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Today I found out the Sun's rotation is slowing down. I can see that the slight angle (on average) of the photons leaving a star, combined with the incoming nature of the surface as seen from that perspective, should transfer some energy into making the star's radiation a higher frequency. But does that alone carry away all of the star's rotational energy -- or is there energy somehow associated with the angular momentum we can see is retained in the pair of diverging photons? (Part of what is perplexing me is how the angular momentum of the universe, which I had been thinking of as a constant, seems to depend dramatically on the precise positioning of the photons of ever-expanding starlight...)

There is a related question about the angular momentum here, which gets into rotational braking by solar wind... but for the moment, to keep things simple, I don't mind if you give an answer for the Poynting-Robertson effect on a hot black ball that doesn't have any particles around it.

In the Poynting-Robertson effect, a star's outer layer emits light in all directions, but from its own rest frame. So the light flies out ahead of the star's rotation, and the reaction slows it down. The missing angular momentum can be seen in the offset between photons (green) from opposite sides of the star. The yellow ring represents either the surface of the star or particles further out in orbit that are in equilibrium with it.

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  • $\begingroup$ can you add a link to the PR effect? I've never heard of it. $\endgroup$
    – JEB
    Commented Nov 8 at 13:27
  • $\begingroup$ also: isn't the angular momentum of the universe coordinate dependent? $\endgroup$
    – JEB
    Commented Nov 8 at 13:28
  • $\begingroup$ @JEB - I added the link that I found most helpful, though I tried to simplify the concept further. As far as the universe... I'll admit, I have no clue. Two moving objects with a torque arm between them have a fixed angular momentum, but when two photons go outside of each other's observable universe where does it go? Is there some holographic recollection like where the gravity comes from in a black hole? Well, anyway, that's for some other question, unless there's a way to tie it in to make this make sense. $\endgroup$
    – Mike Serfas
    Commented Nov 8 at 14:49

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Light can carry angular momentum. So when a rotating object is slowed by the Poynting-Robertson effect, it is the electromagnetic field that carries away the angular momentum.

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  • $\begingroup$ Sorry, I was actually asking about the rotational energy. I did get sidetracked on the angular momentum only because I wondered if there was energy associated with it. Also, the pair of photons moving in opposite directions with a torque arm between them clearly have angular momentum that is not on any single photon... I think? (Oh no, I did it again... hard to think about one without the other coming up) $\endgroup$
    – Mike Serfas
    Commented Nov 8 at 14:51
  • $\begingroup$ @MikeSerfas - If the angular momentum goes (and the object does not change its moment of inertia), then of course the rotational energy goes too. $\endgroup$
    – Anders Sandberg
    Commented Nov 9 at 10:16

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