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Time Travel: Probability and Impossibility

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Re: Time Travel: Probability and Impossibility

#41

Post by quantum2021 »

Strilanc wrote: post #39
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.

Why not? Read the particle as there/remove the particle. On/off.

Re: Time Travel: Probability and Impossibility

#42

Post by servytor »

My favorite person in regards to time travel is Professor Ronald Mallett[0] who wrote an amazing book called "Time Traveler: A Scientist's Personal Mission to Make Time Travel a Reality"[1].

[0]: https://en.wikipedia.org/wiki/Ronald_Mallett [1]: https://www.amazon.com/Time-Traveler-Scientists-Personal-Mis...

Re: Time Travel: Probability and Impossibility

#43

Post by _carbyau_ »

tjs8rj wrote: post #9
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 universe is not only queerer than we suppose, but queerer than we can suppose" - J. B. S. Haldane I've seen so many documentaries and articles pessimistically concluding we'll never go to another star much less another galaxy based on the speed of light. Once again, too many smart people conclude we've figured it all out as is the perennial tradition.

Scientists work on the basis of "what we know" because from a practical point, you can't work off a foundation of what you don't know.

When pushed, all scientists will admit "based on current understanding".

This is what makes a "breakthrough" revision of understanding so exciting! It doesn't happen every day!

Re: Time Travel: Probability and Impossibility

#44

Post by nobody9999 »

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,…

>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.

That applies to time travel into the past.

It's obvious, but it bears noting that the converse is a lot simpler. I say that because, IIUC (and that's a real question), quantum mechanics treats the time dimension as symmetrical, which contradicts observation, at least to this point.

Is that because QM is wrong on that point? Which seems unlikely, since it's one of the most precisely confirmed (by observation) theories we have.

Or is there something about the "macro" world that destroys that symmetry?

I don't have the answer, but it's clear that in a particular reference frame, we move forward in time at a rate of 1 sec/second.

Again, that's obvious, but based on my (limited) understanding, that seems to contradict QM.

Re: Time Travel: Probability and Impossibility

#45

Post by quantum2021 »

roywiggins wrote: post #36
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

"An important assumption going into the theorem is that neither Alice nor Bob is allowed, in any way, to affect the preparation of the initial state"

The assumption is that the past can't change the future, which it obviously can to any observer. You change what you do today and it will change the future. If the two particles are entangled, one particle would change the initial state of the other wouldn't it? Sort of a two way street/two way radio going back and forth potentially?

Seems like you'd be sending information to the future about the changes the information coming from the future was making to the past at the same time as the information from the future was making those changes, so the theory about not changing the initial state would be flawed.

How to read those changes with a machine?

Re: Time Travel: Probability and Impossibility

#46

Post by GistNoesis »

It all boils down to how the simulation was written. If we can make it crash, time travel being possible is very likely.

If the parent universe is running an exploration of possible universes according to consistent laws of physics, upon crash it will probably backtrack the simulation to a previous timestamp, fiddle a little in the phase space, then continue forward.

One way to make it crash is making the simulation crawling to a stop by requiring always more computation to be done by preventing the simulator to take shortcuts in the computing process.

A backtracking mechanism to preemptively avoid crashes is probably deeply rooted in the form of an implicit solver in the time-stepping mechanism to guarantee the consistency of the tangled causal chains.

Which mean it is probably possible to exploit it from within the simulation. By deciding to do heavy computations in the future, depending on what you observe of the past and its implied future according to your previsions, you can pick your past among the physically possible universes. To guide the simulator to the future you desire, you have to create a consistent alternative past toward your possible future.

To not violate the second-law of thermodynamics which has to always increase (monotically) in your universe, you therefore have to reduce the entropy of the past by minimizing it (down to the origin of time, up to your current view point from inside the simulation), aka simplifying history, to give your physical self some room to shape the path from past to future the way you want.

But by the time, you are able to get your time-machine to work, you'll have realized the futility of it and went to the upper plane of existence.

Re: Time Travel: Probability and Impossibility

#47

Post by roywiggins »

quantum2021 wrote: post #45
roywiggins wrote: post #36

Earlier quoted context omitted.

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

"An important assumption going into the theorem is that neither Alice nor Bob is allowed, in any way, to affect the preparation of the initial state" The assumption is that the past can't change the future, which it obviously can to any observer. You change what you do today and it will change the future. If the two particles are entangled, one particle would change the initial state of the other wouldn't it? Sort of…

The point of the no-communication theorem is that if you transport one half of the entangled pair to Alice, and one half of the entangled pair to Bob, neither of them can use measurements on their half of the entangled pair to communicate. It's forbidden by QM as we know it.

If Alice can affect the initial state, of course she can use that to communicate something to Bob, because half of that state then gets transported to Bob. That's exactly as interesting as sending a qubit to Bob in the normal way, so: not very.

> one particle would change the initial state of the other wouldn't it

It wouldn't, or at least not in any way that is accessible to Alice or Bob. If Alice or Bob pokes their particle too hard it will stop being entangled, and there's no operations you can do on one half of the pair that will communicate any information to the other half. Widely-separated entanglement has already been demonstrated, the reason nobody has been able to use it to transmit information is because it's forbidden by QM and so almost nobody has tried, and nobody expects it to work because it that's not how entanglement is understood to work.

A lot of people have a mental model that entanglement means that there's an invisible see-saw between the two particles, so if you could just force one particle to be spin-down then the other one would be spin-up, and that would let you communicate. Unfortunately, the connection is much less durable than that; interacting enough with the particle to force it into a definite state is an observation, and observing the particle cannot transmit information (that's what the no-communication theorem says).

If it does somehow work, it will be because QM is totally wrong. Also it would immediately be used to cheat the stock market.

Re: Time Travel: Probability and Impossibility

#48

Post by pontus »

ajuc wrote: post #34

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 who…

Really interesting article! I just read the first few pages but it seems like a really interesting approach. Thanks for sharing. I'll be reading it more carefully over the next few days.

Re: Time Travel: Probability and Impossibility

#49

Post by [deleted] »

quantum2021 wrote: post #41
Strilanc wrote: post #39

Earlier quoted context omitted.

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.

Why not? Read the particle as there/remove the particle. On/off.

[deleted]

Re: Time Travel: Probability and Impossibility

#50

Post by calvinmorrison »

wyager wrote: post #17

Strong rec for Greg Egan’s Clockwork Rocket series, which explores the role of the +++- metric tensor in chronology protection in our universe, by contrasting it with a universe with a ++++ metric tensor. As per his style, it’s a fiction book set in a universe with these different physical rules, as the characters work out the consequents (and consequences) of these rules, which of course have been thoroughly worked…

WithoutWithout delving into physics, this is why I fell in love with SciFi, Asimov in particular as a youth. Okay - take humans, humanity and all we know and adjust some base conditions. Maybe we can use math to predict rough futures (foundation? ), or what about the three laws? Its so interesting to see how we think we will react as a species when faced with new realities.
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