Implications of self-consistent time travel, or Luck and Divination in hard sci-fi

[Douglas]

It works even worse with a purely causal mechanism. Which number the machine tries first would then be fixed by the design of the machine and if it was not right the only resolution would be for the machine to break. A non-deterministic component on the other hand is free to take whatever value is needed. Essentially we're forcing the otherwise random decay of the test particles to be non-random, similar to the quantum zeno effect

You're not correctly applying the consistency principle. There is no "first" number that gets tried. There is one number that gets tried, total, and it is produced by the universe avoiding the paradox that would result if it were wrong.

Here's what would actually happen with a causal system:

1. A message from the future arrives saying that the solution is X.
2. The system checks that X is indeed the solution, and finds that it is.
3. The system sends a message back in time saying that the solution is X.

The rules about trying some number if you don't receive a time travel message, or sending back a different number if the message is wrong, are only there to constrain which timelines are consistent. They do not actually get used at any point.

[Bohandas]

You're not correctly applying the consistency principle. There is no "first" number that gets tried. There is one number that gets tried, total, and it is produced by the universe avoiding the paradox that would result if it were wrong.

That's exactly what I'm saying. It doesn't check multiple numbers, it checks one number which must always coincidentally be right because the system is rigged so that any wrong number must be paradoxical. But in order to work it must be possible for the value worked on by the computer to be coincidental.

[Douglas]

The coincidence in my system is part of the stable time loop itself, not the device that's doing the calculations. This inherently allows the universe to pick any value that will not produce a paradox, with probabilities depending only on the abstract distribution of the solution space.

I think I was missing the scale of the component you were talking about due to where the discussion started. If you build a hardware random number generator that works by measuring quantum wave function collapses, I think that could theoretically work as an alternative source for the "coincidental" correct answer. The scale of the non-causal coincidence you are trying to get from such a thing could feasibly be smaller than the minimum scale of a non-causal breakdown of the system.

The "luck" phenomenon you initially described, with people coincidentally not taking the route that would hit a pothole, is much much larger scale, however. Any macroscopic phenomenon happening without a cause is far too big for any reasonable this-or-paradox system to produce, with the single exception of the smallest possible way for the system to break. To produce something like that, you need to have a mechanism in place to amplify a quantum scale random event into the desired macro scale outcome. Putting the randomness in the time loop itself does that without needing any actual quantum scale hardware.

[veti]

It seems to me there's an unstated assumption here, which might be worth mentioning...

You assume that the cost of time travel - in whatever resources you choose to measure it - is negligible, and therefore you can afford to just nip back a day or so to warn yourself about a pothole. But if the cost is significant, then the threshold for an event that would justify that sort of intervention would increase appropriately.

And the cost could be very significant, depending on what the "laws of time travel" are.

Imagine, for instance, there's a "law" - well, an effect deduced from observation - that no one person can be in two places at the same time. (Any attempt to go back to a time where "you" already exist results in your time machine malfunctioning, and either just not working at all, or depositing you at a completely different destination.) Then it would take a lot of work to send someone back to "last week". You'd need to have a necessarily-finite pool of people who weren't "here", then, presumably because they'd previously been sent back to before they were born and left there for - long enough to create an interval to bring them back into.

Or, imagine that the time machine will only work reliably on matter - energy states are preserved only erratically. Every time a human travels to the past, they run a serious risk of losing their memory or losing their mind completely. And computers just lose it, period.

Add in that there's an inherent uncertainty in trying to intervene in the past - all the factors I mentioned in my previous post that might frustrate the attempt. Then you've got a scenario where, although time travel is kinda possible, it's horrifically expensive. And the "divination computer" would only work on problems that someone was really committed to solving - because if no-one is willing to pay the price of the time travel to correct it, then no paradox is created by getting the answer wrong.

[Bohandas]

I suppose I should have been more specific. My base assumption is a large network of stable wormholes similar to the "Juncture of Eternity" I described in the Weird Races and Strange Places thread

[Anymage]

A wormhole-connected empire criss-crossing space and time sounds pretty far from "hard sci-fi".

Also, since time will pass on both ends of the wormhole, causation can be meaningfully maintained even if there are theoretical ways to break it. If the locales are normally separated by wide gulfs of space and/or time that wouldn't let them interact without wormhole assistance, it isn't meaningfully different from having a universal newtonian clock and just having all the spatial locations linked by portals. And if some different rates of time passing means that you could arrange a trip through portals and arrive before you left during certain rare confluences, coincidence would just have to limit what people could do during those confluences. Having wormhole networks that allowed people to interact with their own timelines sounds like a special case.

Most relevant to a fictional setting, maintaining self-consistency in your time travel story is hard, and often requires the rest of your story to have to bend towards that goal. As soon as you stop maintaining strict control, it's likely for something to slip through. In these cases, the concept of fragile timelines is probably more dramatically useful. There is only one proper timeline (maybe preordained, maybe built as it goes along), paradox is possible but inflicts metaphysical damage, and you have some enforcer (anywhere from time cops to an actual goddess of time) who tries to maintain consistency at all costs. You're free to try writing a self-consistent empire spanning time and space, but understand the self-inflicted headache you're walking into if you try to maintain it over any period of time.

[danzibr]

I’ve seen (in this thread) phrases about the universe making decisions and willing things. As a physics noob, this strikes me as odd. Does it really seems the universe has a will?

I remember my ol’ chem professor saying water behaved strangely if you zoom way in, like it had a will.

[NichG]

I’ve seen (in this thread) phrases about the universe making decisions and willing things. As a physics noob, this strikes me as odd. Does it really seems the universe has a will?

I remember my ol’ chem professor saying water behaved strangely if you zoom way in, like it had a will.

The term 'decision' at least could be misleading, in that you can have a situation where the equations of motion don't uniquely specify the outcome. In which case, if it ends up being one outcome rather than the other, there's the possibility for all sorts of other factors that weren't taken into account in the original theory. So you might say 'there needs to be some process by which the branch that things are on is decided' - whether it's random, or some unseen physics, or satisfies a minimization principle, or whatever.

[Anymage]

I’ve seen (in this thread) phrases about the universe making decisions and willing things. As a physics noob, this strikes me as odd. Does it really seems the universe has a will?

I remember my ol’ chem professor saying water behaved strangely if you zoom way in, like it had a will.

When you create odd and exotic constraints, obeying the "real" rules the universe seems to run on (like conserving or minimizing certain quantities) will often create situations that seem strange or even ornery to our usual intuitions. In that way it seems like the universe is "wiling it" even with the lack of a conscious actor.

GIven the universe put forth in the initial thought experiment (where macroscopic time travel is possible, but where there is one cohesive timeline instead of being able to create branching ones when you jump back), that will by definition require extraordinary coincidental leaps in order to maintain that consistency. That doesn't necessarily reflect on our universe, where there are way too many unknowns.

[veti]

I’ve seen (in this thread) phrases about the universe making decisions and willing things. As a physics noob, this strikes me as odd. Does it really seems the universe has a will?

I remember my ol’ chem professor saying water behaved strangely if you zoom way in, like it had a will.

I would put that down to slightly sloppy use of language, which everyone tends to fall into when discussing time travel.

"Decisions" might be better described as "resolutions" (of a probability function).

We assume that in the presence of time travel, something odd is going to happen to causality. That's - pretty much the whole point of time travel. The speculation is about what forms that oddness might take. Some of it might look very much like an intelligent, but inscrutable, force intervening in reality.

[Bohandas]

A wormhole-connected empire criss-crossing space and time sounds pretty far from "hard sci-fi".

Its theoretically possible if they crack the problem of creating the negative energy density required to stabilize it

[Craft (Cheese)]

A wormhole-connected empire criss-crossing space and time sounds pretty far from "hard sci-fi".

Also, since time will pass on both ends of the wormhole, causation can be meaningfully maintained even if there are theoretical ways to break it. If the locales are normally separated by wide gulfs of space and/or time that wouldn't let them interact without wormhole assistance, it isn't meaningfully different from having a universal newtonian clock and just having all the spatial locations linked by portals. And if some different rates of time passing means that you could arrange a trip through portals and arrive before you left during certain rare confluences, coincidence would just have to limit what people could do during those confluences. Having wormhole networks that allowed people to interact with their own timelines sounds like a special case.

Most relevant to a fictional setting, maintaining self-consistency in your time travel story is hard, and often requires the rest of your story to have to bend towards that goal. As soon as you stop maintaining strict control, it's likely for something to slip through. In these cases, the concept of fragile timelines is probably more dramatically useful. There is only one proper timeline (maybe preordained, maybe built as it goes along), paradox is possible but inflicts metaphysical damage, and you have some enforcer (anywhere from time cops to an actual goddess of time) who tries to maintain consistency at all costs. You're free to try writing a self-consistent empire spanning time and space, but understand the self-inflicted headache you're walking into if you try to maintain it over any period of time.

So long as the wormhole network forms an undirected acyclic graph, no closed timelike curves are possible: To interact with your own timeline you need to make a round trip. All you need to do to forbid time travel is posit some physical principle such that, if a round trip becomes possible, wormhole pairs will spontaneously destroy themselves until the network is acyclic again. This is desirable for an interstellar empire anyway (at least, desirable for the regime) because it means there's a single, central chokepoint that can be squeezed.