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Photon Momentum

  1. May 20, 2008 #1
    Sorry if this is a ridiculous queation, but how does a photon have momentum if it hass no mass?
  2. jcsd
  3. May 20, 2008 #2
    They do it with energy! The equation for energy in relativity is:

    [tex] E^2 = m^2 c^4 + p^2 c^2 [/tex]

    Where m is the mass, p is the momentum, and c is the speed of light. Put in [itex]m = 0[/itex] and you get E = p c, so any massless particle with energy E will have momentum p = E/c.
    Last edited: May 20, 2008
  4. May 20, 2008 #3


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    From the relativistic mass equation - e^2 = (pc)^2 + (mc^2)^2. Since it has no rest mass, the latter term disappears but we still have e^2 = p^2 * c^2.
  5. May 20, 2008 #4
    Ah I see, thanks very much!
  6. May 20, 2008 #5
    what is the relationship between P and E?
  7. May 20, 2008 #6
    Another way of getting the same result is to use the more popular form

    [tex] E = mc^2 [/tex]

    and solve for m (which can be regarded as the "relativistic mass"). Then just multiply by the velocity, which is c, and treat it like any other problem. You'll see that you get the same result, p = E/c.
  8. May 21, 2008 #7


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    See post #2, if you mean P is the momentum?
  9. May 21, 2008 #8
    ...just by chance because the derivation is wrong. Photons are massless. The correct derivation is the one already written in post 2.
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