Many worlds theory, I don't get something...

Samir Duran Duran

So if you consider multiple worlds theory to involve the universe "splitting" upon a measurement rather than having an arbitrarily large number of universes (to account for universe A, universe A with a tiny change, etc) running parallel from the dawn of time...

If you traveled back in time, where would you go? The "splitting" approach requires a cause for any of the countless splits happening every second, but it wouldn't accomadate a time-traveler unless... you were your own cause? So the cause in one universe would have an effect in another?

Would someone creating a time machine mean that all splits from the dawn of time included universes with a traveler stepping through a portal at that very moment?

I guess the best I can come up with is the ever-present traveler split, but I find it unsatisfying.

Maybe someone with a better understanding of the theories of time travel could clear things up for me?

Fantasma

Hello,

Personally, I do not believe in time travel, I cannot comment, or provide you with a theory myself, but reading wikipedia I found this:

The possibility of paradoxes
The Novikov self-consistency principle and recent calculations by Kip S. Thorne indicate that simple masses passing through time travel wormholes could never engender paradoxes—there are no initial conditions that lead to paradox once time travel is introduced. If his results can be generalised, they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that any situation you can set up in a time travel story turns out to permit many consistent solutions. The circumstances might, however, turn out to be almost unbelievably strange.

Parallel universes might provide a way out of paradoxes. Everett's many-worlds interpretation of quantum mechanics suggests that all possible quantum events can occur in mutually exclusive histories. These alternate, or parallel, histories would form a branching tree symbolizing all possible outcomes of any interaction. If all possibilities exist, any paradoxes could be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction, but some physicists such as David Deutsch have suggested that if time travel is possible and the many-worlds interpretation is correct, then a time traveler should indeed end up in a different history than the one he started from. On the other hand, Stephen Hawking has argued that even if the many-worlds interpretation is correct, we should expect each time traveler to experience a single self-consistent timeline, so that time travelers remain within their own world rather than traveling to a different one.

Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel without paradoxes. In quantum theory observation causes possible states to 'collapse' into one measured state; hence, the past observed from the present is deterministic (it has only one possible state), but the present observed from the past has many possible states until our actions cause it to collapse into one state. Our actions will then be seen to have been inevitable.

---

You may read more here:

http://en.wikipedia.org/wiki/Time_travel

http://en.wikipedia.org/wiki/Many-worlds_interpretation

NexusSix

The problem here is that this stuff is so far out on the theoretical fringe that it's starting to overlap with philosophy, and we really don't have the technology to try to find empirical evidence for this sort of thing.

There's an interesting thread in the D&D forum that touches on some of this stuff--be sure to give it a look-see:

Mad D&D Science

Along with Fantasma's links, you could also check out:

http://en.wikipedia.org/wiki/Multiverse

misbehavin

Well, the main consideration is whether you believe in free-will... If time-travel were possible, then it would need to be accounted for in the infinite-histories theory... In other words, you had already gone back to that plane of existance, and already done exactly as you will do, and since you have no free-will, you will behave exactly as previously encountered.

EDIT: I am big into theoretical physics, and if I have time later this afternoon, I'll come back with a much larger explanation.

blincoln

misbehavin wrote: »

Well, the main consideration is whether you believe in free-will... If time-travel were possible, then it would need to be accounted for in the infinite-histories theory... In other words, you had already gone back to that plane of existance, and already done exactly as you will do, and since you have no free-will, you will behave exactly as previously encountered.

Wait, what? That's exactly the opposite of everything I've read about the many-worlds theory.

My understanding (from one of David Deutsch's books) is that according to the theory, if you could travel back in time you would travel back along the same branch of the "tree" that you had always existed on. But as soon as you arrived in the past (and therefore started making changes), you would move into a new set of branches that began at that point. So if you killed your own grandfather, you would simply exist from that point onward in a universe where your grandfather had been killed by someone from a parallel universe. Meanwhile, back in your "home" branch, your friends would think you had disappeared forever, because there would be no way for you to get back to that exact branch.

misbehavin

blincoln wrote: »

misbehavin wrote: »

Well, the main consideration is whether you believe in free-will... If time-travel were possible, then it would need to be accounted for in the infinite-histories theory... In other words, you had already gone back to that plane of existance, and already done exactly as you will do, and since you have no free-will, you will behave exactly as previously encountered.

Wait, what? That's exactly the opposite of everything I've read about the many-worlds theory.

My understanding (from one of David Deutsch's books) is that according to the theory, if you could travel back in time you would travel back along the same branch of the "tree" that you had always existed on. But as soon as you arrived in the past (and therefore started making changes), you would move into a new set of branches that began at that point. So if you killed your own grandfather, you would simply exist from that point onward in a universe where your grandfather had been killed by someone from a parallel universe. Meanwhile, back in your "home" branch, your friends would think you had disappeared forever, because there would be no way for you to get back to that exact branch.

That's the most prominent theory, and arguably the one that makes most sense in line with the infinite histories principle. But it's not the only theory, and I tend to believe that free-will is a myth, and find the pre-existing acts theory in line with that thought process. It solves all paradoxes, as well, as there is no question as to whether or not you killed your grandfather, because you being alive implies you didn't, and given lack of free-will, you can't.

blincoln

misbehavin wrote: »

That's the most prominent theory, and arguably the one that makes most sense in line with the infinite histories principle. But it's not the only theory, and I tend to believe that free-will is a myth, and find the pre-existing acts theory in line with that thought process. It solves all paradoxes, as well, as there is no question as to whether or not you killed your grandfather, because you being alive implies you didn't, and given lack of free-will, you can't.

That may or may not be true, but you implied that it was a variation the many-worlds theory, and it's not. If everything is predetermined, then that makes it an only-one-world theory, which is sort of the opposite of what we're talking about.

Edit: and anyway, the many-worlds theory is based on quantum physics, whose foundations preclude determinism.