Time travel is undoubtedly impossible!
However what is possible is that you can slow down time relative to you, according to Einstein anyway, and the theory of relativity. (I think)
According to Einstein's theories then anyone who is moving is actually in their own mini time warp - the time relative to them is actually slowed down, but at the speeds with which we are fimiliar the difference is negligible. When it does get noticed is at the speed of light where time slows down to such a level that if you took a ten year trip at the speed of light then returned to Earth, you will find that in Earth time you have been away for hundreds of years.
Or something like that anyway...