In the 1920’s, after centuries of dispute over whether light is a wave or particles, Bohr devised the compromise that holds today, that they are complementary views, so both are true, and nothing better has been found since:
“…nobody has found anything else which is consistent yet, so when you refer to the Copenhagen interpretation of the mechanics what you really mean is quantum mechanics.” (Davies & Brown, 1999), p71.
This don’t ask, don’t tell policy let a photon be a wave when we don’t look, as long as it’s a particle when we do, so physics can apply particle or wave equations as convenient. In no pond do rippling waves behave like particles, and on no pool-table do billiard-balls behave like waves, but Bohr managed to sell the big lie [Note 1] that light is a wavicle. As Gell-Mann said in his 1976 Nobel Prize speech:
“Niels Bohr brainwashed a whole generation of physicists into believing that the problem (of the interpretation of quantum mechanics) had been solved fifty years ago.”
Quantum theory describes light as a wave that vibrates outside space, in a quantum world, as Maxwell’s equations specify, but how can the physical world support that? Figure 3.4 shows the possibilities. The first is physical realism, that only the physical world exists, so there is no quantum world with waves of light (Figure 3.4a). But if so, how do physicists who use quantum waves to predict what light does differ from witch doctors, who invoke imaginary spirits to heal people?
Bohr then argued that the quantum world is real for the purpose of physics, so physicists aren’t witch doctors. He proposed the dualism that the quantum world does exist for our equations, but otherwise it doesn’t, so in effect, the physical world and the quantum world co-exist somehow (Figure 3.4b). But that the quantum world exists for the convenience of physics was an admission of failure, not a theory advance (Audretsch, 2004), p14.
Bohr publicly accepted the quantum world but in private, denied that it existed at all. He wanted the best of both worlds, to use quantum theory but deny what it meant. Copenhagen dualism, like Descartes’s mind-body dualism before it, is a mystical marriage of convenience between incompatible domains, accepted because we want it to be true.
Figure 3.4c shows the quantum realism alternative to physical realism and Copenhagen dualism. It is that physical events are a subset of quantum events, because the quantum world generates the physical world within it, so classical mechanics is a subset of quantum mechanics. This then answers Wheeler question, how come the quantum?
Note: A “big lie” is a statement so outrageous that people think it must be right or it wouldn’t be said.