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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

What happens to rotating solid objects in relativity theory?

In special relativity, there are actually no solid bodies. On a rotating disk on a fast enough record player, the center is stationary, but because points on the circumference are moving, their proper time is different that what is measured at the center of the disk. Between the center and the edge, there is a continuous range of special relativistic time and space distortions being generated from the perspective of someone sitting at the stationary center of the disk. This is known as the Ehrenfest Paradox. There will be no disagreement on the length of the radius of the disk by anyone, however, for any segment on the circumference, they will see less than 2 x pi x R for the circumference. Despite the fact that to us the disk appears solid and rigid, as an OBSERVABLE object subject to special relativity, it is an almost plastic object capable of being deformed as judged by the images it presents us which are formed by the propagation of light.

In special relativity, there are no rigid bodies as well, because such a body is defined by the distance between each atom being fixed and not changeable. When one part of the body comes to a stop, every atom also comes to an instantaneous stop at the same instant. In relativity, there is no way in which the information can travel faster than light, so that there is no way that a 'rigid body' can come to an instantaneous stop.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.