Valid models for time travel?

[clairobscur]

@dotchan

Here’s another question that I’ve never heard an answer that made sense to me: if someone did go about and change the past, how would we be able to tell the difference? For all we know, the past has always been what it is (with a huge web of cause and effect that goes all the way back to the very beginning of the universe). It’s not like the universe has some sort of “version history” that we can access right now.

There are a number of stories using this plot. For instance, the time traveler in a suicide mission murders Hitler in 1925. His co-conspirators in the present check it up and notice immediatly that he failed since history didn’t change : Ernst Thalmann somehow survived and led Communist Germany to WWII. They deduce from this that the past can’t be modified.

[Learjeff]

@eburacum45

Hmm. I think it is more complicated than that. Using the wormhole model, the 20 year-old time traveller A can go backwards from 2050[sub]0[/sub] to 2020[sub]0[/sub]; but he can then kill his own father in 2030, creating a new timeline (2030[sub]1[/sub]) so that when the wormhole opens in 2050[sub]1[/sub] a different time traveller B goes back in time to to 2020[sub]1[/sub].

Time traveller B proceeds to kill her own mother in 2030, creating a new timeline 2030[sub]2[/sub]; this means that when the wormhole opens in 2050[sub]2[/sub] yet another time traveller C will go back to 2020 to create yet another timeline and so on towards infinity.

Now the strangest thing about all this is that if you go through the wormhole in one direction you encounter a completely different timeline to the one you encounter going backwards. By waiting till 2050 when both the forward and the backward wormholes are open side by side you can travel at will from 2050[sub]0[/sub] to 2050[sub]3[/sub] and beyond.

In fact there should be an infinite number of alternate universes in both directions, which means there can be no ‘original’ loop, and therefore no original timetraveller; an infinite number of alternate events come into existence at the instant the wormhole first opens, and there is no original version. How do you know who is going to come through the 'hole in 2020? It could be anybody. Do they all arrive at once, and somehow collapse into a single timetraveller when observed?

You could get all the people in all the alternate universes to come through one by one until they were all in one universe- an increasingly crowded universe full of slightly different copiesof the same people.

This is why they call it a paradox, and is probably why time travel cannot ever happen.

You’re definitely right about the infinite aspect. I’d notate differently, the new timeline is created as you arrive in the past, not when you do something “significant”. There is no godlike arbiter of significance: once you arrive, there are atoms there that weren’t before. You could arrive at the bottom of the ocean, promptly die and decompose, but forever there will be trace details that are different because you were there, thus a different timeline. You’ll still be born and still travel back in time and create a new timeline, infinitely. It does seem like a bit of a waste of whole universes, but then, any multiverse theory does that. One infinity, two infinity … no difference!

By this theory, once bifurcated, two timelines never rejoin. Perhaps if black holes destroyed information (which currently the belief is they do not; they obey the 2nd law of thermodynamics), two timelines could end up being identical. Does that mean there are two identical timelines or have they joined into one? Is there a difference?

There also isn’t a problem with those infinite second time travelers, though it was a great thing to consider. Let me set that up. Traveler A jumps back and promptly dies, making no noticeable difference in human affairs of the past, creating an infinite number of timelines. In each of those infinite timelines, traveler B jumps back to a point before A’s arrival in the past. They all “arrive” at the same world, but the arrival of each creates a new timeline, with just one traveler arriving.

That’s the fun thing about the multiverse. You can use up worlds to your heart’s content and never run out! (I admit I’m not a big fan of the multiverse concept, though.)

[msmith537]

@Learjeff

That’s exactly what I described as bifurcation.

There’s the question over whether the old timeline “continues to exist”. It’s a philosophical difference, which is addressed by the next point, or by else the second time dimension, which I’ll touch on again after that.

The first problem with your distinction here is your use of the word “exists” – present tense. When we’re talking about timelines, we can’t fall back on our intuitive understanding of verb tenses. To say that “the future exists” or “the past exists” is nonsense, from a linguistic standpoint. The future will exist, and the past existed.

But once we admit to the possibility of time travel, that kinks things up considerably (not to mention doing serious violence to verb tenses). If we can go somewhere “now”, it does seem to imply that the destination exists “now”. This is discussed in the Wikipedia article on time travel.

But what does that mean? To quote Inigo, “I do not think that word means what you think it means.” I don’t think it means what I think it means either, thus this thread.

A better way to think of the question is presentism vs. eternalism. According to that distinction, if there’s time travel, “presentism” is false. If it is possible to create “closed timelike curves” (which have been discussed very seriously by notable physicists, and might be possible but probably aren’t), then presentism is dead in the water.

Back to tonyfop. What’s the difference between bifurcation of the timeline, and “overwriting” the timeline with a new, independent one?

A) none. Both are equivalent mathematical models. One uses a separate line in a pseudo-space, the other uses a separate chain of events in a “second pass”, which introduces a second dimension of time (albeit possibly a discrete one). You can map one to the other and there are no observable differences, unless …

B) If you can travel or communicate between the two timelines, then clearly there’s a difference. I discount this possibility, but no doubt Dr Who’s TARDIS can navigate any number of threads of timespace. It doesn’t violate any laws of logic that I can figure, but for some reason it goes beyond the pale of amusement for me.

We would also have accepted “wibbily wobbly timey wimey…stuff” as a correct answer.

[msmith537]

@Learjeff

I glossed over this, but it’s a brilliant point. I have no answer. On the contrary, I have a number of other questions and tangentially related thought experiments, that also touch on what constitutes consciousness or awareness. My pragmatic side and my mathematical side are in big disagreement here, which is always unsettling.

Thank you.

The only thing I can think of is that “now” actually isn’t any more or less important except for that is how we perceive the universe. Sort of like how the Earth appears flat when you’re standing on it. The structure of time and space might appear very different if we could perceive all events simultaneously like the Tralfamadorians from Kurt Vonnegut’s books.

[Cartoonacy]

@msmith537

The only thing I can think of is that “now” actually isn’t any more or less important except for that is how we perceive the universe.

I picture the timeline as a tree diagram. From any given point, the future diverges into a myriad of branches, depending on the choices made. From any “now,” you can move forward into many possible futures, but you can only move backward along a single path – the branch that led to your “now.” Once you arrive “there” (excuse me, “then”), your chances are slim to none of being able to return to the “now” that you started from, because of the infinite number of paths going in that direction.

Ever read James P. Hogan’s The Proteus Operation? One of my favorite time travel novels that uses this model.