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Quantum entanglement shows that reality can't be local

arstechnica.com

81–90 of 172 posts

Re: Quantum entanglement shows that reality can't be local

#81

Ok, as far as I can tell, this all just a rehash of EPR paradox and the weirdness of the Copenhagen interpretation of QM. The many worlds interpretation resolves everything with full locality. http://en.wikipedia.org/wiki/Many-worlds_interpretation Just consider, if I take a black marble and a white in hand, shake them up and put them in two boxes so I don't know which marble is in which box and put the two boxes on…

What you are describing is a "hidden variable theory", but it has been shown (See "Bell's Theorem") such a theory cannot be true. QM is stranger than that.

Re: Quantum entanglement shows that reality can't be local

#82

One thing I've never understood about interpretation of quantum entanglement experiments: Based on what I've learned of these experiments, it seems to me that the least mind-bending interpretation is that the entanglement event results in two particles that have some complementary property k (i.e. one particle has k and the other has ~k) and which one of the two, k or ~k, is had by one of the particles is unknowable…

The way I've usually seen this one presented involves precisely the observation, in that you gain knowledge of the other particle. Of course, saying "because it was always a ~k particle" does not a good explanation make, because that would imply that the resolution of the measurement was somehow predetermined, which is a fancy way of saying that there's a hidden variable. Not that interpreting all of this to mean tha…

> [...] which is a fancy way of saying that there's a hidden variable.

I'd say it's a less fancy (and more specific) way of saying "hidden variable." I read the parent as saying, "I don't understand why there can't be a hidden variable" which is quite a different thing than saying "this can work without a hidden variable: [thing involving hidden variable]".

Re: Quantum entanglement shows that reality can't be local

#83

One thing I've never understood about interpretation of quantum entanglement experiments: Based on what I've learned of these experiments, it seems to me that the least mind-bending interpretation is that the entanglement event results in two particles that have some complementary property k (i.e. one particle has k and the other has ~k) and which one of the two, k or ~k, is had by one of the particles is unknowable…

Physicist here. NegativeK is correct. Bell's theorem is the key to understanding why your explanation doesn't work. Bell's theorem, and the associated experimental demonstration, has been called the most profound discover of science. It's well worth a few hours of your time to try to understand it. You don't need to know any quantum mechanics or advanced math to do so. Incidentally, I'll mention that your argument is…

Non-physicst here. What do you think of Caroline Thompson's work, like:

"Chaotic Ball" model,local realism and the Bell test loopholes

http://arxiv.org/abs/quant-ph/0210150

...I've always wondered if much of the Einstein-was-wrong business is really a category error. I personally liked:

Clearing up Mysteries - The Original Goal http://bayes.wustl.edu/etj/articles/cmystery.pdf

" ...we must keep in mind that Einstein's thinking is always on the ontological level; the purpose of the EPR argument was to show that the QM state vector cannot be a representation of the "real physical situation" of a system. Bohr had never claimed that it was, although his strange way of expressing himself often led others to think that he was claiming this.

From his reply to EPR, we find that Bohr's position was like this:

"You may decide, of your own free will, which experiment to do. If you do experiment E1 you will get result R1. If you do E2 you will get R2. Since it is fundamentally impossible to do both on the same system, and the present theory correctly predicts the results of either, how can you say that the theory is incomplete? What more can one ask of a theory?"

While it is easy to understand and agree with this on the epistemological level, the answer that I and many others would give is that we expect a physical theory to do more than merely predict experimental results in the manner of an empirical equation; we want to come down to Einstein's ontological level and understand what is happening when an atom emits light, when a spin enters a Stern-Gerlach magnet, etc. The Copenhagen theory, having no answer to any question of the form: "What is really happening when - - - ?", forbids us to ask such questions and tries to persuade us that it is philosophically naive to want to know what is happening. But I do want to know, and I do not think this is naive; and so for me QM is not a physical theory at all, only an empty mathematical shell in which a future theory may, perhaps, be built."

Re: Quantum entanglement shows that reality can't be local

#84
post #64
post #36

Earlier quoted context omitted.

"If B has property ~k, then we've proven that B had an internal state before we performed the measurement." This part is the flaw in the logic. That doesn't prove that B has an internal state. All it proves is that, in some sense, B's state is related to A (assuming you repeat the experiment until you reach statistical significance). Entanglement is nothing more than saying one particle is connected, in some sense, t…

Why not choose the mundane and least astonishing explanation? If an object A is moving (with a known momentum) and hits a stationary object B, we can measure the momentum of either A or B and instantly know the momentum of the other object without having to measure it. The two objects are somehow "connected"; their state (momentum) is "entangled". Why is quantum entanglement different?

Because we can measure a difference empirically http://en.wikipedia.org/wiki/Bell%27s_theorem#Practical_expe...

Re: Quantum entanglement shows that reality can't be local

#85

Ok, as far as I can tell, this all just a rehash of EPR paradox and the weirdness of the Copenhagen interpretation of QM. The many worlds interpretation resolves everything with full locality. http://en.wikipedia.org/wiki/Many-worlds_interpretation Just consider, if I take a black marble and a white in hand, shake them up and put them in two boxes so I don't know which marble is in which box and put the two boxes on…

Your description is wildly incomplete. What you've just described is merely ordinary probability theory.

To see the effects of QM, build a device to flip a quantum-mechanical coin. If it comes up heads, put the marbles into the two boxes. If it comes up tails, put the marble into the two boxes in a slightly (but unmeasureably) different way. I.e., whichever procedure you chose, P(white in right)=0.5.

However, when you actually mix in the coin flip, the white marble always in the right box.

This cannot happen in ordinary probability theory, since:

    P(white in right) = P(white in right|heads)P(heads) + P(white in right|tails)P(tails) = 0.25+0.25
Quantum mechanics isn't weird because it has many worlds. Ordinary probability theory does too. Quantum mechanics is weird because those worlds interfere with each other.

Re: Quantum entanglement shows that reality can't be local

#86

This article completely ignored the Many Worlds Interpretation, which preserves all of realism, locality, and Relativity.

Can you explain how MWI is local? In the period of time before the observer is 'entangled' into the same system as the observed particles, the different possibilities can interfere with each other. If this can happen even if the entangled particles are separated by a lot of space, surely that is the same as nonlocality.

Maybe my confusion is that I'm not sure a which point the universe splits. Suppose I entangle two particles, then separate them by a large amount, my universe hasn't split yet, because before I make the measurement, I haven't become entangled with the state of the particles, but when I do, surely the spreading entanglement of myself with the possibilities centers at both entangled particles (i.e. it is nonlocal)?

Re: Quantum entanglement shows that reality can't be local

#87
post #3

This is for me the most interesting part of the whole teleportation / entanglement research. When these papers started coming out we debated the notion of 'faster than light' communication. The counter argument was that you had to move the particles apart and that was constrained by the speed of light. Then the question of "when" the state was resolved was pondered. There were two thought experiments proposed at this…

The most interesting question to me is - what happens when one of the entangled particles get annihilated. If the other one does too, instantly, then we can use that to communicate faster than light.

Re: Quantum entanglement shows that reality can't be local

#88
Bohmian mechanics [1] is a deterministic interpretation of QM that's empirically equivalent to traditional quantum theory. However, it requires non-locality (instantaneous action across vast distances).

From a recent paper [2]:

"Bohmian mechanics reproduces all statistical predictions of quantum mechanics, which ensures that entanglement cannot be used for superluminal signaling. However, individual Bohmian particles can experience superluminal influences."

This paper [3] referenced in the Ars Technica article shows that finite superluminal velocities (c Very interesting result. I assume BM is still consistent with this. The paper does mention BM:

"Bohmian mechanics and the collapse theory of Ghirardi, Rimini, and Weber [...] reproduce all tested quantum predictions, [however] they violate the principle of continuity mentioned above (otherwise they would not be compatible with no-signalling as our results imply)."

The principle of continuity described in the paper:

"In both cases, we expect the chain of events to satisfy a principle of continuity: that is, the idea that the physical carriers of causal influences propagate continuously through space."

"Clearly, one may ask whether infinite speed is a necessary ingredient to account for the correlations observed in Nature or whether a fi nite speed v, recovering a principle of continuity, is sufficient."

Actions can happen instantaneously under BM ("infinite speed"), so BM is still consistent with QM and doesn't allow FTL communication.

[1] http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory

[2] http://arxiv.org/pdf/1207.2794.pdf

[3] http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys...

Re: Quantum entanglement shows that reality can't be local

#89
post #66

Can someone provide or point to a layman's definition of "realism" in quantum mechanics? Recently on a call-in podcast I listen to, a caller has claimed that the following article shows that a) realism has been disproven and b) that proves we are all in a computer simulation: http://physicsworld.com/cws/article/news/2007/apr/20/quantum... >Now physicists from Austria claim to have performed an experiment that rules o…

Philosophically (as written by Einstein, Heisenberg, etc.) there are three assumptions about the world - of which one is broken by the quantum world. We do not know which one. Depending on your interpretation of the Schrodinger equation you can interpret the breaking in any of the three ways. This article says its realism - but as far as I know there is no way to prove its realism and not one of the others. One of these cannot hold:

- Realism, Locality, or Separability

Realism - Einstein's idea of 'elements of reality' - the idea that things actually are material and unchanging. Their properties are constant unless acted upon (ie a Hydrogen atom at time1 and point1 has a constancy of properties at time2 and point2). You could imagine a universe that had, as a property of the universe, atoms change mass every other day - just for the hell of it.

Locality - The idea that a system can only be affected by things within its lightcone - no effect can travel faster than the speed of light. And two objects must be 'local' before they can interact.

Separability - any system separated by non-trivial space/time can be considered independent of each other. The idea that 1/2mv^2 does not include every other part of the universe in trying to figure out what m or v are. Not quite the same as locality - as it would still require local interactions, but that net of interactions could be so thick and pervasive as to prevent any meaningful isolation of any system.

As is stated above, these concepts fall out of the EPR experiment.

Re: Quantum entanglement shows that reality can't be local

#90
post #3

This is for me the most interesting part of the whole teleportation / entanglement research. When these papers started coming out we debated the notion of 'faster than light' communication. The counter argument was that you had to move the particles apart and that was constrained by the speed of light. Then the question of "when" the state was resolved was pondered. There were two thought experiments proposed at this…

The experiment you all came up with sounds great, a lot like Bell's experiment but without the detail of setting up correlations and their implications.

Information cannot be transferred faster than light using entanglement. This is already proved by the no communication theorem. Essentially, because since the outcome of measurements is random/probabilistic, ambiguity is introduced on the receiver end. So since we do not allow [local] hidden variables, there is no way for person B to know whether or not they need to apply a further transformation to get to the original state without person A transferring classical bits.

I am not sure what the news in this article is because I found nothing surprising in it.

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