The least unlikely method of time travel I know of involves wormholes, as described by John Cramer here
this method involves travelling to a specific location in space-time, via a method that doesn’t exist until you set the wormhole up as described in that article. Until you do that you won’t get any time travellers at all, so there should be no surprise that we don’t see any time travellers today.
Once you open the wormhole for two-way traffic, though, goodbye causality.
Terms like “before” and “after” don’t mean the same here as what we normally mean by them. Stay tuned; I’ll come back to this.
Yeah, me too. I must be a masochist.
That’s identical to my bifurcation hypothesis, except that instead of going back to a “close” grandfather, by time traveling you’re creating a duplicate, and affecting the duplicate.
Well, yeah, sorta. You can’t go back in time and change your own history that way, but you can play “what if” games, which are a lot of fun. But it doesn’t fix the problem that in the world I came from, the nasty guys did the nasty stuff to a lot of nice people. They’re still there, in an “alternate” future that I’ve just changed to something else. Meanwhile, though, the nasty guys are defeated in this new branch of reality. Is that a good thing? I think so. But my true love is still back there suffering. Oh darn.
I started out with a model like that, but since the “single timeline” isn’t immutable, you have to introduce a second time dimension, which is what superhal’s suggestion requires.
After more thought, I realized that I couldn’t find any distinction between the multiple timeline (“bifurcation”) model and the “single timeline that changes” model.
However, the “single timeline that changes” model is still interesting in that it helps illustrate some amusing possibilities. One of those amusing possibilities is the lighter that is “never” created. Another is what I’d call a “bistable multivibrator” (aka “flip-flop”).
First, the flip-flop. I murder my mother. This changes the timeline, when the new “present” catches up to when I would have jumped to the past, I’m not there, so my mother lives; I’m born, and I live to jump to the past to murder my mother.
No contradiction here, just a “single timeline” with two possible states (one where I exist, one where I don’t). I’m pretty sure that this is indistinguishable from the multiple timeline model. If there is any observable difference, please point it out!
But it does lead to thinking of a possibility that the multiple timeline model doesn’t tend to suggest. How about a stable loop that evolved from an unstable one? Is that even possible (hypothetically, without internal contradictions)? I suspect it is but haven’t constructed one. I bet some sci-fi author has, though. It’d be a hard story to tell unless the evolution happened in only a few loops.
If it’s possible, it seems to me that one could construct a sequence of events where the result is a stable loop where I give my younger self the lighter – the same one I received from myself when I was younger. There is now no creation of this lighter, but there was a creation of it on an “earlier” timeline (“earlier” in a second time dimension).
At this point my head starts hurting too much to continue. Especially when I think about how old (worn out, liquid level) that lighter is.
Yeah, that’s a great point. It’s pretty much the only refutation of the claim mentioned above, that if time travel existed, we’d know about it already. Harry Sheldon should have considered that possibility!
BTW, the description of using time travel (sending signals to the past) to do NP-complete operations in polynomial time is wild! That’s in the Novikov self-consistency article, I believe.
Dammit, my head is hurting so I might as well continue with the lighter.
Let’s say that when I receive that lighter from my older self, I say “Cool” and flick it once, to see it burn. Then I set it aside and never touch it until I’m about to head back. Each time through the loop, the liquid level is a bit lower. Eventually, it’s no longer a lighter. Now, if I continue to pick it up before hopping back, the timeline is fairly stable still. My younger self gets it, tries it, and it fails. Do I keep it? If not, end of loops stability on this point.
Even if I do keep it, the plastic keeps getting more brittle with age. Eventually, it’s junk, and I no longer keep it to take to the past, so it disappears. Let’s say it has no effect on any other events.
The number of times through the loop is finite, but the number of times reality goes through the loop is infinite. (That’s an axiom; take it or leave it.) So, the probability of seeing the lighter is zero. It happened, but we’ll never see it. Problem solved, sorta.
The apparently stable loop wasn’t quite stable, not until we got rid of that pesky lighter.
The biggest problem I have with the second time dimension is that either there was a first instance or there wasn’t.
Put that in your pipe and smoke it.
Is that a difference between the mutable-single-timeline model and the multiple timeline one? I’m not sure.
That reminds me of the one about the blonde who got an abortion because she didn’t think it was hers.
I haven’t read all of the responses, so forgive me if this is already covered. I think there may be a way to test the predestination option with time travel allowed.
First, let’s talk about the predestination option. Now, to me, that means everything that has already happened cannot be changed because* it has already happened*. If you create a time machine and go into the past, anything you change has already happened and your change is already accounted for in this timeline. So, for whatever reason, you can’t go and kill your own grandfather because that didn’t happen in the timeline.
Anyway, let’s posit that time travel to the past from some future time IS possible. They have no reason to be careful because they can’t change the past in anyway that it hasn’t already been changed.
Ok, that’s the world I hypothesize.
But there MAY be a way to test if time travel is possible in this option. Assuming time travel is possible and not exceedingly rare, we can look for its effects in our present and past.
The simplest method would be to just look for someone who appears, then disappears. Look for events that have an unknown actor for whom his past and future are not accounted for. Similarly, we can look for events that we can’t reasonably explain with the technology present at that time. Or devices that require knowledge that wasn’t present at that time. (What was that ancient brass device found underwater that either accurately predicted time, or was a sophisticated sextant? Sorry, the name escapes me.)
I’m sure others can come up with other, better options for these tests.
If we DO find something like that, we can infer that time travel is possible. That goes a long way toward trying to create a time travel device – just knowing it is possible.
Anyway, just an idea.
J.
I believe you’re right that if we had time travel (or even communication to other times), we could test whether the timeline is immutable. If it is, that would be a pretty strong argument for determinism. That would be at odds with the fact that QM is nondeterministic, but there might be a way to harmonize the new data point with QM.
If changing the past changes your own memory of the past (you as the time traveler), … well, I can’t make that kind of model work at all. Someone with bigger brains will have to field it.
See the Novikov self-consistency principle - Wikipedia , which explains a (practically tautological) aspect of reality if there is time travel and the past is immutable. It has consequences, which are testable.
A couple of questions came to mind while I was reading this thread. Why does a time traveller have to cause a bifurcation(two possible timelines)? A person goes back in time, the timeline changes, and time proceeds normally from that point on. All events occur again, but are not influenced by the former timeline. The lottery question can be answered by this. What gaurantees that the ping-pong balls fall exactly the same way in the second time line? It is completely independant of the first. You go back in time, buy the lottery ticket, and the balls fall differently, and you still don’t win. Your odds of winning are now still the basic odds of the lottery.
My theory eliminates all possibility of paradox. Take the grandmother paradox. You go back in time and kill grandma. You create a new timeline, completely independant of the first where you are not born. Period, end of story. Now the “you” in this timeline is 50 years older, lives his life and dies, and the timeline runs along since that is how it always was in this timeline, and you could never go back to the timeline you come from, because it doesn’t exist anymore. I give an example from the reboot “Star Trek” movie. Nero goes back in time and kills Kirk’s father. So Kirk waits and kills Nero before he can go back in time. But it doesn’t return the timeline to normal, becasuse the Nero that killed his dad is from another timeline, and the one you kill is not the same Nero. And if you go back in time with the Enterprise-E and get between Nero’s ship and Kirk’s Dad’s ship so that he doesn’t kill him, the timeline will play out completely differently again, and not necessarily the same. That’s my 2 cents, anyway. So how wrong is tmy theory?
A good point. If time travel requires a receiver as well as a transmitter, it would easily explain why no one could ever go back to a time before its invention.
This sounds like the movie Primer.
Here’s the problem I have with time travel. If you can go back in time, then that means the past exists somewhere such that it can be traveled to. Well if the past exists somewhere, then it stands to reason that the future must also exist somewhere as well. So it sort of begs the question what makes “now” any more or less important than any other moment on the time space continuum? It also raises questions about destiny and predetermination.
Another question to ponder. How “elastic” is the space time continuum. If I go back and step on a butterfly, does Hitler win WWII? Or would it take an atomic bomb to change the course of history? Or even then, would it just be chalked up to some sort of mysterious “Tunguska event”, probably caused by a meteor and history goes on as expected?
They do that to resolve paradox. i.e. a time traveler goes back in time and kills his grandfather. It creates a future where you can’t exist because you were never born yet if you were never born, you couldn’t have gone back in time to kill your grandfather.
Regarding multiple timelines:
Traveler A goes back in time from 2013[sub]0[/sub] to 1950[sub]0[/sub], changes something and creates a new timeline (1).
Traveler B goes back in time from 2013[sub]0[/sub] to 1970. Does he go back in time to 1970[sub]0[/sub] or 1970[sub]1[/sub]?
Our decisions control(ed) that future. Choice is still important.
Experiment, experiment!
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.
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.
In my model, when going backwards, you always go to your own history. B would go to 1970[sub]0[/sub].
BTW, I’m not confident my bifurcation model holds up. If I got it right, there’s no difference between this and the “mutable timeline” model. The latter requires a careful ordering of events in some other dimension of time (which is generally ignored but implied and absolutely required).
If I got it right, the bifurcation model avoids the need for the second time dimension, replacing it with a space dimension.
What’s the difference? Little or nothing; it’s just another dimension. It’s not really even a “whole dimension,” though I’m going over my mathematical head here. It’s representable by a binary tree, which is really a dimensionality between 1 and 2. That is, if I understand fractional dimensions correctly, and I’m sure I don’t.
More specifically, the time threads form a PO-set with a greatest common minimum (if I recall the terms correctly), and that GCM is the point of the farthest back travel in time. Further, this PO-set has no loops or cycles. Though, cycles might be fun to toy with.
One of my favorite time-travel stories is “At the Cross-Time Jaunters’ Ball” by Alexander Jablokov. This uses the “bifurcation” model (no nasty paradoxes) but the travelers use homing devices to get back to their original timelines. The travelers are artists striving to create original and interesting timelines.
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.
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.
Though I am now amused at the idea of the Universe being some sort of massive Wiki being maintained by higher beings. I’m sure the “Adolf Hitler” article must be the center of the edit war from hell. 