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Quartersharp t1_j0fnrfp wrote

Reply to comment by [deleted] in Does rotation break relativity? by starfyredragon

It does… a little bit. A rotating object will have its outer parts under tension because of the centripetal acceleration, which wants to pull them outward. Unless the object flies apart, it is doing a tiny amount of work by staying intact while rotating, because its outer parts have to keep changing direction. Because of this, the object will also radiate gravitational waves, and will very gradually slow down, probably over thousands and millions of years.

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rpetre t1_j0forax wrote

Nitpick: "centripetal" means "towards the centre", so in your example the tension IS the centripetal acceleration, it's what keeps the parts of the body on their respective circular trajectories.

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wasmic t1_j0g22v9 wrote

No. There's no work being done from the rotation itself, if the object is perfectly rigid. Of course, in the ideal case there is work being done due to tiny stretches all over the place, but that should all cancel out because it both stretches and contracts to keep the same shape. With no overall radial motion coaxial with the force, there is no work being done.

Also, a spinning sphere, or a cylinder spinning around its own axis, will not emit gravitational waves. But something like a rotating cog would emit gravitational waves.

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Putnam3145 t1_j0g7zxw wrote

A perfectly rigid object has a faster-than-light speed of sound, among other problems, and is thus unphysical

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PefferPack t1_j0fu27b wrote

>it is doing a tiny amount of work by staying intact while rotating, because its outer parts have to keep changing direction

Really? This is kindof mind-blowing if true. Work is force dot displacement, so if there's no displacement in the radial direction, then there's no work done. If the body can be considered rigid then there is no radial displacement. Even in a flexible body, there would only be a short transient period of radial displacement as it stretches out from the rotational forces.

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wasmic t1_j0g1tt6 wrote

Yeah, I don't think there's a connection between gravitational waves here, and there's no work being done to keep a rotating object together.

Gravitational waves are not emitted from spherically or cylindrically symmetric objects, either (provided the cylindrical axis is the rotational axis).

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