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Homework Help: Simple question about photons turning into electorn positron pairs.

  1. Jan 4, 2008 #1
    1. The problem statement, all variables and given/known data

    This is a really easy one, goodness only knows why I can't think of how to do it, here goes

    " Explain why a photon in free space cannot turn into an electron-positron pair."

    2. Relevant equations

    I assume E^2- P^2 = 0 will be of some help

    3. The attempt at a solution

    I'm stuck - why on earth shouldnt it if it has sufficient energy to provide the necessary rest masses? Why don't photons do this all the time?

  2. jcsd
  3. Jan 4, 2008 #2


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    E^2-p^2=0 for a photon, but it must be positive for an electron-positron pair.
    That is [tex](E_1+E_2)^2-({\vec p}_1+{\vec p}_2)^2>0[/tex].
    Last edited: Jan 4, 2008
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