hmmm…
what do you guys think?
If nothing happened, if nothing changed, time would stop. For time is nothing but change. It is change we see occurring all around us, not time. In fact, time doesn’t exist. - Julian Barbour
Travel to the future
Time travel can be to the past or the future. Because travel to the future is easier and more practical, it will be discussed first. There are two straightforward ways to travel into another person’s future. In the twins paradox, a person speeding away from his twin who remains on earth will, upon reunion, have entered the earth-twin’s future. Second, according to general relativity, if the twin goes to a stronger gravitational field by leaving the dinner table and descending to the cellar for a bottle of wine and then returns, he will have entered the future of his twin who stayed at the dinner table. When someone enters a relatively stronger gravitational field, their time slows down relative to the time of those who don’t enter the stronger field.
Regarding the first kind of time travel to the future, if you have a fast enough spaceship, you can travel to the year 4,500 A.D. and see the future of earth. You can affect that future, not just see it. This is a direct consequence of the time dilation described in the theory of relativity. You can travel to someone else’s future, not your own. You’re always in your own present. Unfortunately, once you go to 4,500 A.D. (as judged in a frame of reference in which the earth is considered stationary), you are stuck in the earth’s future. You can not reverse course in your spaceship and return to the 21st century on earth. You must live with the consequence that all your friends have died centuries ago. Visits to the future are permanent, not temporary.
On this trip to 4,500 A.D., how much time would elapse on your own clock? The answer depends on how fast your spaceship goes, what accelerations occur, and whether gravitational forces are acting. The faster your spaceship goes, the less time it will take–actually take, not just appear to take. As you approach infinitesimally close to the speed of light, the trip to 4,500 A.D. will take essentially no time at all. That’s from your own perspective though; observers who remained stationary on earth and judged your flight from that perspective will have observed your speedy travel for thousands of years.
In science fiction movies, which almost always depict nonrelativistic time travel, time travelers suddenly appear from out of the past, and other travelers suddenly disappear from now and pop into the future. These phenomena have never been observed, despite the parapsychological literature. If they were reliably observed, then we would consider the hypothesis that spacetime has an extra dimension allowing time travel. The discontinuous worldline in ordinary 4-d spacetime could actually be a continuous trajectory in 5-d spacetime…
Rest of the story is at
http://www.iep.utm.edu/t/time.htm#TIME-TRAVEL
