[Michiel]

[color=#000000ost_uid0][quoteost_uid0]Meaning you can go back in time to when your grandfather was born, find his bassinet, and kill him... ON THE CONDITION that you then kidnap a similar looking baby, and put it in his place, because actually he was your grandfather all along...[/quoteost_uid0]

Sounds like a predestination paradox to me, which is, I think, impossible. That similar looking baby couldn't possibly have identical DNA, so if that baby was your grandfather all along, you would never have existed if you never travelled back in time to do all this.

[quoteost_uid0]But what if you don't? What if, say, you kill your grandfather, and don't put another baby in his place. And you make sure no one else does either. What happens then?[/quoteost_uid0]

Then, you killed your grandfather, which means you never existed, so you couldn't have traveled back in time to kill your grandfather, so you DO exist, so you do travel back in time to kill him, which completes the temporal paradox, which, according to doc Brown, will destroy the universe. Granted, that's a worst case scenario. The destruction could be limited to only our own solar system.

(Is it just me, or has this thread gone completely off topic? I'm not complaining, I like this kind of discussion.)[/colorost_uid0]