Ask Lemmy

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

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.

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.

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

New actions, new consequences.

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.

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.

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

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.

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

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.

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

I subscribe to multiverse theory. It's probably the safest route and probably most likely.

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.

I think time travel as a being who perceives one dimensional time linearly is not possible. And for any entity who doesn't perceive time linearly it would be no different from traveling in a spacial dimension. It's just travel. Anything that entity does in that point is a permanent fixture to the entities that perceive it linearly.

So yes, if someone could travel in time in the SciFi sense, they wouldn't be able to change anything in their past experience (direct experience or prior to their perception, but in their event line) because that's already part of that point in spacetime to anyone who experiences it linearly.

But also, it's likely that time is not one-dimensional just like we know space is not only three-dimensional. So it is possible that you could end up in a separate "branch" of time that your past self from your perspective will never experience (directly or as past events), because it's not the same point in spacetime as the event in your direct past timeline. But it's not like there is a specific set of "branches". They likely don't branch off from a single trunk into the other dimension(s) or if they did "branch", it was at the same time as all other "branches", the beginning of the universe, not as specific events occur like in SciFi. And the changes you make in those branches were always part of those branches to people who will perceive the future of that timeline.

Traveling to the past: You can't go into the past because doing so would change who you are and thus your reason for traveling in the first place. For example, killing Hitler when he was a baby would completely change the world as we know it, thus change you as you are. You might not have been born, or if you were, you wouldn't know who Hitler was, so there was no reason to go into the past, thus your time travel never happened in the first place. It's a paradox via butterfly effect. To underscore this further, you couldn't even change the history of another planet's species simply because it's still a part of your timeline. Same universe.

The only scenario where it might work is going into the past as an impartial observer and not having any impact at all (some kind of magical bubble where you are invisible and no effect on the past). That would be fun, because you get to learn about history firsthand.

An interesting time travel alternative is Trunks' timeline from Dragonball Z, where he went to the past, saved their future, but the androids in his timeline still persisted. This leads me to believe it was not just another time but another dimension (a la Rick and Morty).

Traveling to the future is a bit easier. Technically, with the proper spacecraft, you can go into the future (go sit around Sag A* for a bit), but it would be a future where you weren't around to have an influence in it. It would be like temporarily kidnapping yourself. This might be similar to how people came back five years later after being snapped by Thanos in the MCU.

IMO, the best use of time travel would be to go to the future tomorrow to scan ahead and see what happens (as long as you wouldn't have been needed in that future), then going back to the present time just seconds after you left. So little would have changed that your timeline would remain intact (only your biological clock would be off). So, you might be able to prevent incidents in the world by constantly jumping ahead to see what was going to happen. A future-scanning time traveler might have been able to prevent the recent New Orleans tragedy from happening. They could also be lazy and just learn the winning lottery numbers.

The one that exists.

Infinite branches.

It feels more intuitive, and doesn’t involve any strange problems. It implies that the multiverse has infinite possibilities, they are all realized somewhere, and a time machine allows you to jump between them.

Like in Black Science, I think time travel would fuck with the fabric of reality. Make it shreddy.

I do not believe in nexus events; there is a personal reason (experience ) I don't expect anyone else to believe based on something I experienced but I don't. ETA: Unfortunately, everything has happened already, and I was very angry about it.

Just watched Arrival again yesterday and that's my other guess. More like your choice of "you have already done it, you can't alter the timeline" but can't go outside your lifetime, time doesn't work the way we think and we can perceive other "times" because they aren't really linear, just some quirk of our perception makes it seem that way, you really exist concurrently all along your existence.

But if some machine was designed to take you before or after your lifetime, it would tear at the fabric of reality (lifetime not exactly the correct word but your existence that has a beginning and end of some sort).