Paradox-free time travel is theoretically possible • Hacker News Board

Re: Paradox-free time travel is theoretically possible

#141

Post by alisonkisk » Thu, Jan 21, 2021, 1:58 PM UTC

musingsole wrote: post #82

dghf wrote: post #75

Earlier quoted context omitted.

Are they, though? I thought the Novikov conjecture postulated that time-travel would change nothing at all: if you travelled back to the past, you had "always" travelled back to the past, and anything you did there was already part of history before you started on your journey (aargh, time-travel tenses are hard). Michael Moorcock's Behold the Man [0] would be a good fictional example. This, on the other hand, sugges…

If there are such physical laws to allow details to change while broad strokes must happen, I'd be curious what exactly differentiates a detail from a broad stroke. But all the same, this idea imagines a more timey-wimey view of history that doesn't have just one state. This is an idea explored heavily in Neal Stephenson's "The Rise and Fall of D.O.D.O"

The difference is entropy/uncertainty. You can change anything whose effects are too small to measure or distinguish from equivalent energy states.

Re: Paradox-free time travel is theoretically possible

#142

Post by alisonkisk » Thu, Jan 21, 2021, 2:01 PM UTC

danbruc wrote: post #125

at_a_remove wrote: post #69

Earlier quoted context omitted.

The thing about this "self-correction" is that nobody, but nobody has a mechanism to explain this force. Not how it works or worse yet, how it will know how to self-correct. What particular force decides that the friction on some surface will briefly rise and cause the gun to jam? How does it know to do that? In all of the paradox-forbidding time travel scenarios, nobody has ever mentioned anything deeper than "Well,…

I guess the mechanism is just a constraint on the possible solutions for the scenario. As an analogy imagine holding a rope in your hand and wiggling it up and down, this way you can make waves of more or less any wavelength travel along the rope. Now fix the far end of the rope somewhere or close it into a loop and you will no longer be able to make waves of arbitrary wavelength but only integer or half-integer frac…

Like so many things in physics, a differential equation with constraints and initial conditions to satisfy.

Re: Paradox-free time travel is theoretically possible

#143

Post by markus_zhang » Thu, Jan 21, 2021, 2:03 PM UTC

paxys wrote: post #16

MetaWhirledPeas wrote: post #3

> Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen. I'm sure these researchers are pretty smart, but nothing in this article goes beyond the level of a grade-school debate. Maybe there's something enlightening in the paper. But if there was, you'd think it woul…

Exactly. The article mentions the grandfather paradox, but doesn't really address it at all. Can't "self correct" shooting your past self in the head.

I think it's still a lot easier to treat each going-back-to-the-past as a fork().

Re: Paradox-free time travel is theoretically possible

#144

Post by vidarh » Thu, Jan 21, 2021, 2:08 PM UTC

wongarsu wrote: post #126

ricardobeat wrote: post #121

Earlier quoted context omitted.

That option implies time travel is not possible, since your mere existence at a past moment is already mutating it. Which is the opposite of what the article claims.

Why? If time is immutable (essentially a Block Universe where everything is predetermined) then you traveling in time doesn't mutate anything, just as you experiencing the current time isn't mutating anything. Of course it (seems to) follow that you don't have any free will. But there's nothing in science or philosophy contradicting that (if anything we seem to find more and more evidence to support that idea).

Exactly.

E.g, a possible example immutable scenario that doesn't violate anything (with each number representing a discrete period of time)

    1. An old man appears and says hi to me and disappear.
    2. 20 years pass.
    3. I obtain a time machine.
    4. I travel back in time and return (in my absence from this time period, I'm at time step 1 saying hi to myself)

The key being that we tend to see this as:

    1a. Nothing happens.
    1b.An old man appears and says hi to me and disappear.
    2. 20 years pass.
    3. I obtain a time machine.
    4. I travel back in time and return (in my absence from this time period, I'm at time step 1b saying hi to myself).

At which point 1b looks like a change from 1a, so we conclude there's mutability. But if the universe is immutable, then 1a never existed in the first place. I always, immutably had will have (sorry, I could not resist) travelled back in time to greet myself

And that removes e.g. the grandfather paradox, because a static, immutable representation of it that is consistent with our other rules (not having people spontaneously come into existence, for example) isn't possible.

Re: Paradox-free time travel is theoretically possible

#145

Post by d1sxeyes » Thu, Jan 21, 2021, 2:11 PM UTC

rightbyte wrote: post #105

littlestymaar wrote: post #85

Earlier quoted context omitted.

Would you mind expanding on that?

If you send an apple back in time the conservation of energy is broken. The energy sum of everything increases in the past.

Only if you consider conserving the energy sum of all possible spatial locations to be the rule we need to keep.

If instead the rule is that we need to conserve the energy sum of all possible locations in space-time, then no law is broken.

Re: Paradox-free time travel is theoretically possible

#146

Post by mamcx » Thu, Jan 21, 2021, 2:23 PM UTC

mpalmer wrote: post #27

paxys wrote: post #16

Earlier quoted context omitted.

Exactly. The article mentions the grandfather paradox, but doesn't really address it at all. Can't "self correct" shooting your past self in the head.

Think what they're getting at is that it won't happen that way. Gun will jam, you die of a brain aneurysm. We don't exist in a universe where impossible things happen, so we can't bring them about. But yes, the article doesn't really sink its teeth into it.

I know nothing, so this instead is my theory:

The universe is a VM alike smalltalk.

What you have now, is the action of the chain of actions of the past.

The VM of now is done.

You travel back.

Arrive at the VM of the past, WITH THE STATE of the past.

You mutate state. It work as expected, you die.

But in the STATE of now, you are alive.

When the time traveler exit the VM, the VM go nowhere. It already happened now, but ONLY "remember" the changes of IT, not of the OTHER vm.

Assorted musings:

For a short time, the universe get splitted(?)

The STATE run on RAM, and that is why it not "push to the future" because is NOT persisted?

The universe is multi-core, and time travel is possible because unused computation time that is assigned to the past VM when suddenly it get awakened?

If the STATE is not persisted (to make all this work), how this make sense at all? Or it have snapshots (backup) and you load for a moment the past?

Re: Paradox-free time travel is theoretically possible

#147

Post by schwartzworld » Thu, Jan 21, 2021, 2:31 PM UTC

littlestymaar wrote: post #97

schwartzworld wrote: post #95

Earlier quoted context omitted.

Ah yes, can you change the past or was there a time traveler version of you running around failing to change things the whole time? See season 5 of Lost for endless discussion on this point. You also can avoid paradoxes if it turns out that time travel just puts you on an alternate timeline. There's a different paradox with this version of time travel though. Suppose I'm a kid and future-Me comes back and tells me a…

Your story allows a “closed timelike curves” to exist, then it must be considered as a physical constant. Where do c (the speed of light), G (Gravitational constant) or π comes from?

Can you elaborate?

Re: Paradox-free time travel is theoretically possible

#148

Post by ku-man » Thu, Jan 21, 2021, 2:33 PM UTC

throwaway2245 wrote: post #138

at_a_remove wrote: post #69

Earlier quoted context omitted.

The thing about this "self-correction" is that nobody, but nobody has a mechanism to explain this force. Not how it works or worse yet, how it will know how to self-correct. What particular force decides that the friction on some surface will briefly rise and cause the gun to jam? How does it know to do that? In all of the paradox-forbidding time travel scenarios, nobody has ever mentioned anything deeper than "Well,…

> The thing about this "self-correction" is that nobody, but nobody has a mechanism to explain this force. Not how it works or worse yet, how it will know how to self-correct. You could think of it that the universe where we exist must have a stable/consistent state. If we go back in time and change the state, then that universe can't (didn't) exist, so it can't be the universe that we are in. From our point of view…

If time travel was possible, this proposition is the most logical (you put it very well, by the way). But at the same time, it's almost useless for the time traveller. Imagine a man that suffered a terrible accident when we was 10 years old, and had a miserable life from that point on. When he is 60 time travel is possible, so he decides to travel 50 years back to prevent the accident. He succeeds, but this new timeline doesn't mean the previous timeline won't/didn't happen. This new him will have a better life but the other him will still be miserable.

Re: Paradox-free time travel is theoretically possible

#149

Post by AnIdiotOnTheNet » Thu, Jan 21, 2021, 2:48 PM UTC

zwaps wrote: post #136

AnIdiotOnTheNet wrote: post #130

Earlier quoted context omitted.

> The thing about this "self-correction" is that nobody, but nobody has a mechanism to explain this force. I have a hypothesis: it's about stability. First, let us discard the notion of time "happening" from a perspective of the universe as a whole. Now, let us posit that only universes with stable structures can exist the same way that quantum superpositions can only collapse into certain states based on conditions…

I like this explanation, but it essentially doesn't solve the paradox. It just shifts it to knot-stability. The question is: can I create a situation which can not lead to stability? And if so, what happens? Will it simply not work? Or will every situation lead to the possibility of stability? Say, I travel back in time at the moment the universe "ends", and from that point onward in the past I destroy all time machi…

> The question is: can I create a situation which can not lead to stability?

From the perspective of the universe there is no "create" or "destroy" because time is not experienced, it is just one dimension of its existence. The concept of "create" only exists from the perspective of a 3 dimensional being experiencing the time dimension. We believe we can choose the future because we believe it hasn't happened yet, but that's an illusion of perspective. In a sense, this means that there is no free will, but that's a different argument.

So to answer the question: no, because the fundamental nature physics drops the probability that you would choose such an action to 0, the same way electrons have 0 probability of choosing paths that lead to the gap regions in the double slit experiment.

> And if so, what happens? Will it simply not work? Or will every situation lead to the possibility of stability?

From your perspective, every situation will lead to stability in the end, because the universe where it didn't has a probability of 0.

> Say, I travel back in time at the moment the universe "ends", and from that point onward in the past I destroy all time machines with a certain technology. Then, no one can fix what I did, and no stable loop could exist.

> I mean, my guess is that such a situation simply can not exist, therefore I can not experience it - I can not live in this universe where this is possible. So, I can only live in the universe where my technology somehow fails exactly when someone travels back in time to correct my loop.

Pretty much yes. Though universes in which the situation you describe is impossible to achieve regardless (either because time travel does not exist or because there's nothing you could do to prevent it from being invented again) are significantly more likely.

Re: Paradox-free time travel is theoretically possible

#150

Post by dcanelhas » Thu, Jan 21, 2021, 2:59 PM UTC

I like William Gibsons version of interactions with the past, as featured in The Peripheral.

Since no-one altered our past we can't be in a timeline where people from the future had influence on the outcome. Therefore any timelines after the point of contact run parallel to ours and we cannot change our own present by traveling back in time.

Interesting to imagine the different "solutions" to the paradox though.