Friday, January 3, 2014

Mermin taking Bohr seriously

N. David Mermin writes in QBism as CBism: Solving the Problem of "the Now":
In a Physics Today Commentary,1 and more carefully, extensively, and convincingly with Chris Fuchs and Ruediger Schack,2 I argued that stubborn longstanding problems in the interpretation of quantum mechanics fade away if one takes literally Niels Bohr’s dictum3 that the purpose of science is not to reveal “the real essence of the phenomena” but to find “relations between the manifold aspects of our experience.”
I have mentioned QBism and Mermin's coauthored paper, with the comment that people should have just listened to Bohr in the first place.

The 2012 Physics Today article starts:
Quantum mechanics is the most useful and powerful theory physicists have ever devised. Yet today, nearly 90 years after its formulation, disagreement about the meaning of the theory is stronger than ever. New interpretations appear every year. None ever disappear.

Probability theory is considerably older than quantum mechanics and has also been plagued from the beginning by questions about its meaning. And quantum mechanics is inherently and famously probabilistic.

For the past decade, Carl Caves, Chris Fuchs, and Ruediger Schack have been arguing that the confusion at the foundations of quantum mechanics arises out of a confusion, prevalent among physicists, about the nature of probability.1 They maintain that if probability is properly understood, the notorious quantum paradoxes either vanish or assume less vexing forms.
I agree with most of that, except that I say that quantum mechanics is not more probabilistic than any other scientific theory.

Many times on this blog I have defended the Copenhagen interpretation against those who say that it is incoherent, unscientific, and obsolete. I repeatedly trash big-shot physicists who promote many-worlds or nonlocal interpretations of quantum mechanics as being somehow necessitated by disproving traditional interpretations. Mermin and Lubos Motl are about the only living physicists where I have agreed with their views on this.

To me, the paradox is why so many smart people find quantum mechanics so paradoxical, when the essentials were so clearly explained by Bohr, Heisenberg, and von Neumann 80 years ago. Mermin's theory for this has to do with a refusal to accept a probability interpretation. I have some other ideas, and I will be posting them.

Mermin defends his article with the opinion of another quantum founder:
a 1931 letter from Erwin Schrödinger to Arnold Sommerfeld:3 "Quantum mechanics forbids statements about what really exists--statements about the object. It deals only with the object-subject relation. Although this holds, after all, for any description of nature, it appears to hold in a much more radical and far-reaching sense in quantum mechanics"
There is also a May 2013 SciAm article on QBism where it is again treated as something new, but it is really just the Copenhagen interpretation.

Anyone proposing a new (or old) quantum interpretation should at least recognize the Bohr-Heisenberg position, as that was considered standard for decades. People today act as if Bohr's position was nonsensical, as can be seen in the recent Dilbert cartoon.

I am not sure why a new name is needed to reiterate what Bohr said. After all, Bohr won those Bohr-Einstein debates back in the 1930s.

Update: Mermin previously promoted some opposing views of quantum mechanica in what he called the "Ithaca interpretation" (he is a professor in Ithaca). He said in 1996:
To live with so many requirements I need room for maneuver. This is provided by adopting, as my sixth and final desideratum, the view that probabilities are objective intrinsic properties of individual physical systems. I freely admit that I cannot give a clear and coherent statement of what this means. The point of my game is to see if I can solve the interpretive puzzles of quantum mechanics, given a primitive, uninterpreted notion of objective probability. ...

It therefore appears that the view of probability underlying the Ithaca interpretation must be anti-Bayesian.
And he said in 1997:
I shall not explore further the notion of probability and correlation as objective properties of individual physical systems, though the validity of much of what I say depends on subsequent efforts to make this less problematic. My instincts are that this is the right order to proceed in: objective probability arises only in quantum mechanics. We will understand it better only when we understand quantum mechanics better. My strategy is to try to understand quantum mechanics contingent on an understanding of objective probability, and only then to see what that understanding teaches us about objective probability.10

10 That objective probability plays an essential role in the quantum mechanical description of an individual system was stressed by Popper, who used the term "propensity". See Karl Popper, Quantum Theory and the Schism in Physics, Rowman and Littlefield, Totowa, New Jersey, 1982. Heisenberg may have had something similar in mind with his term "potentia". While I agree with Popper that quantum mechanics requires us to adopt a view of probability as a fundamental feature of an individual system, I do not believe that he gives anything like an adequate account of how this clears up what he called the "quantum mysteries and horrors".
I do not agree that there is any such thing as "objective probability" or propensity in quantum mechanics, any more than probability figures into other scientific theories.

Apparently Mermin tried to improve quantum mechanics with his own Ithaca interpretation and objective probability. Then he realized that was all a mistake, and switched back to completely subjective probability, and calls it QBism. I don't want to blame him for changing his mind, but it would be nice if he explained that QBism is a throwback to Bohr and its novelty is mainly in correcting errors by himself and others.

A comment below disagrees with me, and cites R.P. Feynman for how illogical QM is. I cannot agree. Feynman's textbook on quantum mechanics is now online, and it is not nonsensical, absurd, or illogical. He gives a coherent exposition of the theory. Yes, he has also been quoted as saying QM is strange and mysterious.


  1. @Roger,
    I enjoy your blog, but seriously, your dislike of Einstein does strange things to your reasoning.
    Bohr did not really 'win' the debates with Einstein, Bohr was declared a winner by some, more as a function of popular following (not logical reasoning) rather than argument (he didn't really have much argument). As for why so many find QM nonsensical, it is because it is nonsensical. Bohr said so. So did Feynman. They said so and took pride in bragging about how absurd and illogical nature was (Read Feynman's QED, he says this several times). You should not pride yourself however in subscribing to a nonsensical probabilistic explanation where physics is concerned, as it does nothing for advancing actual understanding, it only advances very complicated fudged math that to this day offers no insight into any actual underlying 'mechanics' of what is going on at the quantum level. I will put my own bias out front: matter and energy have physical extension and does not turn into self causing (a priori) math even at the sub-atomic, quantum or Planck level, that's called magic or Platonism and isn't science. QM is a hodge podge of heuristic math trying to make sense of experiment without any physical mechanism or underpinnings whatsoever. It is actually no different than describing dice with probability, you have a heuristic calculation which tells you nothing about how the results are generated except by imagining probabilistic math calculations acting as 'a priori' physical causes. Logically, math is not a physical cause, it can describe something, but not be the cause of it, which is why you need good logical and mechanical theory to position your math around. If you want to understand how you got the result from a pair of rolled dice, you have to understand how the dice are shaped, how they move and interact with each other and the surface they are rolling on (much less explain their size, weight, gravity and atmosphere under which they operate). You can't get such an answer from probabilistic reduction ad absurdum, any more than you can tell an individual test score from a school wide class average.
    The problem with QM is that the wave model being used is wrong. You are trying to describe a cycle of physical movement of an actual thing with not enough degrees of freedom. Considering that actual particles do not exist in two dimensional planes (they never have, and nothing physical does), physicists should start trying to study particles as they move through three dimensional space, and maybe notice those particles definitely spin, and are moving about in directions their two dimensional models are pretty inadequate at describing without being nonsensical. If you want physical clues, look at your picture below of the Bubble Chamber tracks. The reason you get those spirals is because the particles are spinning at the moment of collision (play with a top sometime and watch what happens when it loses spin) they aren't just extension-less mathematical points in a non physical math matrix.
    Once again, I enjoy your blog, but please, don't put down Einstein in relation to Bohr. Bohr was by far the greater phony and has done the most damage to actual scientific understanding.

  2. I say that Bohr won the debates because all of the QM textbooks teach the Copenhagen interpretation, and dismiss Einstein as a misguided skeptic.