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Very often it is said that the Bell inequality has ruled out the possibility of hidden variables (HV) in QM. This is incorrect since Bell inequality has ruled out local hidden variables: nonlocal HV are still possible. However there is one inequality, the so called Leggett ineqality (not to be confused with the Leggett-Garg ineqality). In the inequality the assumption of locality is dropped: however it contains other assumptions about the ensembles of hypothetical hidden variables. I wonder what is the reaction of Bohmians to this result:

How Bohmian mechanics is compatible with the Leggett inequality?

Very similar question was asked here but there are only two comments in the discussion from which one is pointing to yet another discussion which is no longer available; the second points out that possibly spin in Bohm's theory is also undetermined (the only quantity which is always determined being the position).
I would be very grateful for any help

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    $\begingroup$ arxiv.org/abs/1202.1695 $\endgroup$
    – alanf
    Commented Jul 16 at 11:41
  • $\begingroup$ Thank you, I will take a look on this paper! $\endgroup$
    – truebaran
    Commented Jul 16 at 17:25

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There are several things to consider in the debate between realistic vs. indeterministic and Bohmian mechanics vs. no-go theorems (which includes the Leggett inequality). First of all, Bohmian mechanics is not a classical or alternative theory of quantum mechanics, but an interpretation of it. It reproduces what Schrödinger's equation and Born's rule predict, but gives us an explanation of why the universe seems to obey these laws. Second, the only ontological (i.e., real and measurable) property of the system (or the universe as a whole) is the position of the particle (Bell called it Beable to avoid over-interpretation of the term "particle"). Even measurements of spin, polarization and so on are then reduced to mere position measurements. This is called contextuality. To give an example of what this might look like, consider the Stern-Gerlach experiment: we shoot a beam of spin-$\frac{1}{2}$ particles in a directed magnetic field onto a screen and observe a splitting of the beam; we get two separate measurements of particles on the screen, one above the center line and one below it. One might be tempted to say that we have measured the ($z$-component of the) spin of the particle. But that's not correct! In fact, all we did was measure the position of the particle by having it hit our screen and seeing where it landed. Our equations and analysis now tell us that this is due to the particle's spin. But even then, you might ask, "Well, what's the difference?" The difference is that the particle never "carried" spin, the wavefunction did. The wavefunction, through the pilot-wave guiding equation, guided the particle according to the spin of the wavefunction; our measurement didn't naively reveal the spin of the particle, but through a positional measurement revealed the spin of the wavefunction.

So what does this mean for Bohmian mechanics vs. the Leggett inequality? The inequality (and all related inequalities) assume a rather generous notion of macrorealism, i.e., quote: "A macroscopic object, which has available to it two or more macroscopically distinct states, is at any given time in a definite one of those states." In short, an object is real if it has different possible states that it can be in uniquely at any given time. No mixtures, no superpositions, no collapse, etc. The problem now is that Bohmian mechanics doesn't give you "macroscopically distinct states", but positions. And every measurement is reduced to positions. One of the experiments that shows violation of the Leggett inequality uses polarization, and I even read in discussions of the inequality that "polarization measurements" (in the sense of naively revealing the polarization of the particle wave) refute Bohmian mechanics. But you cannot measure the polarization, you measure the position of the particle, which is guided by the polarization of the particle wave function. Bohmian mechanics does not satisfy the requirements of macrorealism (as put by Leggett and others), so a violation of the inequality does not rule out non-local realistic hidden variable theories (of which Bohmian mechanics is a part).

Hope this helps :)

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    $\begingroup$ Thank you for your answer! I believe that when you are referring to macroscopic realism you mean Leggett-Garg inequality instead of Leggett, is it right? Or maybe it doesn't matter since the overall upshot is that ,,only positions have definite values"? If only positions can be assigned definite value that automatically Bohm's mechanic would be immune to any kind of no-go inequalities since they always use noncommuting operators (which would be not allowed by Bohmian mechanics)... $\endgroup$
    – truebaran
    Commented Jul 16 at 15:39
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    $\begingroup$ And also, if it is so that only positions have definite values than Bohmian mechanics is very much less realistic then it is believed to be! $\endgroup$
    – truebaran
    Commented Jul 16 at 15:40
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    $\begingroup$ @truebaran You're right that I based my argument on the Leggett-Garg inequality. But it wouldn't change the argument, since, as you already pointed out, many (if not most) of the realistic QM no-go theorems are based on measuring rather general properties/states of the system (and their evolution in time, hence the need for non-commuting operators), and Bohmian mechanics does not fulfill this assumption, since we only have position there. $\endgroup$
    – Caesar.tcl
    Commented Jul 16 at 16:10
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    $\begingroup$ And tbf I wouldn't say that BM is immune to these kinds of no-go theorems. You would just have to show that no arrangement of measuring apparatus and system (contextuality...) allows a positional measurement to describe the state of the system in all detail. But I think this is impossible. $\endgroup$
    – Caesar.tcl
    Commented Jul 16 at 16:14
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    $\begingroup$ @truebaran To comment on your last remark: I think this is indeed the case! Bohmian mechanics is much less "hidden variable" than many people assume. It also comes down to what you expect a realistic theory to do in terms of realism. I (and many others) would argue that it all boils down to having an objective reality "without anyone looking". The state of the universe does not depend on measuring that state (although, thanks to contextuality, the precise result of the measurement does indeed depend on how you set up the measuring apparatus). $\endgroup$
    – Caesar.tcl
    Commented Jul 16 at 16:19

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