why? i think in principle, the party traveling into the future couldn't come back and share what happens with the present

If you travel at a relativistic velocity for a decent amount of proper time, relative to some frame, ie Earth, then return to the observer frame (Earth), you'll have technically travelled significantly into the future, since more time will have passed for that observer frame than for you.

This post was edited by Ayah on Dec 15 2011 11:26pm

What Ayah says it right. People ask about "time machines" without realizing what "traveling through time" means. Space and time aren't really different. They are, as Einstien said, the 'space-time continuum'. But humans don't seem to have a problem conceptualizing a "space travel machine".

Any spaceship moving anywhere is "moving through the space-time continuum" and would therefore be 'time traveling' if you moved fast enough and far enough to where you could see a noticeable difference in time. But humans can more easily notice differences in space (e.g. a ship moving from Earth to mars) so we don't have trouble asking the question about a space travel machine. It all comes down to just a matter of how humans have trouble perceiving time.

This post was edited by kayeto on Dec 15 2011 11:41pm

clocks that were once synchronized but are now not , is not what is commonly meant by " time travel "

what is commonly meant by "time travel" IS a complete misunderstanding caused by humans lack of ability to perceive the passage of time.

All you need is a flux capacitor powered by 1.21 gigawatts.

You make it sound as if time was just another dimension in the 'space-time continuum' which is the natural description, and thus time and space the same (in some sense) and it's just our human problem that we percieve space and time so completely different because of some human limitation.
Be sure that this is wrong: even mathematically time and space are totally different, compare the metric used in GR/SR (the special character of time is also clearly emphasized in the Schroedinger equation). Keep in mind that GR/SR is 'just' a theory which is only an okayish approximation of the real world - it only works in the low energy/large distance limit and only there the space-time continuum concept really applies, so it is only some approximation in some limit, and not the universal 'truth'.

but doesn't time dilation itself stem from travelling at relativistic speeds? The acceleration just determines whose time will slow down. I might be wrong though, I just thought of the flashlight-on-a-rocket idea.

The only real problem with GR/SR is that it doesn't account for quantum effects. That's why people are looking for a "quantum theory of gravity". However, it's greatly exaggerating the limitations of GR to say it's "only an okayish approximation" and that they're only applicable in "low energy/large distance" situations.

It is possible to travel to the future if you can manage to get to the speed of light, but it is physically impossible to go back in time.