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

Re: Paradox-free time travel is theoretically possible

#151

Post by darkcha0s » Thu, Jan 21, 2021, 3:24 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…

>I'd love to see how this works when you go back in time and kill your toddler self.

Logically, this would be consistent, as you are no longer present in the future, because you have travelled to the past. What if killing the toddler means you can't travel back to your understood future?

Your 'gotcha' doesn't really disprove anything, beyond raising another question which we can't conceivably answer.

Re: Paradox-free time travel is theoretically possible

#152

Post by yrimaxi » Thu, Jan 21, 2021, 3:28 PM UTC

alisonkisk wrote: post #139

yrimaxi wrote: post #135

Earlier quoted context omitted.

I think free will is orthogonal to mutability/immutability of time.

Free will is a nonphysical illogical concept. It's impossible to define free will inside physics in a way that feels "free".

Your comment is orthogonal to my comment.

Re: Paradox-free time travel is theoretically possible

#153

Post by AnIdiotOnTheNet » Thu, Jan 21, 2021, 3:31 PM UTC

riffraff wrote: post #89

im3w1l wrote: post #57

Earlier quoted context omitted.

I could see how a universe with such rules could exist. But is it our universe? In the universe you describe, wouldn't we already have seen time travellers from the future? The only explanation I can think of for why we haven't seen any, if we live in such a universe, is that we need to build an "arrivals hall" first, and that as soon we manage to do that, people from all over start popping in for a visit.

I have a short story for this! My solution to "where are all the time travelers" is that we are unable to correct for motion, we send people in the past but they end up million of kilometers away in space. So the past is perfectly accessible, but we can't get to any useful spacetime coordinate.

That seems as though it could be solved by making very short trips back and allowing the local gravity to continually correct your velocity and positioning until you reach your destination.

Re: Paradox-free time travel is theoretically possible

#154

Post by yrimaxi » Thu, Jan 21, 2021, 3:31 PM UTC

vidarh wrote: post #144

wongarsu wrote: post #126

Earlier quoted context omitted.

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. 1…

I don’t think this really gets at the paradox. What if you shot yourself?

Re: Paradox-free time travel is theoretically possible

#155

Post by kstenerud » Thu, Jan 21, 2021, 3:32 PM UTC

darkcha0s wrote: post #151

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…

>I'd love to see how this works when you go back in time and kill your toddler self. Logically, this would be consistent, as you are no longer present in the future, because you have travelled to the past. What if killing the toddler means you can't travel back to your understood future? Your 'gotcha' doesn't really disprove anything, beyond raising another question which we can't conceivably answer.

> What if killing the toddler means you can't travel back to your understood future?

How would killing a toddler prevent me from time travelling back? Killing someone will not affect the functionality of my time machine.

Re: Paradox-free time travel is theoretically possible

#156

Post by AnIdiotOnTheNet » Thu, Jan 21, 2021, 3:36 PM UTC

me_me_me wrote: post #140

mpalmer wrote: post #27

Earlier quoted context omitted.

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.

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

Re: Paradox-free time travel is theoretically possible

#157

Post by EliRivers » Thu, Jan 21, 2021, 3:49 PM UTC

at_a_remove wrote: post #69

mpalmer wrote: post #27

Earlier quoted context omitted.

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.

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,…

If I open a box and find a bag of apples inside, how did the universe in the past "know" that the bag of apples needed to somehow end up in that box?

Re: Paradox-free time travel is theoretically possible

#158

Post by dnautics » Thu, Jan 21, 2021, 3:54 PM UTC

stevenhuang wrote: post #90

nautilus12 wrote: post #41

Earlier quoted context omitted.

Depends if the system is open or closed and if mass is converting to energy or visa versa. Also conditions near big bang are not representative.

More fundamentally, energy is a conserved quantity because the laws of physics are time invariant. See Noethers theorem and gauge symmetries.

that's wrong. Noether's theorem says that "energy is conserved" is equivalent to "the laws of physics are time invariant". If you assert that "energy is conserved" because "the laws are time invariant" that risks begging the question. At the very least you should be asking "ok, then, why are the laws of physics time invariant?"

We know that all of the laws of physics we observe at the local scale are time invariant, but that is not necessarily true for any law we haven't discovered yet. While on the local scale that feels so unlikely as for it to be basically considered "inviolable without extraordinary evidence", that is not necessarily the case in the community for laws at the cosmic scale.

Re: Paradox-free time travel is theoretically possible

#159

Post by elil17 » Thu, Jan 21, 2021, 3:58 PM UTC

pdonis wrote: post #32

elil17 wrote: post #12

The article makes it seem like "the timeline corrects itself." What the scientific paper actually says (as best I can understand it) is that, if the universe is entirely deterministic, the conditions necessary for time travel can only occur when the time traveling wouldn't cause a paradox. As the authors note, this isn't super relevant to our universe because: - the conditions necessary for time travel probably don't…

> What the scientific paper actually says (as best I can understand it) is that, if the universe is entirely deterministic, the conditions necessary for time travel can only occur when the time traveling wouldn't cause a paradox. It's actually even more general than that: under non-deterministic laws (such as the way most people currently understand quantum mechanics), if time travel is possible at all, the probabili…

Thank you! That explanation actually makes the paper make a lot more sense.

Re: Paradox-free time travel is theoretically possible

#160

Post by account42 » Thu, Jan 21, 2021, 4:02 PM UTC

tremon wrote: post #117

Uehreka wrote: post #74

Earlier quoted context omitted.

I can’t see how a universe like that could exist. Perhaps if some time-traveler was unlucky, their gun might jam, but I can’t figure out how you could successfully thwart a “well-prepared time traveler” who knows all the details of the situation they’re jumping into and has double-checked their gun after making the jump.

The logical extension of this would be, given how local interactions might affect events across the globe over time, that the entire past becomes immutable. That would reduce time travel to a read-only experience, so it wouldn't have any utility beyond time tourism.

Read only time travel would still be immensely useful as there are so many situations where having a clear answer to what happened is interesting - history, solving crimes, analyzing accidents (or bugs as we are on HN), re-measuring experiments, ...