What does this mean for mass measurements? Basically the reason we can't travel faster then C (afaik) is because of the way relativistic velocities add. ie .8c +.2c +.2c +.2c = .935 c or something ...

A theory developed by Albert Einstein stating that the laws of motion are the same for all inertial (non-accelerating) frames of reference and that the speed of light (in a vacuum) is the same for all ...