A quantum mirage in a pool of oil

An experiment with droplets of silicon oil raises tantalising questions about the foundations of quantum mechanics. Michael Lucy reports.

Over time, the chaotic motion of the droplet builds a coherent pattern of positions.
Over time, the chaotic motion of the droplet builds a coherent pattern of positions.
Saenz et al. / Nature Physics

Physicists have used droplets in a vibrating pool of silicon oil to replicate an effect known as a ‘quantum mirage’, in the latest in a series of experiments that create analogues of quantum-mechanical behaviour at a macroscopic level. The discoveries have gone hand in hand with a resurgence of interest in a century-old interpretation of quantum mechanics known as pilot wave theory.

Pedro Saenz and colleagues at MIT, writing in Nature Physics, believe their study represents “a significant advance”, not only because of the result but also some of the methods they used to reach it.

The pilot wave interpretation of quantum mechanics was first proposed by Louis de Broglie in 1923 to explain the famous wave-particle duality displayed by subatomic particles, in which they sometimes appear to behave as particles and sometimes as waves. He argued that if an electron created ripples in the electromagnetic field around itself, these ripples could in turn act as a “pilot wave” that would steer the particle’s movement in a way that would produce wavelike effects such as interference patterns.

Two droplets bouncing on a vibrating pool of silicon oil.
Two droplets bouncing on a vibrating pool of silicon oil.
Couder et al.

While the theory had no less than Albert Einstein in its corner, it was eventually discarded in favour of the Copenhagen interpretation of quantum mechanics, which holds that we can’t know what is “really” happening at the subatomic level. We can only use a mathematical construct called the wave function to predict the probability of finding a particle at a certain place and time.

Though unpopular, the pilot wave idea has occasionally been updated, notably by David Bohm in the 1950s – who refuted some earlier arguments against the idea – and more recently by Luis de la Pena and others, who have brought the theory into line with some modern developments in particle physics.

In 2005, French physicist Yves Couder and colleagues built a model pilot wave system in the lab. [LINK: https://www.nature.com/articles/437208a ] They discovered that if they took a shallow bath of silicon oil and shook it up and down at the right speed, they could add another droplet of oil to the surface and it would “walk” along it. When the droplet hits the surface of the bath, it bounces off, creating ripples. The ripples mean that the droplet, when it lands again, encounters a slightly tilted surface that pushes it off in one direction or another. In this way, the droplet moves over the surface of the oil, creating a “pilot wave” that directs its own future motion.

While the motion of the droplet appears chaotic in the short term, over time complex patterns appear that correspond to the probability densities that are the currency of quantum theory. This set-up has since been used to demonstrate analogues of quantum behaviour including the double-slit experiment, the orbits of electrons in atoms, and tunnelling through seemingly impenetrable barriers.

In the latest finding, Saenz and his fellow researchers created an analogue of an effect known as a “quantum mirage”, in which manipulating a single atom at a certain point in an electron trap can affect the behaviour of electrons elsewhere. They also used a much shallower bath of silicon oil than has been used in the past, which may allow finer control of the properties of the waves.

While pilot wave theory is not poised to displace the standard interpretation of quantum mechanics any time soon, the steady accumulation of experimental evidence – even if indirect and working by analogy – is worth keeping an eye on.

Michael Lucy is features editor of Cosmos.
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