As I understand it (I don't study physics) any object with mass gets heavier and heavier as it accelerates, so it never reaches c. 'Light' has no mass, but can be regarded as essentially a fluctuation in the electric/magnetic field? From the definition of c^2=1/epsilon0/mu0, I interpreted that c is the fastest speed that a perturbation at one location can spread through the E/B fields, in a vacuum. But, if it was possible to avoid interacting with these fields, and had no mass, what would stop this thing from travelling faster than c? Am I missing something to do with time dilation? Also, would it be impossible to detect?