How the century-long dispute over the true nature of light ended

The following is an excerpt from ours Lost in space-time bulletin. Each month we delve into fascinating ideas from around the universe. You can login Lost in space-time here.

When physicist Clinton Davisson received the Nobel Prize in 1937 for the discovery that electrons, which had been thought of as particles, could sometimes behave unexpectedly as waves, he decided to strike light. He said: “The perfect child of physics [had] he was turned into a gnome with two heads.” It was already known that it was neither, but both wave and particle. Physicists used to think that being a particle and being a wave were mutually exclusive, but in light and now electrons we had two examples that contradicted this. A somewhat confused Davisson couldn’t help but reach for a grotesque metaphor.

He was in good company – 10 years earlier, Albert Einstein had famously argued with Niels Bohr over this apparent absurdity. The two progenitors of quantum theory went at each other armed only gedankenexperimentsor thought experiments because they did not have the technology to implement them in the laboratory. But their dispute is no more. In the year 2025, the experiments that Einstein and Bohr wildly dreamed of were carried out in the laboratory, more than once. Light emerged with both heads intact.

The question of the true nature of light has always been controversial. In the 17th century, it divided two other great scientists. Mathematician Christiaan Huygens argued that light is a wave, while physicist Isaac Newton argued that it is a stream of particles. Huygens published his A treatise on light in 1690, near his death, but it was overshadowed by Newton’s arguments and reputation.

Light’s other head could only stay hidden for so long. In 1801, physicist Thomas Young devised the now-famous double-slit experiment to force light to reveal its true nature. What he did was the equivalent of shouting “I am a wave” to any physicist who would listen. For some time the field was bought. But in 1927, Einstein and Bohr again not only argued about the true nature of light, but also argued about the double-slit experiment itself.

In this experiment, a barrier with two narrow, parallel slits is placed in front of the aperture. What comes next is simple. Shine a light on the slits and then watch the screen. If light were a particle, the screen would show two spots of light, one behind each slit. But what Young and many physicists saw after it was more complex—a beautiful interference pattern that leaves dark and light streaks alternating across the screen. This is a characteristic of light waves. Light waves spill through the slits, and where they meet at their peaks, their brightness intensifies to form a bright streak. Pairing peak and trough leaves a dark band.

So, what were we arguing about a century later? First, Einstein held fast to the previous results of an experiment in which light was shone on a piece of gold, in which he explained its mysterious tendency to eject gold’s electrons by claiming that light is made of particles called photons. This experiment showed only one of the light heads and a different one than Young’s experiment – but Einstein kept looking for signs of the particle nature of light across experiments.

Quantum theory made this even more difficult because it claimed that an interference pattern would appear even if the double-slit experiment was performed with one photon at a time. Physicists tried to imagine how one photon could simultaneously pass through two slits. The details of the interference pattern eliminated the possibility of the photon somehow splitting in two, so it looked like the gnome had performed some kind of magic trick.

Bohr proposed that one way to deal with this is through the principle of complementarity. The wave and particle nature of the photon could be captured in experiments, but never simultaneously. Einstein didn’t have it. Enter gedankenexperiments.

In Einstein’s thought experiment, there is another slit that light passes through before the usual pair, and it is spring-loaded so that it bounces back when a photon passes through. He imagined that physicists could observe whether the springs compressed or lengthened after being hit by a photon, and then determine whether the photon passed through the upper or lower slit. That way, Einstein argued, they could learn which slit the photon had passed through, a very particle-like behavior, but still see a telltale wave pattern on the screen. He thought he had figured out a way to glimpse both photon heads.

Bohr’s counterargument was based on another classical feature of quantum theory – Heisenberg’s uncertainty principle. According to this principle, certain measurable properties of objects are in pairs, such as momentum and position – and there is a trade-off in the precision with which we can know it. For example, if researchers measure a particle’s momentum very precisely, their knowledge of its position will end up being very imprecise. In reality, the particle will appear as a fuzzy, spread out blob. Bohr argued that photon-slit interactions, even Einstein elastic ones, would change their momentum. Measuring the change the photon makes in the motion of the springs—the change in momentum of the slit—could be used to infer the change in momentum of the photon, which would blur its position and destroy the interference pattern, “washing out” its fringes.

Einstein and Bohr never agreed, but their debate became famous. “Every researcher in the field of quantum science has encountered this in one way or another,” he says Philipp Treutlein at the University of Basel in Switzerland. I called him after learning that two separate research teams had made this famous gedanken experiment a reality. The results of the experiments were beautiful, he says—they mimicked exactly what Bohr and Einstein had predicted.

But Treutlein also told me that contemporary physicists usually consider the debate already settled. Yet it took a hundred years before it was concretely tested in the laboratory. This is because photons are small and immaterial, so creating meaningful slits for the experiment required remarkable control of the tiny quantum components. Whatever you imagine when you read “narrow slit” is probably a quadrillion or more times too large to work in this experiment, he says Chao-Yang Lu at the University of Science and Technology of China (USTC). To prevent this, his team at USTC and others at the Massachusetts Institute of Technology (MIT) engineered their slits at extremely low temperatures, allowing individual atoms to be manipulated with laser beams and electromagnetic pulses to turn them into useful slits.

Both teams used two different designs to create their ultracold, flexible slits. And 21st century atomic physics has well-established tools for measuring how an atom is affected by a passing photon. Wolfgang Ketterle, who led the MIT team, liked to detect a slight breeze by looking at tree leaves. “In Einstein’s picture, a photon passes through a slit. Does the slit sense that the photon has passed? Does the slit rustle? We have now been able, with modern techniques, to prepare atoms in such a state that when a photon passes through the ‘slit,’ the atom rustles,” he says. Both teams found that Bohr predicted a trade-off between the sharpness of the interference pattern and how the photon affected the momentum of the atoms. The interference pattern would actually disappear exactly as he predicted.

So we can see that a photon behaves as a particle or as a wave in the same experiment. But thanks to advances in atomic physics, we can do even more: we can capture its dual nature in real time.

Both Ketterle and Lu told me that the most interesting findings came when they measured only a certain amount of information about the recoil of the atoms—just a faint rustle—and also observed a blurred interference pattern. Even partial information about the recoil meant they saw a photon doing something like a particle. Even a hint of an interference pattern similarly revealed its waviness. “Wave interference visibility and particle-like path resolvability are no longer mutually exclusive yes or no options,” says Lu.

As it turns out, you can actually see both heads of light – just not very well.

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