Two important points.
First, we cannot travel at light velocity, so it’s pointless to wonder (at least within the context of the known laws of physics) what would happen if we did something that is manifestly impossible.
Second, any time dilation due to relativity theory is about what other observers see, not about what happens to you. Your time does not slow down when you travel at a high rate of speed. In fact, relative to yourself, you never travel at any speed; you are always at rest. So why would your watch, your heartbeat, etc., care about the fact that some distant observers far away from you travel at a high rate of speed relative to you?
The answer is, they don’t care. Your watch, your heartbeat will continue as usual. Time does not slow down when you travel at a high rate of speed. Your time appears to slow down for those observers relative to whom you move at a high rate of speed (or conversely, who move at a high rate of speed relative to you.)
As to why this is so, well, we found that in this universe of ours, there is such a thing as an invariant speed. This speed is the same for all observers. We call this speed the vacuum speed of light because light happens to travel at this speed in a vacuum, but the concept is more fundamental. What the existence of this invariant speed ensures, among other things, is that the properties of empty space (the vacuum) are the same for all observers, regardless of their speed. And that no observer has a “privileged” reference frame that could be said to be at rest; everyone is at rest relative to their own reference frame, and no reference frame is special. This is a fundamental property of our universe.
As to why we live in just such a universe instead of one, say, in which a privileged reference frame exists, that, I am afraid, is a question best left to priests or philosophers to answer.
By: Viktor T. Toth ( Physicist)