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Time Travel: Probability and Impossibility

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Re: Time Travel: Probability and Impossibility

#31

Post by trevyn »

Koshkin wrote: post #22
vanillax wrote: post #4

This is like saying we will never fly a plane. Do you really think people in the 1200's could fathom a phone or the internet or access all information that ever has been on a device that fits on your phone? Its incredibly naïve to think that time travel isn't possible purely based on "laws of physics".

It is incredibly naïve to think that a thought based on knowledge has the same value as a thought based on ignorance .

Knowledge of one’s ignorance is also valuable.

Specifically, saying that something is “impossible” outside of a well-defined formal system would more correctly be classed as hyperbole than a statement based on knowledge.

Re: Time Travel: Probability and Impossibility

#32

Post by lazide »

tjs8rj wrote: post #15
hn_throwaway_99 wrote: post #6

Earlier quoted context omitted.

> Its incredibly naïve to think that time travel isn't possible purely based on "laws of physics". Huh? Would you argue that it's "incredibly naïve" to think perpetual motion machines aren't possible purely based on "laws of physics"? Because I think it's incredibly naïve to think perpetual motion machines would be possible, precisely because they do violate the laws of physics.

The most we can really conclude is that they don't currently appear to be possible. We have a sample size of 1 with how many universes we've studied. As far we know, this could be a simulation, or a brief moment in time when the ever-shifting laws of physics look the way they do, or the dream of a supernatural being. By all means, our physics works (now) and its the best we've got so lets bet on it and drive our soci…

Except that when people try to use fantastical stories of perpetual motion machines to bilk investors or the like, not calling bullshit does have a net negative.

There is ‘plausible imagining’ and then there is also ‘scammy bullshit’.

Re: Time Travel: Probability and Impossibility

#33

Post by lazide »

new_guy wrote: post #28
hn_throwaway_99 wrote: post #6

Earlier quoted context omitted.

> Its incredibly naïve to think that time travel isn't possible purely based on "laws of physics". Huh? Would you argue that it's "incredibly naïve" to think perpetual motion machines aren't possible purely based on "laws of physics"? Because I think it's incredibly naïve to think perpetual motion machines would be possible, precisely because they do violate the laws of physics.

> Because I think it's incredibly naïve to think perpetual motion machines would be possible, precisely because they do violate the laws of physics. What do people mean by 'perpetual'? 1 year, 100 years, 1 million, is it expected to outlast the universe? And 'laws of physics' is such an anachronistic term, it carries such baggage with it, better to say 'what we think we know so far'. And a 'perpetual energy machine'…

If someone was selling a perpetual motion machine for $1 million dollars, would you be a genius or a rube to buy it?

Re: Time Travel: Probability and Impossibility

#34

Post by ajuc »

Recently an interesting paper came out that starts by accepting all solutions to special relativity (usually the faster-than-light solutions are ignored) and goes on to derive the axioms of quantum mechanics from just that. It turns out what you need to prevent timetravel paradoxes is exactly quantum mechanics.

https://www.eurekalert.org/news-releases/540753

https://arxiv.org/abs/1910.02780

I don't understand the whole derivation (especially the way they move from 1+1 to 3+1 spacetime), but it feels quite elegant. It could be interpreted as universe having a separate family of particles going backward in time synchronizing entangled particles over distance.

Re: Time Travel: Probability and Impossibility

#35

Post by xwdv »

Is there any sci-fi story where time travel is possible, but the government has mandated that the only legal purpose for it is to offset the effects of time dilation from near-lightspeed voyages?

Basically as you travel to your destination, you are also traveling backward in time so you can arrive at the same time you started relative to when you left, essentially making a sort of “warp drive”. To an observer on Earth, a ship leaving to Alpha Centauri would seem to jump and arrive instantly. To the crew on board the ship, the universe would seem to “pause” while they move through a timeless space. And when they arrive at the destination and turn off the engine, everything outside starts moving again.

To me this seems like it’d be the only safe use for time travel, but I wonder if it could still be exploited somehow through some edge cases.

Re: Time Travel: Probability and Impossibility

#36

Post by roywiggins »

quantum2021 wrote: post #30

Quantum Entanglement in Time https://arxiv.org/pdf/quant-ph/0402127.pdf Add in a little on/off manipulation of the entanglement and voila! You've got yourself a 'radio' to the past! (Don't ask me how to build it)

If QM is correct, you can't use entanglement alone to communicate, let alone in a way that violates causality.

https://en.wikipedia.org/wiki/No-communication_theorem

Re: Time Travel: Probability and Impossibility

#37

Post by roywiggins »

vanillax wrote: post #4

This is like saying we will never fly a plane. Do you really think people in the 1200's could fathom a phone or the internet or access all information that ever has been on a device that fits on your phone? Its incredibly naïve to think that time travel isn't possible purely based on "laws of physics".

There's prior art for flight. There's no prior art for time travel.

Re: Time Travel: Probability and Impossibility

#38

Post by moron4hire »

Doesn't the grandfather paradox necessarily assume only a single dimension of time? And 1D time leaves no room for decision branching. So it necessarily also assumes a lack of free will. Which then gets into an irreducible problem of arguing semantics.

But if free will exists, then time must be multidimensional. Multidimensional time agents could then go back and kill their own grandfathers. They'll then be traveling forward in a decision branch that doesn't include their grandfather, but the one that does is untouched. There are other branches where this grandfather doesn't exist, and all people in all those branches would argue the impossibilty of that time agents existing, as he has no path that they can see to get there.

But we might as well argue the impossibilty of a coconut showing up in medieval Europe, just because we never saw the swallow that brought it there.

Re: Time Travel: Probability and Impossibility

#39

Post by Strilanc »

quantum2021 wrote: post #30

Quantum Entanglement in Time https://arxiv.org/pdf/quant-ph/0402127.pdf Add in a little on/off manipulation of the entanglement and voila! You've got yourself a 'radio' to the past! (Don't ask me how to build it)

You can't do on/off manipulation of entanglement.

Side note: timelike entanglement is kind of funny to read about. It's a bit like describing "just leaving an atom in the same spot for a bit" as a "timelike quantum teleportation". It's applying fancy words to make the normal case sound like the strange case.

Re: Time Travel: Probability and Impossibility

#40

Post by short_sells_poo »

nonameiguess wrote: post #27
vanillax wrote: post #4

This is like saying we will never fly a plane. Do you really think people in the 1200's could fathom a phone or the internet or access all information that ever has been on a device that fits on your phone? Its incredibly naïve to think that time travel isn't possible purely based on "laws of physics".

The laws of physics as we know them today don't forbid time travel. In fact, they propose a fairly specific means of doing it via accelerating one side of a traversable wormhole to near the speed of light. The only reason this isn't actionable is it requires exotic matter that may not exist, and if it does, we certainly don't know where to find it or how to make it (and we don't currently know how to make a wormhole,…

> and we don't currently know how to make a wormhole, but we know they're physically possible

I'm not aware of any actual observational evidence or even strong theoretical prediction that wormholes exist or that they are physically possible.

Some parameterizations of our models don't rule out their existence, but it's all hypothetical. Including the question whether they are traversable.

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