The great debate

Bohr answered that the first screen is also an object to which the uncertainty principle applies. To read the path from its recoil, you have to know its momentum very precisely. Then its position is uncertain – by just about the spacing between two stripes. The bright and dark stripes of many electrons then fall on top of each other, and the pattern becomes evenly grey. Path or stripes: you cannot have both at once.

A historical footnote: we know this exchange mainly from Bohr's account of 1949, which he wrote from memory. In the printed proceedings of the 1927 conference, Einstein's objection takes a different form – a single slit and a hemispherical screen. There he asked how it is that the particle arrives at only one point when its wave spreads over the whole screen, an early form of his worry about action at a distance.

Léon Rosenfeld, who was there, later reported that the objection came as a real shock to Bohr, who was very agitated that evening and only found the answer the next morning. It lies in the balance. To read the weight precisely, you have to know the position of the box on a scale precisely. The more precisely you know the position, the more uncertain its momentum – and with it how precisely the weight can be determined in the time available. And because the box hangs in the Earth's gravitational field, a discovery Einstein himself had derived from his equivalence principle (1907/1911) applies: clocks at different heights run at different rates. An uncertainty in the height of the box therefore becomes an uncertainty in the reading of its clock. Put the two together and you get exactly the energy–time uncertainty.

After 1930 Einstein no longer attacked the internal consistency of quantum mechanics. His new question was: is it complete?

Bohr replied the same year, in the same journal and under the same title. He disputed the phrase “without in any way disturbing”. The second particle is not disturbed mechanically, he argued, but the choice of measurement on the first determines which kinds of prediction about the second are possible at all. Position and momentum require mutually exclusive measuring arrangements, so you cannot treat both values as reality that is simply there. Many physicists found Bohr's answer hard to follow, and the two never convinced each other. For almost thirty years the question seemed to be one of philosophy – until John Bell turned it into a measurement.

In 1982 Aspect's group in Orsay measured S = 2.697 ± 0.015; for that setup, quantum mechanics had predicted 2.70 ± 0.05. Later experiments closed, one after another, the loopholes that sceptics still saw. In 2015 tests in Delft, Vienna and at NIST in Boulder closed all the important ones at the same time. In 2022 Alain Aspect, John F. Clauser and Anton Zeilinger received the Nobel Prize in Physics “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”.