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Time Travel: Probability and Impossibility

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Re: Time Travel: Probability and Impossibility

#71

Post by ncmncm »

Once you bring in "possible", you have to consider whether a copy of "you" could spontaneously coalesce in front of your grandfather with a gun in your hand and shoot him, and then maybe dissipate right after, or not.

Was that really "you"? Maybe just like "you" but with horns. Or just the top half, and lasted just long enough to shoot. Or, nothing like you, really. And then your grandmother spontaneously conceived an exact copy of your father.

Or, almost infinitely more likely, who you thought was your grandfather wasn't, and somebody shot him but nobody told you. And, somebody else who looked like him showed up and your grandmother married him and didn't tell that either.

So you need a more reliable sort of own-grandfather-killing than mere possibility affords.

Re: Time Travel: Probability and Impossibility

#72

Post by breuleux »

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".

> 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?

Yes? People in the 1200s could fathom gods and magic, why not phones? What kind of dullards do you think people were back then?

The futures people imagine are consistently crazier than what actually happens. People thought that by year 2000 we'd have Mars colonies, flying cars and sentient computers. Sure, they didn't really see the Internet coming, and sure, the Internet is nice, but it's not exactly up to par, is it?

Re: Time Travel: Probability and Impossibility

#73

Post by drran »

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…

Imagine that you have pair of socks. You sent one sock to Alice and the second sock to Bob. When Alice opened her package, she sees the left sock, so she immediately knows that Bob received the right sock. This is the FTL communication.

Alice can forcefully change her sock from left sock to right sock and back, e.g. by putting sock to the corresponding foots, but Bob will never know that.

Re: Time Travel: Probability and Impossibility

#74

Post by DeathArrow »

ncmncm wrote: post #71

Once you bring in "possible", you have to consider whether a copy of "you" could spontaneously coalesce in front of your grandfather with a gun in your hand and shoot him, and then maybe dissipate right after, or not. Was that really "you"? Maybe just like "you" but with horns. Or just the top half, and lasted just long enough to shoot. Or, nothing like you, really. And then your grandmother spontaneously conceived a…

>Once you bring in "possible", you have to consider whether a copy of "you" could spontaneously coalesce in front of your grandfather with a gun in your hand and shoot him, and then maybe dissipate right after, or not.

According to Everett of quantum mechanics, it is another you killing another of your grandfather so you won't dissappear from the current world.

According to the Copenhagen interpretation, there's a high chance you dissappear.

Re: Time Travel: Probability and Impossibility

#76

Post by mattowen_uk »

djmips wrote: post #75

Backwards travel in time of a human is essentially in the realm of fantasy and magic. It's at the point where I don't even watch movies where that's the lazy trope used to solve some dilemma.

Agreed it's on the 'magic' scale.

For the universe to work, everything (down to the sub-atomic level) has to be moving all the time[1]. To observe movement, you have to have a timeframe. And everything needs to be moving in the same direction in that timeframe[2]. This is spacetime.

To travel backwards in time, you have to 100% break spacetime. As we understand it right now, that's simply impossible.

Stephen Hawking didn't believe in backwards time travel, and I'm pretty sure that he knew what he was talking about.

---

[1] Even if the item is held at 0 degrees kelvin (absolute zero) and atomic movement has stopped, it's still moving through the universe.

[2] i.e. an atom moves X distance in Y seconds.

Re: Time Travel: Probability and Impossibility

#77

Post by caf »

nobody9999 wrote: post #44
nonameiguess wrote: post #27

Earlier quoted context omitted.

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…

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.

As far as I'm aware, the weak force is known (and experimentally demonstrated) to break time-reversal symmetry.

It's only combined Charge-Parity-Time symmetry that QM maintains.

(This still does not explain how a macro arrow of time arises, though)

Re: Time Travel: Probability and Impossibility

#78

Post by simonh »

zoratu wrote: post #59

I did not see mention in the link of the problem of space. You might very well move through time, either forward or backward, but the solar system is moving through the galaxy is moving through the universe is still expanding. Therefore, maybe you moved in time, and when you’ve phased to no longer has the where you needed.

The Strontium Dog series in the 200AD comic had T-weapons. They're grenades that displace everything in a bubble around them a few minutes forward in time. The problem for the target is that by then planet Earth has moved on in it's orbit and they're dumped into the vacuum of space.

The article and the book it's reviewing are discussions of the philosophical issues raised by time travel, in particular the logical paradoxes and metaphysical implications. They are not really looking at the practical aspects of it or the physics, except to the degree that those might impinge on the logical and metaphysical issues.

Re: Time Travel: Probability and Impossibility

#79

Post by echelon »

I'm not interested in "time travel", ie. moving matter through space and time. I'm interested in information travel. We don't need to move matter at all - what's the use? It seems pointless, high energy, and wasteful.

What we need to move in time are bytes. That's infinitely valuable. Perfect markets, infinite series of experimental choices that collapse recursively into one. O(1) compute. Constant time access to many world outcomes.

We're still thinking as if our human bodies matter in the limit, and that holds our imaginations back. Bodies will be extinct in a thousand years or (much?) sooner. (If not, then we fucked up and got ourselves stuck in the gravity well with nuclear Holocaust or something.)

AGI or BCI (whichever wins) will be working on this problem. Information travel would be very useful for them. Assuming the physics on this pan out, which currently appears as though it won't.

Re: Time Travel: Probability and Impossibility

#80

Post by PeterisP »

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.

Entanglement "simply" means that the other party will have a correlation with your measurements.

Without receiving information of what you measured (inevitably in a slower-than-light way), none of their observations can tell whether you did that thing or not. The behavior of a single entangled particle is perfectly indistinguishable from a non-entangled particle, it's just that for entangled particles certain combined results are impossible or unlikely.

Imagine being given a magic pair of dice which always adds up to 7 when thrown at the same time. If you take them to different rooms and get a 6, you know the other party should have gotten a 1, but you can't use it to transfer any information to them because no matter what you do, in isolation their results are indistinguishable from normal dice.

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