Ask Lemmy

I like the one where the motion of the universe is not accounted for, so the travelers drop into empty space. But someone figures out how to use that to travel through space.

Time Wars are fun though. Each prime timeline moving others toward them.

Whichever one is objectively correct based on empirical evidence.

Fun fact: time travel does exist, and I am myself a time traveler. The fact that I'm travelling at one second per second along with everyone else is just a minor detail.

I have always been a fan of stable time loops so I guess option 2 is the best one for me.

One trope I'd like to see more of is loops which are not stable themselves, but are stable as a group. Eg a 2-loop has loop A in which someone goes back in time and changes history leading to a new timeline loop B. Someone in loop B later goes back in time and changes history in a way that turns the timeline back into loop A.

My headcanon is that your option 3 is basically an n-loop that we only see the first few loops of.

Its a one way trip. You can never go back to the original universe.

I really like the nothing is changeable and travel is possible and anything you do while traveling has already happened / was already going to happen concept

If it actually existed, then obviously I would subscribe to whatever theory most accurately described how it worked. That's science.

If you're asking which theory I would predict is most likely, knowing only that time travel was possible as a starting point, then there are only two that I'm aware of that are logically consistent. Either:

• Single fixed timeline, whereby if you go back in time then whatever you do there was already a part of history from the start. You won't be able to "change" anything because you were always there. This is the approach described by the Novikov self-consistency principle.

• Multiple worlds, in which if you go back in time you just end up following a different "branch" of history forward from there.

Any of the models that let you "change your own history" are logically inconsistent and therefore utterly impossible. They just can't exist, like a square triangle or 1=2. They may be fine for entertaining movie plots but don't take them seriously.

I like the persistent present. We simply live with the paradoxes.

"Remember when Hitler was assassinated in 1919, 1933, 1936, and 1939, then off'd himself in his bunker in '45?"

Primer because Primer. (Video warning and some spoilers for a bunch of different films.)

I don't know if I would subscribe to it, but it is one of the more interesting ideas for time travel.

From a quantum perspective the Deutschian and similar models are honestly pretty compelling. They essentially require matching up the past and present in a consistent way that can remove paradoxes.

These make the most sense because it's entirely possible to write down spacetimes that contain "closed time like curves" (CTCs) ie. paths connecting past and present and you can then just let physics play out on these models (or more commonly using black box quantum circuits). The only consistent way to do it is to make sure the past and future side of such curves agree. It's not my area at all, just something colleagues of mine did, but from memory there are nice approaches using the path integral formalism that work really nicely in these scenarios.

All that's to say that I don't think time travel leads to anything changing, the past will have always agreed with whatever time travel happens in the future.

Having worked very briefly with the spacetimes that produce CTCs, I don't expect we'll be able to time travel because they usually violate the weak energy condition.

https://en.m.wikipedia.org/wiki/Quantum_mechanics_of_time_travel

• You branch off into another timeline and your actions make no difference to the previous timeline

New actions, new consequences.

I believe it's impossible in the real universe.

Sure there are solutions of general relativity that contain time loops, but they require stuff like an infinitely long cylinder, or escaping a spinning black hole, or negative energy. I just don't believe beings made of finite matter and with finite energy will ever be able to time travel (except into the future at various rates) and that's the only kind of beings I think exist.

The past, present, and future do not exist as separate states.

Imagine a vast array of all possible states of matter in the universe. Imagine reality has a finite spacial resolution. With a series of numbers, or even a single very large number, you could provide a unique identifier for every possible arrangement of matter in the universe. The positions of every star and galaxy. The detailed interactions of every quark. Imagine a list or array that would have a number of entries equal to some indecent multiple of "ten to the ten to the ten...." Imagine all these possible states, every possible configuration the matter of the universe could occupy.

Then realize...All of these possible states exist at once. They are all as real as any other. There is no preferred state. They all exist in some vast "10 to the ten to the ten" dimensional spacetime. What we perceive as the flow of time is simply us moving from one of these states to another. But our consciousness cannot move arbitrarily between states. There are elaborate rules on which states you will be able to observe dependent upon the states you previously observed. We call these rules the laws of physics.

So when you travel through time, you are simply altering your path on this vast multiverse of possible realities. There is no "real" reality. They are all real. Every possible configuration of the matter and energies of the universe physically exist concurrently.

There are no timelines to split or erase, because there are no timelines. There are just conscious minds moving through a near-infinite array of possible "nows." And all of the nows exist simultaneously. There is no real one. From the perspective of a "time traveler," it will seem like they changed "the future." But the truth is the very idea of a past, present, and future as distinct entities is madness. We're just consciousness drifting through the continuum, from one of the near-infinite nows to another.

The one where you can only jump forward, not backward. It avoids the common paradoxes.

My personal favorite?

Space and time is an infinite number of parallel realities that constantly compress and unravel at every possible random chance. We are 4th (or 3.5th) dimensional beings that experience the most probable result aggregated from an infinite existence. If you time travel back in time, and change the past, it would not affect the your past, but it would affect your future, if you time traveled back to your current time.

You can only change things you don't know about in advance. You know Hitler became chancellor of Germany, so you can't change that. But you can change the name of his dogs if you don't know what they were, and nobody who knew sent you back in time.

I was just reading this article about a mathematical understanding of closed time-like curves.

In essence, the argument is that time travel to the part is possible with a degree of free will, but you would not be allowed to alter the part in such a way as to remove the motivation for traveling back in time. E.g., it would be like Futurama where Fry kills his grandfather, but he impregnates his grandmother, this allowing himself to be born. The idea is that the timeline would correct itself and ensure that your future self will always return to the past.

The first one makes the most sense to me, which is why I think time travel should be used to make significant changes. Go big or go home

Whatever Primer did, cuz that movie scratches my brain real good.

Probably something like attractor field theory from Steins;Gate. In my view it's basically timelines with a bit of topological though thrown on it to combine closely related timelines into bundles, similar to some algebraic topology concepts I guess.

Something else entirely, I don't think we're capable of understanding time (yet?)

Are you asking which system I think is the most plausible, or which is the most desirable?

Plausibility:

Well, I'd guess that time travel probably isn't possible, and if it is, it's probably under extremely limited conditions that render it impractical for viable exploitation. But if you're operating under the assumption that it is, I'd say the "your actions do not affect this timeline" or similar type.

Why?

We have had no record of time travel or seen phenomena likely resulting from it. If at time T, time travel is discovered, it seems unlikely that someone after that time wouldn't have come back in time and done something that we'd have noticed.

And it's not just us. If self-timeline-affecting time travel is possible, then you consider all the possible civilizations out there in the universe who might discover it at some point in time and want to take advantage of it. Yet we've seen nothing from them. It's the Fermi Paradox on steroids. The Fermi Paradox asks why intelligent aliens, half of whom statistically probably evolved before us and should have colonized the universe if they're out there, aren't visible to us. The time to travel over even huge distances, though it is large, is small compared to the time required to evolve a spacefaring civilization. But in the presence of self-timeline-affecting time travel, then even the evolutionary time becomes a non-factor, since civilizations from the future could also show up, and roll back in time with their advanced technology and make use of it from then. The question is no longer just "where is everyone", but the even harder to explain "where is everyone from all time?"

Okay, that's the plausibility question. How about the desirability one, which system I'd like to exist?

Hmm. I guess I'd give the same answer, the "no affecting your own timeline" form. I think that if you could affect your own timeline, that probably some kind of incident in the future -- only takes one -- would be likely to have mucked up things sufficiently to wipe out civilization, and we probably wouldn't be around to even be pondering the matter.

Something else, namely: Time isn't real and uncaused events are not only possible but more common that most people think.

Paradoxes slowly disintegrate the timeline you're in. Thousands of years, Deadpool 3 rules.

The most interesting one to me, and the one that makes the most sense, is that changes propagate forward in time at the same speed as everything else, so 1 second per second. Why would causality suddenly decide to go any faster than that? This effectively means that all "alternate timelines" exist on the same timeline, and overwrite each other as they move forward.

You can visualize this by coloring the original timeline red. When you time travel backwards, you arrive at an earlier point on the timeline and it begin overwriting it orange, with the "head" of the orange section expanding into its future, which is previously red. If someone travels into the orange area again, it turns yellow, etc. If the instant where you time travelled backwards to make the orange region gets overwritten, the color of the timeline to the left of the orange region would begin expanding to overwrite it at the same speed as any other change.

This does lead to some interesting things, like two time travel loops that include the same point in time literally slowly corrupting the timeline. One loop, where you travel back, wait until when you left, then travel back again, would cause the future from your departure point to continually be overwritten by each new loop color, sending constant-width "bands" of colored time forward before they're overwritten by the band from the next loop. Two loops' bands would almost certainly not be commonly divisible, so you'd eventually end up with "bands" moving forward and within the loop that get smaller and smaller, fragmenting the timeline into colored noise. If you lived on the timeline, though, you wouldn't notice-- even if you're in a timeline band that's only 1 second wide, you move with it, so nothing seems out of the ordinary. But if you travelled back to the same point in time repeatedly to check on it, or could freeze yourself in time and watch the bands pass through your point in time, things would be changing incredibly quickly. This also means that waiting time in the future before travelling backwards in time would let the past have time to be overwritten by a different band, so the same point in time would be different depending on when you left the future. All timeline damage would be repaired (at band-expansion speed) if you could remove all instances of time travel backwards to the offending loops, though.

IRL, the speed of causality depends on your speed, too, and in theory, timeline changes would expand outward at the speed of light. My brain is not big enough to think through all the potential consequences of relativistic weirdness and time travel at once, though. I suspect it would allow for "bands"/fragmentation not only in time but in space as well.