Wednesday, March 02, 2011

Bohmian mechanics threatend by Occam's razor

Last semester, I have been running a seminar on "Foundation of Quantum Mechanics" (wiki page) for TMP students that had been disappointed that "Mathematical Quantum Mechanics" was not on foundations.

Overall, I am quite satisfied with the outcome. We had covered several approaches to foundational issues, in particular the relation of quantum to classical physics and here specifically the "measurement problem" (which I am convinced is not a problem but is explained withing quantum theory by decoherence). We will produce a reader with all the contributions and I myself will write some introduction (which I will post here as well once it is finished).

But today, want to discuss Bohmian mechanics which was one of the topics and which has strong support by some local experts. I never really cared about this approach (being one of the Gallic villages where a small group of people know they are doing it better than the rest of the ignorant world, much like algebraic QFT or loop quantum gravity) being satisfied with quantum physics without any extras.

But now was the time to find out what Bohmian mechanics is really about and in this post I would like to share my findings. The big question everybody asks really is "do they make any predictions that differ from usual quantum mechanics i.e. can it be distinguished by some sort of experiment or is it just an alternative interpretation?" but unfortunately I do not have a final answer. But more below.

Before I start, let me put it a bit in perspective: Inequalities of Bell type (an I would include the Kochen-Specker theorem and GHZ type experiments) show in effect that the world cannot be both "realistic" and "local". Realistic means here that all properties have values at any instant of time irrespective of whether they are measured or not while local means that any decision I take here and now (for example whether I measure the x or y component of the spin of my half of an EPR singlet state) cannot influence measurements that are so far away that they cannot be reached even at the speed of light.

Thus one has to give up either realism or locality. The common interpretation of quantum mechanics gives up realism, the x component of the spin does not have a value when I measure the y component but is local. In some of the popular literature you will find statements to the contrary but they are mistaken: It is true, there can be non-local correlations. But this is no different from classical physics: Most of the time the color of the sock on my right foot is correlated with the color of the sock on the left foot, even at the same instant of time (when they are space-like to each other). But the question of locality is not about states (which are always global) it is about operators or measurements. And measuring the color (as compared for example to the size) of one of the socks does not influence the other sock, the local operators do commute.

Bohmian mechanics insists on realism and the price it has to pay is to give up is locality. It does not violate causality in an obviously measurable way but doing the x- or y-measurement here influences what happens far far away. But enough of these philosophical remarks, let's look at some formulas.

In its pure form, Bohmian mechanics is about non-relativistic systems of N particles with Hamiltonian of the form H=\sum_i p_i^2 + V(x_1,\ldots,x_N). Everybody knows that the norm-squared wave function in position representation \rho(x_1,\ldots,x_N)=|\psi(x_1,\ldots,x_N)|^2 gives the probability distribution of finding particle 1 at x_1, particle 2 at x_2 etc. and there is a conserved current j(x_1,\ldots,x_N)=Im(\bar\psi\nabla\psi) for this density. That is if you start with some distribution \rho at an initial time then wait a bit while you flow according to the current you end up with the new \rho at a later time.

The new thing for the Bohmians is to interpret this current as an actual current of particles with velocities \dot Q(x_1,\ldots,x_N) = v =j/\rho= Im(\nabla\psi/\psi). According to the Bohmians, these particles with joint coordinates Q are dots that for example show up on the screen of a double slit experiment. Obviously, if you start with a probability distribution of particle positions given by |\psi|^2 at an initial time and follow the deterministic flow equation for Q above, then at any later time the particles will be distributed according to |\psi|^2. The Bohmians claim, that there are really particles and at any instant of time their position is Q and the velocity is \dot Q no matter whether they are measured or not. That's it.

A few trivial remarks: This theory is non-local as the velocity of the i-th particle does depend via the wave function on the positions of all the other particles. Bohmians say that this is not to worry about since their theory is non-relativistic and this is like for example the Coulomb interaction in non-relativistic quantum mechanics where the force on one electron depends on the instantaneous positions of the other charged particles.

The next remark is that in Bohm's theory there is also the wave function that follows the same Schroedinger equation as in usual quantum mechanics. Thus any question involving only the wave function trivially gives the same answer as in quantum mechanics. The equation of motion for the particle positions Q which are the new ingredient in the Bohm theory depend on the wave function but not the other way around. There is no feed-back and the wave function does not know about the Q. Any quantum mechanical measurement that in the end measures position (like for example Stern-Gerlach) gives the same result as the q follow the wave function that determines the outcome in the usual interpretation.

All observables that are functions of the coordinates x_i at one instant of time do commute with each other and one can thus give them all sharp values at that instant of time. Thus there is no problem with claiming those positions are Q even in the usual interpretation.

Position measurements at different times in general do not commute and thus they have no common meaning. Thus the only hope to find disagreement is in experiments that in the Bohmian interpretation require sharp positions at different instants of time.

When it comes to spin I have the impression that the Bohmians cheat a bit: They declare that "spin is no a property of a point-like particle" meaning that realism does not apply to the different components and like in the usual interpretation, the components do not have a meaning unless measured. One can read this as a manifestation of the preferred role the Bohmians give to observables that are a function of the position operators over all other operators. In effect they claim only those position observables deserve realism.

Of course, one can reformulate the Bell type experiments mentioned above in terms of positions (e.g. by translating spins into positions via Stern-Gerlach set-ups) but then the non-local flow equation seems to prevent any obvious contradictions with quantum mechanics.

There are more formal problems: For time-reversal invariant Hamiltonians, one can always choose the eigenfunctions of the Hamiltonian to be real. Thus for the wave-function to be such an eigenfunction \dot Q=0, the particles don't move, even in i.e. the Coulomb field of a hydrogen atom. You may say that this is not the classical world but the quantum world and there are other equations of motion but I must say I find particles standing still even in the presence of forces a bit strange.

That that brings us to my main criticism: It is not clear to me how to observe the particle at Q. Do experiments measure the wave function (via \langle O \rangle= \langle\psi|O|\psi\rangle) or do they measure Q? And if so, can I prepare (and later measure) Q without significantly disturbing the wave function? If that is the case I can of course check whether I put an electron in a hydrogen atom in an energy eigenstate at some Q and later check whether I find it at some other place (which quantum mechanics would predict).

There are if course ways to wiggle out: You could argue that this experiment is impossible since I would always disturb the wave function significantly by placing a particle at Q and thus everything get screwed up.

But this excuse is pretty much equivalent to "you cannot observe Q (directly)". But then we are adding something (the particles at positions Q) to our theory which is not observable. And that sounds to me to be directly threatened by Occam's razor.

Anyway. Unless somebody explains to me how to measure Q, I maintain that adding Q to the theory is as good as adding invisible angels.

Update: The promised write-up is here.


wolfgang said...

It is pretty well understood by now that decoherence alone does not resolve the measurement problem.
See e.g.

Some additional (stated or unstated) assumption(s) are needed and the Bohmian interpretation is one example.

But as most would agree it is quite ugly and does not work well for the relativistic case.

Robert said...


I have read such statements like made by Adler in the paper you refer to a number of times but I do not agree with the conclusion.

I will discuss this at quite some length in the preface I intend to write but let me give you the brief version here: Adler claims that the final state is not the superposition (3) in which the system and the apparatus+environment are entangled but one has to proceed to a state which is either this or that outcome.

I think insisting on a definite outcome is wrong as it is based on an illusion: There is no definite outcome, only the observer perceives one. In the true final state there one has a superposition of two outcome states in either of which there is the system, the apparatus+environment and the brain of the observer entangled. Decoherence makes the interference between the two go away so the reduced state of the brain is a classical mixture of two states each of which sees exatly one result with certainty but in the two states of course the two different results.

So the observer always has the impression of a definite result and the total (statistical) state of the world is a mixture of the two.

You may call that many worlds if you like but you don't have to.

Robert said...

And you are right to mention what I forgot to say in the post:

Bohmian mechanics does not generalize easily to either

1) More general Hamiltonians
2) the relativistic case
3) QFT

There are attempts but those are not too convincing.

wolfgang said...

>> I think insisting on a definite outcome is wrong

since this is what we experience one has to somehow get from the state/mixture (which is symmetric in the outcomes) to our experience (which is not).

Many-worlds or variants of it do this, but at a steep cost: the usual probabilities are lost and as far as I know it is an open problem how to recover the Born rule for no-collapse interpretations.

I am looking forward to your preface.

Anonymous said...

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Robert said...

Thanks, that's great. See (just too busy at the moment to convert old posts).

Schmelzer said...

Nor the relativistic case, nor QFT is in itself a problem for Bohmian mechanics, for a scalar field this has been done already in the original paper. Gauge fields and fermion fields are a little bit more problematic, but solutions already have been proposed. (My proposal to handle them is part of arXiv:0908.0591, even if Bohm isn't mentioned there, fermions and gauge fields appear there as effective fields only.)

What we are, and, that means, also the states of our minds if we observe something, are described in BM by the position Q, and not the wave function. So, what we measure is always some Q, usually of some macroscopic pointer.