Some ideas are:
- You branch off into another timeline and your actions make no difference to the previous timeline
- You’ve already taken said actions but just didn’t know about it so nothing changes
- Actions taken can have an effect (so you could suddenly erase yourself if you killed your parents)
- Only “nexus” or fixed events really matter, the timeline will sort itself out for minor changes
- something else entirely
Removed by mod
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.
You’ve already taken said actions but just didn’t know about it so nothing changes
12 Monkeys did this one perfectly.
You can’t change things because if you undid the thing, then there wouldn’t be a reason to undo the thing. If you go back in time, you are just going to do what you already did because that is in the past.
Logically speaking it’s the only way time travel can be done, and for bonus points physics wouldn’t have a problem with it.
Any Back to the Future shenanigans is just creating alternate realities, which may or may not instantly destroy the original.
I’d totally forgotten about 12 monkeys. I had that VHS of this when I was 11 or 12 years old, I probably watched it 30 times and I never fully understood it. 25 years later I think it’s time for me to rewatch this
Rewatched it recently, it held up really well!
Oh, I’ll have to watch that
"Default behaviors is undo, no redo. You control undo the undo. Emacs.
I recommend undotree, which is also a non-destructive undo, but for some cases makes it easier to reach those points.
If you go back in time, you are just going to do what you already did because that is in the past.
Only if the Universe is deterministic. If not, random rolls having different outcomes may completely change the course of events and decisions made by people.
Nothing is truly random, including the weather. It is extremely complex and difficult to predict, but once it happens that is what happened. As long as dice fall with the exact same speed and hit the same surface in the same spot at the same angle it will always end up with the same result. The randomness of dice comes from how the very small differences influence the outcome.
Going back in time with the knowledge of what happened the first time means that either you will choose the same thing because something led to that original choice or something will keep you from interfering. Free will exists because we don’t literally know the exact outcome of our actions or the things outside of our control in advance.
Nothing is truly random
Modern physics says otherwise. Einstein also thought exactly like that with his “hidden variables” theory which was later disproven.
Edit: I was interested to read some relevant discussions and here’s some links with quotes
https://philosophy.stackexchange.com/questions/29364/does-true-randomness-actually-exist
It seems very likely that every deviation from perfect homogeneity and isotropy in the universe is due to amplified quantum fluctuations. (That’s true in inflationary cosmology, and I’d expect it to be true in practically any alternative to it.) For example, the shape of Earth’s land masses was probably determined by quantum fluctuations, and has had an enormous influence on human history.
Quantum fluctuations is basically true randomness on quantum level.
The randomness is largely canceled out, except in the case of unstable systems which magnify the effects of any perturbations, no matter how small.
You’re assuming that time travel is equivalent to “rewinding” the intervening time span as if it had never occurred—in which case, yes, nondeterministic events are likely to happen differently.
But that’s not the case if time travel is a closed time-like loop (which is implicit in the “immutable-past” scenario in OP’s point 2). In that case everything happens only once, so it makes no difference whether or not the universe is strictly deterministic.
Maybe this is the same as what you’re saying but my issue with the idea that “You’ve already taken said actions but just didn’t know about it so nothing changes” is that it means time travelers don’t have any free will once they go back in time. If that’s the case, then it bring up existential concerns and that might extend to non backwards time travelers (i.e. us)
I think from a physics standpoint, strict free will is already an illusion and the only useful definitions of free will basically boil down to “choices can be made”, perhaps as far as “Slight differences in initial conditions can lead to different choices” (but somehow excluding random processes). That kind of definition doesn’t even require consciousness, and is compatible with a deterministic universe like ours seems mostly to be. Would also be compatible with the time traveler unwittingly doing everything as must happen, but still via individual choices.
Choice is one of the slight differences that can lead to different outcomes. A rock falling down a hill will always fall downhill because of gravity. An animal can choose to slow itself or even work against gravity to move uphill. Instead of gravity, there are a ton of prior experiences that will influence that choice, but choice is still a distinct part of the process.
Exactly. That’s why I think the only useful definitions of free will are those that are weak enough to distinguish between the animal and the rock in a situation like that.
Are you saying that even without time travel, free will is an illusion? Surely there has to be a time travel scenario, like going back 1 second in time and shaking hands, where all information is known to both travelers, and the future self would know what was done previously, and can choose to take a different action.
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.
But my question was if it was possible. Not do you believe it’s possible.
The only saving grace of GR based time travel is that we don’t actually know if the weak energy condition is physical. It probably is, but technically it could be a false assumption.
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:
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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.
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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 just imagine if life is a simulation and everytime someone travel back a new branch created but then coming back to present timeline you have to fix all the merge conflicts.
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Time travel to the future is possible if you travel fast enough. For example, traveling to the nearest galaxy at near-light speed wouldn’t take long for you, though it would take significantly longer for those observing you from Earth.
As for traveling to the past, I imagine it might involve the many-worlds interpretation of quantum physics, where every possible event that can happen does happen in a separate timeline. In this view, you wouldn’t be “changing” the past but rather experiencing an alternate version of it.
I don’t believe in free will, so I’m not concerned about the idea of altering the future by changing the past. If you traveled back in time and killed Hitler, it wouldn’t affect this timeline’s future; instead, you’d simply enter a timeline where that event occurred. The future of your original timeline would remain unchanged.
Travelling to the future is so easy that you can’t not do it
I think it would be like the first one, except instead of you going back to that time, you would be making a copy of that time to traverse to from your time, similar to how moving a file between devices causes it to be copied.
A relevant quote from a physicist is “some will say it’s easier to predict the future than the past, since a single effect can have multiple possible causes but a single cause can only have one possible effect.”
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
The one that exists.
1
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.
Imagine if someone just naturally traveled through time at like… 1.0005 seconds per second… What would that look like? Would we be able to tell? Would they be able to tell? Would their perception be different?
This is a Relativity thing, isn’t it? Like the astronaut twins sort of…
That’s exactly like being a bit closer to the bottom of a steep gravity well.
Yes, we could tell if they took a precise clock with them. In fact, we have to account for an even smaller discrepancy in order for GPS to work: we here stuck further down in Earth’s gravity well travel through time and extra ten milliseconds or so per year vs. an orbiting satellite.
Are we traveling through time? Or is time simply a universal constant of entropy? Everything you experience is energy flowing from a higher potential to a lower potential, with some “loss” to heat. Without that downward shuffle, a rock balanced on it’s tip is indistinguishable from a time-stopped version of itself.
Basically, time is your body’s sensation of the inevitable terror that is the heatdeath of the universe.
Why would it be less terrifying to be further away from a gravity well?
I think you meant this comment for a different thread.
Nope.
Okay. What did I say about gravity wells?
You implied that life would be more terrifying the faster you traveled through time, like what would happen at the bottom of a gravity well.
Where?
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.
You may like the show Dark, if you haven’t already seen it.
Dark
The season 3 story was pretty much why I’m interested in this trope! Although I maintain Dark was not a stable time loop story, it just had the appearance of one.
Something else, namely: Time isn’t real and uncaused events are not only possible but more common that most people think.
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.
Isn’t jumping forward and then changing stuff in your timeline just traveling to the past with extra steps? If doing something in the past changes the now changing something in the now based in future outcome would also change that outcome.
Something else entirely, I don’t think we’re capable of understanding time (yet?)
