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Photon and its momentum

  1. Mar 16, 2005 #1
    Momentum can be categorised under electromagnet force, and electromagnetic force carrier is photon. But photon have momentum. Isn't this paradoxical?
  2. jcsd
  3. Mar 16, 2005 #2
    The classical electromagnetic field has momentum. The photon is the
    quanum mechanical minimum unit of this field, and it also carries this

    There is no paradox.
  4. Mar 16, 2005 #3
    Mhh, how can it carry momentum if it has mass of zero? Momentum is p=m*v. Or is here the meaning of the quantum mechanical operator?
  5. Mar 16, 2005 #4
    According to E=mc^2, energy and mass are eqivalent in this case.

    Moving energy must carry momentum. It does not require the
    presence of a classical mass.

    A compressed spring is a little heavier than a loose one. If they move at
    the same speed, the compressed one has more momentum.
  6. Mar 16, 2005 #5


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    Staff: Mentor

    No, in relativity theory it's possible for a particle to have momentum and energy even though it has zero mass. The general relationship between mass, energy and momentum is

    [tex]E^2 = (pc)^2 + (mc^2)^2[/tex]

    Set [itex]m = 0[/itex] and you have [itex]E = pc[/itex] which in fact has been verified for electromagnetic radiation.
  7. Mar 16, 2005 #6
    So for photons this equation can be written as:
  8. Mar 16, 2005 #7

    Yes. E=hf, p=f/h where h is Planck's constant, when you are talking about
    photons. For classical fields, momentum/m^2=ExH/c^2, energy/m^2=ExH.
  9. Apr 27, 2010 #8
    Yeah, you are thinking in terms of classical physics.
  10. Apr 28, 2010 #9
    5 year old thread
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