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Homework Help: Energy of a massless particle

  1. Feb 21, 2010 #1
    1. The problem statement, all variables and given/known data

    Show that if mass = 0, then E=pc and u=c.

    2. Relevant equations

    E^2 = (pc)^2 + (mc^2)^2
    B = u/c = pc/E

    3. The attempt at a solution

    I understand that if m=0, then E^2=(pc)^2 => E=pc.

    But isn't p = Ymu? then:
    E^2 = (Ymuc)^2 + (mc^2)^2

    Plugging in m=0 sets E=0...

    It seems strange to me that we can remove (mc^2)^2 from the equation due to m=0, but we can leave (Ymuc)^2 in.

    I'm thinking that this definition of p is not the same as Ymu?

    EDIT: wait... Y would equal infinity. Which multiplied with 0 makes ??? I'm just plain confused now.
  2. jcsd
  3. Feb 21, 2010 #2


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    Science Advisor
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    Gold Member

    p = ╬│mu does not apply to massless particles.
  4. Feb 21, 2010 #3
    Ahhh simple as that lol, thanks.
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