Thursday, September 20, 2018

Mermin defends Copenhagen Interpretation

N. David Mermin has written many popular essays explaining quantum mechanics, and now he summarizes his views on how to interpret the theory in Making better sense of quantum mechanics.

He prefers something called QBism, but nearly everything he says could be considered a defense of the Copenhagen interpretation.
Much of the ambiguity and confusion at the foundations of quantum mechanics stems from an almost universal refusal to recognize that individual personal experience is at the foundation of the story each of us tells about the world. Orthodox ("Copenhagen") thinking about quantum foundations overlooks this central role of private personal experience, seeking to replace it by impersonal features of a common "classical" external world.
He is drawing a fairly trivial distinction between his QBism view and Copenhagen. He illustrates with this famous (but possibly paraphrased) Bohr quote::
When asked whether the algorithm of quantum mechanics could be considered as somehow mirroring an underlying quantum world, Bohr would answer "There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature."
Mermin's only quibble with this is that he prefers "each of us can say" to "we can say". That is, he doesn't like the way Bohr lumps together everyone's observations and calls it the classical world.

Okay, I guess that distinction makes a difference when discussing Wigner's Friend, a thought experiment where one observer watches another. But for the most part, Mermin likes the Copenhagen interpretation, and successfully rebuts those who say that the interpretation is deficient somehow.


  1. "There is only an abstract quantum physical description."
    This is basically an admission that QM as presently formulated is not about describing 'reality', it is about using a mathematical 'abstraction' that gives you useful answers with little understanding of how.

    This can be also seen in the following sentence:

    "It is wrong to think that the task of physics is to find out how nature is."

    "Physics concerns what we can say about nature."
    You could apply this attitude towards epicycles. Hey, they have nothing to do with actual planets or how they move, but what the hell, the answers are useful. So much for reality.

    1. Physics has gone nowhere. It's in the last of its funding hay days. Boring answers and even more boring applications.

    2. Quantum mechanics is the least successful theory of all time: "the only exact solutions to the Schrödinger equation found so far are for free-particle motion, the particle in a box, the hydrogen atom, hydrogen-like ions, the hydrogen molecular ion, the rigid rotator, the harmonic oscillator, Morse and modified Morse oscillators, and a few other systems [2,3]. For more complicated systems, however, approximation techniques have to be used (such as the variational method or perturbation theory), which sometimes give poor results compared with experimental ones, and practical calculations with them are usually very difficult, even with the use of powerful computers [4]. The difficulty is that in a system made of N interacting particles (where N can be anywhere from three to infinity), the repeated interactions between particles create quantum correlations. As a consequence, the dimension of the Hilbert space describing the system scales exponentially in N. This makes a direct numerical calculation of the Schrödinger’s equation intractable: Every time an extra particle is added to the system, the computational resources would have to be doubled [5]."

  2. Found another fantastic skeptic quote, I'm fond of collecting them as they breath fresh sanity into a mentally constipated world:

    "There is no more common error than to assume that because prolonged and accurate mathematical calculations have been made, the application of the result to some fact of nature is absolutely certain."

    A.N. Whitehead