Sound, under certain circumstances, can travel faster than light speed. Just thought I should throw in that recent discovery.

I think the only way time travel is possible is through a form of space travel or quantum travel. Other than that it is physically impossible because time is invariant.

Now, people, aren't we forgetting good ol' overkill?

Theory: Time travel could be achieved by reversing the events of the Universe itself, as opposed to sending the traveller back legitimately to another coordinate in the time dimension.

This is done by either producing a 3D "photograph" of the Universe at this point, waiting some time, erasing the current Universe and using the previous Universe's energy to replicate the "photograph" (no suggestions on how to make the photograph, or replicate it, though it would have to be accurate down to the last electron); or initiating a "reversal reaction" to theoretically send every event backwards.
You would exclude yourself from the reversal ofcourse, though you would have to create an identical energy structure to your presence so it could be reversed if you opted for the reversal reaction.

This could be greatly implified (though still a colossal project) if you could limit the reversal to a smaller section of the Universe, say our solar system; though you would have to be careful of the impacts of external influences, or the reversal would be off.
It may also be considered that inaccurate reversal/replication isn't of too significant an impact, depending on your requirements, though you may also cause some particularly disasterous abnormalities if it's more than 0.000000000000000000000000000000000001% (approx.) off.

This is "cheating" - time still has remained the same, or even moved forward as you proceeded with the change. You may've also unaligned the solar system with the timeline of the galaxy and the rest of the Universe, though if it's not too far it won't be too significant (except for a few interstellar asteroids etc, which I would attempt to accurately immitate). This "time reversal" would then be able to develop another sequence completely unique, ofcourse, andif you kept a photograph of the "time" you left, if you've gone this far with your technology you could probably go back, too.

If the process of existence is predetermined (which is a theory I subscribe to), it makes time reversal more possible, though ofcourse, there is a reason why I call these theories "Overkill".

I also theorise that, if computer simulation could be produced both efficient and powerful enough, this sort of thing could be done entirely in computer simulation; photograph of the current Universe taken, time reversed in simulation, data in simulation produced into reality over top of our current reality, whatever.
Hell, for more practical purposes, ie in order to advance technology, we could create a simulation of Earth today, send them forward 1000 years and collect data on their technological progress.
Oh, and a few points:
-Your mere presence say, 60 years ago, would probably wipe you from existence.
-Once you have mastered everything else and are travelling back in time, don't forget that moving to a different coordinate in the 4th dimension would also require adjusted 3D coordinates - otherwise you'll land yourself a linear distance of several trillions of kilometres away from Earth, extended from the line connecting Earth and the Universe's centre.
Real time travel is risky business.

Or, you could've read my post in the other Time Travel thread...

What if we could use warp-holes for time-travel?

That would require so much energy that it would destroy the universe trying to get it.

You can actually travel through time very easily, to see it just get 2 people, both with watches on that are synchronus to each other. one gets on a plane or some other fast moving object, and move for a predetermined distance, at a set speed, then when you compare watches, they will show different times. As the one in the fast moving object will have had time move slower than that of the person on the ground