Do electrons think? (1949)
quantumlifescience.wordpress.com
quantumlifescience.wordpress.com
It's indeterminate according to the mainstream Copenhagen interpretation, but de Broglie–Bohm theory [1] ("pilot wave theory") is an interpretation that is entirely deterministic, and its assumptions result in exactly the same final equations as the Copenhagen one. But every particle always has a definite position+momentum and there's no notion of wave collapse or philosophical trickiness of what constitutes an "observer". For more on the historical reasons why Copenhagen became mainstream and de Broglie-Bohm didn't, see [2].
[1] https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory
[0] https://news.ycombinator.com/item?id=13198025
[1] http://www.tcm.phy.cam.ac.uk/~mdt26/PWT/lectures/bohm7.pdf
The double slit experiment has been used in various forms to, over time, eliminate these channels. I'm not a physicist, but the language that they are using quite a few papers leads me to believe that these channels have been closed, and the realists are out of remaining options for maintaining realism.
For instance:
An experimental test of non-local realism, 2007
https://arxiv.org/abs/0704.2529
Wherein the authors show, using previously untested correlations between two entangled photons, that maintaining realist interpretations of quantum mechanics (such as pilot-wave theory) as a fundamental concept require the introduction of locality-defying 'action at a distance' actions.
Quantum erasure with causally disconnected choice, 2012
https://arxiv.org/abs/1206.6578
Wherein the authors, using an interferometric quantum eraser experiment, eliminate past and future communication channels by enforcing Einstein locality via a mechanism that I don't understand yet. They conclude that, "No naive realistic picture is compatible with our results because whether a quantum could be seen as showing particle- or wave-like behavior would depend on a causally disconnected choice. It is therefore suggestive to abandon such pictures altogether."
Experimental non-classicality of an indivisible quantum system, 2011
https://arxiv.org/abs/1106.4481
I understand even less of this one, looks like it has to do with performing the kinds of simultaneous measurements of various properties that one would expect to be able to, if realist interpretations were true. They conclude, "Not only is a single qutrit the simplest system in which such a contradiction is possible, but, even more importantly, the contradiction cannot result from entanglement, because such a system is indivisible, and it does not even allow the concept of entanglement between subsystems."
UPDATE:
Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km 2015
https://arxiv.org/abs/1508.05949v1
"For more than 80 years, the counterintuitive predictions of quantum theory have stimulated debate about the nature of reality. ... In the past decades, numerous ingenious Bell inequality tests have been reported. However, because of experimental limitations, all experiments to date required additional assumptions to obtain a contradiction with local realism, resulting in loopholes. Here we report on a Bell experiment that is free of any such additional assumption and thus directly tests the principles underlying Bell's inequality. ... This result rules out large classes of local realist theories, and paves the way for implementing device-independent quantum-secure communication and randomness certification."
I'm not aware of any significant challenges to these studies.
On the other hand formalism works seamlessly mathematically, which cannot be said of standard quantum mechanics. Even this year's Fields medalist Figalli has a paper on how good things are in Bohmian land.
Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km 2015 https://arxiv.org/abs/1508.05949v1
"...This result rules out large classes of local realist theories, and paves the way for implementing device-independent quantum-secure communication and randomness certification. "
No known experiment can rule it out because it's predictions equal those of Copenhagen.
Is a philosophical stance something you believe that a physicist shouldn't have?
Bohmian formalism would require something perhaps no less aesthetically repulsive, but surely more alien: a preferred foliation of spacetime to avoid a preferred reference frame. Turns out it wouldn't have to be an artificial choice, as it can be derived from the wave function itself https://arxiv.org/abs/1307.1714 (that would also mean this preference existing within QM irrespective of any interpretation).
> Physicists also compulsorily reject Bohm’s construction because it explicitly builds nonlocality into its framework—even though violations of Bell’s inequality have conclusively shown that the stage of our universe is nonlocal. This is perplexing. Nonlocality is unavoidable in any theory that recovers the predictions of quantum theory. Therefore, any criticism of a theory that displays Nature’s nonlocal feature in an obvious way is both unfounded and counterproductive. Despite this, Bohm’s inherent explication of nonlocality continues to be obnoxiously mistaken as a strike against it instead of for it.
They have not. What they have shown is that the universe cannot be local and deterministic at the same time.
(In my book, the Copenhagen interpretation is a non-realistic(∆), non-deterministic and local theory, which would contradict your statement.)
(∆) Assuming, of course, that the wave function is not an object of reality, as I think is standard.
Regardless, if it was possible to be "local + non-deterministic" many of us would be fine with that. But its not - Bell rules out "locality + realism", regardless of whether the realistic theory is deterministic or non-deterministic.
Who? I believe the point of view that the Copenhagen interpretation is local is the standard one.[1]
> but its very hard to even define locality
How so? There are various definitions of locality—in terms of no faster-than-light transmission of information, in terms of commutators of field operators at spacelike distances vanishing as well as in terms of C* algebras—and the Copenhagen interpretation fulfills them all.
> if you are genuinely non-realist about everything
I'm being non-realist only about things whose existence we can't prove, e.g. the wave function.
> Regardless, if it was possible to be "local + non-deterministic" many of us would be fine with that. But its not - Bell rules out "locality + realism"
I don't see how this disproves anything of what I said. While I think you're right[2] about the fact that the violation of Bell's inequality rules out local realism—irrespective of determinism—, I already said that, in my book, Copenhagen is not a realistic theory because the wave function is not an object of reality.
[1]: https://en.wikipedia.org/wiki/Principle_of_locality#Copenhag...
[2]: "However, Fine's theorem shows that, this deterministic assignment of properties is not required to prove Bell's theorem. This is because the set of statistical distributions for measurements on two parties, once locality has been assumed, are independent of whether or not determinism is also assumed." (https://en.wikipedia.org/wiki/Principle_of_locality#Local_re...)
I said "in my book" for a reason. You might of course argue that the wave function is real, which would make the Copenhagen interpretation a realistic, albeit non-local theory (as the collapse of the wave function would happen instantaneously everywhere). I'd be very happy to discuss this but please bear in mind that this was not what the discussion was about.
Their conclusions could be wrong, of course, but if they are not, according to the authors' interpretation physicists should abandon realist theories like pilot-wave theory.
[1] https://en.wikipedia.org/wiki/Pilot_wave_theory#Principles
Not sure what the big revelation is supposed to be here. Everyone knows that Bohmian mechanics is non-local, that's exactly the point!
> The double slit experiment has been used in various forms to, over time, eliminate these channels. I'm not a physicist, but the language that they are using quite a few papers leads me to believe that these channels have been closed, and the realists are out of remaining options for maintaining realism.
No, no and no.
The assertion that Bell's inequality shut down Bohemian Mechanics is absolutely false. This claim has been floating around since the the 60s. The biggest disagreement came from ... J.S. Bell himself. He was a proponent of Bohemian Mechanics and didn't agree that he had dismantled it.
==== The criticism ====
> Recently, however, physicists more commonly cite the Kochen-Specker Theorem and, more frequently, Bell’s inequality in support of the contention that a deterministic completion of quantum theory is impossible. We still find, a quarter of a century after the rediscovery of Bohmian mechanics in 1952, statements such as these:
> The proof he [von Neumann] published …, though it was made much more convincing later on by Kochen and Specker (1967), still uses assumptions which, in my opinion, can quite reasonably be questioned. … In my opinion, the most convincing argument against the theory of hidden variables was presented by J.S. Bell (1964). (Wigner [1976] 1983: 291)
==== Bell's Take ====
> There was, however, one physicist who wrote on this subject with even greater clarity and insight than Wigner himself: the very J. S. Bell whom Wigner praises for demonstrating the impossibility of a deterministic completion of quantum theory such as Bohmian mechanics. Here’s how Bell himself reacted to Bohm’s discovery:
> But in 1952 I saw the impossible done. It was in papers by David Bohm. Bohm showed explicitly how parameters could indeed be introduced, into nonrelativistic wave mechanics, with the help of which the indeterministic description could be transformed into a deterministic one. More importantly, in my opinion, the subjectivity of the orthodox version, the necessary reference to the “observer”, could be eliminated. …
> But why then had Born not told me of this “pilot wave”? If only to point out what was wrong with it? Why did von Neumann not consider it? More extraordinarily, why did people go on producing “impossibility” proofs, after 1952, and as recently as 1978? … Why is the pilot wave picture ignored in text books? Should it not be taught, not as the only way, but as an antidote to the prevailing complacency? To show us that vagueness, subjectivity, and indeterminism, are not forced on us by experimental facts, but by deliberate theoretical choice? (Bell 1982, reprinted in 1987c: 160)
> Wigner to the contrary notwithstanding, Bell did not establish the impossibility of a deterministic reformulation of quantum theory, nor did he ever claim to have done so. On the contrary, until his untimely death in 1990, Bell was the prime proponent, and for much of this period almost the sole proponent, of the very theory, Bohmian mechanics, that he supposedly demolished.
As for the epicycles comparison, it's actually apt. They could have given complex plane and Fourier analysis for free if they had a chance (not to mention Copernicus actually used more of them than Ptolemy)(which incidentally is also the case with Many-World-necessiated landscapes and parameter spaces - just as much additional layer but with additional problems that are, in contrast, untractable by current math).
What's your justification for that? I remember Arthur Eddington pointing out that it could be interpreted as modeling our limited knowledge of a quantum system just as well as its 'physical reality'. But there's obviously justification for the first one, while the second one appears to be a large unjustified leap.
Occam's razor. The wavefunction is the simplest construct that explains our experimental results (it appears in every interpretation to some degree or another, certainly in pilot wave theory where it is the pilot wave), so it's simplest to assume it just is what's really happening.
> I remember Arthur Eddington pointing out that it could be interpreted as modeling our limited knowledge of a quantum system just as well as its 'physical reality'.
Hidden variable approaches are incompatible with our experimental results unless you introduce some seriously undesirable properties (e.g. nonlocality).
I didn't say that wavefunction realism is the only reasonable approach, just that it's a reasonable approach. Pilot wave theory isn't - it introduces an extra layer of complexity justified by nothing more than human intuition (not even innate intuition, but the experience of classical mechanics).
You are addressing a straw man. I never said anything about an alternative to wavefunctions—I brought up how they should be interpreted. Additionally, your claim, "so it's simplest to assume it just is what's really happening" has no more justification to it than the first time you said it.
> Hidden variable approaches are incompatible with our experimental results ...
He wasn't talking about a hidden variable theory, just about how to interpret the wavefunction, holding all theoretical elements constant: does it model our knowledge of physical states, or the physical states themselves.
The first one is simpler because we already know we are observing the physical states through an intermediary and that our knowledge is limited; the second one introduces a new capability to the universe.
Unless you're denying that physical reality exists at all, to suggest that the wavefunction isn't physical reality is implicitly to suggest that something else is.
> He wasn't talking about a hidden variable theory, just about how to interpret the wavefunction, holding all theoretical elements constant: does it model our knowledge of physical states, or the physical states themselves.
If you're assuming there are "physical states themselves" that the wavefunction is merely our knowledge of, that's practically the definition of a hidden variable theory.
> The first one is simpler because we already know we are observing the physical states through an intermediary and that our knowledge is limited; the second one introduces a new capability to the universe.
Nonsense. Observation is an ordinary physical process that happens inside the universe; anything that "observing" can do must already be something the universe is capable of.
> According to pilot wave theory, the point particle and the matter wave are both real and distinct physical entities (unlike standard quantum mechanics, where particles and waves are considered to be the same entities, connected by wave–particle duality). The pilot wave guides the motion of the point particles as described by the guidance equation.
What do you consider justification to treat something as physical? Do you consider e.g. electrons "just a mathematical abstraction to describe reality", or do you consider them "something physical"? If so, why?
The Schrödinger (Dirac) equation itself is deterministic. Given a wavefunction at time t=t0, you can calculate its evolution to time t=t1. What is indeterminate is the observation of the observable. The distribution of observations is entirely determined. Only the result of a single observation is indeterminate: in the limit of infinite measurements, you would get the probability distribution back which itself is determinate. The various "interpretations" concern themselves with what happens at the time of measurement. For example, the Copenhagen interpretation says that the wavefunction, which was evolving deterministically, mutates into a new wavefunction which itself evolves deterministically. This act of going from one to the other is considered indeterminate and is given the name of "collapse". However the dynamics of each wavefunction is entirely determinate.
Copenhagen is also probably not correct, though it is still frustratingly mainstream. Copenhagen has the well known problem of lacking a physical definition of "observer", but it also has conflicts with delayed choice quantum eraser experiments (which suggest that the wavefunction never collapses), and contradictions that arise when you try to locate the proposed wave collapse in spacetime.
The Everett many-worlds interpretation is gaining favor, and has none of the problems of the other two pictures. In the Everett view, scientists and experiments and instruments and universes are ensembles of particles like any other, and enter superpositions upon interacting with other systems in superposition. In a sense, every possible outcome of a quantum measurement actually happens in due proportion to the probabilities described by quantum mechanics, but upon becoming entangled with the experiment result, the different outcomes can no longer influence or measure each other. It is the most ontologically minimal theory, since it proposes no other events or laws beyond the already-known laws of quantum mechanics.
> every possible outcome of a quantum measurement actually happens in due proportion to the probabilities described by quantum mechanics
This cannot work, because those probabilities are defined by a basis in Hilbert space and that basis is defined by details of the experiment, which does not actually happen in the Everettian view.
The Born rule is a fundamental feature of all quantum theories, and I don't see why Everett is an exception.
If you want an intuitive picture of what the squared amplitude means, it seems that it would suffice to think of it as the fraction of phase space which evolves into the configuration associated with that amplitude.
See e.g. https://arxiv.org/abs/1405.7907
Let the experiment measure spin projection s_y of an atom previously prepared in spin state |s_x+>. It is known empirically that this experiment has two possible mutually exclusive results, +1/2 and -1/2, with probability of each being 1/2. Standard quantum theory accepts this and uses mathematics to predict those probabilities from data on the mutual arrangement of preparator and analyser of the spin.
Such predictions are difficult to be made in Everettian theory, because that theory describes both the particle with spin and those forming the measuring apparatus in the same way. When that is done, the physical process of measurement can be mathematically described by the Schroedinger equation, which may seem like a win, but it has a big downside: all it gives is a ray in the big Hilbert space of particle plus apparatus at later times; it is not clear what this calculation result is good for. It is not the prediction of real experiment's result; it is not even clear how to use it to calculate probabilities of those results. Certainly not by Born's rule - because what is the eigenfunction to be used in the Born formula? Such eigenfunction is, in standard theory, determined by the classical settings of the apparatus, such as orientation of the SG magnet. But there is no such thing in Everettian theory, there is just psi function.
I don't agree. I would say that the biggest difference-- really, the fundamental difference-- between Everett and Copenhagen is that Everett says that the state evolution of the wave function is always unitary, while Copenhagen says that it's only unitary sometimes but not others (and don't ask about how to calculate which way it will act).
Under unitary state evolution the volume of phase space is preserved and no information is globally created or destroyed. So the "branching" picture of Everett is not precisely correct, though it is handy language for building intuition.
This should not be surprising because of complementary observables-- confining the state of one property will cause some other property to become proportionally unconfined, per Heisenberg.
[0]: https://www.youtube.com/watch?v=r0plv_nIzsQ&t=2s [1]: https://www.youtube.com/watch?v=Qz4CHI_W-TA
I imagine a computer made of silicon with the trillions of grooves that the mind has in biological form would find itself to have a conscious just the same. Remove the silicon from the computer or biomass from the brain leaving just electrons behind and you would have a network I would argue gives rise to the seat of consciousness.
> To think that such a particle can think is so absurd, that I might give the answer “No” and have my talk over. However, (...)