A question about time travel

[Umbran]

Actually, if one follows the whole series, yes, the actions in the various times are a singular metaplot - each little correction is needed before we can see the protagonist return home.

Two things:

1) We are told very explicitly that Dr. Sam Beckett never returns home.

2) He is given the option to do so, but that isn't a logical result of time travel. That's just for doing enough service. Where and when that service was done is not relevant.

This, as compared to Star Trek: Enterprise, in which Scott Bakula is wrapped in a time travel plot that resolves by Captain Archer, at the end, stopping the time travel that starts the conflict, and the fact that it is time travel and events can precede their own causes is relevant.

 

[S'mon]

AFAICT the most mainstream view among physicists is the many worlds hypothesis - if time travel to the past is possible, the act of travelling to the past creates a branching timeline/universe. If you then go 'back to the future' you find yourself in a different timeline/universe than the one you left. But the traveller never gets 'deleted' or 'fades away'; they've simply shunted themselves out of their original timeline into the new timeline they created.

 

[Eltab]

The Back to the Future model has the benefit of being fun to watch while you figure out the 'rules' of time travel, and in-world examples are provided.

Set a few 'critical points' (so you can keep track). Start small - 'Biff Bosses Dad Around' future vs 'Dad Stands Up To Biff' future - and build up to more important changes later.

Fun option: "Non-Nazi Nationalist Government Takes Power in 1930s Germany" (from the board game Third Reich) allows the German invasion of the Soviet Union to cause the USSR to break up, sorta like in the 1990s.

 

[Umbran]

AFAICT the most mainstream view among physicists is the many worlds hypothesis - if time travel to the past is possible, the act of travelling to the past creates a branching timeline/universe. If you then go 'back to the future' you find yourself in a different timeline/universe than the one you left. But the traveller never gets 'deleted' or 'fades away'; they've simply shunted themselves out of their original timeline into the new timeline they created.

Not quite.

The Many Worlds Hypothesis* originally doesn't concern time travel. It first came up to resolve a troublesome bit of quantum mechanics - when a quantum waveform collapses - when we find out whether Schrodinger's Cat is alive or dead - why does it collapse one way, and not another? The Many-Worlds Hypothesis is that... it doesn't. It collapses in all ways, but the life-experience that is you only sees one of them. In the ensemble of all universes, every choice that could have been made, every result that could have happened, does happen, and are seen by the different versions of you.

So, the act of time travel really doesn't need to create a branching universe - the branching universe already exists.

Time travel came into it when someone realized that all the paradox issues of time travel are resolved if you cannot travel back to the exact timeline you came from, but you can only jump back into the past of one of the nearby branches - which is probably exactly like your own, because it only differs in how some John Q Public in Des Moines decided to get onion rings instead of fries with his burger that day.




*It is actually the Many Worlds Interpretation of quantum mechanics, as opposed to, say, the Copenhagen Interpretation. I am very loose with my language above, as it just makes it easier for people who don't want to get into the math.

 

[S'mon]

Time travel came into it when someone realized that all the paradox issues of time travel are resolved if you cannot travel back to the exact timeline you came from, but you can only jump back into the past of one of the nearby branches

I don't think that's correct. As I said, it's the act of travelling into the past which in this case creates the new branch. The time traveller isn't travelling to a pre-existing branch; at least I've never seen anyone posit this before (and I watch a lot of PBS Spacetime!)

 

[Umbran]

I don't think that's correct.

With all due respect, the Many Worlds Interpretation of quantum mechanics dates back to Erwin Schrödinger himself in 1952, followed by a formal approach to it by Hugh Everett in his doctoral thesis in 1957*. The Many-Worlds version of time travel is definitely an extension of Everett's work. The time travel notions have been argued over by physicists, and abused by authors, since that time, but that's the original theory.




*The scorn Everett received for this drove him out of theoretical physics. Later folks redeemed his notions and it is taken as a significant addition to the field at this point.

 

[S'mon]

With all due respect, the Many Worlds Interpretation of quantum mechanics dates back to Erwin Schrödinger himself in 1952, followed by a formal approach to it by Hugh Everett in his doctoral thesis in 1957*. The Many-Worlds version of time travel is definitely an extension of Everett's work. The time travel notions have been argued over by physicists, and abused by authors, since that time, but that's the original theory.




*The scorn Everett received for this drove him out of theoretical physics. Later folks redeemed his notions and it is taken as a significant addition to the field at this point.

Yes, I wasn't disagreeing with the point that Many Worlds theory wasn't about time travel and was applied to time travel later as a way to remove paradox. Just your idea of moving back in time to a pre-existing 'close parrallel', rather than the act of moving creating the parrallel. The point of MW theory is the constant divergence & creation of new universes whenever anything happens.

 

[Umbran]

Yes, I wasn't disagreeing with the point that Many Worlds theory wasn't about time travel and was applied to time travel later as a way to remove paradox.

Okay...

The point of MW theory is the constant divergence & creation of new universes whenever anything happens.

Not really. Many worlds isn't just the constant divergence and creation of new universes. It is the full exploration of all possibilities. I the MWI, anything that is possible does, in fact, happen.

We humans are used to thinking that the past is a set thing, and the future is not yet determined - there is a question as to what the future holds. We, in our current position, are at the forefront of the universe's development in time. That's not a meaningful statement in the MWI. There is no question what happens after this moment - all things that are possible after this moment will happen. Which means... the future is already determined.

So... why do we figure it doesn't exist? In the MWI... the idea that the future has yet to be made is the product of your incomplete perception. In the MWI, the future of the universe is already fully determined... and you cannot change it, because it is everything. So... in any real sense the phrase matters, the future already exists.

The MWI universe is deterministic... and that makes it pretty boring stuff, really. There are no questions.

 

[GMMichael]

Well, that's how it goes.

Thanks for the input, guys.

I think OP figured it out - this post clearly comes from the future.

Time travel came into it when someone realized that all the paradox issues of time travel are resolved if you cannot travel back to the exact timeline you came from, but you can only jump back into the past of one of the nearby branches - which is probably exactly like your own, because it only differs in how some John Q Public in Des Moines decided to get onion rings instead of fries with his burger that day.

I'm not sure this realization actually solves anything, because it requires not only many worlds, but many worlds that can have seemingly random events occurring (like a time traveler popping up out of nowhere).

The nearby branches are likely to follow the rules of physics, in which John orders onion rings or fries, and not in which a person (probably with a full mental picture of an alternate universe) appears out of nowhere, feeling like a military academy. (I know, that's matter-transference and not time travel, but why should they feel differently?)

 

[Umbran]

I'm not sure this realization actually solves anything, because it requires not only many worlds, but many worlds that can have seemingly random events occurring (like a time traveler popping up out of nowhere).

Well, the thing is, in MWI... there's no such thing as a random event. Randomness is only a perception from the point of view of a creature with limited perception of the whole. If you live in a refrigerator, and have no concept of the universe beyond, the appearance of a jar of Vegemite may seem "random" to you. But, there's a larger context in which that makes sense.

At least, as much as Vegemite can ever make sense.

The nearby branches are likely to follow the rules of physics

So, in science fiction, there's no such thing as "breaking the laws of physics". We are positing that time travel is possible - it is therefore within the laws of physics, and not breaking them. In the MWI, everything that is possible happens in some world (and perhaps in many of them). The fact that some inhabitants of a speck of rock within a nigh-infinite (possibly actually infinite) ensemble of infinite universes doesn't know all the laws of physics, such that some happenings seem weird to them, is not meaningful.