“One can, damn it, not reduce the whole of philosophy to a screen with two holes."

That was Danish philosopher Jørgen Jørgensen’s exasperated retort in 1936, when confronted with the double-slit experiment and the questions it posed about the nature of quantum reality. The experiment, disarming in its simplicity and confounding in its implications, caused Richard Feynman to claim that it embodied the “central mystery” of the quantum world. A version of the experiment that laid bare this mystery was realized in the 1970s and 1980s, a tortuous path that started with serendipitous double vision, spider silk, and gold.

The double-slit experiment was first done in the early 1800s by English physicist and polymath Thomas Young. He used regular, classical, not quantum light—unsurprisingly, since the quantum world was yet to be discovered. The light of our everyday experience is classical. It can be mathematically described as a wave and it indeed behaves like one. Young’s experimental setup was sinfully simple. You can perform it at home with a little do-it-yourself chops: Shine light of one frequency (say, using a red laser pointer) at an opaque sheet with two fine openings or “slits.” The light can only pass through those slits. On a screen on the other side, you’d intuitively expect to see two strips of light right behind the slits. Instead, you see alternating bands of light and dark.

The light acts like a wave passing through the two slits, not unlike an ocean wave encountering a coastal breakwall with two openings. Waves emerge on the other sides of the slits, propagate, and interfere, creating bright bands where the crest of one wave lines up with the crest of the other (constructive interference), and dark bands where a crest coincides with a trough, canceling out the light (destructive interference). We get an interference pattern—one which can only be explained if something wavelike goes through both slits at once. A beam of red laser light does indeed act like a wave.

But quantum physics says that light, at its feeblest, comes in quanta, or particles, which are small, indivisible packets of energy called photons. Imagine dialing down the intensity of your laser pointer, such that it starts emitting one photon at a time (easier said than done; and you certainly can’t do this at home).

Now, imagine the single photons going through the two slits, one at a time and landing on a photographic plate on the other side. Our intuition screams: surely, the photons will go through one slit or the other, since each photon cannot be divided any further, and the photons over time will form two strips of light on the photographic plate?

Quantum theory says otherwise. It predicts that we will get the same interference pattern as before, only that it will form slowly over time, as most photons will land only in certain regions on the photographic plate, forming bright bands, and fewer or no photons strike the regions that make up the dark bands. The interference pattern builds up photon by photon. Crucially, this is possible only if each photon acts like a wave that goes through both slits. And that’s the mystery: something that is regarded as an indivisible particle somehow acts like a wave and goes through both openings at once.

Theorists understood this in the 1920s, but experimental verification remained a distant dream. Any such experiment would have to guarantee that there was only one photon going through the experimental setup at any moment. Even in the 1960s, when Feynman began lecturing about the experiment, no one had a clue how to do this experiment with single photons.

Feynman, however, talked up the same experiment done with single electrons, which are particles of matter. While some minds may—mistakenly—allow for photons of light to somehow magically go through two slits at once, they tend to recoil at the idea of an indivisible lump of matter doing the same. Surely, matter cannot indulge in such shenanigans. Well, again, quantum theory tells us otherwise: matter is no different. According to the theory, a fundamental particle is a quantum—the smallest unit of energy—of some field. A photon, for example, is a quantum of an electromagnetic field, a type of force-carrying field. An electron is a quantum of another type of field, broadly classified as a matter field. Quantum mechanics treats all fields and their quanta the same. That’s why even electrons fired one-by-one at a double slit should form an interference pattern on the far screen.

What Feynman didn’t know in the early 1960s is that German scientists had already begun assembling some of the basic paraphernalia needed for doing the double-slit experiment with electrons. It began with a chance observation. In the 1950s, Gottfried Möllenstedt at the University of Tübingen was looking through an electron microscope. There was a thin tungsten wire stretched across the microscope’s objective lens. Sometimes the microscope was creating two images, where there should be one. It turned out that two images formed when the tungsten wire was electrically charged. Möllenstedt figured that the charged wire was causing the electron beam to split into two, giving the microscope its double vision. The finding triggered a question: Could such a charged wire be used to split a fine beam of electrons and then recombined to create an interference pattern, à la the double-slit experiment? It’d be as if the electrons had gone through two slits and interfered on the other side.

Such an experiment required some pretty thin wire. Möllenstedt and his student Heinrich Düker had the brilliant idea of using gold-plated spider silk. Historian of science, Robert Crease, in The Prism and the Pendulum: The Ten Most Beautiful Experiments in Science, writes that Möllenstedt “kept a collection of spiders around the laboratory for this purpose.” After ostensibly experimenting with spider silk, Möllenstedt and Düker settled on gold-plated quartz wire (three microns in diameter, about 30 times thinner than human hair). They showed that electrons aimed at such a suitably-thin charged wire would split into two beams, which would recombine downstream, giving rise to an interference pattern. The duo had invented what’s called an electron biprism.

The interference pattern couldn’t be seen with the naked eye. Möllenstedt and Düker had to first image the electrons on a photographic plate, and then magnify the photograph under an optical microscope to see fine interference fringes. The electrons were demonstrating a duality: The indivisible particles of matter were behaving as waves, interfering and forming their own wave pattern.

Still, this wasn’t quite the double-slit experiment that Feynman envisaged. It showed that a beam of electrons was acting like a wave. Feynman was interested in the wave-like properties of a single electron. Such a quantum version of the experiment would require individual electrons going through the apparatus one at a time, and still form an interference pattern. Just like an electron would have the choice of going through one slit or the other, in the case of a biprism the electron would have the option of taking one path or the other around the thin wire. An interference pattern would imply that a single electron—an indivisible particle—was somehow acting like a wave and taking two paths at once.

It’d take two teams—an Italian team of Pier Giorgio Merli, GianFranco Missiroli, and Giulio Pozzi in Bologna, Italy, in 1974 and Akira Tonomura and colleagues at Hitachi in Japan in 1989—to demonstrate such a double-slit experiment with single electrons, using sophisticated versions of Möllenstedt and Düker’s electron biprism and more elaborate detectors. The Japanese team differentiated itself by claiming to show that the interference pattern formed even though there was indisputably only one electron traversing the equipment at a time.

The series of images shown above reveal the interference pattern forming over time. In the beginning (panels a and b), the dots appear seemingly at random, but as more electrons land on the plate, the dots clearly begin to accumulate in regions of constructive interference, while avoiding regions of destructive interference (panels c and d).

As Feynman said, the experiment encapsulates one of the central mysteries at the heart of quantum physics. At any instant, only a single electron approaches the charged wire. But something splits, takes both paths around the charged wire and eventually recombines. What is that something? In the formalism of quantum physics, that something is a mathematical entity called the wavefunction, which represents the electron’s quantum state. It’s this wavefunction that takes both paths and later interferes with itself. But is the wavefunction real? Or is it a figment of our mathematical machinery, a way to represent our knowledge—or lack thereof—of the quantum underworld?

Differing answers to that question can lead one to fundamentally different views of reality, from those indicating that reality—the electron, in this case—doesn’t exist until it’s measured to those suggesting a multitude of worlds in which every eventuality exists independent of measurement—in one world, the electron goes one way; in another, it goes the other way.

About a century since the invention of quantum physics, we are still in the dark about such questions. Despite Jørgen Jørgensen’s protestations, philosophical questions about the nature of reality can indeed be reduced to a screen with two holes. ♦