Free Farewell to Reality by Jim Baggott Page B

polarizer … is nothing more than (e.g.) a calcite crystal, and nothing in our experience of physics indicates that the orientation of distant calcite crystals is either more or less likely to affect the outcome of an experiment than, say, the position of the keys in the experimenter’s pocket or the time shown by the clock on the wall. 15
    For whom the Bell tolls
    Physicists either accepted Bohr’s arguments or didn’t much care either way. Quantum theory was proving to be a very powerful structure, and any concerns about what it implied for our interpretation of reality were pushed to the back burner. The debate became less intense, although Einstein remained stubbornly unconvinced.
    But Irish theorist John Bell continued to feel uncomfortable. Any attempt to eliminate the spooky action-at-a-distance implied in the Einstein, Podolsky and Rosen thought experiment involved the introduction of so-called ‘hidden variables’. These are hypothetical properties of a quantum system that by definition are not accessible to experiment (that’s why they’re ‘hidden’) but which nevertheless govern those properties that we can measure. If, in the Einstein—Podolsky-Rosen experiment, hidden variables of some kind controlled the polarization states of the two photons such that they are fixed at the moment the photons are produced, then there would be no need toinvoke the collapse of the wavefunction. * There would be no instantaneous change, no spooky action-at-a-distance.
    Bell realized that if such hidden variables were assumed to exist, then in certain kinds of Einstein—Podolsky—Rosen-type experiments the hidden variable theory would predict results that disagreed with the predictions of quantum theory. It didn’t matter that we couldn’t be specific about precisely what these hidden variables were supposed to be. Assuming hidden variables of any kind means that the two photons are imagined to be locally real — they move apart as independent entities and continue as independent entities until one, the other or both are detected.
    Going back to our coin analogy, a hidden variables extension would have the properties of the two coins fixed at the moment they split apart and separate. The coins are assumed to be locally real.
    This seems perfectly reasonable, but quantum theory, in contrast, demands that the two photons or the two coins are non-local and entangled; they are described by a single wavefunction. They continue to be non-local and entangled until one, the other or both are detected, at which point the wavefunction collapses and the two photons or the two coins become localized, replete with the properties we measure.
    This is Bell’s theorem: ‘If the [hidden variable] extension is local it will not agree with quantum mechanics, and if it agrees with quantum mechanics it will not be local. This is what the theorem says.’ 16
    Bell was able to devise a relatively simple direct test. Hidden variable theories that establish some form of local reality predict experimental results that conform to something called Bell’s inequality. Quantum theory does not.
    Bell published his ideas in 1966. The timing was fortuitous. Sophisticated laser technology, optical instruments and sensitive detection devices were just becoming available. Within a few years the first practical experiments designed to test Bell’s inequality were being carried out.
    The most widely known of these experiments were performed by French physicist Alain Aspect and his colleagues in the early 1980s. These made use of two high-powered lasers to produce excited calcium atoms, formed in an atomic ‘beam’ by passing gaseous calcium from a high temperature oven through a tiny hole into a vacuum chamber. Calcium atoms excited in this way undergo a ‘cascade’ emission, producing two photons in quick succession. The physics of the atom demands that angular momentum