In 2005, a student working in the fluid physicist Yves Couder’s laboratory in Paris discovered by chance that tiny oil droplets bounced when plopped onto the surface of a vibrating oil bath. Moreover, as the droplets bounced, they started to bunny-hop around the liquid’s surface. Couder soon figured out that the droplets were “surfing on their own wave,” as he put it — kicking up the wave as they bounced and then getting propelled around by the slanted contours of the wave.
As he watched the surfing droplets, Couder realized that they exactly embodied an early, largely forgotten vision of the quantum world devised by the French physicist Louis de Broglie.
A century ago, de Broglie refused to give up on a classical understanding of reality even as the unsettling outcomes of the first particle experiments suggested to most physicists that reality, at the quantum scale, is not as it seems. The standard “Copenhagen interpretation” of quantum mechanics, originated at that time by the Danish physicist Niels Bohr, broke with the past by declaring that nothing at the quantum scale is “real” until it is observed. Facts on the ground, like particles’ locations, are mere matters of chance, defined by a spread-out probability wave, until the moment of measurement, when the wave mysteriously collapses to a point, the particle hops to, and a single reality sets in. In the 1920s, Bohr persuaded most of his contemporaries to embrace the weirdness of a probabilistic universe, the inherent fuzziness of nature, and the puzzling wave-particle duality of all things.
But some physicists objected, Albert Einstein and de Broglie among them. Einstein doubted that God “plays dice.” De Broglie insisted that everything at the quantum scale was perfectly normal and above-board. He devised a version of quantum theory that treated both the wave and the particle aspects of light, electrons and everything else as entirely tangible. His “pilot-wave” theory envisioned concrete particles, always with definite locations, that are guided through space by real pilot waves — much like the waves propelling Couder’s bouncing droplets.
De Broglie couldn’t nail down the physical nature of the pilot wave, however, and he struggled to extend his description to more than one particle. At the celebrated 1927 Solvay Conference, a gathering of luminaries to debate the meaning of quantum mechanics, Bohr’s more radical views carried the day.
De Broglie’s pilot-wave vision of the quantum world was little remembered 78 years later, when the Paris droplets started bouncing. Suddenly, Couder and his colleagues had an “analogue system” for experimentally exploring de Broglie’s idea.
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