New experimental support for pilot wave theory?

• I
This new research suggesting Debroglie/Bohm pilot wave theory may yet have legs sounded compelling and I would love to hear the thoughts of the PF community:

https://www.quantamagazine.org/20160517-pilot-wave-theory-gains-experimental-support

There is some discussion in the comments by Wiseman (one of the authors of the paper) and by the always entertaining Lubos Motl.

Howard Wiseman says: May 18, 2016 at 7:12 am
The two comments here (by Pradeep Mutalik and Lubos Motl) nicely illustrate the problem with terminology in this area, a problem I've addressed a number of times, most recently in https://arxiv.org/abs/1503.06413 "Causarum Investigatio and the two Bell's Theorems of John Bell" to be published in http://www.springer.com/us/book/9783319389851

For Mutalik, nonlocality is demonstrated whenever a Bell's inequality is violated for space-like separated events. I maintain we should call that a violation of *local causality* (a notion defined by Bell in 1976) rather than of locality. The "causality" element here is appropriate because this notion is built on the assumption that correlated events must have a common cause that explains the correlation. This is not the case in a purely operational interpretation of quantum mechanics.

For Motl, nonlocality means signalling faster than light. I maintain we should call that a violation of *signal locality* rather than of locality.

The nonlocality we address in this experiment is neither of these. It is the violation of *locality*, in the sense (I maintain) that Bell used it in 1964, and that various philosophers of physics (Jon Jarrett, Don Howard) have used it since. It is also known by the ugly name of "parameter independence". You could think of it as signalling at the hidden-variable level. Because we as experimenters don't have access to the hidden variable level, violation of locality does *not* mean we can signal faster than light.

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Answers and Replies

I'm skeptical considering the use of 'weak measurements' in the experiment.

Demystifier
Nugatory
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This new research suggesting Debroglie/Bohm pilot wave theory may yet have legs sounded compelling

Is there a pointer to the actual publication (or an arxiv preprint if it's behind a paywall) anywhere?

George Jones
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atyy
http://advances.sciencemag.org/content/2/2/e1501466 "Experimental nonlocal and surreal Bohmian trajectories". My emphasis.

Page 1: "To explain nonlocal phenomena such as Bell nonlocality (10), any realistic interpretation of quantum mechanics must also be nonlocal, and Bohmian mechanics is no exception (2)."

Page 6: "Indeed, our observation of the change in polarization of a free space photon, as a function of the time of measurement of a distant photon (along one reconstructed trajectory), is an exceptionally compelling visualization of the nonlocality inherent in any realistic interpretation of quantum mechanics."

But "realism" is (from my readings) the UNrealistic view that each photon had the "measured" polarisation before it was "measured". So, on this point, I find it best to stick to the Copenhagen interpretation and reject such false "realism" (such "quantum classicality") and retain locality in all its forms. So (I wonder):

Is the claimed "experimental nonlocality" a by-product of their Bohmian unrealism?
Is my subscription to "locality in all its forms" sustainable?

bhobba
Mentor
Is the claimed "experimental nonlocality" a by-product of their Bohmian unrealism?
Is my subscription to "locality in all its forms" sustainable?

BM was deliberately cooked up to be indistinguishable from the standard formalism.

Because of that it's highly doubtful there is any experimental way to test it. But we have at least one science adviser here that is an expert in BM and he is the appropriate person to comment IMHO.

I really want to hear what he thinks.

Thanks
Bill

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I'm skeptical considering the use of 'weak measurements' in the experiment.
Exactly!
There are by now several weak measurements on Bohmian trajectories, and it is well known that such measurements can be explained also by Copenhagen or any other interpretation of QM. So such measurements do not prove that Bohmian trajectories are real, not any more than standard QM measurements prove that wave function is real.

What these measurement do demonstrate, however, is that Bohmian trajectories are not so meaningless and artificial concept as most physicists used to think. Just as wave function is a very meaningful concept in quantum physics (being real or not), such measurements demonstrate that so are Bohmian trajectories.

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S.Daedalus and bhobba
bhobba
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There are by now several weak measurements on Bohmian trajectories, and it is well known that such measurements can be explained also by Copenhagen or any other interpretation of QM.

Oh no. Not this weak measurement stuff again.

I have lost count of the number of claims that are based on misunderstanding weak measurements.

Its very annoying.

Thanks
Bill

Demystifier
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Conceptually, it can be compared with lines of force associated with a magnetic field. They are a theoretical concept naturally associated with a magnetic field (just as Bohmian trajectories are naturally associated with a wave function) , they can be measured
https://www.google.hr/search?q=line...qSoSCVBEueZdvZX1EerquIUg3LZ3&q=lines of force
but it does not necessarily imply that they are real in a fundamental sense.

atyy
Science Advisor
Exactly!
There are by now several weak measurements on Bohmian trajectories, and it is well known that such measurements can be explained also by Copenhagen or any other interpretation of QM. So such measurements do not prove that Bohmian trajectories are real, not any more than standard QM measurements prove that wave function is real.

What these measurement do demonstrate, however, is that Bohmian trajectories are not so meaningless and artificial concept as most physicists used to think. Just as wave function is a very meaningful concept in quantum physics (being real or not), such measurements demonstrate that so are Bohmian trajectories.

But can this paper be forgiven on the grounds that one bad deed deserves another? ie. ESSW deserved this paper :)

Demystifier
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But can this paper be forgiven on the grounds that one bad deed deserves another? ie. ESSW deserved this paper :)
Not completely forgiven, but it can be taken as a mitigating circumstance.

atyy
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Not completely forgiven, but it can be taken as a mitigating circumstance.

Also, Steinberg is an experimentalist, so he can be forgiven for not knowing Valentini's work (which actually, I did not know about either until reading this forum several years ago).

But Wiseman has no excuse :P

Demystifier
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Also, Steinberg is an experimentalist, so he can be forgiven for not knowing Valentini's work (which actually, I did not know about either until reading this forum several years ago).

But Wiseman has no excuse :P
How is the Valentini's work related?

atyy
Science Advisor
How is the Valentini's work related?

“The universe seems to like talking to itself faster than the speed of light,” said Steinberg. “I could understand a universe where nothing can go faster than light, but a universe where the internal workings operate faster than light, and yet we’re forbidden from ever making use of that at the macroscopic level — it’s very hard to understand.” (At the end of the Quanta article)

I think Valentini's work explains that.

Demystifier
Could someone explain what is meant by "weak measurement" please?

You know, what would be really cool is if every paper published to sites like arxiv.org had it's own discussion forum.

stevendaryl
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But "realism" is (from my readings) the UNrealistic view that each photon had the "measured" polarisation before it was "measured".

I think you need to be a little clearer what you mean by saying that it had the measured polarization beforehand.

I don't know how Bohmian mechanics handles photons, but in the case of EPR with an electron/positron pair, it's not that the spin is determined ahead of time, it's that the spin measurement result may involve facts about the electron as well as facts about Alice's and Bob's detector settings. So Bob changing his detector setting at the last moment may affect Alice's result.

stevendaryl
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Also, Steinberg is an experimentalist, so he can be forgiven for not knowing Valentini's work (which actually, I did not know about either until reading this forum several years ago).

But Wiseman has no excuse :P

Which work by Valentini are you talking about?

Oh no. Not this weak measurement stuff again.

I know, right! I am surprised physicist are still using such measurement and then claiming the result conforms to certain statements.

I think you need to be a little clearer what you mean by saying that it had the measured polarization beforehand.

I don't know how Bohmian mechanics handles photons, but in the case of EPR with an electron/positron pair, it's not that the spin is determined ahead of time, it's that the spin measurement result may involve facts about the electron as well as facts about Alice's and Bob's detector settings. So Bob changing his detector setting at the last moment may affect Alice's result.

In Bell-tests, in expressing the view that a "measured" photon did not have the "measured" polarisation beforehand, I mean this: If the "measurement" outcomes yield V-polarized photons, the photons were not (in general) V-polarized before the "measurement". So the "measurement", in my terms, was not a measurement but a test which revealed how those photons responded to interaction with the detector.

I reject nonlocality. So I reject the idea that Bob, in changing his detector setting, may affect Alice's result.

edguy99
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Could someone explain what is meant by "weak measurement" please?
To understand what weak measurement is, the following analogy from everyday life is useful.

Assume that you want to measure the weight of a sheet of paper. But the problem is that your measurement apparatus (weighing scale) is not precise enough to measure the weight of such a light object such as a sheet of paper. In this sense, the measurement of a single sheet of paper is - weak.

Now you do a trick. Instead of weighing one sheet of paper, you weigh a thousand of them, which is heavy enough to see the result of weighing. Then you divide this result by 1000, and get a number which you call - weak value. Clearly, this "weak value" is nothing but the average weight of your set of thousand sheets of papers.

But still, you want to know the weight of a SINGLE sheet of paper. So does that average value helps? Well, it depends:

1) If all sheets of papers have the same weight, then the average weight is equal to weight of the single sheet, in which case you have also measured the true weight of the sheet.

2) If the sheets have only approximately equal weights, then you can say that you have at least approximately measured the weight of a single sheet.

3) But if the weights of different sheets are not even approximately equal, then you have not done anything - you still don't have a clue what is the weight of a single sheet.

But what if you don't even know whether 1), 2) or 3) is true? Then you have different interpretations of your weak measurement. And that is precisely the case with quantum mechanics: We don't know whether particles have even approximately equal velocities at the same position (with the same wave function), so we have different interpretations. Bohmian interpretation says they have exactly equal velocities, which corresponds to the case 1), while Copenhagen interpretation corresponds to the case 3).

You know, what would be really cool is if every paper published to sites like arxiv.org had it's own discussion forum.
That would be great. One should propose it to the admins of arxiv.org.

Jando, Jilang, ibkev and 1 other person
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I reject nonlocality.
How do you reconcile it with the Bell theorem? Do you also reject reality?

eloheim and bhobba
How do you reconcile it with the Bell theorem? Do you also reject reality?

I reject what is unreal! Bell's theorem is built on the assumption of classicality - via a parameter λ - an unwarranted and over-realistic addition to the properties of a quantum object. So I'm with Anton Zeilinger* (with my emphasis)!

"This inference of nonlocality seems to be based on a rather realistic interpretation of information. If you don't assume this, you don't need nonlocality."​

* in G. Musser's book (2015) - "Spooky Action at a Distance" - p.116; referring to Maudlin's endorsement of nonlocality.

Demystifier
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I reject what is unreal! Bell's theorem is built on the assumption of classicality - via a parameter λ - an unwarranted and over-realistic addition to the properties of a quantum object. So I'm with Anton Zeilinger* (with my emphasis)!

"This inference of nonlocality seems to be based on a rather realistic interpretation of information. If you don't assume this, you don't need nonlocality."​

* in G. Musser's book (2015) - "Spooky Action at a Distance" - p.116; referring to Maudlin's endorsement of nonlocality.
So consider an EPR setup, where spins of the two widely separated particles are measured at the same time, each with another apparatus. Due to the large distance between the two apparatuses, at the time of measurement there is no single observer who can see the outcomes of both apparatuses at once. Therefore, at that time, nobody can see the correlation between the outcomes of the two apparatuses. Now which of the following, in your opinion, is true?

1) Even though no single observer can see it, at that time both outcomes of the separated apparatuses exist and these outcomes are correlated.

2) At that time it is not true that both outcomes exist. Only the observed outcome exists.

3) Whether both outcomes at that time exist is irrelevant for physics because it cannot be verified by an observer. The question whether both outcomes exist or not is a metaphysical question.

Justice Hunter
So consider an EPR setup, where spins of the two widely separated particles are measured at the same time, each with another apparatus. Due to the large distance between the two apparatuses, at the time of measurement there is no single observer who can see the outcomes of both apparatuses at once. Therefore, at that time, nobody can see the correlation between the outcomes of the two apparatuses. Now which of the following, in your opinion, is true?

1) Even though no single observer can see it, at that time both outcomes of the separated apparatuses exist and these outcomes are correlated.

2) At that time it is not true that both outcomes exist. Only the observed outcome exists.

3) Whether both outcomes at that time exist is irrelevant for physics because it cannot be verified by an observer. The question whether both outcomes exist or not is a metaphysical question.

1. Given the i-th run of the experiment, the observables Ai and Bi are correlated.

Justice Hunter, Jilang and Demystifier
stevendaryl
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"This inference of nonlocality seems to be based on a rather realistic interpretation of information. If you don't assume this, you don't need nonlocality.

It's hard for me to understand what it means to reject realism in an EPR-type experiment. Consider a correlated two-photon EPR experiment, in which both Alice and Bob measure polarization using a filter at the same angle $\alpha$. Assume that in Alice's frame, Alice's measurement takes place slightly before Bob's measurement. Alice measures her photon to be horizontally polarized. Because of the perfect correlations, she knows that a fraction of second later, Bob will also measure his photon to be horizontally polarized. During that fraction of a second, there is a fact about Bob's future measurement result that is known by Alice with 100% confidence: that he will detect a horizontally polarized photon using filter orientation $\alpha$. (Okay, since detectors are not 100% efficient, it's probably not really 100% confidence, but let's oversimplify for the sake of argument)

So the question is: Does the knowledge that Bob will measure horizontal polarization represent a physical property of the subsystem composed of Bob's photon plus his detector?

It seems to me that there are good reasons to say the answer to this question is yes. If you can say with 100% confidence what a measurement result for a distant subsystem will be ahead of time, then it sure seems that you know something definite and objective about that subsystem. To me, that's realism: Perfect knowledge about some future event implies knowledge about physical conditions now. This inference doesn't hold in MWI, however. Alice measuring horizontal polarization doesn't tell her anything about Bob's situation that she didn't know already: One "copy" will measure horizontal polarization, and one "copy" will measure vertical polarization.

But

Question 2, though, doesn't have a good answer

maline
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1. Given the i-th run of the experiment, the observables Ai and Bi are correlated.
So you accept that there are spatially separated observables which are correlated. And yet, you do not accept nonlocality. Then how do you explain the correlation? Again, let me offer you a few answers so that you can choose one of them:

1.1) One of the observables (say A) influences the other (say B).

1.2) Both A and B are influenced by a third entity (call it C).

1.3) The correlations are not a result of any influence between the entities. The correlations happen for no reason.

eloheim and maline
naima
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Does BM use operators and eigenvectors?

It's hard for me to understand what it means to reject realism in an EPR-type experiment.

Please define your understanding of "realism". I suspect your difficulty arises from within your definition of this key term. (See below.)

Consider a correlated two-photon EPR experiment, in which both Alice and Bob measure polarization using a filter at the same angle $\alpha$. Assume that in Alice's frame, Alice's measurement takes place slightly before Bob's measurement. Alice measures her photon to be horizontally polarized. Because of the perfect correlations, she knows that a fraction of second later, Bob will also measure his photon to be horizontally polarized. During that fraction of a second, there is a fact about Bob's future measurement result that is known by Alice with 100% confidence: that he will detect a horizontally polarized photon using filter orientation $\alpha$. (Okay, since detectors are not 100% efficient, it's probably not really 100% confidence, but let's oversimplify for the sake of argument)

So the question is: Does the knowledge that Bob will measure horizontal polarization represent a physical property of the subsystem composed of Bob's photon plus his detector?
Alice's knowledge is a property of Alice's consciousness. Given the correlation of the photons, the correlation of the detectors and her knowledge of physics: Alice can anticipate the outcome that will soon be known to Bob.

It seems to me that there are good reasons to say the answer to this question is yes. If you can say with 100% confidence what a measurement result for a distant subsystem will be ahead of time, then it sure seems that you know something definite and objective about that subsystem. To me, that's realism: Perfect knowledge about some future event implies knowledge about physical conditions now. This inference doesn't hold in MWI, however. Alice measuring horizontal polarization doesn't tell her anything about Bob's situation that she didn't know already: One "copy" will measure horizontal polarization, and one "copy" will measure vertical polarization.

But

Question 2, though, doesn't have a good answer

Predicting a future test result means that you know something about the future reaction of a photon when tested by a detector. False realism is to have an over-developed sense of realism (based on an inappropriate classicality - from a typical classical-world-view) and attribute the outcome (H-polarization) to the pre-test photon.

Paraphrasing Zeilinger, "This inference to classicality is based on a mis-interpretation (an inappropriate classically-based interpretation) of the information. If you don't assume this, you don't need nonlocality."

Our discussion might be helped by the mathematical answer to this question: How do you move from LHS of Bell's (1964) equation (3) to the RHS?

vanhees71