Mindscape 241 | Tim Maudlin on Locality, Hidden Variables, and Quantum FoundationsMaudlin is one of the more sensible philosophers of Physics. However, I must caution you.
Last year's Nobel Prize for experimental tests of Bell's Theorem was the first Nobel in the foundations of quantum mechanics since Max Born in 1954. Quantum foundations is enjoying a bit of a resurgence, inspired in part by improving quantum technology but also by a realization that understanding quantum mechanics might help with other problems in physics (and be important in its own right). Tim Maudlin is a leading philosopher of physics and also a skeptic of the Everett interpretation. We discuss the logic behind hidden-variable approaches such as Bohmian mechanics, and also the broader question of the importance of the foundations of physics.
Carroll is a big believer in Many-worlds theory. Maudlin is not. Maudlin pushes Bohmian mechanics.
They both say the Nobel Prize committee, and a lot of others, get Bell's theorem wrong. I explained here why the Nobel site is correct.
I do not agree that the 2022 Nobel Prize has anything to do with quantum foundations. The prize citation said that it was for experimental work only. Those experiments only confirmed what everyone thought for 90 years.
Maudlin says many Bohmians take pride in the theory being just an interpretation of QM, making all the same predictions. However he argues that new research may show a way for Bohmian mechanics to signal faster than light. If so, and experimentally verified, it would contradict QM and relativity.
Carroll and Maudlin both complain that Physics departments do not take these foundational issues seriously. That is because very little worthwhile work has emerged.
They discuss whether electrons and photons are really particles or fields. Maudlin says some believe that the fermions are particles, while the bosons are fields. I am not sure it matters. They are either particles that act like fields, or fields that act like particles.
R.P. Feynman was famously a big believer in particles, as illustrated by Feynman diagrams. Other say the diagrams are just shorthands for field theory calculations. Carroll claims that Feynman was motivated in part by field theories having a vacuum energy, and dark energy had not been discovered yet. Maudlin had not heard that. I am skeptical.
Maudlin made some sensible comments about the arrow of time. I expected Carroll to disagree, but he chickened out.
My main issue with Maudlin is with Bohmian QM. He says that the experts rejected it for faulty reasons. I say they were legitimate reasons.
Bohmian QM is nonlocal. In it, an electron has a position along with a ghost function some distance away. You get the satisfaction of saying the election has a definite position, but it does not do you any good because it is controlled by the ghost function. You can never study an electron in isolation because there is some ghost a thousand miles away that you know nothing about. And the electron might not really be in its Bohmian position.
Carroll said that "hidden variables" is a misnomer because they are observable, not hidden. It is not true. The electron ghost cannot be observed. Science would never make any progress if particle were really controlled by distant ghosts. The whole theory is like having an equation with a mathematical solution that must be rejected for being unphysical.
Maudlin would answer that Bell's theorem, along with the Bell test experiments, have proved that QM is nonlocal, so I should just get over it and embrace nonlocality.
Nope. Bell showed that a theory making certain assumptions must give results differing standard QM results, and therefore be false. Those assumptions are locality, hidden variables, no retrocausality, no superdeterminism, and single outcomes.
Maudlin would say that those are all reasonable assumptions for any acceptable theory, except for locality, so locality must be wrong.
Carroll says that single outcomes must be wrong, so he believes in Many-worlds.
I say hidden variables only make sense in a classical pre-quantum theory, and that is the wrong assumption.
The hidden variable theory assumption is sometimes called realism, commuting variables, classical mechanics, or Bell's beables. The idea is that an intuitively realistic theory would allow the theory to be defined by commuting variables, so that knowing one does not interfere with knowing the others. QM famously has position and momentum as noncommuting observables, so you cannot know both at the same time.
The Einstein EPR Elements of Physical Reality folks say that a theory with Heisenberg uncertainty is incomplete. The Bell-heads say that they are just assuming objective reality.
That would have seemed like a reasonable assumption before Heisenberg declared otherwise in 1925.
Heisenberg uncertainty, and noncommuting observables, seem strange at first, but we see something similar in all wave phenomena. Light and electrons are readily observed to act like waves. So I accept that. All the alternatives, such as action-at-a-distance, are so bizarre that I do not even see how science would develop if they were true.
Mainstream Physics textbooks say QM is obviously right, and there is no need to bother with the sort of theories that Bohm and Bell were pursuing.