Ian Mallett wrote:
However, Physics, as far as I know, does not say what happens with a particle of no mass.
Actually, it does -- a photon is an example of an object with no mass. Such an object always travels at the speed of light -- it doesn't even need a push to get it going. It's fundamentally incapable of standing still. While it has no mass, it does have both energy and momentum, both of which are proportional to its frequency. These are conserved in any collision, so when it bounces off a wall, the wall gains some momentum, just as it would if a massive particle with the same momentum bounced off it. And if the wall starts to move as a result, then it has also gained some energy, which must have come from the photon, so the reflected photon must be red-shifted slightly (longer wavelength = lower frequency = less energy). -- Greg