The Double-Slit Experiment and the Scientists Who Argued About It

Illustrations by Nathan Gelgud, inspired by Through Two Doors at Once by Anil Ananthaswamy

Anil Ananthaswamy has written a book about a single experiment. Take a sheet of paper and cut two slits in it. Put a solid sheet of paper behind it. Shine a light through the paper with the slits on it. In what way will the light be distributed on the solid sheet?

Illustration by Nathan Gelgud, inspired by Through Two Doors at Once by Anil Ananthaswamy

As we learn in Through Two Doors at Once, it turns out that this experiment, which could hardly be more simple, produces questions which could hardly be more complex. Devised in slightly different form by Thomas Young around the turn of the 19th century, the double slit experiment was supposed to have proved Isaac Newton wrong. Newton held that the substance that makes up light is composed of tiny particles, like sand. But if that were the case, the solid sheet would have a distribution of light that roughly matched the shape of the slits on the sheet in front of it.

Instead, we see what are called optical interference fringes, which should prove that light is more like a wave, overlapping and recomposing as it goes around barriers. But a century later, Albert Einstein wasn’t so sure. Light had to be made of photons, because of the things we could do with it, such as converting it into electricity. And photons are like particles. In other words, maybe light was like sand. Or, as Einstein tried to convince Niels Bohr, maybe light could be both like sand and like a wave. Or, as Richard Feynman would say a few decades later, maybe light is just plain wacky.

Ananthaswamy does a better job than I ever could with the intricacies and inherent contradictions of the observations and findings of scientists throughout the centuries since the double slit experiment was first devised. The questions, as he puts it, pile up. Apparently, if we don’t observe a photon it behaves like a wave, and if we do, it behaves like a particle of sand. This, it would seem, challenges what we think we know about reality. Light couldn’t possibly know we are looking at it, right? In Through Two Doors at Once, Ananthaswamy helps us try to understand these profound questions of nature and consciousness.