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The Ehrenfest paradox suggests an object will become smaller at the outer rim due to contraction, When near light speed. Or actually, any object near light speed will contact and appear smaller in its length of direction.

Will this contraction emit photons?

Question 2: Ehrenfest paradox, i quite never understood, probably cos i don't see it as a paradox: if a line is fixed on the diameter on the rotating disk, each point on the diameter will also experience contraction in length if its also to rotate, but since all points on the diameter is connected it would ultimately become smaller too?

And if the diameter is not part of the rotating disk, of course, it would remain the same length?

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    $\begingroup$ Please use standard spelling and capitalization, not text-speak. $\endgroup$ – user4552 Oct 16 '19 at 13:21
  • $\begingroup$ How did the disk start rotating in the first place? If the accelerration is uniform in the lab frame (meaning that the clockwise acceleration always has the same magnitude at every point on the circumferenc) then obviously the circumference cannot contract. $\endgroup$ – WillO Oct 16 '19 at 15:48
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Will this contraction emit photons?

Generally, electrically neutral objects do not radiate when they are subjected to acceleration unless the acceleration highly upsets the uniform distribution of the negative (electrons) and positive (protons) charges inside matter.

if a line is fixed on the diameter on the rotating disk, each point on the diameter will also experience contraction in length if its also to rotate, but since all points on the diameter is connected it would ultimately become smaller too?

And if the diameter is not part of the rotating disk, of course, it would remain the same length?

I think you misunderstood the paradox. Since the diameter is perpendicular to the tangential velocity of any infinitesimally small rim arc, it never undergoes any length contraction contrary to the rim arc itself which is Lorentz contracted due to the tangential velocity. The paradox arises from the fact that if the circumference of a rotating rim is contracted while the diameter remains unchanged, the rim should shatter which seemingly violates the Born-rigidity of the disk as considered by Ehrenfest himself. See the Wiki article. It seems that the paradox is resolved by considering the non-Euclidean geometry of the disk as viewed by a co-rotating observer.

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