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What is photon momentum?

  1. Dec 10, 2014 #1
    While planing some simple experiments us encountered such a problem: what is exactly photon momentum?
    p_ph = ?
  2. jcsd
  3. Dec 10, 2014 #2
    The photon momentum, ##p## is the momentum carried by a photon and is related to the energy of the photon by the equation ## E=pc## where c is the speed of light in a vacuum.
  4. Dec 10, 2014 #3
    Even classically, radiation carries momentum. That's why you get radiation pressure. The whole formalism is derived from the Lorentz forces which implies that E-M field carries momentum. In fact, the quantum field theory of photon arises from covariant quantization of vector potential and therefore photons carry quantized version of the classical E-M field momentum.
  5. Dec 10, 2014 #4
    momentum is a vector .
    Then in which direction does the E-M field travel , say in the case of Cosmic Background Radiation, which is all around us?
  6. Dec 10, 2014 #5


    Staff: Mentor

    What direction does the microwave radiation in a cavity travel?

  7. Dec 10, 2014 #6
    It is not the E-M field that travels, it is the energy that travels. For free space, poynting vector is in the same direction as the wave vector and thus the direction of momentum vector. I am not expert on Cosmic background radiation, but I think they should have momentum, even on the ground of relativity. If you are in some random frame, it is likely that some of the photons are redshifted and some are blueshifted. For comoving fundamental observers, the radiation is isotropic, but that does not mean individual photon does not have momentum.
  8. Dec 10, 2014 #7


    Staff: Mentor

    The CBMR is radiation left over from the early of universe and behaves like blackbody radiation:

    The classical black-body is a kiln with a small hole in it so you can observe the radiation inside. As such direction is not a property of its radiation - if you view it as photons its like a gas that has reached thermal equilibrium - you cant really ascribe a direction to such a situation. In fact its the exact analogue of a gas except it obeys the Bose-Einstein statistics due to photons being indistinguishable and doesn't obey the Pauli exclusion principle of fermions.

    The CBMR doesnt really have a direction - but recently there have been found small departures from uniformity that it is suspected to be left over from the early inflation phase of the universe.

  9. Dec 11, 2014 #8
    Actually, I just attended a talk by a scientist from Astronomy department. He was analyzing some data from a ground based telescope which potentially indicate CMB polarization by the inflatory gravitational waves. Unfortunately, the data is heavily affected by galactic dust and they are currently trying to salvage the actual signal.

    But I am no expert on it. I do not know the details.
  10. Jan 4, 2015 #9
    So E = pc???
    Is it E or p definition?
    Could it be derived?
  11. Jan 4, 2015 #10


    Staff: Mentor

  12. Jan 4, 2015 #11


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    Science Advisor
    Gold Member

    bolded by me

    First Direct Evidence of Cosmic Inflation

    Toward an Understanding of Foreground Emission in the BICEP2 Region
  13. Jan 4, 2015 #12


    User Avatar

    Staff: Mentor

    Or think of a container with a gas inside of it, like air. Each individual molecule travels in some random direction. In what direction does the gas as a whole travel?
  14. Jan 6, 2015 #13
    So, as easily seen, nobody knows what is photon momentum?? That's interesting, while how they calculating over all this experiments?
  15. Jan 6, 2015 #14


    Staff: Mentor

    How you draw such a conclusion has me beat. It a good idea to not to jump to conclusions physicists don't know what they are talking about when the more likely explanation is you don't understand it.

    You may like to acquaint yourself with Noethers Theorem:

    See also:

    See equation 6.16.

    Its the conserved quantity associated with spatial symmetry like energy is the conserved quantity associated with time symmetry

    Did you get what I said about the square in the relativistic equation?

    Last edited: Jan 6, 2015
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