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Photon's Mass and Speed of Light [Confused]

  1. Mar 28, 2010 #1

    When you read, you see that there are different opinions on the question, does a photon have mass?

    From what I think, I know that a black hole's mass increases when it consumes photons, and everybody knows that a photon has energy and momentum. Since energy can be trasformed to mass, therefore a photon has mass.
    The mass of photon is measured by dividing its energy by c2, that measured mass is the mass of a moving photon with its constant speed, the same speed in all reference frames.
    If a moving photon has mass, then it must exert force on other particles. The exertion of force causes acceleration and therefore change in speed, and this violates the idea of constant light speed.

    Can you correct me?
  2. jcsd
  3. Mar 28, 2010 #2
    I was not aware that there was different opinions… The photon has no mass. Take a look on the Particle Data Group site for a lower limit of this mass determined experimentally :smile:
    Oh and by the way : the formula E=mc^2 can be applied to pretty much anything but the photon :biggrin: E=mc^2 is a particular case of the more general formula E^2=p^2c^2+m^2c^4 which reduces to E=pc for the photon :smile:
    And you're right : a photon exert a force on particle but via its momentum p, not its mass.
  4. Mar 28, 2010 #3


    Staff: Mentor

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