what’s all this nonsense about Bell’s theorem and supposed non-locality?He is right, and he cites a Gell-Mann video in agreement, but most of the comments don't.
If I go to the Scholarpedia entry for Bell’s theorem, I’m told that:
Bell’s theorem asserts that if certain predictions of quantum theory are correct then our world is non-local.but I don’t see this at all. As far as I can tell, for all the experiments that come up in discussions of Bell’s theorem, if you do a local measurement you get a local result, and only if you do a non-local measurement can you get a non-local result. Yes, Bell’s theorem tells you that if you try and replace the extremely simple quantum mechanical description of a spin 1/2 degree of freedom by a vastly more complicated and ugly description, it’s going to have to be non-local. But why would you want to do that anyway?
In short, Bell proved in about 1965 a nice theorem showing the difference between quantum mechanics and classical theories. He showed that assuming local hidden variables leads to different conclusions.
In later years, Bell convinced many people that the assumption of hidden variables was not necessary. They are wrong, and most respectable physicists, textbooks, and Wikipedia articles say so. Some say that he would have deserved a Nobel Prize if he had been right, but he got no such prize.
Lee Smolin responds:
I have made what I hope is a strong case for taking seriously what we might call the “John Bell point of view” in my recent book Einstein’s Unfinished Revolution, and find myself mostly in agreement with Eric Dennis and Marko. I would very briefly underline a few points:This is so wrong that I am tempted to cite Lubos Motl's opinion of Smolin.
-This is not a debate resting on confusions of words. Nor is there, to my knowledge, confusion among experts about the direct implications of the experimental results that test the Bell inequalities. The main non-trivial assumption leading to those inequalities is a statement that is usually labeled “Bell-locality”. Roughly this says (given the usual set up) that the “choice of device setting at A cannot influence the same-time output at B”.
Nothing from either quantum mechanics nor classical mechanics is assumed. The experiments test “Bell-locality” and the experimental results are (after careful examination of loop-holes etc.) that the inequality is cleanly and convincingly violated in nature. Therefor “Bell-locality” is false in nature.
-The conclusion that “Bell-locality” is false in nature is an objective fact. It does not depend on what view you may hold on the ultimate correctness, completeness or incompleteness of QM. Bohmians, Copenhagenists,Everetians etc all come to the same conclusion.
There is no experiment where the choice of device setting at A influences the same-time output at B. The Bell experiments only show that the choice of device setting at A influences how we go about making predictions at B, but it never affects the output at B.
There is a big difference. One is spooky action-at-a-distance, and one is not.
It is amazing that Smolin could write a book in this subject, and get this simple point wrong.
I added this comment, but it was deleted:
As Wikipedia says: Cornell solid-state physicist David Mermin has described the appraisals of the importance of Bell's theorem in the physics community as ranging from "indifference" to "wild extravagance".Motl posted his explanation.
Bell assumed local hidden variables. If you believe that the theory of everything should be based on hidden variables, then Bell's theorem is a big deal. If you believe in QM as a perfectly good non-hidden-variable theory, then Bell's theorem has no importance.
As Peter says, "systems don’t have simultaneous well-defined values for non-commuting observables." That is another way of saying that Bell's assumption of hidden variables is violated.
You would need non-locality in a classical theory to fake the quantum results. But that doesn't mean that our Universe is non-local because your classical theory – and any classical theory – is just wrong. In particular, the correct theory – quantum mechanics – says that the wave function is not an actual observable. It means that its collapse isn't an "objective phenomenon" that could cause some non-local influences. Instead, it may always be interpreted as the observer's learning of some new information.One comment cited Travis Norsen to support Bell. I explained his errors in this 2015 post.
Peter Shor comments:
First: quantum mechanics doesn’t just break classical probability theory (as Bell demonstrated); it breaks classical computational complexity theory and classical information theory as well. This is why there are a number of computer scientists who are convinced that quantum computers can’t possibly work.Those computer scientists might be right.
The Bell test experiments were done by physicists with a belief that they would disprove quantum mechanics, and overthrow 50 years of conventional wisdom. All they did was to confirm what the textbooks said.
Most of the XX century was without any firm opinion that quantum mechanics breaks classical computational complexity theory and classical information theory. That opinion developed in the last 30 years, just as experiments were being done. While experiments have confirmed aspects of quantum mechanics that Bell doubted, the jury is still out on quantum computing.