Simple question about photons turning into electorn positron pairs.

1. Jan 4, 2008

cpfoxhunt

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?

Cheers
Cpfoxhunt

2. Jan 4, 2008

pam

E^2-p^2=0 for a photon, but it must be positive for an electron-positron pair.
That is $$(E_1+E_2)^2-({\vec p}_1+{\vec p}_2)^2>0$$.

Last edited: Jan 4, 2008