Hence, said EPR, in one of the most misunderstood, yet simple, arguments in the history of physics, the quantum state is an incomplete description of physical reality. It does predict the correct statistics, but does not describe completely the physical state of individual systems. Stated more precisely, it says that we must describe this pair of particles not only by their joint quantum state but also by other variables, often called "hidden," that determine the behavior of those particles when one measures their spin in a given direction.This is nuts. How do respectable academics get away with writing voodoo papers?
What could be wrong with this conclusion? In 1964, John Bell showed that simply assuming the existence of these variables leads to a contradiction with the quantum predictions for the results of measuring the spin of those particles in different directions, one for the first particle and another for the second one (see  for a simple proof of this contradiction). Those predictions have been amply verified after Bell's publication (see ( for a review).
But what does this imply? That we have no choice but to accept the analogue of the second branch of the alternative proposed about the coin tosses of Alice and Bob: that the measurement of the spin on one side affects instantaneously (if the measurements on both sides are made simultaneously), in some way, the result on the other side. This is what is called nonlocality or "action at a distance."
Jean Bricmont wrote a book with Alan Sokal criticizing Fashionable Nonsense, meaning academics citing pseudoscience to support wacky ideas. Bricmont himself is a prime example of this nonsense.
He is entitled to his opinion, of course, but he is just wrong when he says that no other view exist. Here is how they start the paper:
Let us start with a physically classical situation: consider the proverbial Alice and Bob, situated far away from each other, and simultaneoulsy tossing coins, over and over. One would expect the results on both sides to be random and uncorrelated. But suppose that the results appear indeed random but are also perfectly correlated: each time Alice's toss results in heads, Bob's toss also results in heads and similarly for tails.No, I do not agree. The third possibility is that Alice's and Bob's coin tosses have a common cause.
Obviously such a strange situation would cry out for an explanation. One possibility is the following. First, Alice and Bob are able to manipulate their coin tosses so as to obtain whichever results they desire and second, they agree in advance on an apparently random sequence of results and manipulate their coin tosses so as to both obtain that sequence when they toss their coins.
This looks extravagant, but is there any other possibility? Well, yes, there exists an even more extravagant one: that when Alice tosses her coin, she instantly affects the trajectory of Bob's coin, so that Bob's coin falls on the same side as Alice's coin.
Of course, this looks even more incredible than the previous scenario. But we may still ask: is there a third possibility? We don't see any and we will assume from now on that the reader agrees with us on that point.
That is, someone tosses a coin, duplicates it, and hands one copy each to Alice and Bob. Then Alice and Bob each appear to be getting random tosses, except that the outcomes are correlated.
This is what happens in the EPR experiments. Two distant particles are measured, but they were both emitted from the same source simultaneously.
The paper goes on to argue for Bohmian mechanics, as a nonlocal hidden variable theory. That theory agrees with quantum mechanics for some simple systems at least, so they are free to believe in it if they wish. But in all cases, local theories better job of explaining the data. They are just wrong to deny the possibility of local theories.
Bricmont previous wrote History of Quantum Mechanics or the Comedy of Errors:
The goal of this paper is to explain how the views of Albert Einstein, John Bell and others, about nonlocality and the conceptual issues raised by quantum mechanics, have been rather systematically misunderstood by the majority of physicists.This paper argues that most physicists, textbooks, Nobel prizewinners, etc. are all wrong about quantum mechanics, because they do not believe in spooky action.
No, the textbooks are not wrong.
Bricmont quotes David Mermin:
To those for whom nonlocality is anathema, Bell’s Theorem finally spells the death of the hidden-variables program. ... Many people contend that Bell’s theorem demonstrates nonlocality independently of a hidden-variables program, but there is no general agreement about this.That is correct. If you choose to believe in nonlocality, then it is possible to have a theory of nonlocal hidden variables like Bohm's. If you accept locality, then the hidden variables program is dead. A few physicists believe that Bell somehow proves nonlocality, but they are wrong.
Post a Comment