Friday, May 20, 2011

Varieties of time travel

There are two ways in which backwards time travel may be possible without the usual paradoxes popping (grandfather, auto-infanticide etc) up. One is involving a causal loop and the other involves a branching or forking universe. These two scenarios have been elaborated by David Lewis's famous paper Paradoxes of Time Travel.

But there seems to be other possibilities. Consider the following possibility which I have never heard anyone mention:

Consider a world where the growing-block world is true and God in that world had wanted to "rewind" the course of the world, its entire history to some point before the furthest point the block's edge has advanced. The universe is "reversed" and I'd imagine that things would go like a rewound dvd video to some previous time. Then if determinism is true in that world, it would "replay" like it has the last time to that furthest point again. Now this kind of backward time "travel" is only backward in the "internal" time relative to those in the block universe but it occurs in the regular temporal direction in some external time or their "god's time." So it's "internal backwards" but not external backwards.

I think that this time travel scenario avoids the paradoxes as well but it may remain to be questioned whether this is a "real" case of backward time travel. It seems to fit my understanding for a possibility of time travel.