How a century-long argument over the true nature of light ended

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When physicist Clinton Davison won the Nobel Prize in 1937 for the discovery that electrons, which were thought to be particles, could sometimes behave unexpectedly like waves, he thought of taking a pinch on light. “The perfect child of physics had been turned into a two-headed dwarf,” he said. It was already known that it was not one or the other, but both wave-like and particle-like. Physicists used to think that being a particle and being a wave were mutually exclusive, yet we have, in light and now even in electrons, two examples that refute this. Somewhat bewildered, Davison could not help but reach for a strange metaphor.

He was in good company – 10 years earlier, Albert Einstein had had a famous debate with Niels Bohr over this absurdity. The two progenitors of quantum theory attacked each other with weapons gedankenexperimentsOr thought experiments, because they did not have the technology to realize them in the laboratory. But now their fight is not over. In 2025, the experiments that Einstein and Bohr dreamed up have been carried out in the laboratory, and more than once. Prakash 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 was a wave, while physicist Isaac Newton claimed it was a stream of particles. Huygens published his treatise on light in 1690, near his death, but it was overshadowed by Newton’s arguments and reputation.

Prakash’s other head could remain hidden only for so long. In 1801, physicist Thomas Young devised the now famous double-slit experiment, attempting to force light to reveal its true nature. What it did was the equivalent of shouting “I am a wave” at any physicist who wanted to listen. For some time, shopping took place in the area. But by 1927, Einstein and Bohr were arguing again not only about the true nature of light, but also about the double-slit experiment.

In this experiment, a barrier with two narrow, parallel slits is placed in front of a screen. What comes next is simple. Shine light on the slits, then look at 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 after him saw was more complex – a beautiful interference pattern, leaving alternating dark and light streaks across the screen. This is the identity of the wave power of light. Light waves propagate through the cracks and where they meet at their peak, their brightness increases, creating a bright streak. The pair of peak and trough leaves a dark streak.

So, what was there to argue about a century later? For one, Einstein was holding fast to the previous results of an experiment where light was shone on a piece of gold, in which he explained its mysterious tendency to eject electrons from the gold by assuming that light was made up of particles called photons. This experiment showed only a single head of light, and a different one from Young’s experiment – ​​but Einstein continued to look for signs of light’s particle-ness in all experiments.

Quantum theory made this even more difficult because it claimed that interference patterns would still be visible if the double-slit experiment was performed with one photon at a time. Physicists struggled to imagine how a photon could propagate through two slits simultaneously. The description of the interference pattern eliminated the possibility of the photon somehow splitting into two, making it seem as if the gnome was performing a magic trick.

Bohr suggested that one way to deal with this was the principle of complementarity. Both the wave and particle nature of the photon can be coaxed into being observed in experiments, but never together. Einstein didn’t have it. Enter gedankenexperiments.

In Einstein’s thought experiment, there is an additional slit for light to pass through before the normal pair, and it is equipped with springs so that it is retracted when a photon crosses it. He envisioned that physicists could observe whether the springs compressed or expanded after being hit by a photon and consequently determine whether the photon passed through the top or bottom slit. This way, Einstein argued, they could know which slit the photon had passed through, which is very particle-like behavior, but they would still see the telltale wave-like pattern on the screen. He thought he had discovered a way to glimpse both ends of the photon.

Bohr’s counterargument relied on another classic feature of quantum theory – the Heisenberg uncertainty principle. According to this theory, certain measurable properties of objects come in pairs, such as speed and position – and there is a trade-off in the accuracy with which we can know. For example, if researchers measure the speed of a particle very precisely, their knowledge of its position will be very inaccurate. Effectively, the particle will appear like a blurry, spread out blob. Bohr argued that the interaction of photons and slits, even Einstein’s springy ones, would change their momentum. Measuring the change made by the photon in the speed of the springs – the change in speed of the slit – can be used to estimate the change in speed of the photon and this will obscure its position and destroy the interference pattern, “washing out” its stripes.

Einstein and Bohr never reached any agreement, but their debate became famous. “Every researcher in the field of quantum science has faced this in some way or the other,” says philip trautlein At the University of Basel in Switzerland. After learning that two different research teams had actually created this famous Gedankenexperiment, I called them. He says the results of the experiments were beautiful – they mimicked Bohr and Einstein’s hypothesis very closely.

But Treutlein also told me that contemporary physicists generally consider the debate already settled. Still, it took a hundred years for it to be concretely tested in the laboratory. This is because photons are small and massless, so significant control of small quantum components is required to create meaningful slits for the experiment. Everything you can imagine when you read “narrow slit” is probably a quadrillion or so times too big to work with in this experiment, say. Chao-Yang Lu At the University of Science and Technology of China (USTC). To prevent this, his team at USTC and another team at the Massachusetts Institute of Technology (MIT) fabricated their slits under extremely cold temperatures, making it possible to control individual atoms with laser beams and electromagnetic pulses, turning them into useful slit stand-ins.

Both teams used two different designs to build their ultracold, springy slits. And in 21st century nuclear physics there are well-established tools for measuring how an atom is affected by a passing photon. Wolfgang Ketterle, who led the MIT team, compared it to detecting a slight breeze by looking at tree leaves. “In Einstein’s picture, the photon is passing through a slit. Does the slit indicate that a photon has passed? Does the slit rustle? Now we were able, with modern techniques, to prepare the atoms in such a state that when a photon passes through the ‘slit’, the atom rustles,” he says. Both teams found the trade-off Bohr predicted between the sharpness of the interference pattern and how the photons affected the motion of the atoms. In fact, the interference pattern will disappear just as he predicted.

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

Both Ketterley and Lu told me that the most exciting findings came when they measured only a small amount of the atoms’ repetition information – only a faint rustling – and also saw a faint interference pattern. Even the partial recoil information meant that they were looking at the photon doing something like a particle. Even a signal interference pattern similarly revealed its waviness. “The visibility of wave-like interference and the specificity of particle-like paths 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.