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I Do photons that carry orbital angular momentum have mass?

  1. Feb 11, 2017 #1
    It is known that particles with rest mass cannot travel at the speed of light.
    Can we also say that particles that travel at subliminal velocity, like these OAM photons do, have mass?

    It has been demonstrated [1] that these beams can be thought as made of photons that posses intrinsic OAM, and can be seen as 1/2 spin particles.
    We know that all fermions which are 1/2 spin particles have mass.
    At first I thought that it was straight forward that they have mass, but then I've realised this may not be a general opinion and I've decided to ask.

    http://physicsworld.com/cws/article...ngular-momentum-are-the-latest-twist-on-light [1].
     
    Last edited: Feb 11, 2017
  2. jcsd
  3. Feb 11, 2017 #2

    PeterDonis

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    These photons are not "particles" in the sense that you are thinking of that term. So the correct answer to your question is "mu"--the question is not well posed to begin with.

    This happens to be correct (now that we believe neutrinos have mass), but there is no necessary connection between fermions and having mass; massless fermions are perfectly consistent theoretically (and up until fairly recently we thought neutrinos were such massless fermions, and that caused no theoretical problem at all).

    Btw, the full text of the actual paper is here:

    http://advances.sciencemag.org/content/2/4/e1501748.full
     
  4. Feb 12, 2017 #3
    The mass of a proton is greater than the mass of its quarks constituents but protons are complex systems, not particles. If helical structures are not particles then, can the same thing happen to these structures? Can we say that the mass of the helical structure is not zero?
     
  5. Feb 12, 2017 #4

    PeterDonis

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    What are "helical structures"?
     
  6. Feb 12, 2017 #5
  7. Feb 12, 2017 #6

    PeterDonis

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    Ok, so "helical structures" is just another word for "the spatial part of the photon wave function". But the spatial part is just part of the photon wavefunction, and it's not meaningful to ask whether it, by itself, can contribute to the mass of a system containing many photons. You have to look at the whole wave function.

    In fact, even looking at the whole wave function might not be meaningful in this context. Protons as composite systems have a significant contribution to their mass from the interaction energy of the strong interaction between the quarks. There is no way to make a composite system out of photons that has that property. The examples of composite systems of photons having nonzero invariant mass are classical: you can have systems containing multiple photons whose total invariant mass is not zero (because invariant mass is not additive; what is additive is 4-momentum, and multiple null 4-momentum vectors can add to a total 4-momentum vector that is not null).
     
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