Monday, January 24, 2022

Blaming Von Neumann for Quantum Mechanics

John von Neumann was one of the founders of quantum mechanics, with his immensely influential 1932 textbook. Among physicists tho, he seems to get more blame than credit.

He gets blamed for ruling out hidden variable theories. He was actually correct, as explained by Dieks and Motl.

Now a new paper on Von Neumann's book, the Compton-Simon experiment and the collapse hypothesis blames him for collapse of the wave function.

Few things in physics have caused so much hand-wringing as von Neumann's collapse hypothesis. Unable to derive it mathematically, von Neumann attributed it to interaction with the observer's brain! Few physicists agreed, but tweaks of von Neumann's measurement theory did not lead to collapse, and Shimony and Brown proved theorems establishing `the insolubility of the quantum measurement problem'. Many different `interpretations' of quantum mechanics were put forward, none gained a consensus, and some scholars suggested that the foundations of quantum mechanics were flawed to begin with. Yet, in the last ninety years, no-one looked into now von Neumann had arrived at his collapse hypothesis!

Von Neumann based his argument on the experiment of Compton and Simon. But, by comparing readings from von Neumann's book and the Compton-Simon paper, we find that the experiment provides no evidence for the collapse hypothesis; von Neumann had misread it completely!

I don't know about this experiment, but collapse of the wave function is observed. Not directly, as the wave function is not directly observed, but measurements do put systems into eigenstates that determine future measurements.

This is textbook quantum mechanics.

Believers of many-worlds theory do not want to accept it, as they believe that the wave function splits into a collapsed version that we see, and other pieces in parallel universes that cannot be seen. So they are always complaining about the collapse, as they think it is unfairly discriminating against other universes.

Regardless, we see the collapse in our universe, and we are all indebted to von Neumann for figuring this out.

The paper has some amusing anecdotes about his memory, and this:

When von Neumann’s seminal book appeared in English, Wigner told Abner Shimony: “I have learned much about quantum the- ory from Johnny, but the material in his Chapter Six Johnny learnt all from me”.
So maybe the credit/blame for collapse should go to Eugene Wigner? He supposedly once said that a dog's consciousness could collapse a wave function.

1 comment:

  1. Dear Roger,

    >> "So maybe the credit/blame for collapse should go to Eugene Wigner? He supposedly once said that a dog's consciousness could collapse a wave function."

    Thanks. I didn't know that part.

    BTW, it's a blame. At least a blemish.

    It's at least a blemish, inasmuch as the Collapse proposers / theorists went to the extent that the "von Neumann Cut" --- its existence or non-existence, and the locus/loci in the metaphysical / epistemological reality of the same --- became a subject matter of at least 10^3 peer-reviewed publications in standard Physics journals in the 20th c., including papers authored by Nobel laureates themselves.

    [Google? Would/could you please help me on those questions of *orders* of things? I mean to the base 10? Thanks in advance.]