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[–]gwern 18 points19 points  (10 children)

On a lighter note, I've finished an essay arguing that Ted Chiang's short story "Story Of Your Life" has been grossly misinterpreted and that it's not about time travel at all: https://gwern.net/Story%20Of%20Your%20Life

[–]Works_of_memercy 2 points3 points  (4 children)

Why do you people insist on bringing up interesting topics in the "fun" thread? Not everything that's not Culture Wars is fun, you know...

Anyway, that was a good read, and you even mentioned Dune! It was weird for me because I thought a lot about the way precognition in the Dune universe works as I read it, and I even came up with the Fermat principle as an explanatory tool, while completely forgetting that I read that Ted Chiang's story, so I'm not even sure how much of the following explanation is mine and how much is Ted Chiang's.

Anyways and first of all. I don't quite like your own explanation because it ignores entropy. We can't remember the future because we are living in an always expanding entropy funnel, regarding possible world-states, so our personal past is focused into an exponentially thin beam of our past states, while the future has that exponentially increasing sets of reachable higher-entropy states. So there's that.

But then! I think you failed to give the Chiang's argument its due, especially after considering the Dune.

Clairvoyance is fundamentally different from time travel. When you consider the grandfather paradox, there's this guy who traveled into the past and has now to be prevented from killing his grandfather by an unlikely event (leaving aside QM MWI explanations). That's weird.

Asking the Universe to show you the future is fundamentally different, because now the only constraint on the Universe is that it shows you the future that's guaranteed to make you fulfill it. It can show you a future where you succeed about your goals and then you do stuff and do succeed. It can show you a future where you fail your goals and then you'll overreact and do stupid shit and fail.

Can the Universe find a steady state producing a picture of the future that makes you act in a way that brings that future about? Why not, and moreover, why can't it do that using local rules that in a similarly weird way guarantee that photons travel from the emitter to the receiver along the shortest-time path?

Like, the problem Leto Atreides was facing was that his clairvoyance showed him possible futures that were optimized for making him do stuff that brought them along, not optimized for whatever he wanted about the future.

Oh, you want to have that dead guy to be on your side? Too bad, your clairvoyance only shows you stuff that guarantees that you'll treat him in such way that he'll attempt to kill you. Several hundred times, apparently. Because if that happens then everything makes sense, and the harder you try to exercise your clairvoyance the harder it avoids paradoxes by making you do whatever ends up in the shown future coming true.

And if you try to cheat, like, committing to sending a shitton of spaceships to find the rebel base, then seeing where they found it, then sending a single ship there, well, the Universe always has a way out: showing you the future where you suffered a stroke from the exertion.

In the Dune universe you don't see all possible futures, you only see futures consistent with you seeing them. The entire point of Leto's suicide was that he wanted other people to have a chance to go into a possibly better future that was excluded by that.


Thus: whenever you meet a future-seer on the road, kill them. That releases you from their weird inevitability, and also they must have seen it, so where's the harm.

PS: I really enjoyed the link to the Watchmen chapter, I think I should read the whole thing. On my part I suggest reading "The Invisibles", it's even relevant to the issue at hand because it sort of implies an extra fifth time dimension where Archons' interventions happen, changing the timelines as perceived by the protagonists, but also allowing the latter to maybe steer the world to a better timeline. Or something, I should reread it too.

[–]gwern 4 points5 points  (3 children)

Anyway, that was a good read, and you even mentioned Dune! It was weird for me because I thought a lot about the way precognition in the Dune universe works as I read it...The entire point of Leto's suicide was that he wanted other people to have a chance to go into a possibly better future that was excluded by that. Thus: whenever you meet a future-seer on the road, kill them. That releases you from their weird inevitability, and also they must have seen it, so where's the harm.

Precognition in the Dune-verse is even weirder because, by my reading of some of the obscurer passages in Dune/Dune Messiah/God Emperor Herbert seems to try to imply that precogs can affect the past as well as their present and that there are hierarchies of precogs, so that everything that happens in Dune is actually according to Leto's (not Paul's) plans. So while it looks like a sort of causally-effective precognition scenario, it may actually be a self-consistent time loop scenario, in which everything happens as Leto forced it to by warping lesser precogs' visions to his own vision, ensuring that the Jihad happens so Leto comes into existence. And then he needs to create no-ships and invisible people so as to ensure that no greater precogs can ever undo all this and endanger humanity with machines etc. (I have some notes on this idea but the hints are scattered so far through the Dune books that it would be a huge chore to clear the matter up, and I continue to hold out hope that the despicable son will one day release the Dune 7 notes.)

Anyways and first of all. I don't quite like your own explanation because it ignores entropy.

Not at all. If you want to get into the nitty-gritty, you should read Good and Real's chapter on time and entropy, but the summary is that entropy still doesn't get you a unique arrow of time, even if you assume the Boltzmanian fluctuation into an ultra-low-entropy state of the Big Bang, because the time-symmetry gives you two different arrows of times pointing different directions on 'either side' of the Big Bang away from the low entropy state.

We can't remember the future because we are living in an always expanding entropy funnel, regarding possible world-states, so our personal past is focused into an exponentially thin beam of our past states, while the future has that exponentially increasing sets of reachable higher-entropy states.

As for computationally, this can't be right. Your brain and any fixed region of space can store only a fixed amount of information. This retrodicts only an ever smaller part of the past, as it predicts an only ever smaller part of the future. The problem is symmetrical. If it was asymmetrical, then you would be able to know an indefinitely increasing amount about all of the past as more of it comes into your light cone. Obviously, you don't. What was the exact temperature where you are sitting 100 million years ago? Waiting 1 million years doesn't make that question easier, it makes it harder.

Clairvoyance is fundamentally different from time travel.

It is not. Sending any of mass/energy/information breaks Einstein. Chiang explicitly agrees with me in equating precognition/clairvoyance and time-travel, see the footnote quoting him on that.

PS: I really enjoyed the link to the Watchmen chapter, I think I should read the whole thing.

Definitely. It's justifiably one of the most famous comicbook chapters ever, and the work as a whole is excellent, if not as shocking or innovative now as when released.

(I've already read The Invisibles.)

[–]Works_of_memercy 1 point2 points  (2 children)

Good and Real

Will read, thanks.

because the time-symmetry gives you two different arrows of times pointing different directions on 'either side' of the Big Bang away from the low entropy state.

So? In each of those parts there will be an unique arrow of time, just pointing in different directions =)

If it was asymmetrical, then you would be able to know an indefinitely increasing amount about all of the past as more of it comes into your light cone.

No, why, we are not talking about all of the past/future, we are talking about knowing even some. And it turns out that it's trivial to make a mechanism that stores one bit about some past event, whether the last coinflip turned heads for example, with reasonably high fidelity, but apparently very hard to make it store a bit of information about the future. Despite the physical laws being time-symmetrical. Weird, huh?

And it's not caused by some global considerations, possible paradoxes etc, on the immediate level it's a purely technical difference, I'm not exactly sure, but I think that it's the part where you copy the state of the landed coin (while erasing your own bit of memory) that requires spending a shitton of negentropy which you wouldn't have in the opposite direction.

Clairvoyance is fundamentally different from time travel.

It is not. Sending any of mass/energy/information breaks Einstein.

It's not fundamentally different in that aspect, but it is fundamentally different in other, more relevant to this discussion aspects. For example, traveling to the past to kill your grandfather causes a paradox that's hard to resolve, initiating information retrieval in the present from the future is fundamentally easier to resolve because the universe only needs to restrict the returned data in a particular way, and it's even reasonable to assume that it manages to do that locally, just like it globally optimizes light paths using local equations.

Herbert seems to try to imply that precogs can affect the past as well as their present and that there are hierarchies of precogs, so that everything that happens in Dune is actually according to Leto's (not Paul's) plans.

Oh, yeah, I remember that. It makes much more sense (and works mechanistically even) if you follow me in believing that "Leto's" plans weren't Leto's, it's what the Universe answered when he asked to show him what'll happen in a 5000 years, and it found a timeline that was consistent with him knowing about it. Then other people even from earlier who had the chance to glimpse this rigidified, clairvoyance-consistent timeline were linked in too.

There was not much of a choice on the Leto's part, is what I'm saying. Like, what I'm trying to convey with all of that is that that form of clairvoyance is akin to asking the AI you're playing a Newcomb problem with to show you the future, and it shows you whatever future that's consistent with you achieving it after being shown and minimizes some technical parameter that has nothing to do with any of your wants.

[–]gwern 2 points3 points  (1 child)

So? In each of those parts there will be an unique arrow of time, just pointing in different directions =)

Different directions means there's no true arrow of time. There's just a lot of individual symmetrical ones with pseudo conscious arrows of time due to algorithmic considerations, it being more useful to take the finite set of bits to predict the future than to retrodict the past, despite them being the same thing.

And it turns out that it's trivial to make a mechanism that stores one bit about some past event, whether the last coinflip turned heads for example, with reasonably high fidelity, but apparently very hard to make it store a bit of information about the future.

I disagree. Many things about the future are trivially predictable. It is also difficult to make that one bit stick around indefinitely. What did the coin flip come up as 1 million years ago?

Fundamentally, you still have only a finite number of bits to predict the past and future.

It's not fundamentally different in that aspect, but it is fundamentally different in other, more relevant to this discussion aspects.

Again, Chiang disagrees. So it can't be a part of the story.

[–]Works_of_memercy 1 point2 points  (0 children)

it being more useful to take the finite set of bits to predict the future than to retrodict the past, despite them being the same thing.

Except they are not the same thing, locally. In each entropy-decreasing stretch of the universe it's easier to remember the lower-entropy past than to predict the higher-entropy future.

(on a side note, Orthogonal series by Greg Egan are quite mind-stretching in this respect)

I disagree. Many things about the future are trivially predictable.

I don't get your point. Yes, some things are equally easy, and some things are equally hard. The interesting things though are that which are not the same. For example, it's easy to make a device that remembers the last ten coinflips, it's harder to make a device that predicts the next ten coinflips (or determines them? I don't even know).

Again, Chiang disagrees. So it can't be a part of the story.

Well, yeah. He's wrong on the facts of the matter though, there is a fundamental difference between a person in the future sending information to the past, and a person in the past requesting information from the future. You don't get as bad paradoxes in the latter case.

[–]grendel-khan 3 points4 points  (4 children)

You might be interested in the idea of the 'innovation block' from Egan's The Arrows of Time; it looks relevant to your idea here.

[–]gwern 1 point2 points  (3 children)

Mm. I gave up on the Orthogonal trilogy after the first book when I realized I would need a physics/math degree to appreciate any of the worldbuilding. Is the 'innovation block' really worth slogging through two more books for? Are there any standalone summaries or excerpts?

[–][deleted] 1 point2 points  (2 children)

I read it in response to that post, so I can summarize it: They build a time machine, so the information gradient becomes flat, so the information content of the future becomes equal to the information content of the present, so no one can have new ideas. The day after they blow up the time machine someone invents their universe's version of feynman's path integral quantum graph things, but no one had had any new ideas for three years except for some people who were isolated due to returning from a trip. Once they came into contact, they topped having ideas too.

[–]gwern 1 point2 points  (1 child)

That's pretty crazy. Also I don't think brains work that way.

[–][deleted] 0 points1 point  (0 children)

I thought it was weaker than the first two books, because it revolved around the time travel machine and the physics that enabled it (lots of determinism vs free will talk) as well as the changes in their society after they invent their equivalent of a uterine replicator (spoiler alert: it's greg egan, they become hermaphrodites) and there wasn't nearly as much meaty physics as before.