Some science fiction concepts are impossible... I used to believe Time Travel to be one of these things. ...I think it's impossible to travel back in time at least. It's been proven perfectly possible to travel forward in time at a relatively different speed though.
The other day I was being quite lazy and was wishing for the invention of teleportation. Then I tried to think of how it would be possible. I think it is in theory. Scientist believe that wormholes exist in the universe but have yet to find any. A wormhole is a hole in space-time that would allow you to go from one place in the universe to another. If that's possible then surely it'd be possible to invent something to manipulate the edge of these wormholes, to change their size or to subtly alter their location... Then if that were possible then the technology wouldn't be too far away to create wormholes in the first place. This is how a teleporter would work.
Now I'm thinking that if you can create a wormhole in space-time then you could choose to manipulate it so that time is different at one end of the hole. Thus it actually is possible to travel back in time. However it would still be impossible to travel back to a time before the wormhole manipulator is invented. This explains the lack of any future tourists here now.
