Famous Experiment Dooms Alternative to Quantum Weirdness
Oil droplets guided by “pilot waves” have failed to reproduce the results of the quantum double-slit experiment, crushing a century-old dream that there exists a single, concrete reality.
Yves Couder’s laboratory in 2005
French physicist Louis de Broglie
de Broglie refused to give up on a classical understanding of reality even as the outcomes of the first particle experiments suggested 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.
some physicists objected, Albert Einstein and de Broglie among them.
De Broglie insisted that everything at the quantum scale was perfectly normal and 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.
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.
The physicist Richard Feynman called the double-slit experiment “impossible, absolutely impossible, to explain in any classical way,” and said it “has in it the heart of quantum mechanics. In reality, it contains the only mystery.”
In the [double slit] experiment, particles are shot toward two slits in a barrier, and the ones that pass through the slits hit a sensor some distance away on the other side. Where any one particle ends up is always a surprise, but if you shoot many particles toward the slits, you start to see stripes develop in their detected locations, indicating places where they can and cannot go. The stripy pattern suggests that each particle is actually a wave that encounters the slitted barrier and passes through both slits at once, producing two wavefronts that converge and interfere, cresting in some places and canceling out in between. Each particle materializes in the sensor at the location of one of the crests of this strange probability wave.
Stranger still, when you add a second sensor and detect which slit each particle passes through, the interference stripes disappear, as if the probability wave, known as the wave function, has collapsed. This time, particles pass straight through their chosen slits to either of two spots on the far sensor.
To explain the double-slit experiment, a Copenhagenist will point to quantum uncertainty, arguing that the trajectory of each particle cannot be exactly known and is thus defined only probabilistically, by a wave function. After passing through both slits, as any wave would, and interfering on the other side, the wave function representing the particle’s possible locations is then “collapsed” by the sensor, which somehow selects a single reality from among the possibilities. Questions abound, both scientific and philosophical; Niels Bohr, who tended to answer questions with more questions, welcomed them.
To de Broglie, the double-slit experiment didn’t require an abstract, mysteriously collapsing wave function. Instead, he conceived of a real particle riding on a real pilot wave. The particle passes like driftwood through one slit or the other in the double-slit screen, even as the pilot wave passes through both. On the other side, the particle goes where the two wavefronts of the pilot wave constructively interfere and doesn’t go where they cancel out. De Broglie never actually derived dynamical equations to describe this complicated wave-particle-slit interplay. But with bouncing droplets in hand, Couder and a collaborator, Emmanuel Fort, moved quickly to perform the double-slit experiment, reporting their astonishing results in Physical Review Letters in 2006.
After recording the trajectories of 75 bouncing droplets through a double-slit barrier, Couder and Fort thought they detected rough stripes in the droplets’ final locations — an interference-like pattern that seemed as if it could only come from the pilot wave. Double-slit interference, considered “impossible to explain in any classical way,” was happening without mystery before everyone’s eyes. Drawn by the potential quantum implications, the fluid dynamicist John Bush started up a bouncing-droplet lab of his own at MIT and led others to the cause. Tomas Bohr heard Couder talk about his results in 2011 and later discussed the experiments at length with Bush. He teamed up with an experimentalist colleague, Anders Andersen, to study bouncing droplets further. “We really became fascinated with, in particular, the double-slit experiment,” Andersen said.
Attempts to reproduce these results - Droplets went through the slits in almost straight lines, and no stripes appeared. The French pair’s earlier mistake is now attributed to noise, faulty methodology and insufficient statistics.
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