Talk:Twin paradox/Archive 13

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Switching Frames versus Acceleration

A sentence near the beginning of the article on the twin paradox says:

"One version of the asymmetry argument made by Max von Laue in 1913 is that the traveling twin uses two inertial frames: one on the way up and the other on the way down. So switching frames is the cause of the difference, not acceleration per se.[1]"

The last sentence is incorrect. "Switching frames" cannot be done without acceleration, and acceleration causes the accelerated object to switch frames. If von Laue actually said this, a note of correction should be added. If he didn't say it, a note should be added to the effect that the traveling twin accelerated to change from one inertial frame to another.

Frack99 (talk) 20:38, 15 September 2009 (UTC)

One can also sketch a situation where two frames are used and no acceleration takes place, by using two clocks, one outbound and one inbound. At the event where the clocks meet, the time reading of the outbound clock is transferred to the inbound clock. This way, at the reunion event, the inbound clock shows a lower value than the Earthbound clock. Thus we have the effect without any acceleration but with frame switchting. I can imagine that this is what Max von Laue had in mind, so I see no incorrectness.

If you can find a reliable source that supports some note about the (in)correctess of von Laue's statement, feel free to include it, but look out for wp:synth.

On the other hand, I think that the lead is pretty balanced and complete as it is. DVdm (talk) 21:17, 15 September 2009 (UTC)

Let's examine that theme a little closer. Ship 1 travels distance L (in stationary frame) at speed v. He perceives the distance as L/gamma and it takes time t1 = L/(v*gamma) according to his clock. Ship 2 comes by and sets his clock to t1 = L/(v*gamma) and continues back to the origin point of ship 1 at speed v. When he gets there, his clock will read t1 + t2 = 2*L/(v*gamma). An observer at the origin point started his clock when ship 1 left, and his clock reads L/v + L/v = 2*L/v when ship 2 comes back. The problem occurs when we allow that ship 1 can consider himself at rest, so he perceives the clock at the origin going slower than his, so he claims it reads t1/gamma = L/(v*gamma^2). The same argument applies to ship 2, he perceives the passage of time at the origin as t2/gamma = L/(v*gamma^2). So if each ship had a second clock which kept the presumed time of the origin, and that data was transferred from ship 1 also, ship 2 would claim that the passage of time at the origin should be 2*L/(v*gamma^2). This means that ship 2 would conclude that the observer at the origin is the younger one. Imagine his surprise when he stops there and compares clocks: they don't agree! Now we not only have a "paradox," we have an inconsistency: the observer at the origin is both older and younger than the observer on ship 2! IMHO, this can mean only one thing: transfer of time labels is an invalid procedure for determining who aged more.

Frack99 (talk) 12:24, 16 September 2009 (UTC)

Reply:

• "The problem occurs when we allow that ship 1 can consider himself at rest ... so he claims it reads t1/gamma = L/(v*gamma^2)." => If you draw a spacetime diagram, you will notice that this is the time, attributed by ship1 of an event taking place on Earth, simultaneously, as perceived by ship1, with the turnaround event. This is, for all concerned observers, a physically meaningless event.
• "The same argument applies to ship 2, he perceives the passage of time at the origin as t2/gamma = L/(v*gamma^2)." => On the same diagram you will see that this pertains to another physically meaningless event, namely one taking place on Earth, simultaneously, as perceived by ship2, with the turnaround event.
• "So if each ship had a second clock which kept the presumed time of the origin ..." => This "presumed time of the origin" is a meaningless quantity. Try to express yourself and, more importantly, think in terms of physical events and the times attributed to them by the various observers in their frames. Thinking in terms of "the time of the origin" invariably leads to embarrassing errors.

Please note that an article's talk page is not meant to help readers understand the subject and perhaps get rid of some of their misconceptions, but to discuss the content and format of the article in question, so I really shouldn't have replied to this remark, but I hope it helped anyway.

DVdm (talk) 13:12, 16 September 2009 (UTC)

I agree that the ships' earthclock times are meaningless events. I was concerned that the content of the article was misleading. Thank you for applying your time and talent to clarify this.

Frack99 (talk) 13:33, 16 September 2009 (UTC)

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)

Not sure if there would not be an orthogonal acceleration in the latter case but photons set aside, there is nothing from the outbound ship that is brought back to earth. Is the information that is transferred not irrelevant? Same as, even though it can be arranged that a series of light bulbs that are lit so quickly in sequence that the resulting "light wave" would appear faster than light, still nothing can travel faster than light?

--Felix Tritschler (talk) 22:36, 12 December 2009 (UTC)

Twin paradox in $RxS^{1}$ pseudo-Riemannian space

Generally special relativity is formulated in $RxR^{3}$ pseudo-Riemannian space, t,x,y,z and the examples are in the 2D pseudo-Riemannian section $RxR$, t,x. However, cosmology models suggest a closed physical space $RxS^{3}$. If physical space is of finite extent there's no need for the travelling twin to decelerate, and then accelerate back towards Earth, he can go all the way around the extent of pyhsical space and get back to Earth. Perhaps the conclusion is that it is not possible to construct a spatially closed pseudo-Riemannian space.

--Steve 33025 (talk) 16:03, 29 December 2009 (UTC)

Nope, it's not that bad. ;) In universes with closed dimensions the first postulate of SRT is invalid: It is no longer true that all inertial frames are equivalent. Put differently, SRT holds locally, but not globally in these spaces. The seminal paper for this is Brans, Carl H.; Stewart, Dennis Ronald (15 September 1973). "Unaccelerated-Returning-Twin Paradox in Flat Space-Time". Physical Review D. 8 (6): 1662–1666. doi:10.1103/PhysRevD.8.1662.

the calculus is false

it is an evidence that the calculus in "Difference in elapsed time as a result of differences in twins' spacetime paths" is false:

1/ arcsinh(x) is the primitive of 1/sqrt(1+x²); 2/ if v= au, $\int _{0}^{x}{{\sqrt {1-v(u)^{2}/c^{2}}}du}=1/2{\frac {x{\sqrt {c^{2}-a^{2}x^{2}}}}{c}}+1/2{\frac {c\arcsin(ax/c)}{a}}$ Claudeh5 (talk) 07:58, 24 October 2009 (UTC)

Sure, but $v\neq au\,$, i.o.w. $v(t)\neq at\,$, as a is the constant proper acceleration in the instantaneously comoving inertial frame, whereas v(t) is the coordinate velocity in the original permanent inertial frame. When you use the correct expression $v(t)=at/{\sqrt {1+(at/c)^{2}}}$, you will find the calculus to be correct. See for instance here.

I have added the expression for v(t) to the article, and replaced v with V to make the difference stand out more clearly. Cheers, DVdm (talk) 10:55, 24 October 2009 (UTC)

Ok, i have forgot the law of speed in RR. But this was only a secondary point in the paradox. Claudeh5 (talk) 01:10, 25 October 2009 (UTC)

Maybe we should use more consistent notation. $x_{0}$ be the quantity $x$ as measured by one twin and $x_{1}$ be the quantity $x$ measured by the other twin. Angry bee (talk) —Preceding undated comment added 23:54, 10 January 2010 (UTC).

Asymmetry

How is a symmetrical twin paradox resolved? See, for example, this. Joseph449008 (talk) 16:31, 3 February 2010 (UTC)

Some remarks:

1. The subject of the Twin paradox article is unquestionably an inherently asymmetric situation, so the "symmetrical twin paradox" is certainly off-topic.
2. An article's talk page is not the place for inquiries about the application of an aspect of the article, let alone of a non-aspect of it. A better place for this would be (perhaps) the science reference desk, or in this case for example sci.physics.relativity on Usenet.
3. A brief skimming of this "article" clearly demonstrates its author's complete lack of understanding of the most basic aspects of the subject. It's really almost frightening.

DVdm (talk) 17:27, 3 February 2010 (UTC)

While I share your misgivings about the paper, Wang is a professor of physics at UTC, and the publication venue seems valid. So far, the conclusion seems to be that this is a primary WP:RS on the twin paradox. Our opinion about the quality of the paper is WP:OR, I'm afraid. Paradoctor (talk) 18:12, 3 February 2010 (UTC)

A primary source it might be, but let's keep this in mind, specially in combination with this: no citations by others. I think that soundly disqualifies the article as a wp:reliable source. Cheers, DVdm (talk) 19:06, 3 February 2010 (UTC)

(edit conflict) "Material that has been vetted by the scholarly community is regarded as reliable; this means published in reputable peer-reviewed sources or by well-regarded academic presses.". Of course, if we had cited papers related to symmetrical versions, these would clearly take precedence. WSEAS publications seem to be covered by Web of Science and Scopus. Paradoctor (talk) 19:56, 3 February 2010 (UTC)

Actually, that seems interesting. There are a few other papers on a symmetrical twin paradox, but I'm not sure there are papers that actually resolve it. A lack of citations by others could mean the paper was not found to be relevant or correct enough to address, but it could also mean no one was able to address it. I found a forum where the symmetrical paradox is discussed, and I frankly didn't understand the attempts to resolve it. They appeared pretty confused. Admittedly, I'm a layperson when it comes to this topic. Joseph449008 (talk) 19:31, 3 February 2010 (UTC)

Lack of citations may also mean that is not known. The field as a whole is, IMHO, terribly disorganized. The 50s saw the reappearance of several arguments in the reputable literature that had already been discussed 30 years before, without any sign that the authors were aware of the duplication. I'm not aware of any author ever claiming that their coverage of the topic is comprehensive. Paradoctor (talk) 19:56, 3 February 2010 (UTC)

Further reading

I propose removing the 'Further reading' section entirely. Readers interested in time dilation are already directed to articles by the 'See also' section (and including the speculative paper by Klauber is especially perverse; no general reader of this subject is going to benefit from being directed to that). Excepting the Klauber paper, all the other references in the 'Further reading' section are cherry-picked quote-mining from google books. It would be more useful to tell readers to just go to google books and input "twin paradox" for further reading. Tim Shuba (talk) 18:18, 5 February 2010 (UTC)

• I don't really mind the section, so afaic it can safely stay there. I haven't looked at the Klauber paper (and I don't intend to do so), buf if you think it is inappropriate, by all means remove it. If someone has a very good reason for it to be here, we will find out soon enough - the who's and why's could be interesting. DVdm (talk) 18:48, 5 February 2010 (UTC)

Alright, so I removed the Klauber reference plus another duplicated in the in-line cites. But the structure of external links is baffling and I still think getting rid of the section is best. We have a 'Notes' section for in-line refs; it is clear what goes there. Then we have three separate sections for more external sources: 'References'; 'Further reading'; 'External links'. What belongs where and why? Except for the cheesy-looking all-caps link, the cites in 'References' are to standard relativity sources. I suppose that is okay as a catch-all for parts of the article that are just standard relativity, although it doesn't really help a confused reader know what to check. Now let's look at the cheesy-looking link. Should it be left in the 'References' section? Why or why not? Should it be moved to 'Further reading'? Why or why not? Should it be in 'External links'? Should it be removed entirely? Despite its cheesy appearance, it's not a terrible link. But there are dozens and probably hundreds of such links that might be included. There seems to be no rhyme or reason for these different sections, but if someone has one, I'd like to hear it. My thoughts are to eliminate the 'Further reading', and move the most useful parts either into the text as an in-line ref (preferred), or add to the 'External links' section (okay for a limited number of high-quality refs). The current structure encourages a mishmash of indiscriminate citations. Tim Shuba (talk) 16:26, 6 February 2010 (UTC)

A Calculation Missing from Specific example

In the Specific example there is a missing calculation. There is one for the earth twin calculating his own clock. One for him calculating the ship's clock. And there is one for the ship twin calculating his own clock. There is no calculation for the ship twin calculating the earth's clock. There are two observers and two clocks for a total of four observations. Why aren't there four calculations? --Mr. Shawn H. Corey (talk) 15:23, 4 March 2010 (UTC)

The reason is that the time dilation and length contraction formulae (with the Lorentz factor and its reciprocal) require that "the calculating observer" is inertial. In this setup, the earth twin is inertial, so the "calculation from his viewpoint" is fairly easy. He can reason about three "phyically significant" events (departure, turnaround, return) with easily accessible coordinates, and apply the standard equations in a straightforward way. The traveling twin O.T.O.H. is not inertial, as he must make a turnaround halfway the trip. This complicates the reasoning, since it sort of introduces two additional "artifical" events taking place on the earth twin's clock, simultaneous with the turnaround event, first as seen in the frame of the travelling twin on the outbound part of the trip, and then in that on the inbound part. These two additional events have no physical meaning, i.e. the Earth twin does not "notice" them in any way. I.o.w. nothing phycially happens at these events. Furthermore, there is sort of a "time jump" of the earth clock between these events, due to the fact that the traveller jumps from one inertial frame to another. So, in order to make the "calculation from the travelling twin's viewpoint", the coordinates of these two events are needed. When the jump is taken into account, the calculations match. Of course all this is not easily explained in simple words, so it is usually left to the imagination, or to the talk page :-). DVdm (talk) 19:26, 4 March 2010 (UTC)

OK, I think I get it. When the travelling twin turns around, his velocity swings 180°. But, because the description is given without regard to orientation, it superficially appears the same. That is, the time dilation and the space compression are identical. But they are not; they're equal in magnitude but opposite in direction. --Mr. Shawn H. Corey (talk) 14:48, 5 March 2010 (UTC)

Minkowski diagram of the twin paradox

Not realy. Dilation and contraction don't suffer from direction. They are the same for both directions of the velocity. The complication stems from the pairs of "events between which" time dilation is to be calculated. As I said, for the earth twin, to calculate the "dilated traveller time", the first is the pair (departure,turnaround) and the second is the pair (turnaround,return). The traveller, in order to apply the standard time dilation equation to calculate the "dilated earth time", must imagine and use these two additional ("artificial") events I mentioned. If you look at the image on the right, these events are visible as the two innermost intersection points of the vertical earth twin wordline (marked ct) with the blue and red simultaneity planes crossing the turnaround event. Let's call these events "simturnblue" and "simturnred" respectively. Note the gap between these two events! So the traveller must apply the time dilation equation (1) first to the pair (departure,simturnblue) for the outbound part of the trip, and (2) then to the pair (simturnred,return) for the inbound part. Both these time intervals are of course identical. When these are calculated, we must also add (3) the "time jump" between the artificial pair (simturnblue,simturnred). This time jump is caused by the sudden jump of the traveller from the inertial outbound trip frame to the inertial inbound trip frame. When this time jump (3) is added to the two identical "dilated times" (1) and (2), we get the correct elapsed earth time. DVdm (talk) 16:52, 5 March 2010 (UTC)

You may find this diagram from /Archive 11#Maps of spacetime helpful in visualizing the paths taken by the twins, shown in all three rest frames.

—WWoods (talk) 17:24, 5 March 2010 (UTC)

Excellent:

• departure = O
• turnaround = E
• return = D
• simturnblue = A
• simturnred = C

Thanks. - DVdm (talk) 17:52, 5 March 2010 (UTC)

Finally, something that makes sense. But first, I find that your explanation adds unnecessary details which just confuses things. Those simultaneity planes are not needed. It's the diagrams that add the missing factors (though this is not clear). It's the second and third diagrams which show what is happens from the travelling twin point of view and show where the whole paradox gets started. To explain that, I have to back up a bit. There are three velocities, which give three observers and three clocks. This creates a total of nine observations. Fortunately, three of them are trivial; they are the observer observing his own clock. Two more are accounted for by the earth twin observing the outgoing and return leg in the first diagram. Of the remaining four, only two are described on the article page. Those are the what happens on earth as the travelling twin moves along its own velocity. The other leg of the journey is ignored. It is these missing parts which cause the twin paradox; without them you can never get the calculation right. The calculation of what happens on earth starts with the calculation based on the second diagram for the outgoing leg. Then the inertial frame is jumped to third diagram to calculate the return journey. These two results are added together, which give the wrong answer because they are from different inertial frames. Only the results from the same frames can be added together. Until these missing parts are added to the article (and written in such a way that it doesn't sound like a used-car salesman talking), the controversy not disappear. --Mr. Shawn H. Corey (talk) 19:58, 6 March 2010 (UTC)

You find that my "explanation adds unnecessary details which just confuses things". Remember that you asked for an explanation, so I gave it.

You say that only the results from the same frames can be added together. This is patently false. It does not matter "from which frame a calculation is made", as long as times between well defined events on one and the same clock are added. This is the case with the events O, A, C, D taking place on the earth observer. We add:

• (1) the earth clock time between events (O,A) as calculated with the standard time dilation equation from the traveller's proper time in the outbound frame,
• (2) the earth clock time between events (C,D) as calculated with the standard time dilation equation from the traveller's proper time in the inbound frame,
• (3) the earth clock time between events (A,C) as calculated with the Lorentz transformation and the planes of simultaneity with event E, due to the frame jump at this event.

This addition gives the correct elapsed proper Earth time between the events O and D.

You say that "these two results are added together, which give the wrong answer..." (emphasis mine). So it looks that you either haven't looked at the explanation, or completely failed to understand it.

I don't see a "used-car salesman talking", nor do I see any "controversy". What I see, is someone who clearly fails to understand something after it was explained twice, in increasing detail. At this point it has been explained three times, and if you still haven't understood, I really think you have come to the wrong place. This talk page is for the discussion of the content and the format of the article, which is more or less what we have been doing up to just before your previous comment. This talk page is not meant to chat about the subject, nor does it serve as a place to educate those who have a problem understanding some aspects of the subject, which is more or less what we are doing now, and we shouldn't be doing this. Perhaps a good book would be of better service to you. I recommend Taylor and Wheeler's Spacetime Physics. DVdm (talk) 00:00, 7 March 2010 (UTC)

Two questions

In the Hafele-Keating like experiments there are always two accelerated reference frames. However, in the Twin Paradox thought experiments there is never more than one accelerated frame. How can one consider the Hafele-Keating experiments germane to the paradox?

Could you explain why atmospheric mesons, which are nearly inertial reference frames, decay slowly in the Earth’s nearly uniform weak field? It appears to me that using the weak field transforms their clocks are supposed to run fast. —Preceding unsigned comment added by Emagnus3 (talk • contribs) 04:39, 13 March 2010 (UTC)

First things first: Welcome to Wikipedia! :)

In answer to your first question: 'These results provide an unambiguous empirical resolution of the famous clock "paradox" with macroscopic clocks.'. This is from Science's abstract for the second paper. I know that a lot of other authors agree. I think that your question has merit, though. There are authors who disagree about the relevance of experiments to solving a paradox arising from a thought experiment, so this will have to be mentioned when the corresponding sources arrive here.

Your second question has a simple answer: The special relativistic effect of slowing down the aging rate of the moving particle is several orders of magnitude larger than the slowing down of ground clocks due to gravitational time dilatation. Happy editing, Paradoctor (talk) 12:09, 13 March 2010 (UTC)

It's ridiculous to claim that physical experiments are not relevant to solving a thought experiment. Strictly speaking, it's probably true that authors have made this claim, because people do write ridiculous things. But, for example, Newton's first law of motion is essentially a gedanken, as completely force free laboratories containing frictionless ramps and pulleys are in short supply.

While not explained sufficiently in this article, Hafele-Keating and related experiments are versions of the rotational type of twin paradox mentioned in the final section, and these rotational versions are logically related to the "standard" linear versions. There is a nice, not overly complicated discussion of this in sections 2, 3, and 4 of this paper, which would serve as an excellent reference to flesh out the final section of the article here. Tim Shuba (talk) 17:23, 13 March 2010 (UTC)

"It's ridiculous": So are Time Cube and Paris Hilton. As long as we can source it to reliable sources, and in accordance to WP:DUE and whatever else applies, not only can we insert it, we have to, I'm afraid.

"authors have made this claim": You'll probably find more philosophers than physicists among them, but the answer is "affirmative".

"would serve as an excellent reference to flesh out": I'll be the last to hold you back from adding statements from a reliable source. ;) Paradoctor (talk) 18:30, 13 March 2010 (UTC)

Regarding the Viewpoint of the traveling twin

First, I would like to thank Paradoctor for his warm welcome and wish him well on his anticipated paper.

A successful analysis of Hafele-Keating included an analysis of the effects of Special Relativity. Similarly Paradoctor’s successful analysis of the decay of atmospheric mesons included an analysis of the effects of Special Relativity.

It is not clear to me that, in this section, there has been any analysis, during the period of acceleration, for the effects of Special Relativity. Emagnus3 (talk) 23:07, 3 April 2010 (UTC)

That's kind of the holy grail here. Einstein claimed in his 1918 paper that special relativity is not applicable to accelerated frames of reference. Others (too lazy to look up now) have stated that you can extend SR to accelerated observers, and get the expected results. There are lots of other opinions, ranging from "trivial, no paradox, really" to "ZOMG, Einstein lied to us!". There is no single consensus solution, but among physicists in general, there is a general, if not universal, feeling that the twin paradox is no threat to SR's logical consistency. Philosophers are somewhat more skeptical, and my favorite nutjobs cranks crackpots fringe theorists often use the paradox to prove that their favorite TOE is needed to repair physics in general. If you ask me, uncle Al has left us with another fine mess. ;) Paradoctor (talk) 23:36, 3 April 2010 (UTC)

Decrepitude

I appreciate that the stationary twin will have aged more, but would they also exhibit greater decrepitude? DynamoDegsy (talk) 15:33, 27 April 2010 (UTC)

Not if the time was spent in a nice comfy cryochamber. ;) Paradoctor (talk) 16:38, 27 April 2010 (UTC)

And if there wasn't a nice comfy cryochamber? :o) DynamoDegsy (talk) 07:30, 28 April 2010 (UTC)

There is always S.E.N.S.. ^_^ Paradoctor (talk) 16:13, 28 April 2010 (UTC)

My query results from a discussion amongst colleagues (engineers not scientists). My contention is that Paul Langevin's "striking example" means that, when the travelling twin returns to earth, he will have incurred only 2-years of decrepitude, and all his earthbound contemporaries will have incurred 200-years of decrepitude, i.e. they are dead. Whereas, my colleagues contend that although only two years have elapsed for the travelling twin, he too will have incurred 200-years of decrepitude. I believe that decrepitude is a function of time (and lifestyle), and as time has slowed for him, then so will his decrepitude. DynamoDegsy (talk) 07:41, 29 April 2010 (UTC)

According to Eddington, you have every reason to believe your colleagues are in error. If that doesn't convince them (it won't), ask them to cite their sources, I'm interested. If this argument is something they came up by themselves, please inform them that "decrepitude" does not seem to be a term appearing in relativity. ;) Paradoctor (talk) 16:17, 29 April 2010 (UTC)

Experimental proof

For me, the clocks in planes not really convincing, since the difference is so tiny so that one might say, that other reasons cause the tiny delay. But, I think, experiments with decaying particles in accelerators are much more convincing, since the effect is much larger due the higer velocity close to the speed of light.—Preceding unsigned comment added by 95.222.228.77 (talk • contribs)

Please sign your talk page messages with four tildes (~~~~). Thank you. DVdm (talk) 08:23, 28 April 2010 (UTC)

A remark and a question

Remark You write

They know that the distant star system and the Earth are moving relative to the ship at speed $v$ during the trip.

In my humble opinion that should be

They know that the distant star system and the Earth are moving relative to the ship at speed $-v$ during the trip.

This will have no effect on the calculations.

Question According to the travelers' calculation they will arrive home having aged 5.14 years. How much would the people of the earth-based mission control have aged according to the travelers' calculation? —Preceding unsigned comment added by 84.83.33.64 (talk) 15:09, 9 June 2010 (UTC)

Please put new remarks at bottom of page and sign with four tildes (~~~~)? Thanks.

About the remark. Speed is always positive. Normally when -v is used, we talk about velocity, which has a direction. In that case velocity -v is in the opposite direction as velocity v. When we talk about speed, the direction is either explicitly stated, or silently assumed, which is the case here. So the sentence is correct.

About the question: When the calculation is made from the traveller's viewpoint, they will calculate that the Earth-people age 10.28 years. The calculation is a bit trickier because during the trip the travellers live in two different inertial frames: one frame for the outbound part and another frame for the inbound part. At the turnaround event, there is a sort of "simultaneity-with-the-earth-jump" between the event immediately before the turnaround and the event immediately after. You find a more rigorous treatment and an example (A) in [1] which is a reference for the section Twin paradox#Difference in elapsed times: how to calculate it from the ship. IIRC this question was asked a while ago on Wikipedia:Reference desk/Science. You can search the archives. DVdm (talk) 15:32, 9 June 2010 (UTC)

The bibliographies

The Benton bibliography is now available at s:The clock problem (clock paradox) in relativity. I'll add the bibliographies of Benton (240 entries), Arzeliés (109), Marder (241), and Chang (35) to Further reading later on. Paradoctor (talk) 18:45, 9 June 2010 (UTC)

Correction request

First, excuse me if I'm not getting this right. My request would be to find a better name for so called "simultaneity planes" on this image: http://en.wikipedia.org/wiki/File:Twin_Paradox_Minkowski_Diagram.svg As this other image calls "plane of simultaneity" those points in spacetime that are separated by only space in a given inertial frame: http://en.wikipedia.org/wiki/File:Relativity_of_Simultaneity_Animation.gif So for the first image we can't simply talk about a single plane of simultaneity, since it's relative to an observer's inertial frame, and we have two of them. I think those lines are important, just the name is wrong, because it's ambiguous if we try to Google it. Thanks! —Preceding unsigned comment added by 78.92.229.174 (talk) 23:20, 17 June 2010 (UTC)

Welcome to Wikipedia!

"excuse me if I'm not getting this right": Nothing to apologize for being WP:BOLD. ;)

I updated the the file on Commons as requested. Regards, Paradoctor (talk) 03:58, 18 June 2010 (UTC)

Omg, I wasn't right: We had 3 and not 2 inertial frame, but they were correctly named simultaneity planes. With the new naming, it can be easily seen. Thanks! —Preceding unsigned comment added by 78.92.229.174 (talk) 09:25, 18 June 2010 (UTC)

Ok, so I noticed: At every examined point on the diagram, there is a black line, which is the path of the twin, and since relativity, it's his ct' axis. And we have colored lines crossing are the according x' axis. So coloring the according black lines to blue/red could make the understanding even easier, since every inertial system would have a consistent color. Or not... It's up to you. Thanks for helping! MoZo1 (talk) 13:13, 18 June 2010 (UTC)

experimental set-up

Supposing we choose a different thought-experimental set-up. Twin A sets off and after a brief spurt of acceleration his engine cuts out and he is now coasting at near-light speed. Or to put it another way, he is stationary and twin B is coasting away from him at near-light speed. Their relative velocities are (by definition) identical. An outside observer C chooses a time when the twins are equidistant from him. Presumably they appear to him to be aging/have aged by the same amount. This is presumably also true of any number of outside observers (each equidistant from the twins) at any point in the voyage. Including the time when twin A rejoins twin B. Or is there a point when a sudden discrepancy is observed by an outside observer?Escoville (talk) 15:46, 19 June 2010 (UTC)

Please take this to the science reference desk. This page is for discussing the form and content of the article, not of the subject itself. Thanks. DVdm (talk) 15:58, 19 June 2010 (UTC)

I've watchlisted the science reference desk, and will answer there if you ask your question there. Paradoctor (talk) 15:11, 20 June 2010 (UTC)

I've removed the science reference desk from my watchlist, please notify me when and if you post your question there. Paradoctor (talk) 03:48, 24 June 2010 (UTC)

Specific example

The specific example is wrong.

t = 2d / v

d = 4.45 light years v = 0.886c

therefore t = 10.277 years

I thought it was a bit weird that a star 4 and a half light years away could be reached, and returned, in only 5 years, at 80% of the speed of light. —Preceding unsigned comment added by 203.214.100.226 (talk) 22:02, 13 August 2010 (UTC)

Please sign your talk page messages with four tildes (~~~~)? Thanks.

By the way, the article says 0.866, not 0.886. DVdm (talk) 09:12, 14 August 2010 (UTC)

Three Reference Frames

Mention is made of Max Von Laue's asymmetry argument: "One version of the asymmetry argument made by Max von Laue in 1913 is that the traveling twin uses two inertial frames: one on the way up and the other on the way down. So switching frames is the cause of the difference, not acceleration per se" How about having a diagram showing two different reference frames for the traveling twin? For Instance:

Ah, now I see, WWoods has already tried:

Then my question is, whatever happened to WWoods diagram, and why isn't it in the article now? Was it just for lack of being bold? (JDoolin (talk) 20:20, 16 August 2010 (UTC))

Discussion in archive. It would take a lot of explaining in the article. Note that explanations in images are not appropriate. B.t.w. I have added the 4th view to the image of which I spoke in that discussion. I will also notify WWoods. DVdm (talk) 20:37, 16 August 2010 (UTC)

I see you've modified Woods image so it includes a fourth "frame." I'm not sure what you hope to gain by labeling space and time on nonorthogonal axes. I notice that in your discussion with Woods, you first refer to "the" fourth frame, whereas he is referring to "a" fourth frame. Are you suggesting that there is some specific special frame of reference that needs to be taken into account, or are you just pointing out the infinity of arbitrary inertial reference frames that exist? (JDoolin (talk) 22:21, 16 August 2010 (UTC))

Oh no, don't be afraid, we might have been talking about adding "a" 4th frame to the figure, and as soon as such a frame would be added, we might refer to it as "the" fourth frame (i.e. the one which was added to the figure). I think this is some kind of style figure, called "ellipsis". Anyway, I don't see where I called anything "the fourth frame". I called something "a 4th view where none of the frames coincides with the lay reader". As for the nonorthogonal axes, that is just for generality. There is no reason why the axes should be drawn orthogonally. The only requirement is that the light cone ray bisects the axes. DVdm (talk) 07:59, 17 August 2010 (UTC)

It occurs to me that the Lorentz Transformation might be achieved by two scaling operations; along the x=ct axis and the x=-ct axis. This might produce your "fourth frame" image as an interim step, and would also emphasize the preservation of the speed of light. However, during these interim steps, the skewing of the time and space axes themselves only represent what has happened to the space-and-time axes of the original frame. Also during these interim steps, where you have done a "partial" Lorentz Transformation, you are not really looking at an inertial frame. You're just looking at some arbitrary coordinate transformation of the system. In such an arbitrary coordinate transformation, I agree there is no reason why the axes should be drawn orthogonally. However, if the goal is to show inertial reference frames, we should focus on these; where the full LT has been done, and the time and space axes are perpendicular. (JDoolin (talk) 16:13, 17 August 2010 (UTC))

Why traveling clocks are the "experimental verification" while lifetime of elementary particles is not?

User:DVdm has removed the reference to particle acceleration practice saying that "Time dilation plays a role there. The twin paradox doesn't really." Can I remove the traveling clocks evidence for the same reason? Why the University of Illinois says that particle acceleration does test the "twin paradox"? How is it possible that the prolonged life of accelerated particles does not manifest the twin paradox? --Javalenok (talk) 15:25, 31 December 2010 (UTC)

Technically, the twin paradox (erroneously) says that the twins should have the same age when they reunite, based on a trivial misunderstanding of the theory. It is true that time dilation is experimentally verified with travelling clocks and with particle accelerators, and indeed the paradox is experimentally "de-manifested" by travelling clocks, since, upon reunition, the elapsed time on the travelling clock is actually compared with the time on the lab clock. This i.m.o. differs from what happens in particle accelerators, where particles do not reunite with the lab clocks — they just desintegrate. It is true that their (average) laboratory in-flight-lifetimes are compared with their (average) proper lifetimes (aka laboratory at-rest-lifetimes), but I think mentioning this as another example of de-manifesting the paradox is a bit overdoing it. DVdm (talk) 12:20, 2 January 2011 (UTC)

Why to entail someone's misunderstanding, unnecessary details of verification and concoct a new argument against my edit? Is such evasive answer just a civil way to confirm the mistakes? --Javalenok (talk) 14:45, 2 January 2011 (UTC).

You asked a question and I answered it as precisely as I could. You asked whether you could remove the traveling clocks evidence for the same reason, so I tried to clarify the difference between the two cases as a reason for not removing it. Your somewhat surprising reaction sounds like you assume bad faith from my part, so please note that I had no intention to entail, concoct or evade anything, and you can safely wp:AGF from my part. DVdm (talk) 16:19, 2 January 2011 (UTC)

When somebody adds something and another man removes this note as an inappropriate "demonstration of another phenomenon", it is the 2nd person, you, who needs a lesson. It is funny when the second person, who has made a mistake, starts teaching the topic to the fist. The first one needs no long lecture. To restore his mind from insult, it is necessary that the second just acknowledges his mistake. --Javalenok (talk) 18:46, 9 January 2011 (UTC)

You asked a question. My answer was not intended to insult you and I am sorry if it did. On the other hand, I don't see where I gave you a long lecture and why or how, to restore your mind from insult, I could or should acknowledge your mistake. DVdm (talk) 21:38, 9 January 2011 (UTC)

You insult not by a long lecture but when you state that your false reason is mine mistake. Either prove that that the prolonged life of accelerated objects does not manifest the twin paradox or confirm that you supplied a wrong reason removing my edit.--Javalenok (talk) 13:51, 10 January 2011 (UTC)

I explained the subtle difference between the two situations, and thus the reason to keep only the situation of the airplane clocks. The first example, where actual real living observers fly along with the travelling clock, is more fit to serve as an example^(*) of the twin paradox than the second example, where nobody is actually flying along with the particles to, sort of, say "Hey, the lab clocks are moving, so actually they will show less elapsed time when we will return!". ^(*)Note that the twin-paradox is not really manifested in any situation, since it is nothing more than an act of mistaken thinking. Again, if you feel insulted by anything I said, that was not my intention, so please accept my apologies for having done so unknowingly. DVdm (talk) 14:13, 10 January 2011 (UTC)

Do you always measure the time dilation when clocks are missing? The lifetime of the particles is the clock itself. --Javalenok (talk) 16:47, 10 January 2011 (UTC)

Sure, but the lifetime of one particle is not used as a clock when measurents are made. What is taken as a clock is the average lifetime of many particles, and no single hypothetical particle-riding-observer is carrying this so-called clock, so no such observer can make, not even in principle, the twin-paradox mistake by saying that the lab clocks should show less time upon reunion. This article is not about time dilation, but about the twin paradox, which is just an improper application of time dilation. The particles example is very on-topic in the Time dilation article, but much less in this one. DVdm (talk) 18:45, 10 January 2011 (UTC)

Is time dilation always meant when you have no clocks? I want to know how the lack of conventional clocks allows to measure the time dilation but not the lifespan differences. --Javalenok (talk) 14:52, 15 January 2011 (UTC)

I did not say that "the lack of conventional clocks allows to measure the time dilation but not the lifespan differences." I said that the lack of physical observers riding with particles makes the example much less on-topic in this article. Details in previous reply. DVdm (talk) 23:48, 15 January 2011 (UTC)

At first, I titled this section following the text you stand for. It says that the experimental proof was done by "precise measurements of clocks flown in airplane". There is nothing about the need for "observers". Secondly, throughout all your clarifications here, you develop the importance of moving clocks and pushing the idea that if we have none then the effect of time dilation is demonstrated. You also point out that the "observers (or clocks?) disintegrate". But I still have got no answer on why this disintegration helps to measure time dilation but not the prolonged lifespan of the accelerated twins? --Javalenok (talk) 10:46, 17 January 2011 (UTC)

I have explained the conceptual difference between the two situations, and I don't know how to make it more clear for you. Perhaps someone else can help? DVdm (talk) 10:56, 17 January 2011 (UTC)

arbitrary and arbitrarily

Don't want to be picky, but should not "arbitrary" in

"If we placed a living organism in a box ... one could arrange that the organism, after any arbitrary lengthy flight, could be returned to its original spot in a scarcely altered condition, while corresponding organisms which had remained in their original positions had already long since given way to new generations. For the moving organism, the lengthy time of the journey was a mere instant, provided the motion took place with approximately the speed of light."

be "arbitrarily"? Myles325a (talk) 05:01, 17 February 2011 (UTC)

That would be the case if the phrase is to be interpreted as "(arbitrarily lengthy) flight", but in this case the meaning is different and, perhaps, broader: "arbitrary (lengthy flight)". DVdm (talk) 22:42, 17 February 2011 (UTC)

A different thought experiment for the "twin paradox"

The Wiki article "resolves" the twin paradox by saying that because of the high acceleration of the spaceship, and because of a reference frame jump at the U-turn, the clocks will re-adjust to resolve the paradox and keep intact the prediction of the space-traveling twin being the youngest, upon re-uniting.

If this resolution is a solution to the paradox, the paradox can be renewed by avoiding the conditions needing to be "resolved" in the Wikipedia article. This is done by avoiding having the spaceship experience great acceleration and avoiding the reference-frame-jumping U-turn at the farthest point of travel.

In this thought-experiment the design will stipulate that the spacecraft never accelerates higher than 1G and ensures that there is no U-turn at the farthest point. General Relativity says that acceleration and gravity are indistinguishable in respect to their effects on relativity (gravity and acceleration also cause time dilation). In the presence of either gravity or an equivalent acceleration force, clocks are dilated equally (slowed down the same amount to an outside observer not traveling with the object being viewed, i.e. in each twin's view of the other). So, the spaceship and its clock are held to 1G acceleration as it leaves earth - the same acceleration force experienced by the earth-bound clock and twin due to 1G gravity. As the craft accelerates and approaches the speed of light, we will have it rotate 180° and either coast for a chosen duration of time or just immediately decelerate at a constant 1G rate, eventually reversing direction and then continuing acceleration on its way back towards earth at 1G until again approaching the speed of light. The point of reversing directon is unfelt by the spaceship traveler. At the point of maximum velocity it again rotates 180° and again either coasts for a time or begins a 1G deceleration back to earth, depending on the protocol chosen. The spaceship will never exceed 1G acceleration, exactly matching the gravity experienced by the earth-bound clock. The General Relativity time-skewing caused by 1G gravity or acceleration effects are now equal in both reference frames. Since gravity and acceleration effects are equal, only the Special-Relativity difference in velocities between the two reference frames will affect differences in time dilation as viewed by the other twin. The U-turn is avoided and the craft never stops at the far end, it just passes through zero velocity so there's no opportunistic event for the spaceship to jump into another reference frame. If the protocol choice is for the spaceship to coast for a chosen time in the middle of the journey at its highest speed, then the earth clock will alone experience a greater time dilation (as viewed from the spacecraft) due to its continued exposure to the force of 1G gravity. Coasting is offered as a protocol because it gives an opportunity to go beyond the point of paradox and actually turn the tables by causing the spaceship's view of the earth-bound clock to be even more dilated than the earth's view of the spaceship's clock. This causes the earth-bound twin to be the youngest at their reuniting - and certainly smug about it.

- Gordon Quickstad —Preceding unsigned comment added by 71.59.129.181 (talk) 19:32, 25 February 2011 (UTC)

Please sign your talk page messages with four tildes (~~~~)? This is the second time I asked. Thanks.

Sorry, but this is entirely off topic here, unless you have a proper wp:reliable source for this. We are not supposed to discuss your wp:original research. Please read the wp:talk page guidelines. Thank you. DVdm (talk) 22:50, 25 February 2011 (UTC)

Using a sustained low acceleration doesn't change the 'paradox', it just complicates the calculation of the proper time along the ship's path. You have to use calculus rather than simple algebra. The time dilation correction due to one clock being on the surface of a planet is orders of magnitude less than that of the other clock reaching a relative speed close to that of light, and can be ignored as being beside the point of the thought experiment.

—WWoods (talk) 21:25, 25 February 2011 (UTC)

... and is treated further down in sections 7 and 8. DVdm (talk) 22:50, 25 February 2011 (UTC)

1. The U-turn wasn't avoided at all, since the spaceship still reverses direction of motion and comes back to earth.
2. The principle of equivalence doesn't work the way you think. The gravity on the planet is NOT equivalent to the acceleration of the spaceship and here is why. From the point of view of the observer on the planet he is at the bottom of the planet's gravity well while the ship is outside of the well - that is further up the gravitational potential, and the difference between their gravitational potentials is constant throughout the trip. The planet's gravity isn't very strong so this effect is almost negligible. From the point of view of the observer in the spaceship sometimes the other observer is further up the gravitational potential gradient and sometimes he is further down, depending on whether he is acceleration towards the planet or away from it. He interprets his a acceleration as uniform gravitational field (Yes, that is different from the non-uniform gravitational field of the planet hence the fact that a gravitational field is equivalent to an accelerated reference frame only locally. This problem doesn't satisfy locality since the distance between the observers is at times quite large). Because of that the difference between their equivalent gravitational potential can be extremely large and that effect is NOT negligible. Trying to bring General relativity into that problem is unnecessary and makes the problem much harder to understand. Dauto (talk) 03:52, 26 February 2011 (UTC)

Elapsed Time vs Aging

Thought experiments to resolve the paradox using biological subjects all seem to make the same clumsy error in their verbiage: they move slickly from the concept of "elapsed time" as measured by clocks, which are devices that count repeated states of a physical system considered reliably periodic, to the concept of "aging" of the biological subject, who is not such a device.

This is far from trivial. Time dilation is a consequence of the limitations of measurement and its formulae are in terms of those measurements, so its predictions should be as well. It doesn't model anything involving biological processes. Substituting "aging" for "elapsed time" could be just as erroneous as it would be if your watch ran slow and you figured you aged less, or if you watched someone walk away and figured they were shrinking.

In particular, there is nothing in any of the thought experiments to suggest that the biological processes of the travelers can be correlated to the time-measuring devices in either frame of reference. They sneak the assumption that it can between the lines, so that the reader is presented with a nice image of two very non-identical twins at the end. 75.170.49.213 (talk) 10:46, 8 March 2011 (UTC)

Please note (see wp:TPG) that this article talk page is not a forum for venting opinions about the subject. Anyway, the literature seems to make no conceptual difference between a clock ticking seconds and a heart thumping beats. In stead of material clocks counting seconds, we can imagine heart beat counts of living objects. A living object's age is usually defined as its own hypothetical proper wrist watch's accumulated seconds count, but it can just as well be defined as its accumulated heart beat count—no conceptual difference. In the typical thought experiment, the non-inertial ("travelling") twin's heart beat count is about half of that of his inertial ("stay-at-home") brother. DVdm (talk) 14:55, 8 March 2011 (UTC)

1) how is making the verbiage more precise not about improving the article? 2) hypothetical accumulating seconds aren't a real measurement, even as part of a thought experiment about real measurements. Biological processes like heartbeats aren't a reliable clock either. Even taking them as approximately reliable requires assuming there is no significant biologic effect produced by their acceleration and relative motion. Given the scarcity of observations of highly relatively accelerated organisms, such an assumption needlessly weakens the thought experiment. It is no less illustrative if it retains the precision and concepts of physics without ranging into hypothetical biology. Quotes and sections specifically describing the normal telling of the paradox should keep the "aged" verbiage, but sections analyzing and discussing them should not.75.170.49.213 (talk) 15:36, 9 March 2011 (UTC)

There is nothing hypothetical about accumulating seconds or heart beats. Depending on the type clock used (cesium, wrist watch, or heart) the ages are measured with more or less precision. If, in the typical 2/4 years thought experiment, between the event of departure and that of reunion the travelling twin's heart beat count has increased with roughly 60 million (correponding to roughly about 2 years), then the stay-at-home twin brother's count has increased by roughly 120 million. See for instance this books search, and specifically this paragraph in Hand and Finch, Analytical mechanics, page 555, and James L. Anderson, Principles of Relativity Physics, page 175. See also this books search and make your choice. DVdm (talk) 19:09, 9 March 2011 (UTC)

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 — Preceding unsigned comment added by 88.241.140.254 (talk • contribs) 18:41, 15 September 2011 (UTC)

Section 3

It looks like section 3, "Resolution of the Paradox in Special Relativity", doesn't actually solve the paradox in special relativity - any solution which depends on acceleration uses general relativity. Or do I not understand the proposed solution?

C-14 (talk) 23:14, 27 March 2011 (UTC)

Special relativity can handle acceleration, it's gravity where you need GR.

—WWoods (talk) 23:36, 27 March 2011 (UTC)

Specific Example

In the specific example, the traveler did not calculate the time elapsed in earth using the reference frame of the spaceship. The example suggests that the earth moves at the same speed backward measured in the reference frame of the spaceship. If time elapsed on earth is calculated by the travelers, it will be 2.57 years, half of the time elapsed on the spaceship due to time dilation, which is inconsistent with what the people on earth calculated. So I think this is not a good example. Light Ng Jen Foong (talk) 04:50, 23 April 2011 (UTC)

The people on Earth understand that when 5.14 of their years have passed, the spaceship is at the turnaround point. But it doesn't APPEAR that way because it takes much longer for all of the red-shifted images to get there. Shiggity (talk) 22:57, 2 August 2011 (UTC)

In the paragraph below it is not how the crew calculates their particulars but how their particulars look from Earth so it is wrong: "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 \epsilon d = 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." — Preceding unsigned comment added by 24.254.85.164 (talk) 00:59, 26 November 2011 (UTC)

Unacceptable original research

Michael Fontenot (talk · contribs · deleted contribs · logs · filter log · block user · block log) is repeatedly inserting original research into this article. His claim that two papers, namely Dolby & Gull and Minguzzi are alternatives and are in conflict is not supported by reliable secondary sources. Not only that, he's wrong in his personal interpretation. Dolby & Gull specifically states (e.g. just before the conclusion) that the result is independent of the coordinate frame chosen. Minguzzi never uses any non-inertial coordinate frame in the cited paper.

It is certainly a conflict of interest and perhaps an ethical breach that Michael Fontenot seems intent on edit warring to include his personal paper from a questionable source (Physics Essays). All of his original research should be summarily rejected from this article. Tim Shuba (talk) 00:19, 14 August 2011 (UTC)

• Note: an expanded version of the same material has been posted at The CADO Reference Frame for an Accelerating Observer and is currently the subject of an AfD here. andy (talk) 00:41, 14 August 2011 (UTC)

(My response to Shuba and Smith):

  

In each of the three references (Dolby&Gull, Minguzzi, and Taylor&Wheeler) that I have cited, in the subsection "Reference frames without fictitious gravitational fields, for the accelerating twin" (which I have added to the "Twin Paradox" Wiki article), the authors explicitly provide their answer to the question: "How does the home-twin's age change, according to the traveler, as the traveler's trip proceeds?". Any rational person reading those references cannot fail to conclude that those authors' three answers are all different. It is absurd to contend that some additional published source is needed, in order to reliably come to that conclusion.

In the section preceding my added subsection, there is a reference cited (Einstein, 1918) which gives the "gravitational time dilation" determination of the traveler's viewpoint. That reference answers the above question with the same answer that Taylor&Wheeler give, although the approach used in Einstein-1918 to get that answer is quite different (fictitious gravitational fields are utilized).

My CADO reference (which I cited in the subsection I added to the "Twin Paradox" article) explicitly gives the same answer that both Taylor&Wheeler and Einstein-1918 got: all three of those references say that the home-twin's age will change abruptly during the traveler's abrupt turnaround.

Dolby&Gull, and Minguzzi, clearly do not get that answer: they say that the home-twin's age changes only gradually, over a prolonged period of the trip, even when the traveler's turnaround is instantaneous. But they disagree with one another about how that gradual "her age versus his age, according to him" curve is shaped.

Michael Fontenot (talk) 16:53, 14 August 2011 (UTC)

The reason I provided links to the arXiv abstracts for the two papers above is so people can check for themselves the veracity of Michael Fontenot's absurd claims. In neither of those papers is anything mentioned about "how does the home-twin's age change, according to the traveler, as the traveler's trip proceeds". This is a fantasy made up out of whole cloth. There is nothing contradictory among Munguzzi, Dolby&Gull, and Taylor&Wheeler. Furthermore and more important for the article, there is no reliable secondary source which says so. This, along with the other blatant fabrications introduced by Michael Fontenot, should be removed, and no mention should be made here or anywhere else in the encyclopedia of his uncited paper from the unreliable source Physics Essays, which has failed for over 10 years to generate the slightest interest from professionals in the field. Tim Shuba (talk) 23:24, 14 August 2011 (UTC)

Can the intro be simplified or explained better?

As a non-expert, the intro doesn't really make sense. I think there should be some explanation added as to why that would happen. Or is it just inherent to Einstein's theory? in which case, that should also be added. --AW (talk) 04:51, 29 November 2011 (UTC)

Similar answer as in Talk:Time dilation#I don't understand this. The idea is not that one understands everything by reading the intro, but (perhaps) after having read the entire article and (probably) a number of linked articles. Don't expect to learn special relativity from an encyclopedia though, let alone from this one. That is impossible. - DVdm (talk) 06:32, 29 November 2011 (UTC)

Propose to remove section

We have this (well sourced) section Twin paradox#The time differential explanation in absolute terms at the end of the article, that i.m.o. has nothing to do with the subject of the article. I propose we delete it altogether. Comments? - DVdm (talk) 11:07, 24 December 2011 (UTC)

I have removed the section. If someone thinks there are compelling reasons to restore it, let's first discuss here. - DVdm (talk) 11:29, 28 December 2011 (UTC)

Apparently user Bernardbet (talk · contribs) has restored the section. I don't see any connection between the content of the section and the subject of the article. There is nothing in Taylor and Wheeler's Spacetime physics on page 88 about the twin paradox. So the entire opening paragraph is wp:original research and wp:SYNT. The second paragraph, with the Poincare source, says nothing about the twin paradox. The next paragraph, sourced by Laughlin, says nothing about the twin paradox.

I agree that the last paragraph, although sourced by a mere footnote in the French source, of the section is really relevant for the article, but then again, it is not at its place in a section with a title ==The time differential explanation in absolute terms==. I propose we keep it with a more appropriate section title, say ==Explanation in terms of Mach's principle==. - DVdm (talk) 10:29, 29 December 2011 (UTC)

The point is that 'ether B' folks do not even speak of a twin paradox, therefore the absence of that particular terminology should not be regarded as a deal breaker. See the rewritten first paragraph of section 12 which makes this more clear. The point of including the references is to give weight to 'ether B' point of view. - Bernardbet (talk) —Preceding undated comment added 12:50, 29 December 2011 (UTC).

If these folks don't speak of a twin paradox, then having it here is an even more outspoken example of wp:original research. Your recent addition is also wp:UNSOURCED. Please have a careful read of these policies. The last paragraph is okay with an appropriate modified section header, but I'm afraid there is no place in Wikipedia for the other paragraphs. - DVdm (talk) 13:19, 29 December 2011 (UTC)

Is it? This is an encyclopedia, not a dictionary. If they're talking about the same thing, the language they use is surely irrelevant... Prof Wrong (talk) 14:32, 29 December 2011 (UTC)

The assumption that it is about the same thing is precisely one of original research. If they are indeed talking about the same thing, a reliable source for that fact should be easy to find, but then again that still would be wp:SYNT. Anyway, nothing in the sources that I consulted (Taylor, Poincare, Laughlin) suggests that they are indeed doing so, except for the last paragraph of course. - DVdm (talk) 14:41, 29 December 2011 (UTC)

Yes it is completely OR if you aren't sure if a reliable source does not indicate that they are talking about the same thing. It seems undue to have the fringe views on the page also. IRWolfie- (talk) 15:28, 29 December 2011 (UTC)

The paragraph is an example of WP:Synth. Poincaré never talked about the twin paradox. The specific Wheeler and Laughlin references concern the ether, and do not directly mention the twin paradox. Btw: "ether B" is Lorentz ether theory. There are in fact some like Dingle, Ives, Builder, or Prokhovnik who favor this view in connection to the twin paradox, but this is outside the mainstream. --D.H (talk) 17:09, 29 December 2011 (UTC)

To DVdm: You wrote "If these folks don't speak of a twin paradox.." The fact that the twin paradox does not come up in 'ether B' is relevant. The reader may want to know that there is an interpretation of relativity which makes the twin paradox a moot point. And your claim that the recent addition is not sourced makes no sense for the same reason. See the paragraph in question. To D.H: "Outside the mainstream" is not a basis for complete omission from a Wikipedia article. I will not prolong the debate. --Bernardbet (talk) 23:53 29 December 2011 (UTC)

As it is clear that the addition is both wp:original research and wp:fringe, and lacks wp:notability, I have made the proposed change. Readdition is welcome with proper solid subject-relevant sources. Thanks all for your input and best wishes for 2012! - DVdm (talk) 10:10, 30 December 2011 (UTC)

Media references

There are two science fiction movies concerning this subject: The_Time_Travelers_(1964_film) and Journey to the Center of Time (1967 Remake) - zmin_inc (talk) 12:53, 9 September 2012 (UTC)

Gravitational effect explanation

Paul Langevin told about the Twin paradox at Bologna in 1911. I think that is the first mention of the "Twin paradox". How can a paper from 1907 give an explanation to the Twin paradox (it's in reference one). And it also seems strange to associate Born and "gravitational effect" to something which is supposed about SR only. I tried to google a bit and all mentions of this, point to this Wikipedia article, so I find this suspicious. Any idea? JPLeRouzic (talk) 22:39, 29 December 2012 (UTC)

How do I contribute

I am new to this. I would like to contribute to the discussion. Have I started correctly? R F Norgan — Preceding unsigned comment added by 2.96.243.252 (talk) 12:57, 11 February 2013 (UTC)

Please sign your talk page messages with four tildes (~~~~). Thanks.

Looks like you have forgotten about

• Talk:Twin paradox/Archive 3#The separation distance
• Talk:Twin paradox/Archive 3#The impossibility of an sr explanation of the twins paradox
• Talk:Twin paradox/Archive 3#Further questions re the article
• Talk:Twin paradox/Archive 3#A more detailed criticism of the article
• Talk:Twin paradox/Archive 3#A different version of the Twins Paradox
• Special:Contributions/86.129.192.176.

Cheers - DVdm (talk) 15:01, 11 February 2013 (UTC)

Distance travelled.

My apologies if I am covering now what has already been covered in the past. The current resolution of the Twins Paradox with SR employs a consideration of the distance traveled as seen by each clock. This is surely totally unnecessary as the experiment is designed to compare the measurement of elapsed time between two events (separation and re-union) as measured by two different clocks. Also the employment of distance in the Resolution appears to be faulty. R F Norgan — Preceding unsigned comment added by 2.96.243.252 (talk) 13:06, 11 February 2013 (UTC)

Please sign your talk page messages with four tildes (~~~~). Thanks.

The article talk page is for discussions about changing and improving the article, not for discussions about the subject. For questions about the subject you might try the wp:Reference desk/Science. Anyway, in our article, the sections Twin paradox#Resolution of the paradox in special relativity, Twin paradox#Difference in elapsed time as a result of differences in twins' spacetime paths and Twin paradox#Difference in elapsed times: how to calculate it from the ship don't seem to mention distances. - DVdm (talk) 15:00, 11 February 2013 (UTC)

Specific example

I am rather confused after reading the above replies. If an article is wrong then putting it right is surely an improvement. The distance that I referred to was mentioned under Specific Example. I was in error when i said previously that it was under Resolution. Nevertheless i did expect a reply to my point.I would appreciate one now from someone.

I still do not understand what one does with the four tildas 2.96.243.252 (talk) 10:37, 12 February 2013 (UTC) R F Norgan 12 Feb2013

In the section Twin paradox#Specific example distances are used, not specifically to resolve the paradox, but to explain to the reader what the observers would measure and to show that this is consistent with length contraction. The distances are correct. I have added two sources supporting the analysis in the section.

About the four tildes: when you type them, the Wikipedia system replaces them with your signature. Look at the top of the edit window. It says:

"This is a talk page. Please respect the talk page guidelines, and remember to sign your posts by typing four tildes (~~~~)."

DVdm (talk) 11:23, 12 February 2013 (UTC)

My point is a simple one. The twins paradox is a time dilation experiment. That is it is the comparison of the time keeping of two identical clocks measuring the elapsed time between the same two events. The distance traveled by either of the two clocks is totally irrelevant to the experiment. No argument, in the archives or not, can logically deny that point. Thus this section is irrelevant and only serves to confuse the reader. It should be removed.

However, if you have an argument against this point please make it.

I still dont understand the four tildas.2.96.243.252 (talk) 09:49, 13 February 2013 (UTC)

I don't think the section is irrelevant. It has been here for a long time, and it is backed by reliable sources, so the de-facto consensus seems to be that it is appropriate to keep it.

About talk pages, please have a look at the wp:talk page guidelines and specially at wp:Talk page formatting, where it is explained how we format discussions here. I have removed the new section header and indented our remarks according to the guidelines.

About the four tildes: you typed them and the system has replaced them with your signature. Good! - DVdm (talk) 11:39, 13 February 2013 (UTC)

I am afraid that I cannot yet see how to add indented comments to an existing topic.

As to your reply to me I cannot accept that you and many others disagreeing with my point, or indeed the length of time that Specific Example has existed, are scientific arguments. If you have a scientific argument to counter my point then please make it. If you do not then it is only proper for you to concede.2.96.243.252 (talk) 13:20, 13 February 2013 (UTC)

I have reformatted our comments again, using colons. Each colon increases the level of indentation. At the bottom of the editing window, you find a link to Editing help.

About the Twin paradox#Specific example section: our articles are supposed to report the current mainstream scientific view. It is not our job to comment on it or to make, propose, or even discuss changes based on our own "original research" — see wp:NOR. The section is backed by two mainstream sources, so there is no compelling reason to remove it at this point. Perhaps other contributors will comment on this — see wp:consensus. Cheers - DVdm (talk) 14:13, 13 February 2013 (UTC)

I cannot see a way to get into the page as it does not have an edit facility. Would you assist me further on that point.

I will be very clear on this next point. I am not changing accepted scientific theory in the slightest. You are improperly accusing me of doing so. In special relativity time dilation and length contraction are two separate effects. If you disagree with me then state your scientific case--or concede. It appears that you have no argument. Where do we go from here?

Wiki is asking that this subject should be made more understandable. Removing irrelevant text helps towards that end.2.96.243.252 (talk) 14:33, 13 February 2013 (UTC)

About editing: perhaps you have made your comment by clicking the New section link on top of the talk page. The idea is to edit this current section, not to create new sections. You can edit the current section by clicking the [edit] link next to the section header.

About the section: the best thing we can do at this point, is wait for other contributors to comment.

Meanwhile, it might be a good idea to sign up for a username. I have put a welcome message on your talk page with some handy links. One link explains how to create a username and why it's a good idea to have one. Good luck. - DVdm (talk) 15:03, 13 February 2013 (UTC):

I hope that I have this right for posting. I do not see what new contributors are going to give if they too do not provide scientific explanations of their position, as you have not done. I will not be swayed by 1000 contributors who are merely trying to support the status quo but without scientific justification. I suggest that 'other contributors' are dispensed with and we move on to the next step -- whatever that may be. I have to say that I am very disappointed in your lack of solid replies to my criticism. On the other hand you have been very helpful. I will look into the username situation. I am already a financial contributor to Wiki.2.96.243.252 (talk) 15:57, 13 February 2013 (UTC)

Well, Wikipedia is not really a place where we, as contributors, are supposed to provide scientific explanations of our positions. We are supposed to adequately reflect the mainstream —status quo, like you say— scientific viewpoint. Given the sources, I think the current section does that very well.

Also note that Wikipedia is about collaborating and consensus, and I don't think that your suggestion "that 'other contributors' are dispensed with" would be appropriate - see wp:consensus. - DVdm (talk) 17:00, 13 February 2013 (UTC)

The question is this. Should Wiki reflect the views of the majority of its contributors when that majority mis-applies current accepted scientific theory where that theory is accepted as crucial to the explanation of the Wiki topic in question. It would appear that you would rather have a false Wiki explanation as long as the majority believed it. That approach is undoubtedly the easy way out as it avoids argument and critical thought. But by taking that course you are helping to perpetuate false beliefs via Wiki.Perhaps some of the contributors who accept the current false version of the twins paradox explanation did so simply because they too do not wish to challenge what other people have written before. As a contributor to Wiki it is surely your duty to see that wiki explanations are correct -- not to limply pursue the consensus.2.96.243.252 (talk) 10:29, 14 February 2013 (UTC)

You talk about "the current false version of the twins paradox explanation". First, the section is not an explanation of the twin paradox, but a mere introductory example. Second, it looks perfectly correct and appropriate to me, and probably to all who contributed to it in the past. Third, it is properly sourced. So, without a wp:consensus there really is no compelling reason to remove it. - DVdm (talk) 11:31, 14 February 2013 (UTC)

First I did not think that Wiki was in the business of offering introductory examples. Wiki is asking for an explanation which is more understandable.You are in disagreement with Wiki in that you think no changes are required.

Furthermore, even though I point out the indisputable fact that time dilation and length contraction are two separate and distinct effects you continue to support their muddling together in the current Wiki explanation. The position that you take is as an unshakeable guardian of the status quo regardless of Wiki's desires or any other criticism however sound. This is not the position of a scientist rather that of a politician. Is their a channel within Wiki to which we can now take our dispute?2.96.243.252 (talk) 12:06, 14 February 2013 (UTC)

Usually the next step is to wait for other contributors to comment, and then take it from there. I already pointed to our policy wp:consensus a few times. Please read it, and you will also find other "channels within Wiki" there. Cheers - DVdm (talk) 14:19, 14 February 2013 (UTC)

Yes I can fully understand the merits of a policy of consensus. But if nearly everyone affirmed that the Earth was flat, as at one time they did, does that mean that, in the face of evidence to the contrary, Wiki would state that the world was flat in its description. Because most people do not understand special relativity ( and I include many academics here) are those that do understand it condemned to conform to the majority?. I have made a very simple statement that the measure of the time interval between two time displaced events requires only identical clocks to make that measurement. Yet you argue that I am wrong.2.96.243.252 (talk) 14:43, 14 February 2013 (UTC)

I don't argue that you are wrong about that statement. I argue that distances can be mentioned in the introductory example. The sources do it, so Wikipedia can do it. - DVdm (talk) 15:49, 14 February 2013 (UTC)

Yes, but what is the point of it? Distances are totally irrelevant to the measure of time. This section merely serves to confuse the article. Wiki want to make the article more understandable so removing irrelevant stuff is a step in that direction. You are fighting against the wishes of Wiki. Your whole argument seems merely to be the defense of the status quo, warts and all.92.17.73.236 (talk) 10:13, 15 February 2013 (UTC)

I don't agree that it serves to confuse the article. Per long-time status quo and cited sources, quite on the contrary. Assuming that both 2.96.243.252 and 92.17.73.236 belong to you (R_f_norgan (talk · contribs · deleted contribs · logs · filter log · block user · block log)), let's see what others, if any, have to say. Cheers & patience. - DVdm (talk) 10:29, 15 February 2013 (UTC)

Well, something is confusing readers according to Wiki. If it is not Specific Example perhaps you would care to give me your own opinion as to what it may be. I would also like to point out that just because something is accepted for a long time does not make it correct. Furthermore why do you trust your sources when they are in disagreement with the originating source, Albert Einstein. The problem with patience is that it is a euphemism for the manana syndrome. Nothing ever gets done.92.17.73.236 (talk) 11:33, 15 February 2013 (UTC)