you wonder why nobody has come back
Time goes one way, time travel is possible, we do that all the time (relative to something) just it is one way journey.
The interesting thing about timespace is that you can never go to the past but you can go to the future, you can't see the future but you can see the past. How can you see the past? I do have an idea how. All you see in the universe is old, the farther you look, further you see. You can even see yourself (at some point), by looking to the black hole you can see yourself, it's hard to explain but black hole can bend photons back to you, so basically it's a mirror (wiki gravitational lenzing effect) (there are billions of blackholes! with different images, so ALL the past is visible to the whole universe)
Example: you think time dilation effects are really small? LHC http://en.wikipedia.org/wiki/Large_Hadron_Collider can spin particles at the speed of 0.999999991 c (pretty fast ha?) Lorentz factor at such speed is 7450 (Knoble's formula) that means that if LHC was spinning 7450 years, only 1 year would pass for the matter that was spinning.
If the universe is rotating, then if you travel at the opposite direction, around the entire universe, faster than the universe is traveling, then you will arrive on earth before you left.
This is stupid because speed is relative.
Another is a loop hole. If you put two dots on separate ends of piece of paper, then fold the paper, the dots are on the same spot. So if you could somehow create a loop hole then you'd be able to be at two times at once. Or something like that.
This is stupid too because you can't put any two space points together, you can only bend (or should i say stretch) spacetime.
Knoble gave the right formula to calculate Lorentz factor, time dilation, mass increase, and length contraction effects.
let's say a man from the left goes towards me at the 0.8c speed, so does the man from the right, how would that look for the man from the left? 1.6c? no
s = (v + u) / (1 + ((v * u)/c^2)
that's how (0.8c + 0.8c) / (1 + (0.8c*0.8c)/c^2)) = 1.6c / 1.64 = 0.9756c
where the speed gone? it's relative.