User blog comment:ProfessorMcG/Time Travel. Is it possible?/@comment-1375165-20100812184554

He's referring to something called "time dilation", though Smallpox is right too =P The stronger the force of gravity around you is, or the more closely you approach the speed of light, time around you starts to flow more slowly than it did before, but to yourself time is flowing as it normally does.

Some astronauts have lived about 0.5 nanoseconds less than they would have had they stayed on Earth. On the flip side, if you approach the event horizon of a black hole, where gravity is immensely powerful, time slows until it stops...Crazy stuff, eh?