Re: Does time stop at the speed of light and does time go backwards at faster?

If you are standing on the Earth, using a telescope to watch a clock on a rocket ship, that clock will seem to go slower than a clock next to you here on Earth. Amazing but true... (this has been observed by comparing clocks on real rocket ships after they come back to Earth).

Special relativity tells us that the faster the rocket ship goes, the slower their clock goes relative to ours. As the rocket ship approaches the speed of light, their clock seems to all but stop. A rocket ship can never actually reach the speed of light, it turns out, because that would require an infinite energy. The equations tell us that if it *did*, the clock on that rocket ship would stop all together.

So it's not true that light travels instantly from one place to another... unless you're talking about the trip from the perspective of the light itself! Let's go back to the rocket ship just shy of the speed of light. From the perspective of people on the ship, they can travel anywhere nearly instantly -- but remember that their "clock" is working differently from ours here on Earth. We still see the ship take a long time to get to the next star (measured on our clocks), but the people on the ship might hardly experience any time to pass at all.

Now, as for what happens when you go faster than light... First off, you can never get to the speed of light, so presumably this is not possible unless you were *always* travelling faster than light. Even so, this is probably impossible. If you press on beyond this impossibility, it is true that the equations say that the clock on such a faster-than-light rocket ship would appear to be going in the other direction than the clocks here on Earth.

The "faster than light" light experiment is a bit misleading -- the leading edge of the laser pulse didn't travel faster than light, just the peak. And because what came out was much less than what went in, you can show that none of these experiments actually send information faster that light. So no time-travel yet, I'm afraid...