If photons have no mass, how can they have momentum?

Question: If photons have no mass, how can they have momentum?
- Keywords:
  mass,
  momentum,
  photon
Asked by freddie to Adam on 16 Mar 2011 in Categories: General.
- Adam Tuff answered on 16 Mar 2011:
  
  Fantastic question Freddie! The answer comes from the crucial fact that our equation for momentum p = mv, only holds in non-relativistic cases i.e. when something is moving at a good fraction of the speed of light! The true equation for energy and momentum says that even if an object has no mass and is travelling relativistically, it has some momentum. E^2 = m^2c^4 + p^2c^2 (E = energy, m = mass, p = momentum, c = the speed of light). If you have no momentum, i.e. are at rest, then you get the famous equation, E = mc^2!
  This is a really good question – can I ask which school you are from and who your class teacher is?

freddie commented on 17 Mar 2011:

I’d rather not say on here – could I have your e-mail address?

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freddie commented on 18 Mar 2011:

I’m an Institute of Physics 16-19 member – if that’s what you mean.

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Question: If photons have no mass, how can they have momentum?