mumble, mumble... E=mc2??... mumble...
The mass of an object in motion is
m0
m = ----------------
sqrt(1-v2/c2)
where m
0 is the
rest mass, v is the
velocity of the object, and c is the
speed of light.
(and sqrt represents the square root function)
As you can see, as v approaches c, v2/c2 approaches 1, and the mass of the object approaches infinity. Clearly an object with infinite mass would be trouble, thus it is speculated that objects with a non-zero rest mass cannot travel faster than light. (Not to mention that it would actually have an undefined mass rather than infinite...)
An interesting (for sci-fi at least) theoretical type of particle is the tachyon which would have a negative rest mass, and consequently it can't move slower than light. I don't believe there has ever been evidence that these actually exist.
Ok, that's right, the mass squared of a tachyon would be negative. Assuming you can't actually have a complex mass value, m0 would have to be complex to cancel the complex value in the denominator. Thus, tachyons have to go faster than light, and slow down as you add energy.