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.
Heh heh. Try hitting that with a paddle.
While it has no mass, it does have both energy and
momentum, both of which are proportional to its frequency.
Momentum is defined as mass*velocity. If mass is zero, how does a photon have momentum?
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).
All this is true, but how exactly does a massless particle have momentum?
--
Greg