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Thread: photon zero mass and e=mc^2

  1. #1 photon zero mass and e=mc^2 
    Forum Freshman
    Join Date
    Sep 2011
    ok so i was wondering about photons having zero mass.
    if e=mc^2 then c^2=e/m and e/c^2=m
    if mass is zero you find yourself dividing by zero which is impossible
    1/1 times the reciprocal of zero (lets say zero is 0/1) 1/1*1/0=1/0 as you can see its a paradox (as all of you already knew)
    so how can zero mass have energy. well that's what i asked so i used a scientific calculator and e/c^2=m to try to calculate what the mass would be if e=mc^2 were true using the energy in joules(4*10^-19) of a photon and the speed of light (2.99792458*10^8) and got this
    (i used the parentheses to make it less hard to see)


    how could that be that would mean 0*c^2=4*10^-19
    i am no physicists i am just a freshman in high school but i don't see how this could be possible and know i must have made an error could someone please just show me where its driving me crazy.

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  3. #2  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    The reason is that the famous E=mc2 equation is actually a slightly simplified version. The full equation is
    where p is momentum.

    The photon has momentum but no mass and so the equation still works: E = pc

    (You may have learnt that momentum = mass x velocity, but that is a classical mechanics calculation. In the case of quantum mechanics, it is better to think of the momentum of a photon as just an "intrinsic" value it has related to its energy).

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