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

Re: Paradox-free time travel is theoretically possible

#171

Post by yrimaxi » Thu, Jan 21, 2021, 6:45 PM UTC

EliRivers wrote: post #169

yrimaxi wrote: post #166

Earlier quoted context omitted.

Ok. I guess verb tense falls short in this case. :-)

It does, but that's also the point. The past is fixed in this model. If you go back in time, that's not new. You always did that. How the present looks now is a function of you having always done that. We know that you didn't shoot yourself, because you didn't.

Yes of course, except for the last part. Their temporal experience, if they went into a time machine, is:

1. Time machine backwards

2. Shoot themselves

3. Time machine forwards

And that’s how they would have experienced it. The whole premise seems to be that this is allowed:

1. Time machine backwards

2. (do something)

3. Time machine forwards

Of course step 2 always happened since it’s the past. And yet they shot themselves. But they couldn’t shoot themself and exist now as well. And that’s the paradox.

Two choices at this point:

1. You can’t really do anything in step 2. In which case time travel is impotent, and hence why even dignify calling it time travel at all?

2. You have some potency in step 2, but you can’t do things that lead to paradoxes. But then how would you handle those special cases?

Just stating that “the past is fixed” doesn’t solve the paradox, it restates one of the premises.

Re: Paradox-free time travel is theoretically possible

#172

Post by Digit-Al » Thu, Jan 21, 2021, 7:06 PM UTC

schwartzworld wrote: post #95

littlestymaar wrote: post #68

The time-travel paradox arises only if you presuppose the existence of free will: if we don't, and just consider human beings as physical objects acting deterministically, then there can be no paradox: the time traveler will simply do what was already had been done in his past.

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…

Have you ever read "All you zombies" by Heinlein? https://gist.github.com/defunkt/759182/ad44c6135d168ae54503a...

Re: Paradox-free time travel is theoretically possible

#173

Post by me_me_me » Thu, Jan 21, 2021, 7:13 PM UTC

AnIdiotOnTheNet wrote: post #156

me_me_me wrote: post #140

Earlier quoted context omitted.

So this is essentially magic. Any self correcting mechanism implies magic. How does 'universe' know what needs to be done to fix timeline?

How do particles in delayed-choice quantum eraser [0] experiments know whether or not their path will eventually be observed? If we accept "magic" as an explanation, then it would appear based on experimental evidence that the universe is, in fact, magic. [0] https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser

We don't know how it work.

That's it. Unless someone comes up with testable idea.

But inserting as a cause magical forces that we cannot even begin to explain as means of correcting time discrepancies.

So whats the point concocting hypothesis that there is no issues with time paradox all based on unknown? Its a plot for a SciFi novel not a paper.

I don't want to come across as a party-pooper, but those are outrageous claims with no meat and no bones.

Re: Paradox-free time travel is theoretically possible

#174

Post by vidarh » Thu, Jan 21, 2021, 7:15 PM UTC

yrimaxi wrote: post #171

EliRivers wrote: post #169

Earlier quoted context omitted.

It does, but that's also the point. The past is fixed in this model. If you go back in time, that's not new. You always did that. How the present looks now is a function of you having always done that. We know that you didn't shoot yourself, because you didn't.

Yes of course, except for the last part. Their temporal experience, if they went into a time machine, is: 1. Time machine backwards 2. Shoot themselves 3. Time machine forwards And that’s how they would have experienced it. The whole premise seems to be that this is allowed: 1. Time machine backwards 2. (do something) 3. Time machine forwards Of course step 2 always happened since it’s the past. And yet they shot the…

You're still thinking of this as a chain of events unfolding, rather than as a playback of a set of immutable, self-consistent events.

> The whole premise seems to be that this is allowed:

No. That was not the premise. The premise was an immutable representation of self-consistent state transitions, so what occurs in step 2 will always have occurred at that point in time, because otherwise the system isn't immutable. Step 1. just links step two into the causal chain of events in a way necessary to make the existence of 2. consistent.

Again: Think of a graph. An immutable graph. The flow of time in this model simply follows the graph. It doesn't do anything at all, it just replays the events.

The problem here is thinking about this as if there are choices being made as time passes.

> Just stating that “the past is fixed” doesn’t solve the paradox, it restates one of the premises.

It's not stating the past is fixed. It's stating everything is fixed.

Re: Paradox-free time travel is theoretically possible

#175

Post by AnIdiotOnTheNet » Thu, Jan 21, 2021, 7:30 PM UTC

me_me_me wrote: post #173

AnIdiotOnTheNet wrote: post #156

Earlier quoted context omitted.

How do particles in delayed-choice quantum eraser [0] experiments know whether or not their path will eventually be observed? If we accept "magic" as an explanation, then it would appear based on experimental evidence that the universe is, in fact, magic. [0] https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser

We don't know how it work. That's it. Unless someone comes up with testable idea. But inserting as a cause magical forces that we cannot even begin to explain as means of correcting time discrepancies. So whats the point concocting hypothesis that there is no issues with time paradox all based on unknown? Its a plot for a SciFi novel not a paper. I don't want to come across as a party-pooper, but those are outrageous…

> But inserting as a cause magical forces that we cannot even begin to explain as means of correcting time discrepancies.

That is my point though: there is no need for magic for this mechanism. The double slit delayed choice quantum erasure experiment is analogous to time travel as there is no way the particles could "know" that their path would be observed at some point in the future, yet if they were, those particles mysteriously only took paths that created the gaps. Given that this is a real, testable phenomenon in the real testable universe, it would not require magic to explain that idea that only timelines in which a time traveler doesn't cause a paradox can exist. The time traveler, like the particles in the experiment, chooses paths as they see fit and it only later turns out that they necessarily were ones in which paradoxes were avoided.

Re: Paradox-free time travel is theoretically possible

#176

Post by yrimaxi » Thu, Jan 21, 2021, 7:59 PM UTC

vidarh wrote: post #174

yrimaxi wrote: post #171

Earlier quoted context omitted.

Yes of course, except for the last part. Their temporal experience, if they went into a time machine, is: 1. Time machine backwards 2. Shoot themselves 3. Time machine forwards And that’s how they would have experienced it. The whole premise seems to be that this is allowed: 1. Time machine backwards 2. (do something) 3. Time machine forwards Of course step 2 always happened since it’s the past. And yet they shot the…

You're still thinking of this as a chain of events unfolding, rather than as a playback of a set of immutable, self-consistent events. > The whole premise seems to be that this is allowed: No. That was not the premise. The premise was an immutable representation of self-consistent state transitions, so what occurs in step 2 will always have occurred at that point in time, because otherwise the system isn't immutable.…

And I’ll reiterate: I accept all of these premises! I’ve never denied them! All you and Eli have managed to do at this point is try to nitpick the linguistic deficiencies of my explanations, which are inevitable anyway since I’m not a physicist with a specialization in space-time theories.

And whether I “get it” or not is secondary to the point of whether you have truly managed to confirm or disconfirm the time travel paradox, which I still claim you have failed to do. It is simple to wave away a time travel where you do something compeletely benign, like frickin’ wave or greet your former self. Try dealing with something of consequence, like killing your former self (as in: the person in that point of time-space which carries the mutable signifier of “yourself” which is younger than yourself) or giving yourself a kilogram of gold from the future, which I think would violate the law of preservation of energy or whatever.

Is time travel backwarks possible or not? If it is, see the questions in my previous comment.

> No. That was not the premise.

And yet if time travel backwards is possible you simply cannot deny that one would experience (!) that time travel as steps 1–3. Temporal experience. Hah!

There’s absolutely nothing hard about the concept of time being immutable; that’s how children’s cartoon handle time travel, not as two timelines where the first one gets erased right after the second one “happens”. (Okay, okay, they probably use both of these tactics.) The true puzzle is time travel which introduce paradoxes, which your original comment says absolutely nothing about. But maybe you’ve answered that in another comment (I have only kept up with this subthread). If not, you should probably try to do that instead of dewelling on this minor philosophical insight of yours.

Re: Paradox-free time travel is theoretically possible

#177

Post by yrimaxi » Thu, Jan 21, 2021, 8:03 PM UTC

d1sxeyes wrote: post #132

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,…

Of course there's a mechanism to explain it: determinism. It's got nothing to do with 'knowing' anything, it's just that we experience life as a vivid playthrough of causes and effects that could never have started or ended any differently. Now, you might not agree with that explanation, but it can't be precluded, and it solves all of your concerns above.

This explains absolutely nothing. You time travel back and kill your former self. Paradox. How to resolve? Determinism.

That’s just invalid.

“Knowing” here just referred to the magical force that will keep time travel suicide from happening. But without a magical force you’re even more stumped!

Re: Paradox-free time travel is theoretically possible

#178

Post by mjsir911 » Thu, Jan 21, 2021, 8:06 PM UTC

kstenerud wrote: post #116

> In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time. > "No matter what you did, the salient events would just recalibrate around you," Tobar said. I'd love to see how this works when you go ba…

Nothing changes in the past, including your arrival, because your arrival would have always happened. The past stays consistent because it was already determined to have happened.

How this would work going back in time to kill your toddler self is that, well, you don't. Gun jams, you have an aneurysm before you can, or maybe you killed the wrong toddler and there was just always a toddler killed in the past.

The fact that you are alive to go back in time means you were not killed in toddler form.

Re: Paradox-free time travel is theoretically possible

#179

Post by yrimaxi » Thu, Jan 21, 2021, 8:10 PM UTC

littlestymaar wrote: post #68

The time-travel paradox arises only if you presuppose the existence of free will: if we don't, and just consider human beings as physical objects acting deterministically, then there can be no paradox: the time traveler will simply do what was already had been done in his past.

This explains absolutely nothing. “Will simply do what was already done in his past” is the non-sequitur: if they shoot themselves in the past then that was what was done in the past. Sounds paradoxical? Because it is, and determinism can’t stop a human from shooting a gun any more than free will can because this has got nothing to do with volition.

Re: Paradox-free time travel is theoretically possible

#180

Post by Dylan16807 » Thu, Jan 21, 2021, 8:22 PM UTC

yrimaxi wrote: post #176

vidarh wrote: post #174

Earlier quoted context omitted.

You're still thinking of this as a chain of events unfolding, rather than as a playback of a set of immutable, self-consistent events. > The whole premise seems to be that this is allowed: No. That was not the premise. The premise was an immutable representation of self-consistent state transitions, so what occurs in step 2 will always have occurred at that point in time, because otherwise the system isn't immutable.…

And I’ll reiterate: I accept all of these premises! I’ve never denied them! All you and Eli have managed to do at this point is try to nitpick the linguistic deficiencies of my explanations, which are inevitable anyway since I’m not a physicist with a specialization in space-time theories. And whether I “get it” or not is secondary to the point of whether you have truly managed to confirm or disconfirm the time trave…

> Try dealing with something of consequence, like killing your former self (as in: the person in that point of time-space which carries the mutable signifier of “yourself” which is younger than yourself) or giving yourself a kilogram of gold from the future, which I think would violate the law of preservation of energy or whatever.

You can make up impossible scenarios without time travel too. Like someone that shoots themselves (lethally, thorough and immediate) and then a month later they go out to lunch and eat a ham sandwich.

The ability to write out a scenario doesn't mean the scenario is possible.

The theory is that "I go back in time and kill myself" is just as impossible as "I kill myself then a month later eat a ham sandwich". And for largely the same reason: both scenarios violate causality, so the universe can't be shaped that way.