# Why can a photon not decay?

1. Mar 22, 2012

### nonequilibrium

Note: I'm only talking about real photons, no virtual particles.

The argument that Griffiths gives, is (p79)
and although I have respect for Griffiths, I'm wondering if this isn't a fallacious argument: the photon might not have mass, but it has energy...

Another argument might be (I'm guessing): "the kinematics does not allow it", but I think that kinematically it would be allowed, for example, for a photon to decay into three new photons (with longer wavelength).

Yet another argument might be: "the concept of half-life is not well-defined for photons, since they have no reference frame where they stand still". That might be true, but that still doesn't explain why photons do not decay.

What do you think is a convincing reason for the impossibility of the decay of a single photon?

2. Mar 22, 2012

### M Quack

For a single photon it is conservation of momentum and energy. The only possible "decay particle" would again be massless.

A high energy photon can hit another photon and produce a particle/antiparticle pair it the photon's energy is high enough. Look up pair production. Note that you need two photons for this to happen, but one can be virtual, e.g. from an electric field.

The lightest pair would be electron/positron, so that the photon has to have an energy above 1.022 GeV (hard gamma radiation).

3. Mar 22, 2012