[btilly]

They claim to have found a classical system that reproduces quantum mechanical effects. But if they manage to extend it to many particles, interacting, they will find that they have just come up with another interpretation of QM which is experimentally indistinguishable from the rest. And it wouldn't even be the first one. (Bohm's hidden variable theory has precedence.)

Furthermore the "incompressible fluid" they postulate sounds like it enables non-local behavior (which it has to to match current versions of the Bell test) so it is unable to help resolve the issue of reconciling GM with QM.

So this does rather less than they claim. Assuming that their claimed result is correct.

[troymc]

Models like theirs predate Bohm by a long shot. From the introductory paragraph of the paper:

"In 1746 Euler modelled light as waves in a frictionless compressible fluid; a century later in 1846, Faraday modelled it as vibrations in ‘lines of force’ … Fifteen years later Maxwell combined these approaches, proposing that a magnetic line of force is a ‘molecular vortex’…"

They basically updated Maxwell's model. From their conclusion:

"We brought Maxwell’s 1861 model of a magnetic line of force up to date using modern knowledge of polarised waves and of experiments on quantised magnetic flux. Our model obeys the equations for Euler’s fluid and supports light-like solutions which are polarised, absorbed discretely, consistent with the Bell tests, and obey Maxwell’s equations to first order."

What's nice is that their model is classical. Even if it "just" makes exactly the same predictions as other models, it's nice to have a model where physical intuition can be brought to bear.

[phkahler]

>> What's nice is that their model is classical. Even if it "just" makes exactly the same predictions as other models, it's nice to have a model where physical intuition can be brought to bear.

A classical model can also be simulated on a classical computer, so if it produces the same results as QM then quantum computation would be... fiction or just redundant?

[tjradcliffe]

The only caveat to this is the ontological status of Faraday's "lines of force". Their model is based on these, and so far as anyone knows they are just a conceptual or pedagogical convenience. They have a quantitative meaning (you can actually calculate the "density of lines of force" between two charges or magnetic dipoles) but they aren't really good for much. They are generally mentioned in passing in intro or intermediate E&M courses, but mostly as a matter of historical interest.

If they could be shown to have independent effect of the kind that the vector potential was shown to have via the Ahronov-Bohm effect, then this whole approach to quantization would become extremely interesting. Otherwise, you're right: it's just another interpretation of QM, and not a very interesting one at that (despite their claims, as I explained in a separate comment, they can't reproduce the experimental violations of the CHSH inequalities in Aspect's and other experiments that introduce time-variation to precisely rule out the kind of prior communication they are arguing for.)

[ars]

> Furthermore the "incompressible fluid" they postulate sounds like it enables non-local behavior

It says compressible not incompressible.

[davyjones]

Usually a pretty common mistake. Fluid is always compressible (else sound won't travel in STP water). It is whether one chooses to model the flow as compressible or not.

[cordite]

They really ought to stop teaching that fluid is incompressible in middle school.

[chinpokomon]

I think that is a simplification of the model for high school level physics. It's like when we use Hooke's law to describe a spring constant that only holds true for part of the range. Compared with a volume of gas, fluids are "incompressible."

[fixermark]

What's the current status of Bohm's hidden variable theory? Does it stand up in light of the Bell test (I was under the impression that the Bell results suggest an arbitrary number of hidden variables would be necessary)?

[sampo]

> What's the current status of Bohm's hidden variable theory? Does it stand up in light of the Bell test

In "normal" quantum mechanics we have the wave-particle dualism. A particle behaves also like a wave, whatever that means.

In Bohm's hidden variable theory, also known as "pilot wave theory" the wave and the particles are separate. The pilot wave is a wave of unknown making (the theory does not say what it would be made of), and this wave follows the normal quantum mechanical behaviour. Then particles "ride" on this wave. So all the quantum mechanical wave effects happen in the pilot wave, and then the classical particle-like particles just follow theirs paths, already laid out by the wave.

Although "spooky action at a distance" is also experimentally verified, explaining this by postulating a wave that fills the whole universe, and explicitly reacts spookily over distances, makes the whole "spooky action at a distance" uncomfortable explicit in this theory, so it doesn't appeal to most physicists.

"The de Broglie–Bohm theory makes the same (empirically correct) predictions for the Bell test experiments as ordinary quantum mechanics. It is able to do this because it is manifestly nonlocal."

http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory#...

[bkcooper]

What's the current status of Bohm's hidden variable theory? Does it stand up in light of the Bell test?

Yes. Bell's theorem doesn't forbid hidden variable theories (in fact, he has a publication preceding his famous result explicitly demonstrating a hidden variable theory that could reproduce QM measurements on a two-level system), just local hidden variable theories. In his collected papers, Bell often remarks that he feels Bohm's interpretation should be more widely studied.

It's a good question why it's not. My understanding, though I'm not well versed in the subject, is that there are some pretty severe shortcomings to it, particularly when you start considering systems with many particles. I've read some of Bohm's writings to try to understand it and as far as I got I found it pretty underwhelming; it seemed more like a bookkeeping trick than any sort of real insight.

[jostylr]

Try reading some of the stuff from http://www.bohmian-mechanics.net/ I was a student of Sheldon Goldstein of that group and can attest that it has no problems with many particles.

In fact, part of my thesis was explaining how identical particles are handled in Bohmian mechanics. Basically, one considers a configuration space whose points are sets of points in physical space rather than some arbitrarily ordered points. One then immediately gets bosons and there is a principle which guides one to fermions. This works beautifully with spin as well, where the spin is associated with the physical point in space, not with the particle label as is usually done. I find it to be very elegant.

There is also an extension to quantum field theory in which the particle creation and annihilation operators actually represent those events with the Bohm particles. So that is not a problem either. It becomes an indeterministic theory at that point (creation is a random event while annihilation is simply when the particles collide).

The main conceptual question is dealing with instantaneousness of the theory. It is a merit of BM that it brings this to the forefront. One can always put in an arbitrary (hidden) foliation of space-time to deal with it, but it is a bit distasteful even if, as there are, foliations that can be derived the other existing structures.

One final note is that Bell was a very strong proponent of Bohmian mechanics even when Bohm had forgotten about it for a time. Bell's formulation was based on the first order probability flow and not the quantum potential. Bell's version works for spin while the quantum potential does not (sadly for numerical work).

[btilly]

I don't know and don't keep track. I lost interest after realizing that it can be made compatible with any possible observation.

I'm personally a fan of the Everett interpretation. Also called the many worlds. Which is what you get if you assume that quantum mechanics applies to the observer. Then the act of observation throws the observer into a superposition of possible states where different things were observed. And those states cannot meaningfully interact later for thermodynamic reasons.

Unless someone comes up with good reason to believe that quantum mechanics does not describe humans, I see no reason not to accept it. And if quantum mechanics is replaced by something different, to the extent that quantum mechanics is an accurate description of us, that interpretation remains correct.

[IsTom]

There's a problem with with many-worlds that I haven't seen convincingly refuted: when collapsing superposition of two states you might need to (or rather almost always) need the ratio of worlds with outcome A to worlds with outcome B to be irrational. So unless you create a continuum of new worlds each time it won't work.

[lmm]

The wavefunction is a superposition of n independent wavefunctions at different amplitudes, like you can divide a piano's sound into a bunch of pure sine waves. "From the inside" each feels like a self-contained world (in a physically rigorous sense), but the ratio between their amplitudes and phases can be an arbitrary complex number - just as even if you're only playing a C and a G, the ratio of their amplitudes can be anything.

[btilly]

Why is this a problem? This complaint looks to me much like someone complaining about the Pythagorean theorem because it results in irrational numbers. (And yes, historically there were such complaints. And no, that is not a good reason to discard that theorem.)

A system described by physics evolves according to the laws of physics. We find the result surprising. But that is because we have bad intuition, not because physics is fundamentally broken.

[davorak]

> when collapsing superposition of two states you might need to (or rather almost always) need the ratio of worlds with outcome A to worlds with outcome B to be irrational. So unless you create a continuum of new worlds each time it won't work.

Has this been shown? I would want any such proof inspected for hidden continuum assumptions existing in the probability calculations, thus leading to irrational probabilities.

[jostylr]

Try reading http://arxiv.org/pdf/0903.2211.pdf This basically presents many worlds as a mass density obtained by integrating out the wave function. The worlds we see can be traced through the evolution, like two videos overlain on each other can be deduced over time.

[Symmetry]

The Everett Interpretation is best thought of as the idea that collapse just doesn't happen. Which means that there aren't discrete integer worlds but only higher and lower amplitudes. If you had discrete worlds that would also be a problem because you could change how many worlds you had by how you did the math.

[Dylan16807]

>good reason to believe that quantum mechanics does not describe humans

One simple idea is that the more energy and matter you shove into the wave function, the easier it is to collapse, so that at the macro level quantum effects are rendered impossible.

[btilly]

The problem is that there is absolutely no experimental or theoretical reason to believe that quantum states ever collapse. And there is an explanation for why we would perceive collapse even if there is none.

If you take those ideas seriously, you're forced into the Everett interpretation.

[tjradcliffe]

But the "explanation" assumes what it sets out to prove.

If there is no reason to believe in collapse, there is no reason to believe that we can only be conscious of our state of entanglement with one component of a wavefunction rather than both.

That is, the Everret interpretation assumes that for some unknown reason we can only be conscious of the classical world, and uses this assumption to "explain" that we are conscious only of the classical world.

Consider a polarizing beam splitter with detectors in either arm. We are only ever conscious of a photon being detected in one arm or the other. But why not both, since the matter of our brain is necessarily entangled with both components of the photon wavefunction?

All Many Worlds does is push the central mystery around, from "Why do photons prepared in the same initial state collapse into different final states?" to "Why aren't we conscious of being entangled with both photon polarization states rather than just one?" It won't do to simply say, "Well, consciousness doesn't work that way." We know it doesn't. The question is, given the otherwise completely continuous physics describing the world, why is the physics of the brain such that it can't generate consciousness of that world?

Decoherence and similar approaches have the same problem, because they assume that for some reason the brain is unable to detect the quantum world without the aid of such classical phenomena as interference patterns in photon detection, but there is simply no warrant for that assumption.

If you restrict your description of the universe to non-collapsing QM you would never guess at the existence of the classical world. Ergo, a brain fully-described by non-collapsing QM is a quantum brain, and there is no particular reason why it shouldn't be in all states at once. That is it not in all states at once is manifestly true, but the question is "Why not?" It won't do to simply assume it, as all these alternative interpretations of QM do.

Getting the brain to be aware of only a single classical world is exactly the same problem as getting a wavefunction to collapse. It has just moved the problem around, not solved it.

[btilly]

If there is no reason to believe in collapse, there is no reason to believe that we can only be conscious of our state of entanglement with one component of a wavefunction rather than both.

Absolutely true. The process of cognition is addressed by science. Consciousness, not so much.

That is, the Everret interpretation assumes that for some unknown reason we can only be conscious of the classical world, and uses this assumption to "explain" that we are conscious only of the classical world.

No such assumption is made. In fact you are the one adding an implicit assumption that consciousness obeys classical rules, when we have no data suggesting that such is the case.

Consider a polarizing beam splitter with detectors in either arm. We are only ever conscious of a photon being detected in one arm or the other. But why not both, since the matter of our brain is necessarily entangled with both components of the photon wavefunction?

This is a non-issue if consciousness can exist in non-interacting superpositions? Indeed there is indirect evidence that this is the case.

While science is currently unable to explain the phenomena of consciousness, we are able to say a lot about the process of cognition. And to date nobody has ever demonstrated that we can be conscious of something we did not learn about through physically understood processes. (If you have such a demonstration, there is a million dollar prize waiting for you, courtesy of James Randi.)

Quantum mechanics predicts that entanglement will cause that physical system to separate into a superposition of non-interacting states. And therefore all evidence is that there is no way for your consciousness of one state to affect cognition and therefore any awareness of any other.

Again, this is now a non-problem.

[saalweachter]

Ah, but macro-level systems can amplify quantum effects. For instance, the human eye -- under the right conditions -- is sensitive to individual photons.

[skygazer]

A packet of energy absorbed and exciting a molecule in my eye, causing a cascade through a complex system still feels entirely classical. I'm not sure merely invoking "photon" is enough to imply quantum effects. It's when the behavior breaks from deterministic to probabilistic (independent of initial conditions) that we switch paradigms.