If we find quantum computers are no more powerful than classical ones, well then perhaps CTCs exist.
Although what if quantum computing is actually a result of CTC under the space time fabric? (Probably a nonsensical proposition eh?)
If you could travel back in time, that would mean that future and the past must co-exist with the present. If the pasts exists for us, the future exists, which means events have already happened, which means if time travel was to be invented then it would have already happened and someone would have traveled back to the past.
But if traveling back to the past, and changing something, then any changes to the past would alter the future, but changing the past could prevent time travel being invented which would mean the person never traveled back to change the past to alter the future to begin with...
Unless changing the past splits time into 2 with both outcomes.
If the purpose of traveling back in time is to fix a problem that occurs in the future, but the act of changing the past splits time, then the time you knew would not be altered, only the alternative timeline you created.
But if the past and future already exist, and time travel is possible, then all possible futures have ready happened and all possible pasts have already happened which would mean you have infinite timelines of every possible outcome of time itself. Because any change to the past in a given timeline only splits time to another outcome for the timeline, we would never know if our future changed or not making the entire event pointless.
I only believe it's possible to travel to the future, but not 'skip' in the same way back to the future does. Only freeze ourself and awake in the future which is kind of just pausing and letting time pass by.
^ my opinion.
If some particular observer's definition of "now" could be determined to be the "right" one by some experiment, that would reject the equivalence principle, which is the foundation of relativity. In other words, not likely to happen.
Using this principle one can build a conceptual "chain of simultaneity" between observers that extends arbitrarily far into the future or past.
It is no surprise, then, that physics doesn't have a concept of "now". In all physical equations, time is a free parameter, which we can set to any value, and those equations will tell us what we should expect to observe at that time. Physical equations do not reify any particular "t".
The fact that time seems to "flow" subjectively for us is an artifact of the fact that information can't (that we know of) flow from the future to the past, therefore at any given slice of time for a given observer, the past appears "set" (the observer has information/measurements about it that confine its possible states), but the future appears undetermined (there are no available direct measurements of it).
Why information can't flow from the future to the past is an extremely interesting question that is not terribly well answered, especially given that the laws of the universe are time-symmetrical. I think most physicists would posit that it has something to do with the perplexingly low entropy of the big bang, statistically mandating that later states must have higher entropy, thus birthing the second law of thermodynamics and giving rise to the "arrow of time".
Why specifically the Second Law would prevent the flow of even a single bit of information travelling into the past is not clear to me. As the article points out, Deutsch 1991 showed us that grandfather paradoxes don't exist once you factor in quantum mechanics. This is a question that has captivated me for many years.
I took a whack at it a while back:
I agree with basically all of your article. It certainly explains the dichotomy of physical laws allowing a spacetime to be "played backwards", while our experience must always be that of it being "played forwards"-- the time reversal of learning something is forgetting it, and this would invert the topological sort of knowledge dependency that places slices of our experience in the "future" or the "past". As such an observer in an isolated part of the universe which was being "played backwards" would label our future as their past.
If we buy into the Second Law, it makes sense to me that there would be some direction where there are more messy entanglements between all the parts of the universe, and in the other direction there would be fewer, and we would name the first direction "the future" and the second direction "the past", since there would be information embedded in all those mutual entanglements that would over-constrain states in the past and under-constrain states in the future.
What I don't get is what prevents me from setting up a circumstance where a measurement I make in the present is correlated with the result of (say) a coin flip in the future. That is easy to do if the coin flip is in the past, but impossible the other way around. I agree that by your logic if I were getting more information on the whole from the future, then I would almost definition label that "the past". But why not even one bit from the future? I can make a meaningful mark on a particle at time t0 and read it at t1, but not if t0 > t1. How come?
> there would be some direction where there are more messy entanglements between all the parts of the universe
Yes, that's exactly right, except for the word "messy". It's not "more messy", it's just "more".
> what prevents me from setting up a circumstance where a measurement I make in the present is correlated with the result of (say) a coin flip in the future
Nothing. If you flip a coin with enough precision you can make it come up reliably on one side or the other. ("But that's cheating!" you say. "I want the flip to be random." Well, you can't have it both ways: if the flip is random, then by definition it's not going to be correlated with anything in the past!)
> why not even one bit from the future
Because then it wouldn't be the future.
Think about this: how would you distinguish "receiving a bit from the future", which is apparently not possible, and "making a reliable prediction about the future", which is possible in many cases?
> I can make a meaningful mark on a particle at time t0 and read it at t1, but not if t0 > t1. How come?
You can't actually "make a mark" on a particle the way you can on a classical object. You can prepare a particle in a particular quantum state, but that's not the same as putting a mark on it.
Yes, of course. That was loose wording for "affect a particle a way that is meaningful", i.e. prepare it. :)
> "But that's cheating!" you say. "I want the flip to be random."
Actually, no, I'd rather the bit be useful!
> If you flip a coin with enough precision you can make it come up reliably on one side or the other.
Sure, if I know enough about the universe at some moment in time, by unitarity, I know its state at all other times. Having enough information to run the laws of physics forward to compute the outcome of the 2020 super bowl coin flip isn't much different, then, than running them backward to get the 2019 flip (and just as impractical). But what is interesting is that I could correlate (say) the spin of an electron with the result of the flip and read it later to tell what the flip was, but not earlier to tell what the flip will be.
(Obviously information about the past coin flip is available in many more places than just in my prepared particle, but I don't need anything more than that one qubit to precisely know the result. That's a big difference from needing to know everything in the coin's past light cone! )
If we peel away the human-imposed notions of time and causality (as you do in your article), and see spacetime as a "block" with microscopic time symmetry, or perhaps even further dismantled into only basis states of Hilbert space, it's still obscure to me why particles-- even individual ones-- seem to "carry" information only from the past and not from the future.
On an intuitive level it seems perfectly natural. On the level of (time-symmetrical) fundamental physics, I can't pin down why it would be.
Yes, but these details matter. Preparing a particle in a quantum state is fundamentally different from making an identifying mark on a classical object.
> > If you flip a coin with enough precision you can make it come up reliably on one side or the other.
> Sure, if I know enough about the universe at some moment in time, by unitarity, I know its state at all other times.
That's not what I meant. I'm not talking about trying to measure the state of the coin and the flipping apparatus in order to predict the outcome, I'm talking about building a precision flipping apparatus that allows you to control the outcome.
> I could correlate (say) the spin of an electron with the result of the flip and read it later to tell what the flip was, but not earlier to tell what the flip will be.
Actually, you can do both, and the procedures are essentially identical: to do the former, you look at the coin and manipulate the electron state to match. To do the latter you look at the electron and manipulate the state of the coin to match. Easy-peasy.
Is that not what you wanted? If not, why? (Remember that when I suggested you would want the coin flip to be random, you denied it.)
Don't want to sound pedantic, but we do this all the time. Also, time travel to the future (in a "when you come back 80 years have passed" sense) is something that happens at relativistic speeds; this is fact as far as physics is concerned.
Therein lies the problem. What makes you think the past can be changed? If the past can’t be changed, there’s no problem with time travel (at least on these grounds).
Moreover, if you adopt the block universe view, as general relativity seems to demand, there’s no such thing as change at all. There’s no becoming, only being. This was the conclusion reached by Parmenides in ancient times. Julian Barbour wrote a book on the subject called The End of Time, for those who are interested.
The only way time travel consistent with known physics is if past past travel is impossible. You can visit the future, but if you do, you can't travel back. Besides paradoxes, time travel to the past would make perpetual motion machines possible.
Exactly. The whole timeline already exists and is self-consistent. Whatever you do in the past already happened, and it did so in a way that was consistent with the future you travelled back from.
Time travel to the past is not inconsistent with known physics (see the various GR spacetimes with CTCs). There are reasons to think it’s probably impossible in our universe, however.
To be more explicit, the grandfather paradox is not an argument against time travel , precisely because the past cannot be changed (as you assumed in your example).
The only way for the timeline to defend from such inconsistencies would be to proactively thwart all your attempts, but then, Time/Fate has become a conscious entity.
No, you can't. Whatever it turns out you do already happened. You're assuming that being in the past means you can somehow "change" it, which is incoherent nonsense.
As David Lewis pointed out , the idea that self-consistency in the face of time travel requires some kind of "thwarting" of your actions by "the universe" is just a poor (and unnecessary) plot device employed by some science fiction authors. You should read up on Novikov self-consistency . Read the Lewis paper too, it doesn't require a physics background and will illuminate the heart of the matter. See in particular the discussion on compossibility.
I understand why I shouldn't be able to accidentally cause a paradox, but if I know how to cause one and have the means to do it, and the universe is mindless, what could stop me?
Our universe appears to lack the necessary extreme curvature or negative mass to sustain CTC’s. In the absence of such natural curvature or means to create it artificially and stabilize it, the whole question is a non-starter.
The timeline would shift in such an odd way too, because the past and future co-exist with each other, if you changed the past to fix the future the moment you travel would alter the timeline instantly and possibly prevent the need to ever travel in which case everyone would be shifted all of time from the moment the change in the past occurred...
It becomes very confusing, think I need a migraine pill and some sleep now...
Really, what is it about quantum physics that causes people to make these weird connections all the time?
(P.S.: The many-worlds interpretation is better called universal wave function theory. Less sexy, but more accurate.)
Time travel, if possible, would be entirely orthogonal.
There is no movement between the "worlds" in "many worlds" (which is part of the reason why "many worlds" is such a misleading label). At best there's interference between them.
Not necessarily disagreeing, but does something being pointless mean that it's not actually possible?
1) If, as you suggest, going back to the past to change something splits time, then everything is fine, albeit in a sort of selfish way: you went back to change something for yourself, but the timeline where the (presumably bad) event happened will still exist, you just won't have to experience it anymore. That is, of course, assuming that when you go back to change time, when you go forward again to return to your own time, you actually move forward on the new timeline, not on the old one. And this also doesn't address what you do with the you that exists in that timeline already. Even when you go back to your original time (just on the new timeline), it's still a problem: when you arrive back at your original time, there should be two of you (because the future you on the new timeline didn't need to go back in time to change something, because it was already changed).
2) The other possibility is that you don't actually need to change anything. If the many-worlds theory is true, then those other possible universes already exist. So if you want to change something, you just use some bit of technology to find the universe that has the change you want, and then transport yourself to it. You still have the problem of what you'll find there: will there be another you that you have to murder and hide the body (leaving your old universe with a never-to-be-solved missing-person report)? Or will you have to send him to your old universe, where he'll be confused about events being different? But then what stops him from just using that same technology to return to his universe to kick you out?
Sorta tangential: time travel might not even need to be what we think it is. If many-worlds is true, then it stands to reason that those many possible universes include one where the big bang happened a year later (or any other arbitrary time sooner or later) than it did in our universe. So if you want to go have a do-over for an event that happened a year ago, you just find a universe that's delayed a year, and switch to it. You still have the aforementioned problem of what you do with that universe's you. And in that case your body is also a year older than it should be in your new universe, so you probably wouldn't want to pick a universe so far back such that you look significantly different than the you that belongs in that universe.
At any rate, this is all really fun to think about.
I agree it's unlikely to be possible that we could send a person or object back to another time or forward in an movie trope kind of way...
What if everything exists at once, but there's an unalterable sequence of events. So, like you say everything has already happened past, present, and future including any kind of time travel.
But, let's say time travel is possible, but is limited to small state changes in the building blocks of the sequence of record (dare I say, the blockchain of time).
In theory we could make these changes and they would be completely undetected until we understand both how to create the change and know when and where to look for the changes.
Now suppose in the future we find a way to do this and collectively agree on some kind of location to start a linked list of information from other "times".
Thus begins some kind of technological singularity. Instantly we gain access to all the knowledge anyone at any time in existence that has the means wants to share.
Now our problem is how to process that much information in a way to make it useful and trustworthy in a reasonable amount of time.
This isn't well thought out and I'm sure there's some kind of paradox in there somewhere, but it was a fun Friday musing. I think this kind of allows for "time travel" to be possible via encoding information instead of killing Hitler while also allowing for those changes to have already happened without altering our currently observable past.
If anyone knows of any sci-fi stories or movies that explore a similar concept I'd be totally interested in the recommendation.
Now it's two months out and it's two months back, when you're pushing the speed of light
Twenty years on your homeworld's track, pushing the speed of light
And your friends are gone and your lovers too
And there's damn-all left that you can do
And you try to lie, but you know it's true, pushing the speed of light
Pushing the speed of light
I think they ended up returning 12000 years later to find the entire planet waiting for them with a light display across the Earth that says "Welcome Home".
Well, isn't the universe scheduled to "die off" due to low entropy in 10^100 years or so?
How much of science is done on weekends? I feel that investigations without capital investment is done in a hobby-type schedule. Probably these discovers (if any) might don't change our reality immediately, but are not worth the investment?
Yes that's a valid point and well-taken.
But there is a big difference between "impossible" and "just really really hard". And that's where modern physics is right now.
And if we were able to get a better handle on the hard problem of consciousness, the riddle of "what is time" might also yield.
So it's back to the metaphysical debate about whether the strict materialists are the ones who are "right" in all of this.
Matter goes through "events" that have cause and effect relationships, but can effect ever precede cause?
I don't see how you can empirically claim to have sufficient knowledge to answer that question.
But that's just me, I'm sure you see it differently.
> 
What do we want?
When do we want them?
It doesn't matter!
So to disagree it's enough to proof time existence or link to such proof.
First, if we're realists/utilitarian/pragmatists we don't need some fancy proof for time existing. We can see that the material world changes and we call that time. We even measure that passage with clocks. We even know (and have proven experimentally) that things going faster slow down clocks on them. So, whether time "really exists" or not is a meaningless question when we're concerned with time as we experience and know it.
It's like someone shooting a bullet at you, and you are asking "yes, but does this bullet _really_ exist?". Well, it might not (e.g. we might all live in a simulation, or the nature of physical objects might be just some probability distribution of information, and not "exist" in the plain sense we understand it), but in this life as we know it, a bullet hitting you will still have the consequences we know and expect.
Second, time exists in Relativity equations, so there's that.
>Time is a useful abstraction for calculating physical models, making forecasts. As we have other useful abstractions in math or programming - it doesn't make them existing in real world.
Doesn't matter. It's enough that we can measure it, we don't care for the "thing in itself" or if it's merely some illusion. We live in a universe where whether it's an illusion or not, it has the same effects for us (e.g. we grow old, things change state, our clock dials rotate).
I don't need a fancy proof either - any proof would be enough. What we both can agree is that things or processes are happening. They don't need time for that. Constant change, constant mutation doesn't require past or future, but now only. But as I said, we - intelligent species - need a concept of time to predict how the change will proceed.
What we measure in clocks depends on a clock - a sand falling down in a sand glass, a pendulum or a spring cycle and so on. After this we make a relation from this event and start to measure other events with it. Still it's just things happening, not a prove of a time existence.
If you say that you experience time. Well. That's how neural network works - they are shaped by events/impulses which happened to the network and accessing this information gives you illusion of a past - but this information encoded in your brain exists now.
> Doesn't matter. It's enough that we can measure it, we don't care for the "thing in itself" or if it's merely some illusion. We live in a universe where whether it's an illusion or not, it has the same effects for us (e.g. we grow old, things change state, our clock dials rotate).
So we agree in a part that things change state and this is enough to imply growing old and rotating clocks. I don't agree it doesn't matter - as if the time doesn't exist, we don't need to have a pointless discussion about "travelling in time".
The change itself might not, but to perceive something as having changed, e.g. to understand that what is in N state now was in some N - 1 state before, and N - 2 before that, presupposes time (or rather, can be considered the same thing as time).
>So we agree in a part that things change state and this is enough to imply growing old and rotating clocks. I don't agree it doesn't matter - as if the time doesn't exist, we don't need to have a pointless discussion about "travelling in time".
It wouldn't be pointless as to the actual phenomena we'd experience. It would the same as asking "since we see we can grow old, or see the clock hand rotating, can we somehow manage to go to the now moment when we were young, or where the clock hand was 1 hour before?".
Though I suppose you didn't really travel; you reset everything -- including yourself -- to a previous state/time. If you could isolate yourself, and then return the rest of the universe to some previous state (while finding a coherent place for your personal newer state in the old rest-of-universe state, possibly editing out the parts of the universe's state that contain the old you), then "traveling" would make more sense.
> but only that somehow the same state occur one more time
Assuming that state transitions are deterministic (apart from your meddling that allowed you to return to a prior state, that is), then this is equivalent to rolling back time, so the distinction doesn't really matter. If you pick a quantum state that occurred 10 years ago (by your subjective perception of time), then everyone will be 10 years younger, people who were born in the last 10 years will no longer exist, and people who died in the last 10 years will be alive again.
Also remember that everyone has their own reference frame when determining time. There's no such thing as "absolute time", or even the absolute passage of time. But that's ok; we don't need that to be the case for this to work out.
>If the moment is different in this way, can it still count as the past?
Well, several answers here.
a) If it's the same in every other way, then it's good enough as "the past" to me.
b) We understand things as substantially the same all the time even though they've changed in small or big ways (e.g. we consider ourselves the same person as the child we once were, we consider a city to still be the same city even though new buildings have come up/gone down as time passes, etc.). So why not consider the past + that difference as "the past"?
c) What else could it be? It's surely not the future, and it looks a hella lot like the past. At worst, we could say it's a new divergent version of our original past.
(But how would we even know it's a divergent version? Nothing might have changed, our trip might be a closed loop, where we always were to visit the past -- that is, there was never a past at time X without us visiting it from the future).
You're traveling at the speed of light. You have your perception of entropical change and an observer has theirs. There is no singular time that "exists". There are multiple perceptions that "exist". Time itself is not existing, as much as it is a perception of process(es). Humans mark states and compare later. This is not time, this is a measure of states. The term time is a shorthand for that, to the detriment of comprehending/understanding what it (the term Time) is describing.
"This kind of argument" you mean "argument"? Arguments applies to everything, agree. This kind of argument applies first of all to time and possibly to something else by analogy. Beauty of reasoning.
But if in some hypothetical universe, you realise by arguments, that nothing exists - what's the reason of "giving back the word" which in this hypothetical universe has no meaning? I don't follow this logic.
I think you'll find that your argument that time does not exist also applies to literally everything else. Space does not exist. Matter does not exist. People do not exist. There is no clearly defined moment of birth or death, or boundary between a person and the rest of the world; thus "people" don't exist. To use your words:
If you say that you experience seeing distinct people. Well. That's how neural networks work - they categorize the events/impulses which happened to the network into distinct objects that didn't exist in the events/impulses themselves.
> But if in some hypothetical universe, you realise by arguments, that nothing exists - what's the reason of "giving back the word" which in this hypothetical universe has no meaning? I don't follow this logic.
If you define a word such that it never applies, you're communicating badly. By the above arguments, nothing exists. And by the definition of the word "exists" that you're using, that's correct: nothing exists.
But that's a dumb way to define "exists". It's a perfectly useful word. And you should give it a more reasonable definition such that time, space, and people exist.
What the parent says is take your argument for time not existing:
"If you say that you experience time. Well. That's how neural network works - they are shaped by events/impulses which happened to the network and accessing this information gives you illusion of a past - but this information encoded in your brain exists now."
By the same logic, matter doesn't exist either.
Our brains  perceive a material world, but it's only a as thoughts an impressions they do so. There doesn't need (nor do we have of there existing) an actual material world. A neural network as complex as our brain that gets the same weights and is fed the same events/impulses as raw information (e.g. digits of input) would see the same "material" world.
No more reason for the material world to exist, than there's for time.
In fact, perhaps it's just my own brain, alone, as perhaps I'm the only actual person in the universe. What proof do I have that anybody else is not a "non playing character"?
About Solipsism - for sure in the universe you're living in you're the only one, as your perception is unique. We all have our own universes of thoughts. There's no proof for the question, that's why you should accept such possibility.
A reconfiguration of the matter on the world on the same state would still be exactly equivalent to a travel back in time (to that "now") in that case.
And such a time travel might not even take active agency to happen:
Occurring the same state is obvious on condition of finite amount of state and infinite amount of mutations. The question is if amount of states are finite.
For time travel to the future, I think it's about predicting future events of a few seconds before they happen, based on past knowledge we've gained or learned. An example of this for me is seeing a situation where somebody is about to crash their vehicle, as the driver doesn't see another vehicle then it happens. I recall a recent fender bender like so in Manhattan during January. I knew it was about to happen given there wasn't the space then crash.
So those are my definitions of time travel.