# Homework Help: Conservation problem

1. Sep 26, 2009

### quasar_4

1. The problem statement, all variables and given/known data

Show that although gamma rays can have energies greater than the energy of a e+e- pair at rest, the process gamma --> e+e- is forbidden.

2. Relevant equations

Noether's theorem

3. The attempt at a solution

I know that to show that the process is forbidden, I should be able to show that it violates a conservation principle. But I'm not at all sure what formalism to use for the problem. Can someone give me a nudge in the right direction?

2. Sep 26, 2009

### G01

Energy may conserved in this process but what other major conserved quantities do we have? HINT: The one that isn't conserved here, you learned about in introductory physics. Remember, the resulting particles are at rest.

3. Sep 28, 2009

### quasar_4

Ok, so it turns out that I was misreading the problem. All that I need to do (if I am now reading correctly, lol) is to prove that pair production can't occur in empty space!! That's not bad at all; the requirements of energy conservation demand that another nucleus be present to absorb some of the photon's linear momentum (without another nucleus, conservation of linear momentum is violated). It's a quick proof. Thanks for the help. Sometimes I just need to actually read my problems more carefully...