Most people would say that special relativity prohibits us from travelling at the speed of light, for, as you stated, time ceases to move when you are moving through psace at the speed of light. But, in reality, you are always moving at the speed of light. The question is whether you are moving through space, time, or (much more commonly) through some combination of the two. Simply put, the (squared) sum of your speeds through space and time is always equal to the speed of light. I have always preferred this way of thinking about special relativity. I highly recommend Relativity Visualized by Lewis Carroll Epstein for an explanation of special and general relativity. Also, the equation Richard cited, E=mc2, referrs to a particles _rest_ energy. Particles in motion follow a different equation, which utilizes momentum. Photons are particles with momentum but not mass, if they stop moving, they disappear. The momentum is determined by frequency and velocity. In this sense photons are like waves in a rope, they don't have mass per se, but they can carry energy. This is probably a pretty weak analogy but the best I can think of getting ready for work. T.R. Stratton, A.B. Physics -- T.R. Stratton [log in to unmask]