Time travel.

[ledzeppie]

So I was watching a video on youtube. http://www.youtube.com/watch?v=QM28eErikAo

It says that the faster you travel through space, the slower you travel through time, and that if you went faster than light, you could go back in time. Is this true? Forget about it being impossible to go faster than light, and assume that it is. Wouldn't this only be relative?

[HallsofIvy]

Yes, If you were to observe a person traveling at, say, 90% the speed of light, relative to you, you would see their clock moving more slowly, their heart beating more slowly, etc. And, yes, this is "relative". If that same person were observing you, he would see you moving at 90% the speed of light relative to him and would observe everything in your frame moving more slowly.

I don't see why it would be important to "forget about it being impossible to go faster than light, and assume that it is". This has nothing to do with going faster than light. In fact, the effect is one of the reasons it is impossible to go faster than the speed of light: as something approaches the speed of light, relative to you, its "time" comes to a stop.