[AnimalMuppet]

Things that move at the speed of light carry momentum, even though they have zero mass. For something moving at the speed of light, the momentum is the energy divided by c. (You can't have something with mass moving at the speed of light.)

[mech422]

Thanks - seems like it was the momentum thing I was missing. I got a far as nothing with mass can move at c and was out of my depth :-P

Thanks