Valid models for time travel?

Factual Questions
1

We’ve all read the books and seen the movies with models of time travel that just plain make no sense. For example, “Back to the Future” is a great fun movie, but the part where our hero watches a photo from his origin time as his sibling fades from it is bunk. It’s perfect from the point of view of a fun movie. My question here is, what reasonable models of time travel do we have?

I can think of two.

The first is “Contradictions just won’t happen.” There is a single timeline, which never “changes”. Anything in the future is determined, and everything in the past already happened and is immutable. You cannot go back and kill your grandfather.

I heard this model discussed in a Nature show on time travel and wormholes. A physicist explained how you could create a wormhole and then manipulate it so that one end was near the other in space but not in time. He talked about placing the ends on a billiard table so that if you roll a ball toward one end, it comes out the other end before it goes in the first end. He said that no matter how you manipulated things, you could never get the “future” ball to block the original one from entering the wormhole.

A second model might be what I’ll call “bifurcations”. Whenever you travel into the past, as soon as you arrive, you bifurcate the timeline, creating a new future. Thus, no contradictions. You kill your grandfather and are never born in that new future timeline. I admit I get a little fuzzy about what happens if you don’t kill your grandfather in this new timeline. In that case, a slightly different you still comes back to the past – the same one? – and does different things than the “first” you. He may run into you. Who knows. Regardless, his arrival creates yet another bifurcation, and things play out.

A problem with that model is travel to the future. One wonders which of several timelines you arrive at. One possibility is that you arrive at all of them (being duplicated by any bifurcations that happen after you push the button, as though you’re somehow present in the timeline while you’re traveling.) Another is you arrive at a random one, or the “default” one which “precedes” any arrivals from the future.

I begin to suspect that this model might have problems, but so far I haven’t found any I can’t wiggle out of. There is some sense of an additional time dimension here, not a continuous one but a discreet one, defined by the bifurcations (because each event has a “with” and “without” future timeline, where the “without” clearly preceded the “with” one.)

I suppose a third model is the multiverse one where everything that might happen does (glossing over a lot here). I have a hard time figuring out what to make of that one, though. It boggles the imagination. I admit that the bifurcation model has a lot in common with it, though.

Do these models stand up to rigorous scrutiny? Are there other models that do?

If this belongs in IMHO, that’s fine, but I suspect that there is serious thought on some of these matters, where experts have weighed in, and it’s not purely a matter of opinion.

2

You’re forgetting the most valid model of all: time travel isn’t possible at all. The preponderance of evidence to this point vastly supports that model.

Which is roughly equivalent to your first case, where you can do it, but can’t change anything. I’d argue that’s semantically impossible: your presence in the past is, by definition, a change, so the only way the “immutable past” trick works is for it not to be possible to travel there at all. (Or for there to be an intelligent, omniscient, omnipotent “preventer of change” watching over it, but even if you’re going to allow the God hypothesis, what’s the point?)

3

I don’t really think any of those models make any sense the way you describe them. In my mind, there are only two possibilities:

  1. Time travel is not possible. The end.

  2. Time travel is possible, but the perspective to understand how it fits into timelines is more complicated than we currently realize. For example, we’d probably have to stop thinking of a “time line” at all. It’s perhaps a little like the change in thinking required when you go from Newtonian physics to relativity and you realize that spatial dimensions and even the speed at which time passes are all dependent on your speed. But unless someone comes up with that theoretical leap, we’re not even in a position to imagine time travel any more than Newton was in a position to imagine time dilation in an apple.

4

No, it works. You being there isn’t a change, because you were always there.

5

Granted, “Time travel is not possible” is the simplest answer, though a proponent would have to explain the wormhole thing. Oh yeah, wormholes aren’t possible, or what you get out has no information based on what went in (which is hard to imagine thanks to the 2nd law of thermo, so … back to they aren’t possible.)

Please explain what doesn’t make sense.

The only thing that doesn’t “make sense” to me about the first option is, what’s the mechanism that prevents you from changing the past? Admittedly, that’s the kind of thinking that doesn’t work for quantum mechanics (and I suspect the scientist I mentioned above was a QM kind of guy, used to arguments like “Well, that’s how the wave function collapses.”)

I bet you’re right that no matter what, any real answer will involve stuff that doesn’t make sense to us based on our current mindsets. But let’s do the best with what we have and what we do know.

6

I thought we already proved time travel one way (forward), but faster then normal time, things like relativistic effects as one approaches the speed of light, and if I understand it correctly frame shifting around a rotating gravitational field.

Though the OP more refers to backwards time travel, but I think we need to look at that is we already know that forwards time travel works.

7

The OP has clearly spent a lot of time (no pun intended!) thinking about this, and I believe all such parties would get a lot out of reading the Wikipedia article on this subject. It examines all these ideas, and others, in great depth.

8

The multiverse theory seems to have some measure of support even outside of time-travel scenarios. It’s a theory, not currently testable, but a theory that makes some sense logically. The idea is that whenever there are two things that could happen, the universe splits, and in one universe, one of the choices happens, and the other choice happens in the other universe. From this theory we get the idea that anything that could have happened actually did happen in a parallel universe. E.g. somewhere out there there’s a universe where Romney won in 2012, and also a universe in which Romney not only won in 2012, he also managed to convince Congress to declare war against Switzerland and declare October 29th to be “National Ping Pong Day”.

9

This is called the Novikov self-consistency principle.

10

On the immutable timeline: You don’t have to kill your grandfather to be introducing changes, and some of those changes are guaranteed to contradict things that originally happened. So the only way it works is if we imagine that the entire timeline was set in stone, with what is essentially predestination of all future and past events. If you preserve causality as we know it, then time travel is impossible. If causality is not preserved, then time travel is either impossible (because it never happened) or it is predestined (because it did, and must, happen). Either way, nobody gets to change anything. You don’t even get to change the future - all of your actions are predestined and free will is an illusion. (This vision actually fits quite a few religions well, but it’s a let down for scientists and fiction writers, especially since you had to violate causality in order to accomplish nothing.)

I see bifurcation and the multiverse as being essentially the same idea. Both require multiple universes. It’s just that one creates multiples on in response to certain events, while one creates infinite mulitples every moment. Either way, you’re not actually time traveling, are you? You’re universe traveling.

11

Have you ever read Niven’s “All The Myriad Ways”, a short story about the consequences of proving the bifurcation principle of time travel? In this story suicides and murders are on the rise because people realize that any decision they make creates a million other worlds where difference outcomes result-in the grand scheme of things, even the illusion of Free Will has been destroyed.

12

Right. Like when Fry went back in time and had sex with his grandmother, thus “becoming” his own grandfather, he didn’t change the past. That was what had just always happened in the past, and now Fry and the others discovered the truth.

13

But that’s exactly the idea. What’s wrong with that?

14

It doesn’t matter what you do anymore-it’s all set in stone. If you hit somebody with your car it’s because you were supposed to. If you rob a bank it’s because you were supposed to. What is going to happen is going to happen, and there’s not a damn thing you can do about it…so why bother?

15

Well, and the whole “what’s the agent?” question. I go back in time to kill my grandfather. Since I know him in the present, something has to prevent me from doing so (the “set in stone” argument). But killing someone is a fairly simple thing, and I can make any number of attempts at it I like: hundreds, thousands, millions. But somehow, they always fail to work? Three times is enemy action, they say. Who or what is imposing this bizarre set of coincidences that save my grandpa all the time?

More simply: Figure out today’s lottery numbers. I didn’t win. I take those numbers, go back to the past, and by a ticket with them. The future is now causative: I know I didn’t win. Still, I’m holding a ticket with the winning numbers. But what agent makes it so that I can’t collect?

16

But that is the Universe we live in. Everything you’ve ever done, there’s not a damned thing you can do to change it. Everything you’re doing now, there won’t be a damned thing tomorrow you can do to change it. Does that make you say “why bother”? How is it any different if we introduce time travel?

17

Of course we’re all familiar with the relativistic twin “paradox” (which is not a paradox). I never thought of that as forward time travel, but you’re definitely right: it could be used that way.

Sci-fi version: the “forward time travel” device just oscillates you so fast that you’re essentially frozen. Of course, it’s different than fictional time travel, because to everyone else, you’re there all the time, just vibrating faster than they can see the vibrations. If they shoot you, you die when you slow down. And let’s ignore the energy consumption! This also avoids the problem of hitting the target in space as well as time. :slight_smile:

Doh! I never considered Wikipedia, the fount of all wisdom, knowledge, and trivia. I’ll be looking into that.

As I understand it, something different only happens if it’s a different collapse of a wave equation. That is, every possibility of any quantum event happens. At the high level, we only get the possibilities that are the result of different quantum outcomes. Whether that encompasses all conceivable high-level outcomes or not is questionable. I’m also curious about the cardinality of the multiverse. Is it a countable infinity, or is it the cardinality of points on a line?

Thanks :slight_smile:

I think that depends on what we mean by “as we know it”. Since we don’t experience time travel, it would definitely require a bit of expansion.

A lot of us are either determinists or nondeterminists, where this is true anyway: you can’t choose to affect the future. But from the standpoint of ethics, that’s irrelevant: you still have the perception of free will, and it’s up to you to use it wisely (and reap the benefits) or not (and reap the perils).

True, except that when going backwards, you’re creating a new universe, starting from a point in your past. (A point in the past of your timeline.)

Interesting.

Because you still have the illusion of free will. IMHO, it’s more than an illusion, but in a way that “free-willers” would find unsatisfactory. Regardless, you pretty much seem to have the choice to be the guy who takes his life seriously and does well, or the guy who figures there’s no point and wallows. You pays your penny and you takes your choice.

18

A lovely thing about that is, if we ever do manage to get two ends of a wormhole to make a time machine, we can test that theory, and it’ll tell us a lot about the world we live in! IMHO, it could go either way, but either way, we’d have a lot of interesting new questions to ask.

Seriously, if I had to make a bet, I’d bet on impossibility. But we’re having fun here. Perhaps it should be in IMHO, but I did get some very informative answers above (thanks!)

19

Oops, caught myself using a casual definition of “causality” above. Tsk tsk! dracoi was more correct than I gave him credit for.

20

I agree, but I’d never say never. Lots of things were once believed, with great fervor and based on all known evidence, to be true, and were later proven otherwise. Time travel and a faster-than-light drive will, I suspect, someday fall into that category of human “certainty.”

“When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.” - Arthur C. Clarke