:37, 1 August 2014 (UTC)
Welcome to Wikipedia. Length contraction is meaningless for point particles, zero length is difficult to reduce further. If you have further questions not related to improving the article, you can ask the friendly folk at the WP:HELPDESK. Paradoctor (talk) 12:19, 1 August 2014 (UTC)
the relativistic Doppler shift VS Constant Speed of Light
The "what it looks like: the relativistic Doppler shift" section illustrates the process of the twins observing each other using video signal. I don't think it's absolutely correct.
Firstly, it's just an illustration based on the principle of time dilation, but doesn't explain why when the moving twin reaches the destination, he ages 3 years but the staying one ages 5.
Secondly, and most importantly, this illustration doesn't take the Constancy of Light Speed into consideration. Assuming the twins communicate with each other using light pulses, the image of the "1 year older staying brother" is sent from the earth when the moving brother is at 1/5 of the journey, that is 0.8 light years away. Here, if we subscribe to the Constancy of Light Speed, and include the effect of length contraction, this image should be received before the moving twin reaches the midpoint the journey.
Thirdly, I think the illustration of twins using signals to see each other doesn't help explain the paradox, but rather complicates it. We assume a GOD observes both twins and can see their changes without delay (here we assume the GOD sees things without the need of light transmission). Then, there's no reason the GOD should see them age at different rates before the moving twin changes direction. If the inertial frame change is to blame for their difference in aging, the GOD would see the moving twin get younger suddenly at the turn of direction.
In the section Relativity of Simultaneity it is stated that from the traveling twin's point of view, there would be a "jump discontinuity" in the traveling twin's view of the time elapsed for the stationary twin at the point where he turns around. However, this does not fit the facts given in the Specific Example section. There it is stated when the traveling twin is traveling away, each one sees the time elapsed for the other transpiring at 1/3 the normal rate, and when the traveling twin is returning, they each see each other's time elapsing at 3X the normal rate. When the traveling twin reaches the turnaround point, he has experienced 3 years of travel time, and sees 1 year having transpired for the stationary twin. The stationary twin does not see the turnaround until 9 years into the trip according to his own clock, and he see 3 years having transpired for the traveling twin. On the return journey, the traveling twin experiences 3 years on his own clock and 9 years on the stationary twin's clock. The stationary twin experiences 1 year on his own clock and 3 years on the traveling twin's clock. Each twin believes that 10 years have elapsed on the stationary twin's clock and 6 years have elapsed on the traveling twin's clock; they just have different opinions about the time on their own clock when the turnaround occurred. There was no discontinuity from the traveling twin's point of view of the stationary twin's passage of time. — Preceding unsigned comment added by Skaphan (talk • contribs) 01:29, 29 November 2014 (UTC)
Please sign your talk page messages with four tildes (~~~~). Thanks.
The Specific example section is about what they actually see, whereas the Relativity of simultaneity section is about what they would calculate or measure. The problem is with the word "view" in your sentence:
... there would be a "jump discontinuity" in the traveling twin's view of the time elapsed for the stationary twin at the point where he turns around.
That should be:
... there would be a "jump discontinuity" in the traveling twin's calculation or measurement of the time elapsed for the stationary twin at the point where he turns around.
The section carefully takes this into account by stating that if he recalculates, he will measure... and that there is a jump discontinuity in the age of the Earth-based twin. If it had been written that the traveling twin's view would show a jump, then you'd have a good point and we'd have to make a little change in the text. DVdm (talk) 11:31, 29 November 2014 (UTC)
Maybe I am misinterpreting what you mean by "jump discontinuity"? If each twin sends a train of identifiable clock pulses every minute to the other twin for the duration of the trip, certainly there would be a discontinuity in the observed clock rate of the other twin at the turnaround point (from 1/3 normal to 3X normal). But I don't see that there would be any gaps in the series of clock pulses as observed by either twin. At the turnaround point the traveling twin will see 3 years elapsed on his own clock and 1 year elapsed on the transmitted clock of the stationary twin. When he (instantaneously) starts the return trip that will not have changed. During his 3 year return voyage he will observe 9 years transpiring on the stationary twin's clock, starting at a reading of 1 year elapsed and ending at a reading of 10 years. That is all consistent with the example given. If the traveling twin could not directly observe the stationary twin's transmitted clock, he could still "calculate" to figure out what it would have been if he could have observed it directly, and if he did it right, it would be the same. 63.228.100.82 (talk) 17:26, 30 November 2014 (UTC)
Per our wp:talk page guidelines, this is really not the place to explain to each other what we mean by certain phrases and concepts, or discuss our personal interpretations of the sources. Five sources have been provided for this particular part, so for a better understanding perhaps you should consult the cited sources. Good luck and happy reading! - DVdm (talk) 17:38, 30 November 2014 (UTC)
making the logical leap
This is not the place to discuss the subject. Proposals to make changes to the article are welcome, provided everything can be traced back to reliable sources. See wp:Talk page guidelines
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.
I am a newbie to this subject, and (shame, shame) I have not yet read the entire article. But from what I've seen here and at Time_dilation#Simple_inference_of_time_dilation_due_to_relative_velocity, I came up with an "explanation" that seems to satisfy my skeptical family members. FWIW, here it is:
The famous Twin Paradox is where a travelling twin ages slower than the stationary twin. (And a moving cesium clock ticks more slowly than a stationary clock.) The "paradox" follows from an experimental fact that matter (i.e. protons and electrons) can not move faster than "c", the speed of light in space. You might be able to convince yourself by the argument that if something could go faster than light, then light would too (which is a contradiction).
Here is the way I have finally been able to make the logical leap from that fact to an actual explanation of the "paradox" (and the "nature of space-time"). Suppose we are in a spaceship, travelling at speed 1.0c. Then we are frozen, because any forward or sideways movement we make would require a net speed faster than c. Similarly, all the electrons orbiting all the atoms of our bodies are also frozen, as is the cesium atom in our atomic clock. We can't experience anything, not even aging, because everything is stopped, except for the fixed motion of the spaceship. And when we get where we're going, it seems (to us) as if the trip took no time and the distance was zero.
If the ship slows to 0.8c, our cesium clock starts ticking again, but the net motion of the electrons is still limited by the speed of light, so the clock has to tick a little slower than the stationary clocks at home. It can be demonstrated, mathematically, that the actual factor by which tick intervals elongate (dilate) at that speed is 1.67, which is given by a formula called the Lorentz factor. So time is moving slowly, but so are we, so we aren't aware of it. If our destination is 4 light-years from where we started, it takes us 4/(0.8c) = 5 years to get there (according to the stationary clocks back at home). But our clock has ticked off only 5/1.67 = 3 years in that amount of time. That explains the twin paradox.
And more generally, it explains the nature of space-time, which is that when we move in space, we move less fast through time. They are connected because of the speed limit on electrons, and periodic motions like that are how we ultimately measure and experience time.
Quite an intuitive explanation. I'll pick one nit, though: "when we move in space, we move less fast through time" is a misleading way to express it. To think of us as "moving through time" is incorrect, since it implies an external measure of time against which to calibrate/measure time. Rather, our measure of proper time along a straight line between two events (the traveler's measure of time) is least of all measurements along the time coordinate of other inertial observers (e.g. the stationary twin). —Quondum01:35, 7 April 2015 (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.
The "Specific Example" section does not relate to the subject of the article
There is a bit of a problem of definition going on: the Twin Paradox is that there is a difference between what the twins would observe and what actually happens.
The fact that they age differently is not a paradox, it's that they BOTH observe the other aging slower but then their ages diverge.
Some more thought on WWoods diagrams in archives 11 and 12 and 13
I made this Twin Paradox Explanation Video today, which features WWoods diagrams and an idea of forming two different versions of a "noninertial reference frame" for the traveling twin. One of these versions uses lines of simultaneity from the turn-around event.... Another version uses the speed-of-light-line from earth to the turn-around event. JDoolin (talk) 01:28, 28 November 2015 (UTC)
Alas, per wp:talk page guidelines, article talk pages are for discussions about improving the article, not about the subject, and certainly not about our own work. As "a new of way of thinking about the twin paradox", that link is pure wp:original research and therefore entirely useless on Wikipedia, both in articles and on article talk pages. As soon as your new way of thinking is quoted and discussed in the literature in relevant reliable wp:secondary sources, that might change. Some five years ago we had a little exchange about that on your talk page: see User talk:JDoolin#External link to your site. - DVdm (talk) 10:18, 28 November 2015 (UTC)
Alas is right. Are there any journals that I might try submitting my idea to? It seems like this is a subject that should be simple enough to figure out, and deserves more thorough peer-review leading to an actual agreement, rather than 14 pages of archived argument on the Wikipedia page. JDoolin (talk) 13:29, 28 November 2015 (UTC)
I've made this animation based on WWoods idea. In this, you can watch the Lorentz Transformation of the Minkowski diagram take place during launch, turnaround, and landing. This has events of simultaneity with event E, as defined by WWoods, events A, B, C. But it also includes some other events that are lined up with the past and/or future light-cones of events O, E, D. In my own opinion, events F and G will be more significant to the traveling twin than events A, B, C. Event F represents the events going on, on earth, which the traveling twin should be able to watch, live, while he is on the space-station. Event G represents the event where earth can watch the turnaround live. JDoolin (talk) 16:09, 13 December 2015 (UTC)
While I may have misunderstood DVdm's suggestion of providing a fourth frame here, I think he meant to show a Loedel diagram alongside the three Minkowski diagrams from the three most-relevant inertial reference frames. I took the liberty of modifying this to show, instead, an in-between-velocity Minkowski diagram. It might be worthwhile to highlight the differences between the construction and use of a Loedel diagram and the construction of a Lorentz-Transformed Minkowski diagram. If so, you may want to revert back to your earlier image to make some kind of clear distinction like that. JDoolin (talk) 16:09, 13 December 2015 (UTC)
JDoolin (talk) 16:09, 13 December 2015 (UTC)
You changed an image that was pointed to in archved talk pages, rendering these threads useless. It looks like you have completely misunderstood these threads. I have reverted the image to the original. Please stop abusing article talk pages to present your private work. You were warned about this before, so I have left another warning on your talk page. - DVdm (talk) 16:43, 13 December 2015 (UTC)
My apologies DVdm. I incorrectly thought your image was a Loedel diagram. Now I believe what you have done is rotated the image 45 degrees to the clockwise, contracted the x-axis by a factor of about 1.1, stretched the y-axis by a factor of 1.1, and then rotated it back counter-clockwise by 45 degrees. That's a nice technique, and in fact, represents a true and accurate Lorentz Transformation of the coordinates of events. (definitely NOT a Loedel diagram.) However, your construction has the problem that the t-axis and the x-axis are skewed along with the coordinates of the events. That should NOT happen. JDoolin (talk) 23:38, 13 December 2015 (UTC)
It looks like you haven't understood what the archived talk threads were about. I have updated the image in a way that the archived talk threads remain in accordance with the image: I added axes and axis labels: green (x,t) for inertial twin frame, red (x',t') for outbound twin frame, blue (x",t") for inbound twin frame. - DVdm (talk) 08:59, 14 December 2015 (UTC)
I took your axes and axes labels as given but placed them all with origin at event E. (I also removed worldline XYZ, events F, G, the shading, and left event E stationary within the diagram.) Check if this animation represents something closer to your desired in-between-frames representation that you have in mind. JDoolin (talk) 20:25, 14 December 2015 (UTC)
Sure, but I don't see why any of this should be important. On this article talk page we are supposed to discuss the article, not some animation that you have created, based on some archived part of the talk page. - DVdm (talk) 21:08, 14 December 2015 (UTC)
Needs better explanation
I have read through a few archives of talk pages and the article, and am still scratching my head. It looks like you guys only want experts to weigh in, so you may ignore me if you wish. However, I think someone needs to explain things in a little more detail.
For example, I see that for the simplicity of calculation, acceleration is left out of the subject. However, I'm fairly sure it is relevant. When the twin leaves the planet, after accelerating, they will both appear to the other to slow down. That is clear. However, while accelerating, the guy on the ship can look at a giant clock on earth (or its rotation maybe) will be able to calculate that time on earth has moved faster than normal. after accelerating it slows down.
This is extremely important in the "turn around" part of the journey. If you leave it out and say a passing space ship copies the twins clock, well the passing ship is already in a different inertial frame. That is very important. I don't think that's clearly illustrated here.
Suppose the traveling twin is on a free return trajectory around a black hole (or neutron star maybe). Yes, I know that is gravitation, not velocity, but the twin will see time catch up and move forward as he swings past the black hole. What I am trying to verify is that this would also happen under self-powered acceleration. I am not asking for you to explain it to me in talk, I know that's not the purpose here, I am suggesting that the article address that if there is a source available to cite.
Acceleration is fully taken into account in the section Twin paradox#Difference in elapsed times: how to calculate it from the ship. At the end of the section, the relevant standard example with a traveller experiencing constant self-powered (proper) acceleration phases, successively given by a, −a, −a, and a. This models the traveller departing with speed 0, accelerating until half-way, then decelerating to standstil at the turnaround point, and starting the entire return trip in a similar way in the opposite direction. If you follow the cited source, you'll find some more interesting examples, including one with "infinite" (aka "immediate") accelerations from speed 0 to v (modelled with a delta-dirac functional), which is of course the standard "triangular" case. - DVdm (talk) 06:40, 22 August 2016 (UTC)
Article needs reorganization
In principle, a wiki article needs to be able to address the needs of everybody from a middle school student needing a brief introduction to a subject, to a college student who wouldn't dare presume to think that Wikipedia could be a source (horrors!), but still needs a jumping-off platform to guide him/her to interesting directions of research.
Unfortunately, this article displays a considerable amount of bloat from the well-meaning efforts of dozens of contributors over the nearly 15 years of its existence. Even the lede (which ideally should serve as a "gentle introduction" to a topic) presents considerable difficulties to an uninitiated reader.
By way of illustration, compare the current lede with the earliest version of this article, which is about the same length as the current lede. Which is more understandable?
The main body of the article is a mixed bag of essay-style writing and encyclopedic contributions requiring varying degrees of mathematical sophistication. Equal weight is provided to deservedly obscure interpretations of little general interest, such as the "Explanation in terms of Mach's principle". The result is an article that a typical reader has no hope of wading through.
Years ago, I faced similar chaos in the Quadratic equation article. I made a bold reorganization by introducing an "Advanced topics" section so that the first 2/3 of the article would only cover material that the a typical student would be interested in, while shuttling off to the end the advanced stuff (which I am sure represented labors of love on the part of the editors who spent many hours of effort writing about these specialized subjects).
I think that the article is in reasonably good shape, and I'm afraid that revamping it might open a Pandora's box. I would not be in favour of what you have in mind.- DVdm (talk) 08:23, 23 September 2016 (UTC)
Request for comment on neutrality of statement about paradoxical symmetrical ageing
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.
Text was already adjusted mid-discussion ("incorrect and naive" in particular) to address concerns and to better reflect references. Feel free to start a follow-up discussion if further concerns. - jc3704:06, 22 November 2016 (UTC)
I believe that the statement in the first paragraph of this article '... according to an incorrect naive application of time dilation and the principle of relativity ...' does not exhibit the neutrality expected from WP articles. Accordingly, I edited it to make it neutral. This edit has been reverted by the moderator. May I please have opinion, not on the correctness or otherwise of the application referred to, but on the appropriateness of such a biased statement in a WP article.Ed Addis (talk) 10:10, 25 September 2016 (UTC)Ed Addis, 25/09/2016
Naïve wording is improper - the problem is mostly that it lacks fidelity to the sources so fails WP:V. It is also not following WP:LEAD, is off WP:TOPIC, and could be better written WP:WBA.
It fails WP:V as this is simply not the wording they use or even hint at so seems adding WP:EDITORIAL that is out of order. Looking at the cites I do not even 'incorrect' as common. By simple Google, I see a 650K count of 'twin paradox', with only 126K (one-fifth) even containing 'incorrect', and only 11K (one-sixtieth) containing 'naïve' -- and looking closer, those hits seem almost totally copies from this Wikipedia entry.
It is not following WP:LEAD direction to provide summary of the article and its most important parts, since the topic and article do not involve Naivety. A label relating to moral idealism just doesn't fit to this, and the article below the lead goes into many things but not the alternative views. The lead shouldn't really have or need cites if it's summarizing the article, so having 5 cites on one line smells funny, like WP:CITEKILL.
It is somewhat off WP:TOPIC as the article is about thought experiment and the lesson in inertial frames, rather than about the alternative confusions. They might deserve a small mention in the body during clarification of points (which sms not that clear in the article) but seems just not the main topic of the article. There are many confusions and wrong answers possible and if there's a reason this particular one is key, that is not presented in the article.
The writing is a bit tangled. The 'incorrect naive' is a weird compound and just not necessary so drop it for ease of reading. The whole line could be simplified in length and comma clauses from the 40 words, 4 commas
"This result appears puzzling because each twin sees the other twin as moving, and so, according to an incorrect naive[1][2][3][4][5] application of time dilation and the principle of relativity, each should paradoxically find the other to have aged more slowly." seems more cleanly said as
"This result is an apparent paradox in time dilation and relativity because each twin sees the other as moving, so each might expect the other to have aged more slowly."
The rest of the paragraph could use some work too, since it calls it a paradox then says it is not, which might be better handled by putting the 'not-a-paradox' line at the top rather than the bottom, and saying it 'this is in apparent paradox but not an actual conflict in logic'? That's all I can think of right now, ope it helps hMarkbassett (talk) 00:45, 27 September 2016 (UTC)
An overwhelming scientific consensus agrees that an incorrect and naive interpretation of the postulates directly leads to the twin paradox, so that is exactly what Wikipedia should report. Removing mentions of essential incorrectness and naivity would entirely destroy what the article is about. - DVdm (talk) 07:44, 27 September 2016 (UTC)
DVdm - the overwhelming scientific discussions have nothing to do with naive. Do not use the word, are not discussing the concept, and neither is the article. The 'incorrect naive' is incorrect. Even saying 'incorrect' is a bit off topic of the paradox itself, it's an opinion remark and talking about one possible sidetrack, when the normal practice is busy with stating and explaining the answer -- the topic that the article focus is on.
Otherwise, note what one sees from search on this topic. Trying Bing for variation, first hit is Wikipedia, then about says it as two paradoxes, one paradox that the traveler age has two answers (e.g. born 90 years ago yet is 25 years in physical age,) and the more important paradox of the differing the twins views of each other contradicting (the point of symmetry that the travelling twin ages slower). That's the "twin paradox" where the article focus is and it could/should be working to do a better job. Needing to be better explained (or the point of 'twin paradox' being TO better explain) was also at the third Bing hit (Scientific American - with better description of the clocks involved). I also see that the twin paradox is in recent times still getting variations and different resolution methods, mentions for simultaneity, and puzzled folks.
Regardless, you've got my RFC input and explanation that it's from 'naive' is not part of the cites and 'incorrect' is an unnecessary small sidetrack. If you need me to explain my input further, post below and otherwise cheers. Markbassett (talk) 16:11, 27 September 2016 (UTC)
In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more. 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.
This way the opening sentence perfectly reflects the documented standing scientific consensus, which is what Wikipedia is supposed to do. Pulling apart the words also avoids the current slightly possible wp:SYNTHy combination of the phrase "incorrect naive application". - DVdm (talk) 20:06, 27 September 2016 (UTC)
Comment Storm in teacup, threatens to create wall-of-text altercation about items that should be easily resolvable. This is not my line, but the matter should be settled by
1) (In view of the disagreement, careful and punctilious, but not ridiculously long lists of) citations of authoritative sources that satisfy all parties (there are enough SR and even GR sources to satisfy all reasonable parties, goodness knows)
2) In resolving disagreements about acceptable sources, Einstein or Feynman trump Scientific American, which in turn trumps Fox News
3) Aiming for clear expression of concepts relevant to the topic, using wording derived as directly as reasonable from cited sources.
4) Where evaluative language (such as "naive") is used, then if there is disagreement, it must come from agreed sources, not from editor interpretations.
5) Aiming for a coherent and lucid logical structure for the article
All those points (and more, but more should be unnecessary) have been implied in the foregoing, because they are long-standing and standard. The question is how to resolve disagreements about the best competing texts. If such a disagreement does not settle itself pretty soon, then the original text should be the default choice.
Naïve is proper and apt. First of all, per DVdm above, the word 'naïve' is fully supported by reliable sources. The objection to the word may simply reflect a misunderstanding of its usage in technical fields. Using the word 'naïve' here is not making a judgment about the people that hold such ideas, it's simply descriptive of 'the first way you might think of approaching this', as opposed to a proper solution with a full understanding of the problem.
Even if the word didn't appear in the relevant literature (as it does), I would consider it a perfectly neutral and apt term in that context, especially as Wikipedia allows paraphrasing from reliable sources, so your argument about editorializing falls flat for me.
Changing what I see as a perfectly reasonable term simply because it rubs you the wrong way gets a bit closer to censorship than I'd feel comfortable with. Arathald (talk) 03:45, 13 October 2016 (UTC)
Delete the word "naive". I don't read the sources stating the words "naive incorrect application", the 3 words seem to be assembled from multiple sources. I think the 3 words together are technically alright, but stating "naive incorrect" is redundant and doesn't add any value or clarity to the next, and seems to disparage people new to the subject.CuriousMind01 (talk) 11:14, 13 October 2016 (UTC)
Good point. I have reworded as proposed above: [3], keeping both naive and incorrect. This way the words are separately mentioned and sourced. - DVdm (talk) 11:35, 13 October 2016 (UTC)
Bad lead. I think it's ok to discuss the naive and incorrect application, but why can't we discuss the actual matter properly first? Its very confusing and not helpful to open with an incorrect explanation. Consider if magnet opened up with a discussion of how magnets don't work via sympathetic connections- I'd hope most of us would reorder things. Lots of things known as "paradox" are not true paradoxes. For what it's worth, I always heard the term "twin paradox" referring to the correct thought experiment of two twins attaining different ages via high speed travel and return of one. It's not truly a paradox, but it seems as though it might be, much like e.g. Banach-Tarski paradox. SemanticMantis (talk) 13:58, 18 October 2016 (UTC)
Ha, but there are two ways to understand the term "paradox": really contradictory, or just seemingly contradictory. Anyway, as the relevant literature clearly states, an incorrect and naive application of the prinicples directly leads to a situation/statement/delusion that is universally called the "Twin paradox". This is just the essence of the matter, and thus it ends up reflected in the lead of the article, as it probably should per WP:LEADSENTENCE. If, as you perhaps might suggest, the lead should reflect a correct and informed application of the prinicples, then the article should be renamed to something along the lines of differential aging in relativity. That term exists in the literature, but it is not nearly as common, and I don't think we're going to rename the article. People obviously tend to focus on the error. - DVdm (talk) 14:58, 18 October 2016 (UTC)
But the paradox does have a role in SR. The lead does not discuss mistakes people make in writing about it. It just states what it is: a persistent mistake—see the sources. That is the whole point. - DVdm (talk) 18:49, 18 October 2016 (UTC)
The wording "incorrect naive" is technically correct, but excessive, over the top, unjustified by the context. "Mistaken" would be better. And the five refences attached to those two words are unnecessary. I haven't look into the history of this article - has it been plagued (like Monty Hall problem) by editors arguing for a mistaken understanding of the paradox? Maproom (talk) 07:56, 20 October 2016 (UTC) Remarks struck in view of following response. Maproom (talk) 09:16, 20 October 2016 (UTC)
The wording "incorrect naive" has already been replaced with "incorrect and naive", with separate sources, 3 for "incorrect" and 2 for "naive". Both exact words do appear in the sources, so there should be no reason to, for example, replace the sourced "incorrect" with "mistaken". That could be interpreted as wp:SYNTHy. Yes, indeed the sources were introduced specifically to avoid a Monty Hall like plague. So far it seems to have helped. Every now and then someone comes to the talk page to discuss, but usually referring to the sources quickly settles things down.
As evidenced by this anon article edit, followed by this, this and this talk page user edit (—clearly the anon is the user—), this RFC was opened by someone who thinks that special relativity is simply mistaken: they removed "incorrect", weaseled out "naive", and deleted the sources, because "the reality is that there are many, myself included, who do not agree with this interpretation" and "It is always wrong to describe variant viewpoints as incorrect, when any differences of opinion exist. It is just academic imperialism." - DVdm (talk) 09:09, 20 October 2016 (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.
How to simplify our vision of the twin paradox.
Time dilation is since Einstein reported in a rather complicated manner.
We look at the transported clock "A" with the "point de vue" from our fixed clock "B". And of course we find that "A" runs slow.
But we forget that "all the laws of physics are the same in every inertial frame of reference". That means ː if we transport , at constant speed a sister clock "A" of "B" , that sister clock runs exactly at the same rate than "B".
And if we measure , with "A" , a certain amount of time between the initial position of "A" and its final position , we will find the correct answers (that is with the Lorentz transformations) , with never having thought that "A" runs slow. That means we dissipated any confusion between the physical clocks and time they measure ǃ
Of course we must also explain what happens in the acceleration - deceleration phases , but it is now well-known that the contribution of these phases are negligible. And also that modern clocks are almost insensible to accelerations ǃ
That interpretation is not only easier to explain , but it simplifies the experimental analysis. We understand how there is in space-time a sort of trading between space and time.
Improvements to the Relativistic Doppler Shift section?
The following sentence in the relativistic doppler shift section of the article confuses me:
"At arrival, the image in the ship screen shows the staying twin as he was 1 year after launch, because radio emitted from Earth 1 year after launch gets to the other star 4 years afterwards and meets the ship there."
If the ship has already come to a stand-still and so his space-dilation already tells him he is 4 light-years away from his source, then the sentence is believable.
However, while the ship is still travelling at 0.8c, it will view its distance traveled as only 2.4 light-years (per the earlier sections of the article).
Accordingly, I would expect the passenger on the ship to only be able to see the radio emitted from Earth 0.6 years after launch. (Since the ship's time is 3 years, and light takes 2.4 years to get to him from earth, at his current velocity and length-contraction.)
If the ship crash-lands into a planet and so instantly decelerates from 0.8c to 0 relative velocity, does 0.4 years of radio data from earth instantly arrive at the ship???
I think the article could probably be improved to explain why it uses earth-time and earth-space to calculate the radio arriving at the ship instead of ship-time and ship-space. (And why the discrepancy?)
The distance travelled is not relevant here. It is just about the images as seen by the traveller, which trivially takes place at the traveller's location. These images are the same immediately before arrival, immediately after arrival, and then again immediately after the start of the return trip. There is no discrepancy in the "remote time images", as visually seen by the traveller. There only is a discrepancy in the so-called calculated current time on Earth, but that is not relevant here. This part is about what it looks like for the traveller. - DVdm (talk) 10:59, 19 February 2017 (UTC)
Paradoctor (talk) 05:02, 16 May 2014 (UTC)
