Talk:Ehrenfest paradox

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To-do list for Ehrenfest paradox: edit · history · watch · refresh

The artcile is wrong. Rather than misinforming the readership, it should be taken offline until it can be repaired.

I am not familiar with Wikipedia's quality assurance guidelines and policy, and I have not examined the article in depth, but I can see that the top right figure entitled "Ehrenfest's train at 0.6 c..." is wrong (note that the connectors between the box cars along with the box cars would appear to contract since the spacetime, in which both the box cars and the connectors are embedded, contracts - hence the paradox). The text and calculations following the figure are also incorrect, but I don't have the time to correct them. Hence my recommendation. (Softcafe 02:33, 21 October 2007 (UTC))

I'd like to see a resolution of this paradox in some clearer language. In particular, how does a rotating disk appear to an outside observer? - (unknown user)

The following page contains links to animations that show several visualization expert's interpretation of this phenomena: http://www.gravitywaves.com/visual/vis_1.html

[s/ softcafe (sign-in doesn't work) 03:50, 13 January 2008 (UTC))]

I agree with Softcafe that the article is completely wrong. The analysis is nonsensical and overlooks the widespread authoritative view (of John Archibald Wheeler among others) that the "contraction" is merely an apparent effect (though the measurement may be real) due to "relativity of simultaneity" causing the forward end of a moving length to be logged slightly before the rearward end. So nothing physically shrinks and there is no paradox. Any sufficiently short segment of the moving rim can be treated symmetrically just like two inertial frames moving uniformly past each other. The historical survey is also erroneous and misleading.

This is the talk page for discussing improvements to the Ehrenfest paradox article.
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[edit] Ehrenfest 1909

The article is in danger as being classified in Category:Articles with way too long talk pages. Let's try to fix the most urgent things and take a break from this for some time, yes?

IMHO, the single most important thing to fix, is the (mis-)representation of the Ehrenfest 1909 paper. As said above, stripped from historical language and from the presentation form of reductio ad absurdu, it simply states:

A disk cannot go from rest to rotation while maintaining Born rigidity.

It doesn't say what a disk will do, let alone a "real disk" with some real material properties. It only says what it will not do. Maintain rigidity.

It may be the case that the presentation is long standing problem, caused by translations issues or only ever quoting some sentences from the paper.

Pjacobi 03:59, 12 June 2006 (UTC)

Hi, Peter, isn't that what the current version says? If not, please make minimal changes as you see fit and I will review when I get a chance. ---CH 19:36, 13 June 2006 (UTC)
I have to log off in minute, but perhaps later.
It's currently: Ehrenfest was trying to argue that Born's notion of rigidity is in fact incompatible with special relativity.
It should be something like Ehrenfest stated the now common fact, that most motions of extended bodies cannot be Born rigid and gave the disk going from rest to rotation as an example
19:58, 13 June 2006 (UTC)
Amen! Harald88 06:18, 12 June 2006 (UTC)
I suggest having some respect for the fact that Chris is not wrong very often, and that this "paradox" seems to drive everyone up a wall since it is so easy to misconstrue. In any case, I am in agreement with you two on what Ehrenfest was trying to say in 1909, and the need to correct this article to properly describe it. If Chris does not modify the article soon then I will take a stab at it. --EMS | Talk 14:35, 12 June 2006 (UTC)

I'd like to do this, although I am approaching exhaustion re all these arguments. I archived most of the discussion as per PJacobi.---CH 23:42, 13 June 2006 (UTC)

I'm currently struggling to find the time to edit this. If you are committed to editing the article soon, I can hold off for a bit. (I would prefer to see how you change the article now and then make any additional changes if there should still be a need.) --EMS | Talk 04:27, 14 June 2006 (UTC)

[edit] Clean-up

I now made a start with the clean-up (see Talk page and archived Talk page). Notably, I included including Pjacobi's intro and merged it with CH's writings, also refining it at some points, and corrected the discussed errors.

- The following text probably still needs reworking and/or corrections:

*1911: Ehrenfest notices that if we try to spin-up an initially non-rotating disk, we cannot do this while maintaining Born rigidity.
*1916: While writing up his new general theory of relativity, Albert Einstein notices something overlooked for seven years: the disk-riding observers measure a longer circumference, C′ = 2π r √(1−v2)−1, not a shorter one, not C′ = 2π r √(1−v2). That is, because rulers moving parallel to their axis appear shorter as measured by static observers, the disk-riding observers can fit more small rulers of a given length around the circumference than stationary observers could.
*1922: Henri Becquerel incorrectly claims that Ehrenfest was right, not Einstein.

- The section "Resolution of the paradox" will likely also need a few minor corrections; and still lacking is a clear account of the contraction factor (the "solution"!) of a rotating disc.

Harald88 21:43, 14 June 2006 (UTC)

I don't consider this "mixed" version a very lucky one. If I had a clear idea, how to consolidate the changes to be done with CHs version I would have done it myself.
I now have clear view of the Ehrenfest 1909 paper, but I would have to read some other of the "old" papers to have a clear vision. And I definitively don't have the time for it now.
Pjacobi 18:43, 15 June 2006 (UTC)
Neither do I; IMHO a complete overhaul is required, and I also would need to read more of the other papers. But time was moving on and I risked to forget all the points that were discussed. The new version is rougher but less wrong.
I already suggested a solution to reduce the "mixed" presentation, also now in the archives: to dedicate a separate article to rotating reference frames, with cross references between these articles. In that case, much of this article would be transferred to that one, leaving the emphasis of this article on the deformation of the rotating disc as compared to the same disc before rotation. IMO such a presentation will be much more kind on the readers and strongly reduce confusion. Harald88 20:05, 15 June 2006 (UTC)
I will go along with the view of the current version being "rougher but less wrong". However, I cannot support a complete rewrite of this article. Instead, what is needed is some follow-through on the current efforts to build on what Chris has done but with the idea of making the article less technical and more readable. The "Resolution ..." section itself is badly in need of a rewrite, however. After that is done we can take stock and figure out where to take this article next.
Overall, this is a tricky topic and one on which a lot of research has been done, and on which said research continues to this day. Even so, I see no good reason why a clear and accurate article cannot be written about it. As vexing as it is, the Ehrenfest paradox is not something like the Riemann tensor. --EMS | Talk 21:44, 15 June 2006 (UTC)

When I deleted ChrisH's "Essence of the paradox" section it was because it is irrepairably incorrect, based as it is on a misconception regarding special relativity. It is a feature of SR as distinct from Lorentz's earlier theory, as we more or less agreed on the BSP talk page, that contraction does not actually occur in the "moving" frame where "rest" lengths are unchanged, but is only exhibited by measurements made from the "stationary" frame. It follows from this that there are two crucial errors in the first paragraph.

(1) All lengths will be measured by static observers to be contracted in the tangential direction by the same factor (gamma) so both boxcars and bungees will be measured at about 32 inches - it is absurd to suggest that the bungees stretch to "make up the circumference". SR makes no distinction between boxcar or bungee.

(2) An observer traveling in a boxcar will find all measurements that he can make in his vicinity to be the same as when at rest. He can be supposed to be able to move along the circumference checking both bungee lengths and boxcar lengths, both of which according to SR will be unchanged at 40 inches - so again it is quite absurd to suggest the bungees would have stretched to 60 inches.

Taking care not to slip into Lorentz's mode of thinking where contraction takes place in the moving frame but is conveniently undetectable, we can say that the intrinsic geometry of the disc for an observer riding on it, is Euclidean - as indeed it must be in the absence of any gravitational field. The "paradox" only exists for the static observer, whose necessarily local measurements of portions of the circumference will show contraction compared to when at rest. Obviously both circumference itself and measuring rods placed along it will be measured identically (shorter by the same factor), so the idea that more measuring rods will fit around the perimeter is also absurd and incorrect.

Almost all the confusion surrounding this problem has been caused by using Lorentz's theory, which is quite unable to cope with this kind of motion and which leads to the false idea of "relativistic stresses" in the material of the disc. To reach relativistic tangental speeds there will certainly be very large centrifugal stresses but there can be no stresses from any "contraction" that according to SR does not take place in the moving disc itself. Rod Ball 10:23, 21 August 2006 (UTC)

You are in severe disagreement with scientific consensus. Please don't edit this article. --Pjacobi 11:29, 21 August 2006 (UTC)

Claims that the circumference is larger than 2πR (due to more measuring rods fitting in) together with claims that it is less, do not represent what is normally understood by "consensus". The continuing contemporary articles show the problem is still unresolved in that sense. Rod Ball 13:27, 21 August 2006 (UTC)

I am in complete agreement with Pjacobi. Rod - You should not be editing this article, or any relativity-related article IMO. You write above that:
Almost all the confusion surrounding this problem has been caused by using Lorentz's theory.
I assure you in the strongest terms that we are using Einstein's theory. The real question is what it is that you are using. --EMS | Talk 01:19, 22 August 2006 (UTC)

It would be amusing if it were not so tiresome, that yet again you avoid any technical remarks that might support your assertions (or not) and resort instead to ad hominem argument. As you have not produced anything for Ehrenfest paradox I can't judge what you may use, but since you seem to have difficulty distinguishing SR from Lorentz's theory it might just as easily be one or the other. I honestly think you should study the subject rather more carefully before venturing an opinion on who should or should not be editing such articles. In fact, a complete rewrite of this one is necessary, as the current version is hopelessly confused and inaccurate. Rod Ball 08:58, 22 August 2006 (UTC)

Can we have an explanation at the level of an advanced undergraduate textbook of what's going on? In particular, when the static observer looks down on the rotating disk, what will he measure? What will it look like as it spins up or slows down? And why will this be? Joe Sept 4, 2006

Try these:
http://arxiv.org/PS_cache/physics/pdf/9808/9808001.pdf
http://www.citebase.org/fulltext?format=application%2Fpdf&identifier=oai%3AarXiv.org%3Aphysics%2F9808001
Rod Ball 14:59, 7 September 2006 (UTC)
That's easy, and sure, the article should state such elementary things more clearly and upfront: If we neglect the centrifugal effect (thus in theory), then the diameter of a rotating cylinder would reduce as calculated by several authors by an amount that is less than the Lorentz contraction factor. This because the central matter prevents full Lorentz contraction. In practice the centrifugal forces are more important.
This article is at the moment indeed still in a rudimentary state. It's work in progress: not enough editors with limited time work on too many articles.
Harald88 18:21, 5 September 2006 (UTC)
Very different approaches should not be confused, as first Max Planck and recently Gron pointed out. That some articles as well as some editors do confuse them, is counterproductive. Harald88 21:52, 7 September 2006 (UTC)

On the contrary there is no justification for supposing any relativistic contraction of the radius (diameter) - for this is what essentially leads to the "paradoxical" conclusions. Only very early incorrect analyses by Lorentz and Eddington claimed radius contraction. ( Most early discussion of this problem was misguided.) AFAIK all modern approaches consider for which observers the circumference may appear contracted. Rod Ball 12:21, 7 September 2006 (UTC)

Very different approaches should not be confused, as first Max Planck and recently Gron pointed out. That some articles as well as some editors do confuse them, is counterproductive. Harald88 21:52, 7 September 2006 (UTC)


Another possibility is that although the radius of the cylinder(or spinning ring of radius R for simplicity) cannot contract(because its perpindicular to the direction of motion) the angle in the Length formula for the circumfrence could contract instead(whoooa... how simple is that!) leading to new circumfrence of a spinning ring(for simplicity) of (2(pie)Y^-1), where gamma is the realivitic factor and the velocity of a point on the ring is R(w) where w is the angular velocity. —The preceding unsigned comment was added by 24.163.95.198 (talk • contribs) 16:04, February 22, 2007 UTC (UTC)

I think that you are very mistaken about what is going on here. The space does not contract, but the contents do. --EMS | Talk 04:21, 23 February 2007 (UTC)


This is complete nonsense ! Special Relativity does not predict either that the rim contracts nor (even more ridiculously) that it expands because "more contracted measuring rods fit along the rim" (as if the rim and the rods could be made of relativistically different "stuff" - obviously the measurement graduations could be marked on the rim itself).

As has been pointed out time and time again over the years, the "contraction" of special relativity - unlike that of Lorentz - is only an apparent effect. It results from a difference in simultaneity in marking the position of the end points. It does not represent a real "physical" shrinkage.

It is blindingly obvious that this must be so (in Einstein's theory) because of the symmetrical relationship such that A's measurement of B's meter rod is less than A's own meter whilst reciprocally, B's measurement of A's meter rod is less than B's own meter. This is perfectly possible as a kind of "perspective effect" with velocity rather than distance causing the apparent effect.

What is clearly completely impossible is that B's meter rod is less than A's meter whilst A's meter rod is less than B's. Clearly anyone who seriously believes that has not only failed to understand SR, but has taken leave of rationality altogether.

Thus using a correct interpretation of Einstein's theory we find that, for limited segments of rim over which "simultaneous" marking of positions may be performed, the rim-riding observer will measure apparently "contracted" sections of stationary surround, whilst a stationary observer will measure apparently "contracted" sections of moving rim.

Thus Ehrenfest's "paradox" (originally due to old Lorentian notions of contraction) can simply (and only) be resolved by applying the correct modern view of "relativistic contraction" as a apparent effect. Only the measurements are supposed to be taken as real in the same sense as a "Doppler effect" appears to show a change in frequency - but of course the "real" frequency is unaltered. —Preceding unsigned comment added by 212.85.28.67 (talk) 12:04, 3 May 2008 (UTC)