Effects on Quantum Physics Interpretation (theory)

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Frenetic Pony
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Effects on Quantum Physics Interpretation (theory)

Postby Frenetic Pony » Sat Nov 17, 2012 10:16 pm UTC

So this paper: http://www.nature.com/nphys/journal/vao ... s2460.html

Proposes that the most accepted (as far as I've seen) notion of nonlocality, or "spooky action at a distance" can't exist without superliminal communication also existing. That most accepted notion of course being that two entangled particles are in a superposition of states, and that they only exit those superpositions when "measured" (should just call it interacted with really).

Or more specifically. We know that two entangled particles, A and B, are going to end up in states X and Y respectively. Quantum mechanics states they're in a superposition when created, and both are in state X and Y simultaneously, XY. And that they only enter X or Y when measured. But this brings up the problem that when we measure one, say particle A, as being state X then we know particle B must be in state Y. This occurs no matter how far away particle Y is, yet how does B "know" to collapse into Y when A is measured?

There have been a lot papers and thoughts as to how and why this happens, and most to show that it's just fine, if a bit fudgy seeming in logic. The paper above purports to show that such is impossible without instantaneous communication, no matter how you want to try and fudge with "hidden" state or such things. Thus, seeming to suggest that it's not fine. That any "internal" or hidden variables that communicate between each other, and are unobservable by us, would still end up allowing all other variables to communicate at least faster than the speed of light if not instantaneously, transferring information and breaking relativity.

To my limited understanding, this either causes trouble with fundamental portion of our understanding of quantum mechanics, or means instant communication is possible. Which I wouldn't COMPLETELY rule out. After all, our universe could be simulated in a giant computer, and this simulation could choose whatever timestep for computation desired, instant communication? It can take a million years "real time" to compute that and we'd never notice in "our" time. Then again I'd thought it better to ask more knowledgeable people first. Being inside the Matrix would just make me want to complain to an Admin and wonder what screwed up game the devs were playing at.

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Re: Effects on Quantum Physics Interpretation (theory)

Postby Tchebu » Sun Nov 18, 2012 1:45 am UTC

The point of quantum mechanics is that there aren't actually any hidden variables... Judging from the abstract, it seems that they just show that any superluminal propagation of a hidden variable will necessarily be exploitable for superluminal communication, further giving us reason to reject these possibilities.
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Re: Effects on Quantum Physics Interpretation (theory)

Postby doogly » Sun Nov 18, 2012 4:18 am UTC

Right. It is very much a classical shackle if you think that a two-particle state is actually two different particles, and one particle "over here" has to find a way to "know" what happens "over there." States don't live in space though, observations do. States live over in Hilbert space, it's a totally different game. You can't use entanglement to effect an observation "over there" by doing something "over here" -- that's what would be spooky action at a distance.

Essentially the paper says, "If you are the kind of person who has a soft spot for non-locality and thinks that would be cool (cause why not futz with quantum mechanics, it's sort of weird, making it weirder is no big deal), but does not want to introduce superluminal communication (because special relativity has extremely tight constraints, and then you are not only non-local but non-causal, and that's gross), terribly sorry good chum, but the former necessarily implies the latter."
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Re: Effects on Quantum Physics Interpretation (theory)

Postby Aiwendil » Sun Nov 18, 2012 5:41 am UTC

The point of quantum mechanics is that there aren't actually any hidden variables


The point of quantum mechancis is to describe phenomena. It's true that Bell's theorem shows that no local hidden-variable interpretation is possible, but it is perfectly possible to have a non-local hidden variable interpretation. The Bohm (or 'pilot wave') interpretation is an example. The paper at hand claims to show that any non-local hidden variable interpretation would allow superluminal communication. But it looks like the models they consider are exclusively ones in which there are hidden variables with finite (but greater than c) propagation speeds. I'm pretty sure Bohm's theory is not like that; it has effects that happen instantaneously between two points, rather than travelling at a finite speed. So I don't know whether such interpretations are affected by this paper's considerations. This actually brings up the point that 'locality' is a term that is used in different ways. When a physicist talks about locality, he or she generally means that effects can only propagate at the speed of light or less - so in that sense, something travelling faster than c is non-local. In more philosophical/foundational discussions, locality often simply means that effects must propagate; something at point A can't affect point B unless it travels to B by affecting the points between A and B. The paper cited seems to be using the first definition of non-locality, while Bohm's theory is non-local in the second, stronger, sense.

I don't know if the paper's claim is valid, but I'm rather skeptical that it implies FTL communication is possible in all non-local hidden variable theories, since I'm pretty sure one can recover standard QM in full from Bohm's interpretation. But in any case, the paper says nothing about interpretations without hidden variables, which do still have the 'spooky action' but in a way that provably does not allow FTL communication.

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Re: Effects on Quantum Physics Interpretation (theory)

Postby doogly » Sun Nov 18, 2012 5:55 am UTC

There isn't actually anything spooky about what Einstein used the phrase "spooky at a distance" to describe though - that behavior, the one in quantum mechanics, is not actually non local.

Bohm's pilot wave is not lorentz invariant, so it appears to be more than a mere interpretation.. Funny business is afoot. I haven't looked into it too deeply though, I will admit.
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Re: Effects on Quantum Physics Interpretation (theory)

Postby gmalivuk » Sun Nov 18, 2012 4:23 pm UTC

Aiwendil wrote:I'm pretty sure Bohm's theory is not like that; it has effects that happen instantaneously between two points, rather than travelling at a finite speed. So I don't know whether such interpretations are affected by this paper's considerations. This actually brings up the point that 'locality' is a term that is used in different ways. When a physicist talks about locality, he or she generally means that effects can only propagate at the speed of light or less - so in that sense, something travelling faster than c is non-local.
This doesn't mean "locality" is used in different ways. It means that Bohm's theory evidently doesn't play nice with special relativity. Because according to SR, propagation that happens instantaneously in one frame happens at finite but superluminal speeds in another range of frames, and actually goes backward in time in a third. Which therefore also implies that anything going even a little bit faster than light in one frame appears to be going instantaneously in another.
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Re: Effects on Quantum Physics Interpretation (theory)

Postby doogly » Sun Nov 18, 2012 6:03 pm UTC

Yeah, locality in physics is always [A(x),A(x')]=0 if x and x' are spacelike separated. With the notable exception of fermionic operators, where you want { , }.

Everything else is colloquialism.
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Re: Effects on Quantum Physics Interpretation (theory)

Postby Aiwendil » Mon Nov 19, 2012 3:34 pm UTC

I think I phrased that badly. Yes, given Special Relativity, faster than light in some frame implies simultaneity in some other frame. But there is still a difference between an effect propagating from point A to point B faster than light (so that, yes, it will propagate instantaneously in some frame) and an effect due to conditions at point A occurring at point B without having propagated through the space between them. If I recall correctly, in Bohm's interpretation the behaviour of a particle at point A depends directly on the behaviour of the pilot wave throughout the universe, without any physical effect mediating between them. In philosophical circles, that's what the term 'non-locality' is sometimes used for: two spatially separated events being directly causally related, without mediation.

The usual formulation of Bohm's interpretation requires you to write things down in a preferred frame. But it's constructed in such a way that no experiment can ever determine what that frame is. (Which rather suggests that there must be a way of formulating it without a preferred frame, though I'm not sure whether that's been done). And despite the formulation being so flagrantly non-local, it's also constructed in such a way as to yield no empirical disagreements with special relativity - and, in fact, to give the same empirical predictions as the standard formulation of quantum mechanics. That's what it's supposed to do, anyway; maybe it fails to do so. It's been a while since I've looked at the formalism of it.

doogly wrote:There isn't actually anything spooky about what Einstein used the phrase "spooky at a distance" to describe though - that behavior, the one in quantum mechanics, is not actually non local.


Yeah, locality in physics is always [A(x),A(x')]=0 if x and x' are spacelike separated.


Yes, that's a pretty standard definition. But the idea of locality - that an object is directly influenced only by its immediate surroundings - is a lot older than quantum physics. The collapse of the wavefunction is something that could reasonably be called non-local, in the latter sense, even if no physical effect (or information) ever travels faster than light. But this is just a matter of definitions.

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Re: Effects on Quantum Physics Interpretation (theory)

Postby mfb » Wed Nov 21, 2012 4:24 pm UTC

ArXiv version of the paper

We have shown that if the non-local e ffects that we observe in Bell experiments were due to hidden influences propagating at any finite speed, then non-locality could be exploited for superluminal communication.

While it is nice to have a proof, I don't think models with such a finite velocity are relevant.

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Re: Effects on Quantum Physics Interpretation (theory)

Postby Tchebu » Wed Nov 21, 2012 10:17 pm UTC

While it is nice to have a proof, I don't think models with such a finite velocity are relevant.


I would wager that you can take the limit of their argument as the velocity goes to infinity, and demonstrate that the Lorentz symmetry group becomes the Galilean group..
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