Talk:Ehrenfest paradox

message: The explanation contains errors which make it impossible to follow, the length of the boxs suddenly changes half way through the text from 40 to 32 long.  —Preceding unsigned comment added by 83.100.232.37 (talk) 19:06, 11 November 2009 (UTC)
thread_title: Untitled

message: I am confused. I am a layman with strong interest in SR, just the kind of person for whom this article should be written.
First I am told that C should be shorter, due to Lorentz contraction. That is not obvious, so I imagine a lot of rods along the rim on the surface of the disk, I know they will appeat shorter (because of Lorentz contraction). If they are numbered (1, 2, ...,n) both parties have to agree on what number is marked on the last rod necessary to complete the circle. If the rods are 1m long, but the Lorentz contraction renders them, say, 0.5m as seen from the intertial frame, the person on the rim will say C is n meters long, and the person on the inertial frame will say it is half that. It should be relevant to explain what things would be like from the view point of the observers on the rim if a similar arrangement of rods is done on the circumference just outside the disk on the inertial frame. 
But then the article goes on to say that "later reasoning" would make it possible to fit more rods along the rim. That is manifestly absurd. Epovo (talk) 19:58, 28 April 2009 (UTC)
thread_title:  viewpoint of a non-expert 

message: The "brief history" section states: "1922: Henri Becquerel claims that Ehrenfest was right, not Einstein."
This is obviously impossible given that Henri Becquerel died in 1908. There are no citations either. Maybe it's referring to his son Jean Becquerel instead? Wopr (talk) 11:20, 27 July 2010 (UTC)
replies: text: Yes, it was Jean, see p.7 of Grõn's 1975 paper.  Vaughan Pratt (talk) 00:34, 31 May 2013 (UTC)
thread_title:  Error(s) in the "brief history" section? 

message: This explanation with the train cars doesn't seem right to me.  If you're moving at 0.6 c, then the track ought to experience a Lorentz contraction of 80% - so it's not a round track, but an ellipse with the long axis passing from your car toward the center.  The next car should therefore be almost 80% closer to you.  The car at the opposite end will be greatly foreshortened.  The idea of totting up the lengths of all the cars regardless of their local frame and getting 1200 is just bogus.  The major source (1) does pose a somewhat similar question, but only in the narrow context of considering the lengths of Born rigid rids from a single frame of reference stationary in regard to the ring. Wnt (talk) 16:28, 3 August 2010 (UTC)
thread_title:  It isn't a round track, is it? 

message: As a non-physicist with mathematical training, I am fascinated by this paradox but unfortunately can't make full sense of the article, and feel that it could improve substantially with some cleanup by an expert. In particular,
# The "Resolution of the paradox" section lacks citations, and seems to read more like an editorial about various resolutions of the paradox than an NPOV synthesis of them.
# That section also isn't comprehensible to a layman. In particular, the central question raised when the paradox is presented -- to an observer in a stationary reference frame, does the ring contract, or no? -- is never answered (or if it is, the answer isn't clear). For the record, from reading this and related articles, my understanding is that the core misconception is the attempt to naively apply relativity to homogeneous rigid bodies, which do not physically exist, and that this same misconception is the source of many other relativity paradoxes (e.g. Bell's.) If we instead view the ring as being made of real material (point masses held together by inter-particle forces) the paradox is resolved: as the angular velocity increases the ring experiences greater and greater stresses due to Lorenz contraction; how much this affects the ring's circumference depends on the material's Young's modulus. There is no contradiction because, although the circumference of the ring does change, the forces causing the shrinking of the ring are indeed perpendicular to the ring's radius. 
# From what I can tell by reading the discussion here, there used to be an intuitive example of the paradox using a train track, that has since been removed for being inaccurate. It would be nice if an intuitive (and correct) illustration of the paradox's resolution were re-added. TotientDragooned (talk) 20:30, 22 August 2010 (UTC)
thread_title:  Cleanup needed 

message: Most of the Resolution section was about errors instead of resolution.  I moved all discussion of errors into a new section.  —Codrdan (talk) 06:30, 27 August 2010 (UTC)
thread_title:  Resolution vs. errors 

message: Please correct me if I'm wrong, but the simplest explanation of the paradox's resolution seems to be missing entirely.  Here are some questions:
#The first section discusses the POV of an (accelerating) observer on the circumference, but shouldn't there be a discussion in the COM frame?
#The second section discusses practical issues of imperfect materials.  Shouldn't it be possible to minimize materials problems by using a huge cylinder (large R) to avoid excessive acceleration?  
#The first section needs to be explained better.  From what I've read so far, my impression is that C=2πR (the flatness of spacetime) breaks down for the noninertial observer and the pieces of the rotating object would have to either stretch or separate from each other.  This is consistent (or at least not obviously inconsistent) with current theory, because acceleration is equivalent to gravity in GR and SR doesn't apply in noninertial frames.  Is that anywhere near the current scientific consensus?
—Codrdan (talk) 07:43, 27 August 2010 (UTC)
thread_title:  Resolution:  Where's the COM explanation? 

message: This talk page resembles a crash site. Oh dear. Zarnivop (talk) 19:33, 3 September 2012 (UTC)
thread_title: ZOMG

message: # The external link to the Gron paper ("Øyvind Grøn: Space Geometry in a Rotating Reference Frame: A Historical Appraisal. In: G. Rizzi and M. Ruggiero, eds.: Relativity in Rotating Frames. Kluwer, 2004.") is broken...  Too bad, because that paper is pretty good.
# The suggestion in item five of the "misconceptions" section that general relativistic correction might be appropriate to deal with the acceleration is wrong or at least very misleading.  GR is not needed just cope with acceleration; it would be needed if gravitational effects (either from the mass of the cylinder the rotational energy contributing to the stress-energy tensor) were significant, but that's not the case here.  — Preceding unsigned comment added by 66.26.66.148 (talk) 21:26, 2 September 2013 (UTC)
thread_title:  some problems on a quick read 

message: quoted:
"a real disk expands radially"
Note: Because SRT is not talking about physical objects SRT is not a material science!
In material science we know that centrifugal forces drive the radius & the diameter & the circumference too!!
Hence you cannot throw such ideas into the discussion when a theory-internal justification & dissolution is needed.
What you have written there is ridiculous!  — Preceding unsigned comment added by 178.4.25.148 (talk) 13:44, 25 December 2015 (UTC)
thread_title:  Scientific Nonsens  

message: The question should be:
Why do we have to shrink the circumference?
Don't answer: "Because of Lorentz-Contraction", because this idea of deformations of physical objects is not part of SRT.
Don't assume the paradox is resolved if you don't have the resolution!  — Preceding unsigned comment added by 178.4.25.148 (talk) 14:03, 25 December 2015 (UTC)
thread_title:  The source of the misconception is still unknown 

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thread_title:  External links modified 

message: "t discusses an ideally rigid cylinder that is made to rotate about its axis of symmetry" A cylinder has two axes of symmetry. Which one are you referring to?  — Preceding unsigned comment added by 2A02:C7F:C405:700:F5A7:D318:452:192F (talk) 07:22, 11 July 2018 (UTC)
thread_title:  Axes of symmetry 

message: The section "Essence of the paradox" states the circumference of the disc is 2\pi r/\gamma, where I write \gamma = (1-\omega^2r^2)^{-1/2} for the Lorentz factor. But this is blatantly against the consensus view, which is that the amount of "disc material", let's say, is 2\pi r\gamma. Colin MacLaurin (talk) 05:45, 12 March 2020 (UTC)
replies: text: By definition and by Wikipedia policy, the consensus view is the one that appears in the sources, and that is reflected in our articles. If you have a problem with the literature, this is not the place to vent it—see wp:Talk page guidelines. You might perhaps get some help at our wp:Reference desk/Science. Good luck overthere. - DVdm (talk) 16:49, 12 March 2020 (UTC)
thread_title:  Completely wrong 

message: "Resolution of the paradox" section talks about the spatial geometry of a disk in steady-state rotational motion, which is non-Eclidean and given by the Langevin–Landau–Lifschitz metric. Further, the spatial geometry being given by the Langevin–Landau–Lifschitz metric is claimed to be the modern resolution of the paradox. But it should be noted that the actual paradox posed by Ehrenfest is about the transition of a disk from rest to rotational motion and not about a disk in steady-state rotational motion. So, analysis of the spatial geometry of a disk in steady-state rotational motion, while being non-Eclidean (Langevin–Landau–Lifschitz metric), does not resolve the actual paradox posed by Ehrenfest. Mrtompkins1 (talk) 04:47, 16 February 2024 (UTC)
thread_title:  Problem with the "Resolution of the paradox" section