How a century-long argument over light’s true nature came to an end

The following is an extract from our Lost in Space-Time newsletter. Each month, we dive into fascinating ideas from around the universe. You can sign up for Lost in Space-Time here.

When physicist Clinton Davisson was awarded the Nobel Prize in 1937 for revealing that electrons, once thought to be particles, could occasionally act like waves, he made a pointed remark about light. He stated, “the perfect child of physics [had] been changed into a gnome with two heads.” This highlighted the already established fact that light is neither solely a wave nor a particle, but both. Initially, physicists believed that wave and particle properties were mutually exclusive, yet light, and now electrons, demonstrated otherwise. Davisson’s bewilderment led him to employ a striking metaphor.

Davisson was not alone in his confusion. A decade earlier, Albert Einstein and Niels Bohr engaged in a renowned debate over this paradox. These pioneers of quantum theory clashed using only gedankenexperiments, or thought experiments, as the technology to test their ideas in a laboratory was unavailable. This debate has since been resolved. In 2025, the experiments envisioned by Einstein and Bohr were replicated in labs multiple times, reaffirming light’s dual nature.

The debate over the true nature of light has long been contentious. During the 17th century, it polarized two eminent scientists: Christiaan Huygens, who argued for light as a wave, and Isaac Newton, who contended it was a stream of particles. Huygens’ publication, Treatise on Light in 1690, was overshadowed by Newton’s influential stance.

Light’s wave-like aspect couldn’t remain hidden indefinitely. In 1801, physicist Thomas Young introduced the famous double-slit experiment, compelling light to reveal its nature. The results emphatically indicated its wave characteristics to physicists. Although this view dominated for a time, by 1927, Einstein and Bohr were again debating the essence of light, including the double-slit experiment itself.

The double-slit experiment involves placing a barrier with two narrow, parallel slits in front of a screen. When light shines on the slits, the expectation is simple: if light were a particle, the screen would display two bright spots. However, Young and subsequent physicists observed a complex interference pattern, characterized by alternating dark and light stripes across the screen. This pattern is unmistakably indicative of light’s wave properties. Light waves pass through the slits, and where they coincide at their peaks, brightness intensifies, forming bright stripes, while peaks meeting troughs create dark stripes.

So, what sparked a renewed debate a century later? Einstein was holding onto previous experimental results where light, when shined on gold, expelled electrons, leading him to propose that light consists of particles called photons. This experiment highlighted one aspect of light’s nature, distinct from Young’s findings, prompting Einstein to seek evidence of light’s particle characteristics across various experiments.

Quantum theory complicated matters, asserting that even a single photon in the double-slit experiment would produce an interference pattern. Physicists struggled with the concept of a single photon traversing both slits simultaneously. The interference pattern’s details ruled out the possibility of splitting, making it appear as though the “gnome” was performing a trick.

Bohr suggested the concept of complementarity to address this. Experiments could reveal both the wave and particle nature of a photon, but not at the same time. Einstein was unconvinced, leading to the use of gedankenexperiments.

In Einstein’s conceptual experiment, he envisioned an additional slit equipped with springs that would recoil when a photon passed through. He suggested that scientists could determine through which slit the photon traveled by observing the springs’ movement, thus exhibiting particle-like behavior, yet still witnessing the wave-like interference pattern on the screen. Einstein believed he had devised a way to observe both aspects of a photon simultaneously.

Bohr countered this idea by invoking the Heisenberg uncertainty principle, a core aspect of quantum theory. According to this principle, certain properties, like momentum and position, are linked, and measuring one precisely affects the accuracy of the other. Bohr argued that the interaction of a photon with the slit, even Einstein’s springy one, would alter their momenta. By measuring the effect on the springs, one could infer the photon’s momentum change, making its position uncertain and erasing the interference pattern.

Though Einstein and Bohr never resolved their disagreement, their debate remains a cornerstone in quantum science. “Every researcher in the field of quantum science has encountered it in one way or the other,” notes Philipp Treutlein from the University of Basel in Switzerland. He shared his thoughts after learning that two research teams succeeded in bringing this famous thought experiment to life. The experimental results closely mirrored what Bohr and Einstein had imagined.

Despite the consensus among modern physicists that the debate is settled, it took a century to test it in the lab. Photons are extremely small and massless, making it challenging to create suitable slits for the experiment, requiring extraordinary control over tiny quantum components. Chao-Yang Lu from the University of Science and Technology of China (USTC) suggests that anything imagined as a “narrow slit” is likely vastly oversized for this experiment. To overcome this, his team at USTC and another at the Massachusetts Institute of Technology (MIT) built their slits at extremely low temperatures, allowing them to control individual atoms with lasers and electromagnetic pulses, creating effective slit substitutes.

Both teams employed distinct approaches to develop their ultracold, spring-like slits. Modern atomic physics provides precise methods to assess an atom’s response to a passing photon. Wolfgang Ketterle, who led the MIT team, compared it to detecting a breeze by observing tree leaves. “In Einstein’s picture, the photon is going through a slit. Does the slit notice that a photon has gone through? Does the slit rustle? We were now able, with modern techniques, to prepare atoms in such a state that when a photon goes through the ‘slit’, the atom rustles,” he explains. Both teams confirmed Bohr’s predicted trade-off between the interference pattern’s clarity and the impact on the atom’s momentum by the photon. As Bohr had anticipated, the interference pattern indeed vanished.

We can, therefore, observe a photon demonstrating both wave and particle behavior in the same experiment. Advances in atomic physics now allow us to capture this dual nature in real time.

The most thrilling discoveries, according to Ketterle and Lu, emerged when they gathered limited recoil data from the atoms—just a faint rustle—and witnessed a vague interference pattern. Even partial information indicated the photon’s particle-like behavior. Similarly, any hint of an interference pattern revealed its wave properties. “The visibility of the wave-like interference and the distinguishability of the particle-like path are no longer mutually exclusive yes-or-no options,” says Lu.

Ultimately, you can perceive both aspects of light, though not with complete clarity.