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Comment Re:This is huge (Score 1) 214

I'm not seeing much of a disagreement with me in your latest reply. For the most part, you appear to be restating in your own words things I've also said. I think we now agree on what the choice is: locality, or common cause. If you want to maintain locality then you have to deny a common cause in these entanglement experiments. That is, even though the 'entangled' particles demonstrate properties that are highly correlated, the correlation nevertheless lacks a common cause. Or, has a common cause that occurs AFTER the experiment is performed (or thereabouts). Do you agree with this characterisation?

Good that we have cleared things up. I can agree with your characterisation if you are more specific about "common cause": what we have to give up is Reichenbach's common cause principle, which is not the only sort of common cause imaginable. In fact, we know the correlations exist because of the entangled state, so the state is some kind of common cause, just not Reichenbach's.

And to conclude, I'd like to bet that you are not a physicist (probably a philosopher?), if you think it is in any way tenable to abandon locality.

I'm in favour of letting people see the results of their bets. I have a background in physics, but my main area is indeed philosophy, so well done :)

I would be interested to hear why you think abandoning locality would be a big problem.

Ahá! For once the stereotypes worked =)

Well, for starters, it is hard to reconcile nonlocality and relativity; it requires the nonlocal influences to be some conspiratorial sort that do not actually lead to any superluminal signalling, and I find this conspiracy distasteful. Furthermore, it makes the scientific endeavour very suspect, if not actually impossible. A key hability in science is to isolate some system, control its variables, and see how changing them affect the system. In a nonlocal world, the first step of isolating the system is already impossible, so you're not going to be able to have much control over your system, and this reduces what you can learn about it.

Comment Re:This is huge (Score 1) 214

The state change only becomes effective when the results from the two
labs are brought together and are jointly analyzed, which can happen
centuries later. Bohmians like Maudlin tend to confuse such changes in
distributions with a change in the world, because the notions of states
and wave functions are reified, and considered as some real thing out

Note here that locality is maintained by not having any appropriate change in the world until the two labs bring their results together! This is what I take Wiseman to be referring to when he talks about giving up on correlation. In the Nature article I linked earlier:

No, it's not. Werner is talking about the nonlocality of the wavefunction collapse, whereas Wiseman is talking about abandoning Reichenbach's common cause principle.

But one can go further, by recalling that local causality rests on two principles: Einstein’s principle of relativistic causality, and the principle of common cause. Thus Bell’s 1976 theorem can be restated as: either causal influences are not limited to the speed of light, or events can be correlated for no reason.


Those who hold Einstein’s principle to be inviolable (the localists) must conclude that some events are correlated for no reason. A challenge for them is: if correlations do not necessarily imply a cause, when should scientists look for causes, and why?

and from the paper,

In conclusion, for a proper appreciation of the foundational importance of Bell’s
theorem to physics, information science, and the philosophy of causation, one should be
familiar with both the 1964 Bell’s theorem and the 1976 Bell’s theorem, even though
they are logically equivalent. The former proves that quantum phenomena are either
nonlocal (in a “causation by agents” sense) or undetermined, while the latter proves
that quantum phenomena violate local causality (in a “common cause for correlations”

Let me clarify what they are talking about: Bell's theorem follows from local causality. Local causality itself can be derived either from the conjunction of determinism and locality, or from the conjunction of Reichenbach's common cause principle and locality. So, if you want to keep locality, you have to give up determinism (as shown by Bell's first theorem) and Reichenbach's common cause principle (as shown by Bell's second theorem, in a more modern reading). Maybe reading this paper of Wiseman will make things clearer.

While Wiseman, Werner, and Maudlin may be all saying subtly different things, their understanding seems to me largely the same. Maudlin shows (as Bell did), that embracing indeterminism isn't enough. What Wiseman points out is that the choice isn't between locality and indeterminism, but between locality and correlation. What Werner says is that the correlation comes from entirely local events, presumably late occuring: when the labs bringing their results together. You have given up on indeterminism, but that isn't one of the options on the table.

As I said before, if indeterminism is the price to pay for keeping lcality, then we're much better off ditching locality. The same goes if one is referring to giving up on correlation of events. But keep in mind the kind of correlation here one needs to give up: it's the correlation we find in the kind of experiment given in this slashdot article. These are very *strong* correlations. How crazy does a view have to be before we give up locality?

Come on, Werner and Wiseman largely agree, but they are talking about different things. Maudlin is in violent disagreement with everybody else. But I'm repeating myself here. What I'd like to point out is what exactly is meant by "giving up on correlation of events". What one needs to give up is a very specific thing, namely Reichenbach's common cause principle, essentially that probabilities of two distant events will factorize when conditioned on their common cause. The mainstream view is that Reichenbach's principle has been falsified, and that we need to develop a true, quantum version of it. See, for example, this paper.

And to conclude, I'd like to bet that you are not a physicist (probably a philosopher?), if you think it is in any way tenable to abandon locality.

Comment Re:This is huge (Score 1) 214

What? No, come on, this doesn't make any sense! The theorems of 1964 and 1976 are the same! In 1976 Bell chose to conflate both assumptions into one that he calls "local causality", but local causality is exactly the same as the conjunction of determinism and locality. Wiseman does not agree with Maudlin at all, he is just charitably explaining where Maudlin made a trivial mistake.

Come on, In my previous reply I thought you had an interesting point about the locality of quantum mechanics, but now this is just an uninteresting wordplay.

Maudlin is wrong, and trivially so. Moreover, he is alone. To see how fringe are his views, please read this paper of Werner, which is a comment on the paper "What Bell did".

Comment Re:This is huge (Score 1) 214

Thanks for sending me the paper, now I know what you're talking about.

First of all, let us clear up something: what Bell showed is that any deterministic and local theory (you can call it local hidden variable theory if you want, it's just a name) will respect Bell inequalities (in the experiment in question, they will achieve a value of the correlation S that will respect the Bell inequality S <= 2). Since we can violate the inequality in Nature (they achieve S = 2.42), the assumptions must be wrong.

So, in principle, you can deny either determinism or locality to allow the correlations to violate Bell inequalities. What most physicists do is to deny determinism, and keep locality. Maudlin's preference is to deny locality, and to go with Bohmian mechanics, which is explicitly deterministic and nonlocal. But Maudlin makes a false claim, that Bell proved that no "local" theory can violate Bell inequalities, independent of determinism. This is simply false, as any simple study of Bell's theorem will show you. This is just Maudlin's "theorem". So, in the end, his claim is that there is no theory that is non-deterministic, local, and can violate Bell inequalities.

To support his claim, he says that quantum mechanics itself is nonlocal, denying the standard way out of Bell's theorem.

The reason he claims quantum mechanics is nonlocal is because of the "nonlocal collapse of the wave function", which is indeed a problem in some stupid interpretations of quantum mechanics, as the Copenhagen interpretation, that make the wavefunction collapse. The standard way out for Copenhagens is to claim that the collapse of the wave function is not a physical process, just a calculating device. This is just a lame excuse that explains nothing (but I mention because is extremely common).

What one does need is an interpretation of quantum mechanics that does not collapse the wavefunction, like Many-Worlds. About this, Maudlin just babbles incoherently:

That does not prove that Many Worlds is local: it just shows that Bell’s result does not
prove that it isn’t local. In order to even address the question of the locality of Many Worlds a
tremendous amount of interpretive work has to be done. This is not the place to attempt such
a task.

Comment Re:This is huge (Score 1) 214

Hmm, polarisation-preserving fibers are commonplace in quantum optics experiments, never heard of any problem with them. One thing you should keep in mind is that there are no "classical light waves", only photons, so one would need a very strange kind of interaction to be able to preserve the polarization of "macroscopic states" while fucking up the polarisation of individual photons.

But this point is moot anyway, because the rules of the game allow the experimenters to do anything before the choice of setting in each individual diamond is decided. So what they do is sent a lot of photons, trying to get the diamonds entangled, failing most of the time, and then when they succeed, they generate the measurement settings and do the measurement.

Comment Re: This is huge (Score 1) 214

Actually, it is being swapped; the initial state is (left electron entangled with left photon) and (right electron entangled with right photon), and the final state is (left electron entangled with right electron), with the photons destroyed. So the entangled was swapped from electron-photon to electron-electron.

Comment Re:This is huge (Score 1) 214

Technically speaking, Bell's theorem needs determinism and locality to apply, so to conclude that the world is not deterministic, one does need to assume that the world is local. Since I know of no serious scientist that wouldn't assume the world is local*, I can safely say the conclusion of the experiment is that the world is deterministic.

When you write

Even stochastic theories violate Bell's inequality when they insist on maintaining locality.

I think you meant that they respect Bell's inequality no? Otherwise your argument wouldn't make sense. I think what you're talking about is a class of theories known as factorizable; they respect Bell's inequality, even though they allow the outcomes to the random. But it is easy to show (shown by Fine in 1982) that these theories are mathematically equivalent to local deterministic theories, so they are simply not relevant. Since you haven't given me any details, I can only speculate (and no, I'm not going to read a book to understand what you're talking about).

But this point is moot, since I know of a local non-deterministic theory that can violate Bell inequalities: quantum mechanics.

* I know personally one crazy bastard that thinks the world is nonlocal, and I have heard rumours about a couple of others. And I know personally almost every scientist working in this field.

Comment Re:This is huge (Score 1) 214

Yes, one can have determinism without locality. This is done, for example, in Bohmian mechanics. But I would warn you against giving up locality in this way. What we need is an extremely conspiratorial kind of action-at-a-distance to be able to predict (actually postdict) the observed results. The choice of measurement they are going to make in one of the diamonds must determine, faster than light, the result of the measurement being done on the other diamond. Needless to say, very few people (mostly philosophers) take this idea seriously.

As for you other questions, I do not know enough about them to dare giving you an answer, but from what I know, they seem hardly relevant to the matter at hand.

Comment Re:This is huge (Score 1) 214

Yes. The outcome of the Wigner's friend experiment (Described in more detail by David Deutsch in section 8 of this paper). In that case, the Copenhagen interpretation predicts that we would see no interference, whereas the Many-Worlds interpretation would say that we would see interference.

These experiments you mention have no chance of testing that, because they are looking for completely different stuff. What the interpretations of quantum mechanics differ on is how very large and complex quantum systems behave. I think it will remain impossible in the foreseeable future to do the experiment as described. But in my opinion, a good enough simplification can be done if we have a universal fault-tolerant quantum computer. That, is not that far. I would guess 20 years, if funding keeps constant at the current level.

Comment Re:This is huge (Score 1) 214

What do you mean by the experiment?

One needs counterfactual definiteness to derive the Bell inequalities, so yes, one needs to assume that (among other things) to conclude the world is not deterministic. But the experiment itself is just collecting measurement statistics, so it does not need to assume anything like that.

Comment Re: This is huge (Score 1) 214

No, the state that describes the system is the electrons together with the photons, you are doing a measurement on the photons, you are doing a measurement in a part of the state.

Look, this is a very fundamental property of quantum mechanics, it just doesn't make any sense for it to fail. The whole thing would go down in flames.

Comment Re: This is huge (Score 1) 214

How do you communicate a bit using this distinction? Let's say I want to send '0', how do I do it?

It is a problem. Let's say the probability of getting them entangled is 0.5 (I'm too lazy to look it up now). Then you do the thing one time, you don't entangled the electrons, awesome, and they read '0'. Then you want to send them another '0', you do the thing, now they do get entangled, and they read '1'. Shit, now what? You cannot control what they are going to read! They do know the results of your measurements (first not entangled, then entangled), but this is just classical correlation, not communication.

Retirement means that when someone says "Have a nice day", you actually have a shot at it.