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Time Travel (2006) (scottaaronson.com)
119 points by panic 32 days ago | hide | past | web | favorite | 82 comments



> if closed timelike curves existed, then quantum computers would be no more powerful than classical ones.

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?)


It's generally considered inelegant but not nonsensical. There exist hidden-variable theories that reproduce all the normal predictions of quantum mechanics (including quantum computations), but they require non-local interactions. I think a restricted set of CTCs could mediate those interactions.


I love movies and TV shows about time travel. But I just don't think it's possible at all.

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.


It is already implied by relativity that time doesn't flow. You cannot choose a definition of "now" that some other observer who is moving relatively to you would agree with. Under relativity, observers will disagree about which events are happening simultaneously. So if there are two observers in the universe, the notion of a "global now" is incompatible with relativity.

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 thought that time was essentially an illusion? Isn't time just things changing from one state into the next. So time travel isn't really logical, it would entail changing the entire state of the universe at once from its current state to one it had in the past


> Why information can't flow from the future to the past is an extremely interesting question that is not terribly well answered.

I took a whack at it a while back:

http://blog.rongarret.info/2014/10/parallel-universes-and-ar...


Great article, thanks for sharing! I actually found your Google talk some years ago and found it fascinating.

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?


Thanks for the kind words.

> 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.


> You can't actually "make a mark" on a particle

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.


> That was loose wording for "affect a particle a way that is meaningful", i.e. prepare it. :)

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.)


I enjoyed this PBS Space Time video explaining this and more with examples: https://www.youtube.com/watch?v=HUMGc8hEkpc


It is worth to note that systems with preferred system of coordinates and "flowing" time can have Lorentz symmetry (for instance see https://physics.stackexchange.com/questions/887/relativistic...) so relativity doesn't tell us about time travel as much as we might hope


>I only believe it's possible to travel to the future

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.


“Primer” is the one exception. It deals with time travel in a physically realistic way (not surprising given it is an independent artsy film school movie by a writer/director with a math background).


> But if traveling back to the past, and changing something

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.


Are you implying traveling to the past doesn't change anything? You could spread antibiotic resistant bacteria, you could walk over a bee that was going to pollinate a flower with a significant mutation, you could breathe their seasonal flu and carry it over to your time before it manifests.

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.


> Are you implying traveling to the past doesn't change anything?

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.


Don't be silly. If you dedicated your whole life to invent a time machine, succedded at your 80s, met your younger self and gave him your research so he can build it in his 30s, how is that self-consistent?


It’s not. You just described an inconsistent timeline, but this has nothing to do with whether time travel creates such inconsistent timelines.

To be more explicit, the grandfather paradox is not an argument against time travel [1], precisely because the past cannot be changed (as you assumed in your example).

[1] https://en.wikipedia.org/wiki/Grandfather_paradox#Philosophi...


Time travel _to_the_past_ has all the potential to create inconsistent timelines. You can deliberately cause severe inconsistencies because you have the crucial component: being in the past.

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.


> You can deliberately cause severe inconsistencies...

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 [1], 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 [2]. 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.

[1] https://www.csus.edu/indiv/m/merlinos/paradoxes%20of%20time%...

[2] https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...


Come on, just humor me. Following my first example, why couldn't I visit my younger self and give him my life research? What physical law could possibly prevent us from working together and building a second time machine in a few years, thus guaranteeing a paradox?

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?


Stephen Hawking posited that the first and only thing a time machine would do is detonate as a direct consequence of virtual particles reaching arbitrarily high energy density through repeated loops. If you don’t like various chronology protection conjecture, fair enough, but building a time machine means something unless you’re inserting magic into it.

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.


First thing that occurs to me as a work around for this paradox is to think of traveling into the past as recreating a certain state as opposed to actually "reversing" the clock so to speak. Like pulling a commit hash from a while a back and then forking the repo. Maybe we're just not on a fork yet?


If you go back in time but don't do anything at all in order to not change any event from occurring the way its meant to then I guess it could be like that. But if we assume there's only 1 timeline and we change the past then the current future changes, you wouldn't know that you changed the future because the timeline you came from is the timeline you altered. But if you altered the timeline preventing you from needing to travel back then you would never travel back to prevent the timeline from occurring.

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...


There's a simple solution here - all pastward time travel (and by extension all FTL travel, since it's the same) results in a parallel timeline.


Unless changing the past splits time into 2 with both outcomes.

Many-worlds [0]

[0] https://en.wikipedia.org/wiki/Many-worlds_interpretation


... which is entirely unrelated.

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.)


Can you justify your statement? Wavefunction collapse would certainly implies there can be multiple coexisting states. Only one is observed by "your" reality.


Wavefunction collapse (which is actually not a thing in MWI/universal wavefunction theory) is an event or transition that moves strictly forward in time.

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.


If all time is flat then perhaps the only time travelers who manage to make it back in time are the ones who don't change anything. Or, to look at it another way, their changes are already "baked in".


> 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.

Not necessarily disagreeing, but does something being pointless mean that it's not actually possible?


So two possibilities that might solve your objections:

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.


Disclaimer, I'm not a mathematician, physicist, or even of notable intelligence... but I too have seen a lot of movies :)

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.


If we solve Cryosleep we don't need to go faster then light, just aim carefully and hope that the computers have not died by radiation when it's time to wake you up in a million or so years. As time seem to appear infinite. But what if when we wake up, all the mass in the universe (except our space-craft) have been sucked into another dimension, will time still exist !?


Except that this means that you are disconnecting yourselves from anyone who is not in cryosleep, as they would have all died. Incidentally, this problem exists even when moving at close-to-light speeds as well :(


that's why you need to set up a lockstep

https://io9.gizmodo.com/the-worldbuilding-in-lockstep-is-so-...


It's the way of life, and actually not that bad. There would be other things to worry about. And you could probably bring your pet, and anyone crazy enough to follow you on such an endeavor.


I quit everyone on Facebook and feel just fine. I am ready.


Or almost as if you move away from friends and family. The human condition.


If you move away, you usually can come back.

  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
https://www.youtube.com/watch?v=ud6LiVJkwyA


This reminds me of the final battle in the anime Gunbuster where they are trying to save the human race while traveling at light speed. They know if they engage the alien race they will never be able to see their friends or family again.

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".


That text sent shivers down my spine. As for our mundane world: You usually can come back, but shit often gets in the way. I guess I'm getting kinda old-ish.


Thank you for sharing this


Not that long ago, people would migrate to another continent and literally never hear again about the people they left behind. I have a great grandfather that did that. It's really not that different from migrating to another star in that sense.


>As time seem to appear infinite

Well, isn't the universe scheduled to "die off" due to low entropy in 10^100 years or so?


Yes exactly. It sounds like a long time unless you're waiting around for your classical computer to brute-force solve a problem for which quantum computers have an exponential advantage.


> This question has a very long history of being studied by physicists on weekends.

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?


https://en.wikipedia.org/wiki/Timeline_of_the_far_future Maybe someone from the future could update this for us.


Time doesn't exist therefore you can't travel in it.


You're making the assertion that it's impossible to say anything about time that's empirical, that isn't upon deeper examination a statement about human consciousness.

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.


Not by itself, but it's part of the space-time continuum (through which you can travel).


You confuse Einstein model of physics which is powerful and help us count and build things with what actually the reality is. We use many different models in math or programming without believing they are reality.


[flagged]


I agree that a citation is useful, but I know that I've seen articles about how the variable for time cancels itself out in many important physics derivations, suggesting that time is at least not an important factor in the reality of the universe, and thus possibly does not exist.


You could say the exact same thing for eg. mass.


>> Time doesn't exist therefore you can't travel in it

> [citation needed]

What do we want?

Citations!

When do we want them?

It doesn't matter!


I heard there are various models of time. It doesn't look easily searchable though.


I wonder if there is one in which time does not exist...


Citation would be needed for a time proof. I don't know about such.


Also it's really weird to ask for citation for a simple implication. Science is based on proofs, not on quotations.


Somehow I get down voted for sharing this simple thought. As we participate the same reality you need to admit time travelling is a must have of a pop science magazines. But this sentence makes it all nonsense - as there's no proof for time existing. 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.

So to disagree it's enough to proof time existence or link to such proof.


>But this sentence makes it all nonsense - as there's no proof for time existing.

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'm glad you've bothered to answer.

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".


>Constant change, constant mutation doesn't require past or future, but now only.

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?".


So the question is, when you broke a vase can you glue it back to the previous state - and the answer is yes. The more exact state you'd like to achieve the more effort it will require. But having the same state again doesn't mean you've travelled in time, but only that somehow the same state occur one more time (or better to say once again ;) ).


I think what the parent is getting at is that the progression of states is time. If you think of time as just the entire universe morphing through a progression of different quantum states, and then find a way to modify the quantum state of the entire universe so that it is equivalent to a previous quantum state, then "traveling through time" would easily be a colloquial way of saying what you've done.

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.


That's for a vase. Why would it have to be the same for time? This presupposes that time is unidirectional -- which it was supposed to prove.


When you go back to this moment. Would you then expect to have the experience of remembering “the future”? Would it still be “the moment we were young” or something different? If the moment is different in this way, can it still count as the past? If it isn’t, what makes you think you’ve only experienced it once?


>When you go back to this moment. Would you then expect to have the experience of remembering “the future”?

Yes.

>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).


At some point you're going to realize that this kind of argument applies to everything else as well, not just time. When that happens, I do hope you'll give us back the word "exists". It's a useful word.


As I understand it,

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.


Word exists is useful, the same like the concept of time. I don't take it anywhere no worries.

"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.


> This kind of argument applies first of all to time and possibly to something else by analogy.

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.


I don't get your arguments about not existing people, matter, and space. I try hard though.


Perhaps not hard enough.

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 [1] 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"?

https://en.wikipedia.org/wiki/Solipsism)


No. I'm saying about technical aspect of neural networks which are shaped by experienced signals - which in other words is encoding information about what we call past. That's why we can tell what happen before - because we have encoded information about it in our brain - but before doesn't exist any more as it mutated to different state and there is nowhere to travel. You can bring back previous state, but it's not travelling in time in a way like Hollywood shows. I'm not going any further here into claiming that everything is an illusion. Not because of limitations of your brain you will not pass through walls. There's not the same logic here as you're saying.

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.


If there's only now - that's zero dimensional time, but it's still existing time, it's just not an extended dimension.


Which also implies there's nowhere to travel in this case.


Not really.

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:

https://en.wikipedia.org/wiki/Poincar%C3%A9_recurrence_theor...


It would be an equivalent but not travelling in time itself. If I will go by car to point B I can't say I was travelling by train just because of the same result.

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.


I would equate time travel to being a memory and thought process. We're able to regenerate the images of events in our minds vividly when we think about past events which make us happy (e.g. child birth) or things that make us sad (e.g. manager screaming at us due to another bug crashing our software). This is time travel to the past by my definition.

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.


Have you watched Stein's Gate? If we could send a little bit of data to the past maybe we could fit our memories.




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