This is a linkpost for https://www.wired.com/2014/06/the-new-quantum-reality/

Have We Been Interpreting Quantum Mechanics Wrong This Whole Time?

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Suppose there's some idea, X, which you think might help to solve a problem, Y. And there's also a dumb version of X, X', which you know doesn't work, but which still has enthusiasts.

And then one day there's a headline: CAN IDEA X SOLVE PROBLEM Y? Only you find out that it's actually X', the dumb version of X, that is being presented to the world *as X*... and nothing is done to convey the difference between X' and the version of X that actually warrants attention.

That is, more or less, the situation I find myself in, with respect to this article. I wish there were some snappier way to convey the situation, without talking about X and X' and so on, but I haven't found a way to do it.

Problem Y is: explain why quantum mechanics works, without saying that things don't have properties until they are measured, and so on.

Idea X is, these days, usually called Bohmian mechanics. To the Schrodinger equation, which describes the time evolution of the wavefunction of quantum mechanics, it adds a classical equation of motion for the particles, fields, etc. The particles, fields, etc., evolve on a trajectory in state space which follows the probability current in state space, as defined by the Schrodinger equation.

The original version of this idea is due to de Broglie, who proposed that particles are guided by waves. This was called pilot-wave theory, because the wave "pilots" the particle.

Pilot-wave theory was proposed in the *very* early days of quantum theory, before the significance of entanglement was properly appreciated. The significance of entanglement is that you don't have one wavefunction per particle, you just have one big wavefunction which provides probabilities for *joint configurations* of particles.

A pilot-wave theory for many particles, in the form that de Broglie originally proposed - one wave per particle - contains no entanglement, and can't reproduce the multi-particle predictions of quantum mechanics, as Bell's theorem and many other theorems show. Bohmian mechanics *can* reproduce those predictions, because in Bohmian mechanics, the wavefunction that does the piloting is the single, entangled, multi-particle wave used in actual quantum mechanics.

All this is utterly basic knowledge for the people who work on Bohmian mechanics today. But meanwhile, apparently a group of people who work on fluid dynamics, have rediscovered de Broglie's original idea - "wave guiding a particle" - and are now promoting it as a possible explanation of quantum mechanics. They don't seem to care about the theorems proving that you can't get Bell-type correlations without using entangled waves.

So basically, this article describes the second-rate researchers in this field - in this case, people who are doing the equivalent of trying to force the square peg into the round hole - as if they are the intellectual leaders who define it!

Isn't this a fundamental logical error? They're trying to show that the Bohmian interpretation is correct by constructing a classical model that exhibit quantum behavior, but we already know that Bohmian interpretation, since it's an interpretation, already has all the feature of quantum mechanics.

"The experiments involve an oil droplet that bounces along the surface of a liquid. The droplet gently sloshes the liquid with every bounce. At the same time, ripples from past bounces affect its course. The droplet’s interaction with its own ripples, which form what’s known as a pilot wave, causes it to exhibit behaviors previously thought to be peculiar to elementary particles — including behaviors seen as evidence that these particles are spread through space like waves, without any specific location, until they are measured.

Particles at the quantum scale seem to do things that human-scale objects do not do. They can tunnel through barriers, spontaneously arise or annihilate, and occupy discrete energy levels. This new body of research reveals that oil droplets, when guided by pilot waves, also exhibit these quantum-like features."

Per the article:

Droplets can also seem to “tunnel” through barriers, orbit each other in stable “bound states,” and exhibit properties analogous to quantum spin and electromagnetic attraction. When confined to circular areas called corrals, they form concentric rings analogous to the standing waves generated by electrons in quantum corrals.

and

Like an electron occupying fixed energy levels around a nucleus, the bouncing droplet adopted a discrete set of stable orbits around the magnet, each characterized by a set energy level and angular momentum.

Yes and yes and yes (those are all examples mentioned in the article). If you have a specific example of a quantum phenomenon that pilot wave theory doesn't exhibit, I'd like to know. Pilot wave advocates claim that pilot wave theory results in the same predictions, although I haven't had time to chase down sources or work this out for myself.

My knowledge of it is pretty superficial, but I'm pretty confused about how it represents states with a superposition of particle numbers. For fixed number of (non relativistic) particles you can always just put the interesting mechanics (including spin, electromagnetic charge, etc!) in the wavefunction and then add an epiphenomenal ontologically-fundamental-particle like a cherry on top. We'll, epiphenomenal in the Von Neumann measurement paradigm, presumably advocates think it plays some role in measurement, but I'm still a bit vague on that.

Anyhow, for mixtures of particle numbers, I genuinely don't know how a Bohmian is supposed to get anything intuitive or pseudo-classical.