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A Friction in Vacuum

  1. Nov 2, 2017 #1
  2. jcsd
  3. Nov 2, 2017 #2


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    2017 Award

    Staff: Mentor

    The article in question: (PRL - 5 pages)
    Follow-up article by the authors: (Journal of Modern Optics - html)
  4. Nov 2, 2017 #3


    Staff: Mentor

    Ignore the pop science article; as so often happens, it doesn't give a good description of what the paper actually says.

    What the paper actually says can be described as follows:

    (1) Consider an atom in an excited state that emits two photons of equal energy in opposite directions. Since the two photons carry equal but opposite momenta, their recoils on the atom cancel and the atom's state of motion should not change.

    (2) If we analyze this process in the initial rest frame of the atom, we find that, indeed, the atom's state of motion does not change; it remains at rest, and simply loses energy, and therefore mass (here and hereafter, "mass" means "rest mass" or "invariant mass"), equal to the energy carried away by the emitted photons.

    (3) If we analyze this process in a frame in which the atom is moving, we find what appears to be a "paradox": the momentum of the atom changes (whereas in its rest frame it didn't--it remained zero). However, a more careful analysis shows that this momentum change is entirely due to the change in the mass of the atom; its velocity does not change. So there is no actual inconsistency with the principle of relativity; it is just that you have to be careful when analyzing scenarios where the rest masses of objects change.
  5. Nov 2, 2017 #4
    What is recoil shift? Does it affect doppler shift?
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