And questions raised in this comment:
Pradeep Mutalik says:
May 19, 2016 at 5:39
"This is definitely a great article with great discussion. Thank you, Dan Falk, and all commenters here.
@Howard Wiseman,
I agree completely with your exhortation to follow proper terminology, and I went through your illuminating paper. I still don’t quite grasp your subtle differentiation of “local causality” and “parameter independence – signaling at the hidden-variable level,” though. Is it that the latter applies only to realist theories, whereas the former applies to operationalist theories as well?
In this connection it is a little amusing to see George Opletal referring to Luboš Motl as a “Czech realist.” No doubt he means a realist in the sense of “pragmatist” and not in the philosophical sense ̶ Luboš seems to be definitely an operationalist of the “shut up and calculate” variety.
I am all for developing realist models like Bohm’s. While I understand the need for practicing physicists to use the mathematically most elegant formulations, we do not need to make QM weirder than it is by confusing mathematical descriptions of reality with reality itself ̶ the map in NOT the territory. I can accept the existence of any kind of weird physical objects/fields that can play within time and space dimensions (any number of them) such as waves that can instantaneously spread out their essence across the universe, entangled composite objects that internally violate relativistic signaling limits in a way we cannot, space quantization a la Roger’s comment, Bob’s whirlpool model and so on. Yes, there could be objects of this type – who are we to proscribe them, with our limited experience in the micro domain? But to claim that mathematical models such as the wave functions existing in multidimensional complex Hilbert spaces, and objective probabilities, are real objects, is to break with all other science, and should not be done unless all else fails. I think we are far from that point. When we postulate sub-quantum objects, it will, of course, be necessary to postulate hidden features that we cannot experimentally access today and may never be able to – that is fine, and to be expected, as long as it is done in a principled and parsimonious way. Who knows, a successful model of this type may actually one day extend QM. That being said, we await a realist model that reflects the elegance of the quantum formalism.
I have some questions for commenters here who are better versed in QM than I am. Please forgive and point out any obvious things that are off-base – I’m trying to understand these things more deeply.
@ Joshua McMichael
Thanks for posting this interesting abstract. The abstract states, “Our result suggests that giving up the concept of locality is not sufficient to be consistent with quantum experiments, unless certain intuitive features of realism are abandoned.” My question is: does the Bohm pilot wave model have features that need to be abandoned or is it compatible with the Leggett inequality?
@George Opletal
As I understand it, decoherence doesn’t explain why one particular alternative is selected, unless you accept the hyperpromiscuous baggage of the Many Worlds theory. Is there any objection to a sub-quantum randomizing mechanism at a scale far below the Planck scale which kicks in whenever the degree of entanglement exceeds a certain threshold?
@John Duffield
If the photon is really the pilot wave, isn’t it correct to say that it’s unlike any wave we know, because the wave completely disappears from everywhere when “something akin to an optical Fourier transform” is applied to it. Don’t we need to have to have a physical model of how something so dispersed becomes pointlike across spatial dimensions instantly? All this seems to indicate that quantum objects are truly timeless or outside of time in some way.
@ G. ’t Hooft
Very interesting comment. I want to echo Jim’s comment. Your answer will probably go over my head, but please try. Thanks!
ETA: Here is 't Hooft's comment:
G. ’t Hooft says:
May 19, 2016 at 5:12 am
"Amazingly, most people still don’t understand that sending signals faster than light requires that commutators of operators do not vanish outside the light cone. In QFT, they do, while correlation functions (expectation values of time ordered products of operators) do not vanish at all outside the light cone. In fact they can be very strong there. It’s the correlation functions, not the commutators, that cause the violation of Bell inequalities and the like. Once you realize this, all weirdness of qm goes away. No need for pilot waves or all those ugly concoctions some people come up with to explain “wave function collapse” etc.
Unfortunately, we probably first have to understand how to quantize gravity in order to discover models that show in detail what happens. As long as we don’t, people will continue to worry about “superdeterminism”, “conspiracy” and the like."
@Anyone who wants to answer
(I might be completely off-base here)
Bohm’s pilot waves are internal features of the model that have to violate relativity in order to be compatible with QM. Why should these internal features need to be made compatible with relativity? They obviously cannot. As long as QM is compatible with relativity, that’s all that matters: after all, on the surface Bohm’s model is compatible with QM isn’t it? Only the surface needs to be compatible with relativity, the internals don't. Is this wrong?"