traveling forward in time might be able to be achieved by traveling at light speed (even though that's physically impossible, but say you got close to light speed, like 99% light speed). however going back in time is entirely impossible. Hypothetically, you could expand a wormhole big enough to open a portal to the past. However there are many problems with this, starting off with the fact that the portal would collapse in on itself from a feed-back like affect but with radiation. Also a commonly known paradox, called the grandfather paradox, and other similar paradoxes cause past time travel to not make any sense. The grandfather paradox describes the theory that if a time traveler were to go back in time and kill their grandfather, they would have never been born, so who killed the grandfather?