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Energy of photons vs classical physics energy

  1. Jun 21, 2013 #1
    Relativity says photons have energy E=pc. Classical physics says E=1/2 pv. There seems to be a factor of 2 missing in one case or the other or does the energy formula change as the speed of light is reached? There must be a simple explanation but I have not found it yet. I did not know where to put this question-under classical physics of quantum physics. Thanks for any help.

  2. jcsd
  3. Jun 21, 2013 #2


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    There are no photons in classical physics, so classical physics cannot say anything about them.

    With the Maxwell equations and classical physics, you can derive E=pc for electromagnetic waves. There is no factor of 2.
  4. Jun 21, 2013 #3


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    The single relationship between energy and momentum that applies to both massive and massless particles is E = √(p2c2 + m2c4). For a massless particle, set m = 0 and it reduces to E = pc. For a massive particle at rest, set p = 0 and it reduces to E = mc2.

    For a massive particle that's slowly moving, keep both terms and expand in a power series in p. The first two terms give you the usual Newonian expression, E = mc2 + p2/2m.
  5. Jun 21, 2013 #4
    Energy of photon vs classical physics energy

    Thanks for the replys. I was trying to understand what happens to energy of a proton as a proton is accelerated from rest to almost the speed of light as in the LHA. I think I can calculate that now.

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