If it is possible, why haven't there been any records of time travelers throughout history?
And then there's the Twilight Zone Effect (since I can't remember the actual name). It states that time will just repair itself as if nothing was wrong. For instance, in an episode of the Twilight Zone, Katherine Heigal (sorry I butchered her name) is sent to the past tokill baby Hitler. She achieved her goal. Though the family maid stole a baby off a peasant in the street and passed it off as the real baby Hitler. This peasant baby went on to enslave and murder thousands. Time repaired itself. Now the only question that remains is what could have happened had she failed her task?
We may just know them by different names. Maybe someone went back into the 15th century, had all the answers, spoke a strange dialect of language and was burned as a witch? Maybe someone went back in time, had all the answers and put himself in a position of leadership and conquered half the world?
Feedback applies to a wormhole that would take you back in time, the best example to explain it is with a speaker system. When you sing into a microphone it amplifies the signal and sends it to to the speakers which play it out loud. But the microphone can hear the speakers so some of that goes back through the system and gets amplified a second time. Too much feedback and the speaker system blows. Same thing would happen with a wormhole except with radiation instead of sound. The wormhole would constantly suck up radiation and send that radiation into the past, meaning the current wormhole would be sucking in more and more radiation, eventually it would be destroyed.
Paradoxes are the whole "change the past" idea. Say a scientist opens a wormhole into his own past. He takes a gun and shoots his past self. So his past self is now dead, but that means his current self can't exist, so who shot the past self? Physicists believe there's a law of nature that makes prevents paradoxes, which in turn prevents time travel into the past. For those two reasons physicists believe it's impossible to travel into the past.
As for travel into the future, relativity says it's possible. Basically you have to think of things as in 4 dimensions instead of 3, the 4th being time. Just as gravity can bend space it can also bend time. GPS satellites have proven this, because they're farther away from the Earth than we are time travels a billionth of a second slower for them. There are systems that correct for that but if those systems weren't there every GPS on Earth would be off by as much as 6km.
The CERN particle accelerator has also proven this. By accelerating particles to 99.99% of the speed of light (anything except light going at light speed is impossible) they observed particles that normally only exist for fractions of a second stay around 30 times longer. Those particles traveled into the future on a small scale.
If you were to get into orbit around the black hole in the center of the Milky Way time would travel half as fast for you as someone on Earth. So for every year you age in black hole orbit I would age 2 years on Earth.
I agree with dhampy the other day I was thinking about time travel and what if someone went back in time, and noticed right about the same time say the lightbulb was invented that nothing was going on so the kind of pushed Humphry Davy into making it.
BUT! at the same time I just realized SuperSkyline89 makes a damn good point!!
[Edited by armedman, 6/4/2010 9:54:35 AM]
Time is a human concept anyway, it's not like you can just enter a date into a machine and have it travel there. The past is not a place, even if you could travel back in time, it wouldn't be instant. It would be the same as traveling into the future; your body would have to be preserved while time reverses.