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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.