Classically quantum

Published in Physics World, 7 Oct 2013

They may look like simple oil drops, but they evoke many of the strange features of quantum theory. Coincidence? Jon Cartwright investigates

Waves can be particles, and particles can be waves. Scientists have struggled to come to terms with this strange feature of the quantum world ever since the French physicist Louis de Broglie first described wave–particle duality in 1926. Are entities waves and particles at the same time – “wavicles” maybe? Or do they switch somehow, depending on the situation? The nature of wave–particle duality seems impossible to understand, because no-one has ever observed something being both a particle and a wave in the everyday?world.

At least, that’s what physicists thought. In 2005 Yves Couder and Emmanuel Fort at Paris Diderot University discovered an odd phenomenon. If they placed an oil bath on a vertical vibrator, any oil droplets released onto the surface would not coalesce with the rest of the fluid, as you would expect. Instead the droplets would bounce up and down – their impact cushioned by a pocket of air – while generating circular standing waves.

Adjusting the amplitude of the vibrations, Couder and Fort noticed something even stranger happening. The droplets began to fall onto the wave crests in such a way that they were propelled across the surface. They bounced off the sides of the bath and one another, but always at a distance and never coming into direct contact. It was as though the waves were guiding the droplets to perform an elegant dance – to flit past and spin around one another, but never to collide.

These wave–droplets – or “walkers”, as the researchers began to call them – appeared to be the first macroscopic example of wave–particle duality. The waves could not exist without the droplets, nor could the droplets move without the waves. When a droplet eventually sunk, its corresponding wave would vanish; similarly, if a wave was damped, the droplet would stop moving around the oil bath.

Eight years on, Couder and Fort have discovered more and more ways in which their walkers can reproduce phenomena previously considered unique to quantum mechanics – from quantized orbits, to single-particle interference in a Young’s double-slit experiment. The similarities are so marked that many researchers have begun to question whether there could be more to it than mere coincidence. Could a simple, classical experiment reveal something about how the quantum world ticks, and even lead to a deeper theory?

“I find these results really, really fascinating,” says Aephraim Steinberg, an experimental quantum physicist at the University of Toronto in Canada. “And I think many people should find them striking.” […]

The rest of this article is available here.