# Photon collision question

1. Apr 24, 2012

I have a few questions regarding matter creation and photon collisions.
When photons collide into each other, can you predict the particles that are created? Is there a limit to the amount of energy photons can carry? Is there a limit to the number of particles that two photons can make after a collision? Thanks

2. Apr 24, 2012

### fzero

Because of the nature of the EM interaction (it is an abelian gauge theory), photons do not directly interact with one another. However, they can interact indirectly by exchanging charged particles. The simplest possible diagram is

where an electron-positron pair is drawn. More generally, for energetic enough photons, more massive particles can appear in place of the electron. Energy conservation requires that $E_\gamma + E_{\gamma'} \geq 2 mc^2$, where $m$ is the mass of the charged particle.

There is no theoretical upper bound on the energy of a photon. Now the particles created can further interact or decay, leading to a more complicated final state. But the diagram above is the lowest-order and therefore dominant process for photon collisions.

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