Before I answer this question, remember this E = mc², where E is energy, m is mass and c is the speed of light Imagine that someone could run at the speed of light. read more
They'd also see your clocks – and your heartbeat, your speech, your computer cycles – running slow. This is true, but completely unobservable in everyday life. Typically on earth, it's an effect of about 1 part in a quadrillion, but at 99% the speed of light, you'll appear to be running at only 1/7th speed. read more