Conservation of angular momentum doesn't seem correct to me. I think this because radiation is present everywhere, no space is completely void, and no object perfect. Thus all objects will have uneven radiation pressure. My question is there anything in space that is not increasing or decreasing in rotation or is fixed in space? Is the rate in which an object spins is determined by surrounding celestial bodies? Or in other words any object in space depending on its position in the galaxy will slow or increase to an ideal rotation?

  • $\begingroup$ I think it is right in a completely static Universe. The radiation pressure of the thermal background will be anisotrop and will slowly decrease the impulse moment (except a perfectly symmetric body). The effect is obviously much longer as the time scale of the decrease of the CMB. Essentially, we can talk about "drag" in the photon gas of the CMB. $\endgroup$
    – peterh
    Commented Jul 5, 2018 at 23:53
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    $\begingroup$ But conservation of angular momentum still holds for the correctly defined isolated system... $\endgroup$
    – DJohnM
    Commented Jul 6, 2018 at 4:17
  • $\begingroup$ Your title doesn't seem to match your text. Can you clarify what you are asking? $\endgroup$ Commented Jul 6, 2018 at 15:03
  • $\begingroup$ @CarlWitthoft is that better? $\endgroup$
    – Muze
    Commented Jul 6, 2018 at 17:57
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    $\begingroup$ Angular momentum is only conserved if we think about masses as points. Since real masses are not points, we have to deal with tidal forces (different amount of force on zillions of particles that happen to have a strong, but not infinite, electrical attraction to each other). $\endgroup$
    – user21
    Commented Jul 8, 2018 at 2:46

1 Answer 1


Angular momentum does not change unless a torque acts upon a body. So your question is just whether the torque on a body could ever be exactly zero. The answer is of course that it can be arbitrarily small but we can never be sure it is exactly zero.

More interesting questions would have been "does the CMB exert a torque on an object?", or "is it possible to define a state of zero absolute angular momentum?".


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