- #26

Jimster41

Gold Member

- 750

- 80

I keep thinking about this one... I can understand that Newton's approximation is a sufficient "how". It will provide an accurate a prediction of rotation (for all practical purposes).

But I think the OP's question was "why" (and I share that question).

We could use Newton if we wanted prediction of "how", but isn't that stopping short? Don't we know that it is a superficial answer because we have accepted that Einstein's "how" is a better... "why".

So it seems to me that to answer the OP, we need to try and explain the initial rotation from the perfectly motionless cloud set up by @Bandersnatch using GR. Am I wrong in invoking "Frame dragging" and the "Lense Thirring" effect to understand it? Is there a more correct/direct GR explanation?

[Edit] Even if it is not a montionless initial setup, the question of how gravity "propagates" angular momentum seems relevant, aside from the Newtonian approximation. Or is it all happening via "collisions"? What about the case where the density is low, and collisions are rare?

[Edit] I just saw the two more recent posts above regarding initial temperature. There was an initial finite temperature correct? If there was, doesn't uniform random motion reduce to motionlessness. Don't all the Newtonian moments cancel for any given particle over any interval of action?

But I think the OP's question was "why" (and I share that question).

We could use Newton if we wanted prediction of "how", but isn't that stopping short? Don't we know that it is a superficial answer because we have accepted that Einstein's "how" is a better... "why".

So it seems to me that to answer the OP, we need to try and explain the initial rotation from the perfectly motionless cloud set up by @Bandersnatch using GR. Am I wrong in invoking "Frame dragging" and the "Lense Thirring" effect to understand it? Is there a more correct/direct GR explanation?

[Edit] Even if it is not a montionless initial setup, the question of how gravity "propagates" angular momentum seems relevant, aside from the Newtonian approximation. Or is it all happening via "collisions"? What about the case where the density is low, and collisions are rare?

[Edit] I just saw the two more recent posts above regarding initial temperature. There was an initial finite temperature correct? If there was, doesn't uniform random motion reduce to motionlessness. Don't all the Newtonian moments cancel for any given particle over any interval of action?

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