The double-slit experiment demonstrates the quantum nature of reality

RUSSELL KIGHTLEY/SCIENCE PHOTO LIBRARY

A thought experiment that was at the heart of an argument between famed physicists Albert Einstein and Niels Bohr in 1927 has finally been made real. Its findings elucidate one of the core mysteries of quantum physics: is light really a wave, a particle or a complex mixture of the two?

Einstein and Bohr’s argument concerns the double-slit experiment, which dates back another hundred years to physicist Thomas Young in 1801. Young used this test to argue that light is a wave, while Einstein posited that light is actually a particle. Meanwhile, Bohr’s work in quantum physics boldly proposed that it can, in a sense, be both. Einstein didn’t like this controversial idea and imagined a modified version of Young’s experiment to counter it.

Now, Chao-Yang Lu at the University of Science and Technology of China and his colleagues have performed an experiment that realises Einstein’s idea, using the best tools of modern experimental physics to reveal that quantum objects are as peculiar in their dual wave-and-particle nature as 1920s physicists suspected. “Seeing quantum mechanics ‘in action’ at this fundamental level is simply breathtaking,” says Lu.

In the classic double-slit experiment, researchers shine light onto a pair of narrow, parallel, horizontally oriented slits positioned in front of a screen. If light were a particle, the screen ought to show a blob of light behind each slit, but Young and countless researchers that followed instead saw an “interference pattern” of alternating dark and light stripes. This indicated that light is more like a wave that spills through the slits, with the screen capturing its ripples clashing into each other. Remarkably, the interference pattern persists even when the light’s intensity is reduced to a single particle of light, or a photon. Does this mean that the perfectly particle-like photon somehow interferes with itself as if it were also a wave?

Bohr argued for the notion of “complementarity” where it is impossible to see the photon’s particle-ness when it is exhibiting wavy behaviour and vice versa. In their debates on whether this truly holds, Einstein imagined placing an additional slit before the usual pair that would be equipped with springs, so it could recoil when the photon entered it. Based on the springs’ motion, physicists could then determine whether the photon went through the top or bottom slit. According to Einstein, this would mean being able to simultaneously describe the photon’s particle behaviour – travelling through a specific slit like a tiny ball would – and its wave behaviour as evidenced by the interference pattern, which would contradict complementarity.