Talk:Twin paradox

simple diagram

Consider adding a diagram like this one: http://www.anselm.edu/homepage/dbanach/st10.jpg no fancy colors or animation needed, this is simple and very clear

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 ${\frac {du}{d\tau }}$ where u is the proper velocity ${\frac {dx}{d\tau }}$. The only thing is, working backwards from the stated result $v(t)={\frac {at}{\sqrt {1+a^{2}t^{2}}}}$ implies that the thing that they are keeping constant is instead ${\frac {du}{dt}}$. 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 ${\frac {du}{dt}}$ 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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Note that an article talk is the place where we discuss the format and content of the article, not the subject. See wp:talk page guidelines. - DVdm (talk) 17:19, 4 September 2013 (UTC)

The First Sentence

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))

Present and accounted for: "The following paragraph shows [...] how to calculate proper time as a function (integral) of coordinate time" ;) Paradoctor (talk) 02:38, 19 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)

"I prefer to say" What you prefer is of interest to Wikipedia only if it is published in reliable sources. If it isn't, neither it nor discussion about it belong here. Regards, Paradoctor (talk) 14:22, 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.

Unauthorized Persons Inc. (talk) 00:57, 5 December 2013 (UTC)]

^{(Off-topic subject discussion)}

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.

I'm afraid we must stop this discussion here, per wp:TPG. - DVdm (talk) 08:53, 5 December 2013 (UTC)

Only for homecomers?

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)

Sure, but that's another story—see Time dilation. The story of this article is about what happens when they meet again and compare their watches. - DVdm (talk) 09:43, 5 January 2014 (UTC)

Quotation of true text of Langevin is destroyed by fascists and called "original research"

Original research[modifier]

(Redacted. Replaced copy of my talk page message. See User talk:Yanick Toutain#Original research - DVdm (talk))

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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Please don't add your personal views on sources to the article. See wp:unsourced, wp:reliable sources, wp:original research, wp:SYNTH. I have reverted your second edit too. - DVdm (talk) 22:54, 5 February 2014 (UTC)

YANICK TOUTAIN (MATERIALIST, NEONEWTONIST, HONEST MAN, ANTIFASCIST, ANTIPOSITIVIST, GNOSEOLOGUE MATERIALIST : BLITZ WAR AGAINST FASCISTS RELATIVIST ! REVOLUTION IS HERE ! HANNIBAL AD PORTAS LUNDI 24 MARS 2014 "paradoxe%20des%20jumeaux"%20Un%20article%20modifié%20par%20un%20matérialiste%20honnête.%20Lisez%20la%20nouvelle%20version%20avant%20sa%20destruction%20par%20les%20fascistes%20relativistes%20escrocs http://revolisationactu.blogspot.fr/2014/03/wikipedia-paradoxe-des-jumeaux-un.htmlYanick Toutain (talk) 18:55, 24 March 2014 (UTC) --Yanick Toutain (talk) 18:55, 24 March 2014 (UTC)