It's official: Time machines won't work

Also, contrary to popular fiction, the past isn't changed only because a blundering human is transported into it, it is already altered by the mere introduction of additional matter into a system that would have evolved differently without it. So, if you want to change the past, transport a single proton back 2 million years into some swamp. Of course, that would be pointless because nobody would ever know what changed...

The real conundrum is what actually happens to the "old" universe at the instant of traveling back into the past. Is everything that happened between those two points in time erased? Or does the old universe continue as if nothing happened and a new parallel one branches out that is of no consequence at all? In the infinite universes theory, time travel would merely amount to "jumping" into the "right" neighboring universe (however, personally this whole business with infinite branching parallel universes always seemed like a wasteful and inelegant model to me).

All these theories have one thing in common though: things change only for the time travelers themselves. So for meaningful time travel, you would have to be able to transport whole beings across a doorway in spacetime. Twice. In the right direction. Given the known conditions near theoretically surmised holes in spacetime, that's not an easy task. Atoms: maybe. Huge fragile collections of unstable molecules: probably not.