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

Re: Paradox-free time travel is theoretically possible

#41

Post by nautilus12 » Thu, Jan 21, 2021, 5:18 AM UTC

dnautics wrote: post #28

jennyyang wrote: post #19

Time travel violates the law of conservation of energy. By travelling back in time, you yourself are injecting more mass/energy into a system. So it's great as a science fiction trope, but if you're going to talk physics, that needs to be addressed.

Iirc there is no requirement for mass/energy to be conserved, and some models of expansion require conservation to be violated.

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.

Re: Paradox-free time travel is theoretically possible

#42

Post by henrysarabia » Thu, Jan 21, 2021, 5:19 AM 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.

"The past is obdurate."

This concept is narritively explored in Stephen King's "11/22/63" if anyone finds this interesting.

The novel is an entertaining read and the audiobook available on the Libby app (requires a local library card here in the US) is high quality.

Re: Paradox-free time travel is theoretically possible

#43

Post by kiba » Thu, Jan 21, 2021, 5:21 AM UTC

suby wrote: post #8

gear54rus wrote: post #6

Earlier quoted context omitted.

Self-fulfilling prophecy, yes. But the article reads like some sci-fi bs tbh. What's considered the 'same outcome'. If I kill baby Hitler, would the universe create another just like him? Or someone else will do same things? If someone else does them, surely it won't (for example) kill as much people. So someone dead would now be alive in the future (or vice versa). And now the outcome isn't the same yeah? Sorry but…

I find the concept absurd as well. If you do something like go back in time and murder Hitler, you're going to be impacting the actions of every single human being alive. Within a few generations there's going to be an entirely different set of people alive than the set of people alive in the universe in which Hitler was not murdered. This idea that you would still have to go back in time because the universe correct…

This is assuming you will be successful in assassinating Hitler and that the timeline is mutable as opposed to the idea that time travel was always 'meant' to happen.

Re: Paradox-free time travel is theoretically possible

#44

Post by greentimer » Thu, Jan 21, 2021, 5:21 AM UTC

Worth noting that you can’t conclude time travel is impossible or irrelevant just because other people have. There could exist time travelers who secretly have control either of your life or the world with advanced technology (potentially up to the level of mind control chips) who are keeping the evidence from getting out.

Re: Paradox-free time travel is theoretically possible

#45

Post by mikemoka » Thu, Jan 21, 2021, 5:21 AM UTC

what if he goes back in time to the moment before leaving and disintegrates the time machine?

if selfconsistency is right it makes me wonder if a time traveler can actually be interacting with the past around him

Re: Paradox-free time travel is theoretically possible

#46

Post by zaroth » Thu, Jan 21, 2021, 5:25 AM UTC

nautilus12 wrote: post #40

loblollyboy wrote: post #29

Earlier quoted context omitted.

I don’t have the details but I’m pretty sure passage of time is due to a tendency for entropy to increase, and that if I go back in time a second by, say, walking backwards, it is no more a violation of the cons of energy as if I walk forward.

I think the point about conservation of mass still applies. There are literally 2x yourself at a certain point in time, mass is not conserved.

Always conservation of mass is maintained by broadening the scope of the system under test. You can take matter out of a local system and make that local system less massive, but it’s not a violation of physics because the broader system maintains a steady state.

So isn’t the simple layman’s answer that the “system” also spans the 4th dimension and you are just displacing mass around in a 4D space instead of the typical 3D space?

Re: Paradox-free time travel is theoretically possible

#47

Post by baking » Thu, Jan 21, 2021, 5:26 AM UTC

TimTheTinker wrote: post #5

Pasting the abstract from the paper: The theory of general relativity predicts the existence of closed time-like curves (CTCs), which theoretically would allow an observer to travel back in time and interact with their past self. This raises the question of whether this could create a grandfather paradox, in which the observer interacts in such a way to prevent their own time travel. Previous research has proposed a…

"Further studies will be necessary to find genuine physical scenarios realising the acausal processes we have discovered."

To bad no one went back in time to tell the authors whether they actually discovered anything or not.

Re: Paradox-free time travel is theoretically possible

#48

Post by 8bitsrule » Thu, Jan 21, 2021, 5:28 AM UTC

I've never seen the past or future. Only physical things still or moving, combining with other things or not ...

What we call 'the past' is a mental picture of where physical things used to be positioned and their condition. But everything in 'that now' has changed. Just as when we 'travel' in physical space, when we get 'back home' it has changed.

If I were to 'time travel' I'd be gone from now. But everyone else would go on living in now. So where was the 'instant of time' I travelled to stored so I could go there? And the infinite number of instants between it and now?

Fanciful idea, but no evidence ... none zero zip nada

Re: Paradox-free time travel is theoretically possible

#49

Post by gremlinsinc » Thu, Jan 21, 2021, 5:28 AM 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.

It also pre-supposes that you aimed to kill your grandfather. Perhaps you accidently did this. The reason for you going back in time still exists.

However, I believe the worst that could come of this would be to split the timeline in the case that string theory and multiverses exist where the timeline we return to is one that is different than the one we came from.

What if we really are living in a simulation?

Could it be possible that any moment we arrive in the past would possibly have a random outcome on the future? What if every choice from the moment you arrive to the moment you left could have a random effect, and thus the world you arrive back at if it is a closed loop could be different as night and day, simply because all events that happened after your arrival were erased and all choices were given a second chance to arrange themselves like coin tosses mixed with end-user free agency.

If that were the case, how would you prove that it's a randomized closed loop vs string theory since to you the time-traveler, everything has changed which the easiest reason would be you're in a different timeline. It never dawned on you that events along the timeline could be shuffled at anytime and are inconsequential to how the universe works because we're actually in a simulation.

Re: Paradox-free time travel is theoretically possible

#50

Post by deviation » Thu, Jan 21, 2021, 5:28 AM UTC

I'm so agnostic concerning the concept of time itself at the end of the day. I feel that we're so busy struggling to find a way to make time travel work, that we forget that time itself isn't a mechanism of the universe, and that it's just a word we use to describe some sort of effect or bi-product that we're observing.

Here's a can of worms... Compare the concept of time travel to something like 'smell travel'. Is it really that different? We are conditioned to know what smell is because our body has a mechanism to "interpret" particles and send signals to our brain which we 'remember'... How do we know that Time is any different? Perhaps travelling back in time is a flawed concept in itself, in the same way that "travel back in hearing" or "travel back in smell" makes little sense at all because we're just describing an effect or mechanism of our biology, rather than an effect or mechanism of the universe.