To Wolf,
Read this physics archive that says -it might not really gain mass. Also some say photons actually do have mass, but are infitismly small and therefore can travel at the speed of light or more. (As I quoted a German experiment previously.):
“Question: How does mass increase when you approach the speed of light?
Answer 1: This question addresses Einstein’s theory of Special Relativity.
If my answer does not satisfy you, there are many clear books on
the topic in any university bookstore.
When a particle/object travels close to the speed of light, and is
thus considered “relativistic”, the energy of the particle is
expressed as E = (gamma)*mass*velocity. Here gamma is a relativistic
factor that is greater than unity. So, one could consider the
factor (gamma)*mass a new mass, one that is larger than the mass
of the particle when it is at rest. This is why you hear that
mass increases when you approach the speed of light. It can be
argued that it is only an appearance of greater mass, or that it
depends on how you look at the problem. In short, it is all relative. 😉
It should be noted, however, that in order for an object to actually
reach the speed of light, it must have no mass, since E=mass*speed of
light^2. This is true of massless particles such as the photon,
the “particle” that transports light. (Notation:* means multiply by
and ^2 means squared).”