How Alain Aspect’s experiment settled the Einstein-Bohr debate: episode 4/4 of the Quantum Physics podcast: the groundbreaking work of Alain Aspect, Nobel Prize in Physics, explained
In May 1935, Einstein, Podolsky and Rosen published in the Physical Review a crucial article. They claim to demonstrate that quantum physics hides things from us. That it does not tell everything about the physical world, so it is incomplete. As soon as he learns of it, Niels Bohr decides to take up arms to defend it. He immediately understands that the refutation of Einstein’s subtle reasoning will not be trivial. So, for six weeks, days and nights, he devotes himself to this single task, as if caught in a sort of trance.
“Clearly, as long as a speed camera at the edge of the highway has not flashed you, the speed of your car is a quantity that is not determined”
He summarizes his arguments in an article addressed to the same Physical Review July 13, 1935, article which bears exactly the same title as the EPR article: “Can the quantum description of physical reality be considered complete? “. The answer he gives is an unequivocal yes, but his argument is muddled. Having failed to detect the slightest error in Einstein’s reasoning, Bohr is reduced to criticizing his criterion of reality: in certain situations, he explains, one must refrain from granting physical attributes to objects. “when we deal with phenomena for which we cannot make a sharp distinction between the behavior of these objects and their interaction with the measuring instruments (1)”. The speed of a particle, for example, would not be a specific attribute of the particle, but a property that it would share with the instrument that measures it… didn’t flash you, the speed of your car is an undetermined quantity.
Einstein, on the other hand, has a diametrically opposed point of view, based on his belief that the physical world possesses an existence independent of measurement: “What we call sciencehe wrote, is for the sole purpose of determining what is.” In short, whether there are speed cameras or not, it doesn’t matter.
At the time, these debates hardly fascinated physicists, because they had no impact on the way they worked or made calculations. But one day in 1964, an event happens that will arouse their interest. John Bell, an Irish physicist, demonstrates in an extraordinary article that it is possible to envisage experiments for which quantum physics as it is gives predictions different from any theory claiming to complete it by adding what these are called “local hidden variables”. Clearly, the debate between Bohr and Einstein, initially purely philosophical, could be settled in the laboratory. In fact, in the early 1980s, Alain Aspect, Nobel Prize 2022, and his team carried out superb experiments with photons at the Orsay Optical Institute (…)
Bibliographic source
(1) Niels Bohr, “Discussion with Einstein on Epistemological Problems in Atomic Physics“, in Paul Schilpp, Albert Einstein: Philosopher-Scientist, New York, MJF Books, p. 234.
