Light does not have mass, although as you point out it
does have momentum. At first this might seem contradictory since you
may be used to the classical physics relation P=MV, which says that an
objects momentum is the product of its mass and its velocity. If light
has no mass, how can it have momentum?
The answer is that
P=MV is not true for things that are moving extremely fast, where
"fast" means close to or at the speed of light. Is such cases we have
to use Special Relativity (invented by Albert Einstein), to describe
the behavior of things. One of the statements of special relativity is
that an objects energy E, it's momentum P and it's mass M are related
E^2 = (PC)^2 + (MC^2)^2
Where ^2 means "squared" and C is the speed of light.
In this case, if the mass M is zero, we get
P = E/C
says that the momentum is just the energy divided by C. We know light
carries energy (the sun warms us) so we see that according to Special
Relativity light should also have momentum.
(republished on 07/23/06)