Upper limit on speeds
According to special relativity, the energy of an object with
rest mass m and speed
v is given by
γmc2, where
γ is the Lorentz factor defined above. When
v is zero,
γ is equal to one, giving rise to the famous
E =
mc2 formula for
mass-energy equivalence. Since the
γ factor approaches infinity as
v approaches
c, it would take an infinite amount of energy to accelerate an object with mass to the speed of light. The speed of light is the upper limit for the speeds of objects with positive rest mass.
[28]
More generally, it is normally impossible for any information or energy to travel faster than
c. One reason is that according to the theory of special relativity, if something were travelling faster than
c relative to an inertial frame of reference, it would be travelling backwards in time relative to another frame,
[Note 5] and
causality would be violated.
[Note 6][30] In such a frame of reference, an "effect" could be observed before its "cause". Such a violation of causality has never been recorded,
[14] and would lead to
paradoxes.
[Note 7][31]
(
http://en.wikipedia.org/wiki/Speed_of_light)