A question about time travel

[Umbran]

Well there is no changing the past unless you invent an event that "broke" the rules and damaged or destroyed all of the possible worlds and collapsed them down into a singularity.

Well, in the MWI, the fact that you went back is already part of the past of that world. That you decide to go back in time is one of the possibilities, and MWI explores ALL possibilities.

You may say, "But I haven't gone back in time yet, how would it know?" And, I'd answer that the "not yet" is nonsensical in the MWI. You are not at the forefront of history in MWI - that entire history, everything you can ever choose to do, is already predetermined.

The verb tenses of English cannot handle this, but... the entire history of the Universe was already set at its creation. The full extend of the universe in the time dimension as already there.
Everything you might choose to do is included in the ensemble at creation. You don't actually have free will to choose to go back or not. You both will and won't, in several different universes of the MWI.

 

[Nytmare]

Well, in the MWI, the fact that you went back is already part of the past of that world. That you decide to go back in time is one of the possibilities, and MWI explores ALL possibilities.

Oh no, I totally understand. I'm saying that as a scripting plot point and not as a matter of theoretical physics, the only way to make it work is to invent a story element that ignores and rewrites the rules that you've set in place about how the world works.

 

[Umbran]

I'm saying that as a scripting plot point and not as a matter of theoretical physics...

Well, from that perspective, the audience doesn't actually know the rules of the world. You don't have to say to the audience, "This breaks the rules". You just write stuff that happens, and move along.

 

[Particle_Man]

Although with MWI doesn't that mean that, if "timeline hopping" is possible at all, then there are an infinite series of worlds where (a) Particle_Man went to each of them (and an infinite number of worlds in which (an) Umbran (but not Particle_Man) went to each of them, and infinite number of worlds in which (an) Umbran and (a) Particle_Man went to each of them, etc.) thus multiplying the infinities by more infinities? Could a paradox be created in which (a) Particle_Man both "hits" and "misses" each time-line? Or does the "Everything is possible" not apply to the act of travelling between time lines itself, so "anything is possible" does not mean "every timeline hop is possible" does not lead to "every timeline hop happens"?

As an aside, I just thought of how Time Travel gives a great "non-origin" to the Joker (assuming you use the "who is this guy? he came out of nowhere!" idea of the Joker). He appeared in Gotham with no past. Either he erased it himself (perhaps driving him mad) or he comes from another time line (so there is no history of him in Batman's time line).

From a gamer point of view, imagine that some NPC encounters the party, seems to know a lot about them, and claims that he was part of the party all along, but his past got erased after a battle with a time elemental that cast him sideways in time. How would the party deal with this?

 

[Nytmare]

Although with MWI doesn't that mean that, if "timeline hopping" is possible at all, then there are an infinite series of worlds where (a) Particle_Man went to each of them (and an infinite number of worlds in which (an) Umbran (but not Particle_Man) went to each of them, and infinite number of worlds in which (an) Umbran and (a) Particle_Man went to each of them, etc.) thus multiplying the infinities by more infinities? Could a paradox be created in which (a) Particle_Man both "hits" and "misses" each time-line? Or does the "Everything is possible" not apply to the act of travelling between time lines itself, so "anything is possible" does not mean "every timeline hop is possible" does not lead to "every timeline hop happens"?

I'm not quite sure where you think the paradox lies in that statement. If you're dealing with an infinite set, an infinite order of magnitude larger or smaller is still infinite.

As Umbran was saying to me before, the specks and motes travelling back and forth between the many worlds aren't operating outside of or even impacting that framework, they are, have, and always will be the part of the framework statically laid out when/as the system existed/s.

I don't know if reading Flatland would help for a lot of the people in this thread or not (Flatland - Wikipedia). As I have always understood it, the line between how you and I experience and comprehend time vs what Many Worlds theorizes it actually is, is the difference between how you or I would experience and comprehend what a three dimensional object was if we existed and were interacting with it WITHIN a two dimensional space. Within two dimensions, with no concrete way of actually experiencing a third, a sphere would be a circle, bounded by 2 dimensions. Not even a circle as we would imagine it drawn on a piece of paper, because there's no third dimension vantage point that we could ever view it from. A sphere passing by us would be like a circle that started off as a dot, would get bigger and bigger, and then eventually shrink and disappear. Our understanding of the sphere would be that growing and shrinking circle, not what it actually was: a sphere.

That disconnect is quite possibly the same disconnect that you and I have when we talk about how we experience time. What if the whole of everything that has happened and will happen, as we see it sequentially and chronologically, is just a where our three dimensions bump into a complex extradimensional structure? Extradimensionally, you would be able to see it in its entirety for what it actually is, but trapped down here in 3 dimensions, all you see are the moments and seconds and months and years dragging along.

Also, the presupposition isn't that "everything is possible" it's that anything that IS possible is. It's not a Rule 34 catchall. Just because you can imagine a world where flying, sentient mailboxes are the dominant form of life doesn't mean that reality had to bother cobbling that set of impossible circumstances together.

 

[Umbran]

...thus multiplying the infinities by more infinities?

Yep. One of the egregious things about MWI, especially with time travel, is the purely ludicrous number of universes you actually end up having. The word "infinite" hardly covers it. Upper mathematics has a concept of "cardinality" - a measure of how big an infinity is. "Aleph-null" is the basic infinity we usually think of - the infinity of countable numbers, like integers. Aleph-1 is the infinity of real numbers. And it only gets worse from there.

Could a paradox be created in which (a) Particle_Man both "hits" and "misses" each time-line?Or does the "Everything is possible" not apply to the act of travelling between time lines itself, so "anything is possible" does not mean "every timeline hop is possible" does not lead to "every timeline hop happens"?

Given the assumptions that MWI is correct, and time travel is possible, then every possible hop happens. Also, every possible instance of not taking the hops also happens. They become separate, distinct timelines, so there is no paradox. The uncertainty of both doing it and not doing it is not available in the MWI - the whole point is to not have that uncertainty. You trade uncertainty for a truly ludicrous number of universes.

From a gamer point of view, imagine that some NPC encounters the party, seems to know a lot about them, and claims that he was part of the party all along, but his past got erased after a battle with a time elemental that cast him sideways in time.

Well, if he got shoved "sideways"... then it isn't that his past got erased. It is that his past wasn't here. It still exists, just over there, where we can't see it. So, yes, maybe he was a part of the party, but it wasn't this party, it was the party in that other timeline, who are different individuals.

How would the party deal with this?

A dude shows up and says he knows you from a separate timeline, and says that over in that timeline, you totally trust him. You have no way to verify this, nor whether that other you is not, in fact, of good motives or judgement.

That's... not much of a credential.

 

[Umbran]

I'm not quite sure where you think the paradox lies in that statement. If you're dealing with an infinite set, an infinite order of magnitude larger or smaller is still infinite.

Yep. Add infinity to infinity (or multiply infinity by infinity) and you just get more infinity.

As Umbran was saying to me before, the specks and motes travelling back and forth between the many worlds aren't operating outside of or even impacting that framework, they are, have, and always will be the part of the framework statically laid out when/as the system existed/s.

Exactly.

I don't know if reading Flatland would help for a lot of the people in this thread or not (Flatland - Wikipedia).

Yes, I think that's a useful way to approach it.

Also, the presupposition isn't that "everything is possible" it's that anything that IS possible is. It's not a Rule 34 catchall. Just because you can imagine a world where flying, sentient mailboxes are the dominant form of life doesn't mean that reality had to bother cobbling that set of impossible circumstances together.

Correct - the MWI is "everything that is possible, happens". If it isn't possible, though, it doesn't happen. If, for example, the physics of the universe do not allow faster-than-light travel, there will be no timeline in the Many Worlds that is Star Trek.

 

[Bohandas]

Yep. Add infinity to infinity (or multiply infinity by infinity) and you just get more infinity.

Taking the powerset of infinity, on the other hand....

Correct - the MWI is "everything that is possible, happens".

A fun thing about the Many Worlds Hypothesis (and to a lesser extent, other deterministic interpretations) is that Murphy's Law actually follows from it as a corollary

 

[Bohandas]

Amnyway, I like Novikov's Self-Consistency Principle, which basically posits that the past can't be changed even if it can be entered and interacted with; Events will conspire to produce the same timeline that has always existed.

The standard example is a ball traveling towards a wormhole on a trajectory such that if it continued its present course it would hit itself and prevent itself from entering the wormhole. Under the self-consistency principle it would strike itself not at the angle that one would naively expect from the starting conditions, but at a new angle that is exactly the angle its past self would need to be struck at to emerge at the new angle.

It would be a nightmare to run in a game but has great potential for stories. And not just stories about people trying to change thing and failing either. If we look at the situation four dimensionally, we can see how if time travel were convenient enough it would actually cause those with access to it to have good luck. It's incorrect to look at as time running past to future and then someone going back to create a new but somehow consistent timeline, there only was, is, and will be the one original timeline. And so, while one way for the timeline to remain consistent would be for events to conspire to prevent the traveler from making a desired change, an equally plausible one would be for events to conspire so that the thing that they would try to change simply doesn't happen in the first place. And so a time traveler would experience extremes of luck, the only bad things that would happen would be those that they couldn't plausibly change in a convenient fashion (with what is "plausible" and "convenient" depending on the extent and soundness of the wormhole network and other inter-temporal infrastructure, the time traveler's personal resources, and their proficiency, cleverness, and industriousness in dealing with this sort of logic; ideally there would be multiple failsafes in place to prevent weird interfering coincidences - or worse, failure of the time travel infrastructure - from being the path of least resistence for self-consistency*)

*Theoretically this could actually be why time travel doesn't exist; it might not be impossible per se, it might just be that the easiest and most probable way for things to stay consistent is for it not to happen.

 

[Umbran]

A fun thing about the Many Worlds Hypothesis

So, a pedantic note: it is not a Hypothesis. Nor is it a Theory. It is called an "Interpretation" for a reason.

Hypotheses and Theories can be tested. The Many Worlds Interpretation is non-falsifiable. It is a sort of narrative that is consistent with the laws of quantum mechanics, but is not testable by those laws.

So, why have it? Well, as a mental tool. When we put together our maps for RPGs, we think in rectangular coordinates (distance from the origin in the X and Y directions). But, if you are trying to do orbital mechanics, those rectangular coordinates suck. Bad. They are horrible to work with. Instead, we typically choose to work in cylindrical or spherical coordinates, and the math suddenly becomes vastly easier to work though.

The MWI is the same - it is a way of thinking about quantum mechanics that can help you manage the ideas without having your brain melt and run out your nose. It is also comforting to people who are not at ease with non-deterministic universes.