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Naive Mass of Photon Question

  1. Mar 24, 2009 #1

    This is probably very naive!

    What is wrong with the following

    The (momentum of light) = E/c so it is then possible to write this as
    (mass of light x speed of light) = E/c

    and thus ... E = mc^2

    ........... this came up after I read the following with my students

    http://www.davidbodanis.com/pages/promised_note_highschool2.html [Broken]

    Which starts with Momentum = E/c but then takes a long route round to E = mc^2

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 24, 2009 #2


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    What's wrong with it, is that "mass of light" is zero, so you are saying 0 = E / c.
    For light, the momentum is related to the wavelength, not the mass. The correct formula is
    E^2 = (pc)^2 + (mc^2)^2
    which for a particle at rest reduces to E = mc^2 and for a photon with momentum p to E / c = p.

    In fact, you may want to read https://www.physicsforums.com/showpost.php?p=1285138&postcount=6 [Broken]).
    Last edited by a moderator: May 4, 2017
  4. Mar 24, 2009 #3


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    "momentum= mass times velocity" does not apply to light.
  5. Mar 24, 2009 #4
    "the momentum is related to the wavelength,"

    and the wavelength is related to the energy. the total energy of any system is the critical element of consideration, and there is a reason why a system is treated mathematically via its total energy, rather than addressing mass and energy separately in a moving or accelerating system. it seems disingenuous to attempt to deal with mass and energy as f they were different entities - they are only different manifestations of the same thing.
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