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The quantum world remains weird: Remembering the doomed “pilot wave”

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double slit experiment

French physicist Louis de Broglie (1892-1987) hoped that quantum mechanics could be brought within the same frame as classical physics via pilot wave theory, which envisioned “concrete particles, always with definite locations, that are guided through space by real pilot waves.” Apparently not.

But a series of bouncing-droplet findings since 2015 has crushed this dream. The results indicate that Couder’s most striking demonstration of quantum-like phenomena, back in 2006 — “the experiment that got me hooked on this problem,” the fluid dynamicist Paul Milewski said — was in error. Repeat runs of the experiment, called the “double-slit experiment,” have contradicted Couder’s initial results and revealed the double-slit experiment to be the breaking point of both the bouncing-droplet analogy and de Broglie’s pilot-wave vision of quantum mechanics.

The physicist Richard Feynman called the double-slit experiment “impossible, absolutely impossible, to explain in any classical way,” and said it “has in it the heart of quantum mechanics. In reality, it contains the only mystery.”

Tomas Bohr attributes his grandfather’s certainty that nature is incurably weird at the quantum scale to Niels Bohr’s most important physics research: his 1913 calculations of the electronic energy levels of the hydrogen atom. Bohr realized that when electrons jump between orbits, releasing quantized packets of light, there was no mechanical picture of the situation that made sense. He couldn’t relate the electrons’ energy levels to their rotational motion. Even causality failed, because electrons seemingly know before they jump where they are going to land, in order to emit a photon of the correct energy. “He was probably more aware than most of how weird that whole thing was,” Tomas Bohr said. “He was just somehow philosophically inclined in such a way that he was ready to accept that nature was that strange — and most people were not.” Natalie Wolchover, “Famous Experiment Dooms Pilot-Wave Alternative to Quantum Weirdness” at Quanta

It may be worth asking, why do we think nature should be clockwork anyway? When did we see clockwork produce life or great ideas or anything of the kind?

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See also: At Scientific American: Quantum theory does not require a conscious observer

First, if de Broglie hadn't proposed that matter waves could exist, what we know as modern QM would have had a much harder time coming about. In fact, Schrodinger's famous wave-equation builds on de Broglie's insights. What Bohr's experiment proves is that oil droplets don't exhibit the quantum mechanical behavior of "single" quantum objects. Is that a surprise to any one given that we're dealing with a 'classical' object? Bohr is simply asking the wrong kind of question. The double-slit experiment involves a quantum object 'interfering' with itself. The oil droplet, OTOH, involves many objects (10^20 = approx. a thousandth of Avogadro's number), and fulfills Schrodinger's quantum treatment of a "many-body" system showing that such a system will exhibit a "classical" outcome, something pointed out by Tomonaga in his The Story of Spin. I don't think anything of worth has been 'disproved' here. PaV
One insurmountable problem for pilot wave theory, i.e. Bohmian mechanics, is that it simply doesn't mesh with Quantum Electrodynamics, (i.e. Quantum Mechanics as applied to Special Relativity), which is regarded as one of our most precisely tested theories ever in the history of science.
Bohmian mechanics, a ludicrous caricature of Nature - Lubos Motl - July 15, 2013 Excerpt: There's no way out here. If you attempt to emulate a quantum field theory (QED) in this Bohmian way, you introduce lots of ludicrous gears and wheels – much like in the case of the luminiferous aether, they are gears and wheels that don't exist according to pretty much direct observations – and they must be finely adjusted to reproduce what quantum mechanics predicts (sometimes) without any adjustments whatsoever. Every new Bohmian gear or wheel you encounter generally breaks the Lorentz symmetry and makes the (wrong) prediction of a Lorentz violation and you will need to fine-tune infinitely many properties of these gears and wheels to restore the Lorentz invariance and other desirable properties of a physical theory (even a simple and fundamental thing such as the linearity of Schrödinger's equation is really totally unexplained in Bohmian mechanics and requires infinitely many adjustments to hold – while it may be derived from logical consistency in quantum mechanics). It's infinitely unlikely that they take the right values "naturally" so the theory is at least infinitely contrived. More likely, there's no way to adjust the gears and wheels to obtain relativistically invariant predictions at all. I would say that we pretty much directly experimentally observe the fact that the observations obey the Lorentz symmetry; the wave function isn't an observable wave; and lots of other, totally universal and fundamental facts about the symmetries and the interpretation of the basic objects we use in physics. Bohmian mechanics is really trying to deny all these basic principles – it is trying to deny facts that may be pretty much directly extracted from experiments. It is in conflict with the most universal empirical data about the reality collected in the 20th and 21st century. It wants to rape Nature. A pilot-wave-like theory has to be extracted from a very large class of similar classical theories but infinitely many adjustments have to be made – a very special subclass has to be chosen – for the Bohmian theory to reproduce at least some predictions of quantum mechanics (to produce predictions that are at least approximately local, relativistic, rotationally invariant, unitary, linear etc.). But even if one succeeds and the Bohmian theory does reproduce the quantum predictions, we can't really say that it has made the correct predictions because it was sometimes infinitely fudged or adjusted to produce the predetermined goal. On the other hand, quantum mechanics in general and specific quantum mechanical theories in particular genuinely do predict certain facts, including some very general facts about Nature. If you search for theories within the rigid quantum mechanical framework, while obeying the general postulates, you may make many correct predictions or conclusions pretty much without any additional assumptions. https://motls.blogspot.com/2013/07/bohmian-mechanics-ludicrous-caricature.html
A more detailed critique of Bohmian Mechanics (Pilot Wave theory) is here,
A Critique of Bohmian Mechanics (Pilot Wave theory) - video https://www.youtube.com/watch?v=pn2hoU4jaQQ
Of humorous note:
“When Bohm expressed “hope” that violations of QM (Quantum Mechanics) would be found later and hidden variables supported, Bohr responded that the strange sentence is almost isomorphic to “I hope that 2×2=5 will be proven at some point which will have a good effect on our finances.” https://motls.blogspot.com/2015/12/how-term-copenhagen-interpretation-got.html
Of related interest to that humorous note, I find it extremely ironic, and especially rich, that in order to try to save a classical materialistic view of reality with pilot wave theory, Atheists are crucially dependent on, ('beyond space and time'), immaterial mathematics in the first place in order to try to argue their case:
What Does It Mean to Say That Science & Religion Conflict? - M. Anthony Mills - April 16, 2018 Excerpt: In fact, more problematic for the materialist than the non-existence of persons is the existence of mathematics. Why? Although a committed materialist might be perfectly willing to accept that you do not really exist, he will have a harder time accepting that numbers do not exist. The trouble is that numbers — along with other mathematical entities such as classes, sets, and functions — are indispensable for modern science. And yet — here’s the rub — these “abstract objects” are not material. Thus, one cannot take science as the only sure guide to reality and at the same time discount disbelief in all immaterial realities. https://www.realclearreligion.org/articles/2018/04/16/what_does_it_mean_to_say_that_science_and_religion_conflict.html
In their use of immaterial mathematics, Atheists just can't help shooting themselves in the foot. As David Berlinski states in the following article,
An Interview with David Berlinski - Jonathan Witt Berlinski: There is no argument against religion that is not also an argument against mathematics. Mathematicians are capable of grasping a world of objects that lies beyond space and time…. Interviewer:… Come again(?) … Berlinski: No need to come again: I got to where I was going the first time. The number four, after all, did not come into existence at a particular time, and it is not going to go out of existence at another time. It is neither here nor there. Nonetheless we are in some sense able to grasp the number by a faculty of our minds. Mathematical intuition is utterly mysterious. So for that matter is the fact that mathematical objects such as a Lie Group or a differentiable manifold have the power to interact with elementary particles or accelerating forces. But these are precisely the claims that theologians have always made as well – that human beings are capable by an exercise of their devotional abilities to come to some understanding of the deity; and the deity, although beyond space and time, is capable of interacting with material objects. http://tofspot.blogspot.com/2013/10/found-upon-web-and-reprinted-here.html
Reports of the pilot wave interpretation's death have been greatly exaggerated. What these results refute is primarily the analogy between the pilot wave interpretation of QM and the behavior of oil droplets on a vibrating bath. Tomas Bohr also gives a gedankenexperiment that shows some simple versions of a pilot wave interpretation cannot work, but the now-"standard" de Broglie-Bohm interpretation (aka Bohmian mechanics) is not touched by this. Here's the relevant paragraph in the article:
Early on, de Broglie did offer a kind of compromise, a version of his theory that was promulgated again in 1952 by the physicist David Bohm, and which is now known as Bohmian mechanics or de Broglie-Bohm theory. In this picture, there’s an abstract wave function that extends through space — an entity that’s just as mysterious in this theoretical framework as it is in the Copenhagen interpretation — as well as real particles somewhere in it. Proofs in the 1970s showed that de Broglie-Bohm theory makes exactly the same predictions as standard quantum mechanics. However, with one element of classical reality restored — concrete particles — new mysteries arise, like how or why a mathematical wave function that’s spread everywhere in space is bolted in certain places to physical particles. “Quantum mechanics is not less weird from that perspective,” Tomas Bohr said. Most physicists agree, but it’s really just a matter of taste, since the experimental predictions are identical.
Personally, I'm with the "most physicists" described here; the de Broglie-Bohm interpretation strikes me as rather ugly, and doesn't really provide a satisfying solution to the weirdness of QM. But I also recognize that my senses of aesthetics and satisfaction aren't good guides to reality, so I don't consider this a significant argument against de Broglie-Bohm. Gordon Davisson

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