Wave-particle duality: How photons and electrons challenge our understanding of reality

🔬 Two centuries of confusion

In the late 17th century, two giants of physics fundamentally disagreed. Isaac Newton argued that light consists of tiny particles — “corpuscles” as he called them. Christiaan Huygens countered that it was a wave, similar to ripples on the surface of water. For over a century the corpuscular theory dominated, mainly due to Newton's prestige.

In 1801, British physicist Thomas Young presented to the Royal Society an experiment that changed everything. He passed light through two narrow slits and observed on the screen behind them a pattern of alternating bright and dark bands — an interference pattern, a characteristic signature of waves. If light were simply particles, it would form two bright lines, one behind each slit. Instead, the light waves interacted: at some points they reinforced each other (constructive interference) and at others they cancelled out (destructive interference). The wave nature of light now seemed indisputable.

💡 Light as a particle again

The wave theory reigned virtually unchallenged for a century, until a series of experiments overturned certainties once more. In 1900, Max Planck proposed that radiation energy is transferred in discrete “packets” — quanta. In 1905, Albert Einstein explained the photoelectric effect by showing that light behaves as though it consists of particles (photons) with energy E = hν, where h is Planck's constant and ν the frequency. Between 1922 and 1924, Arthur Compton proved that photons also carry momentum, by scattering X-rays off electrons — exactly as particles would in a collision.

An impasse thus emerged: the same thing — light — behaved as a wave in Young's experiment and as a particle in the photoelectric effect. Niels Bohr named this dual nature "complementarity": wave and particle do not contradict each other, but reveal different facets of the same reality depending on the measurement.

🏆 De Broglie's bold proposal

In 1924, 31-year-old French aristocrat Louis de Broglie submitted a doctoral thesis that shocked the academic community. If light, which was considered a wave, could behave like a particle, then why couldn't electrons — which were considered particles — behave like waves? De Broglie proposed that every particle with momentum p is associated with a wavelength:

λ = h / p

The examination committee, unsure whether they were dealing with genius or nonsense, asked Einstein. He replied that de Broglie's idea “lifts a corner of the veil.” The thesis was approved and de Broglie was awarded the Nobel Prize in Physics in 1929.

⚛️ Electrons passing through two slits

Experimental confirmation came with striking speed. In 1927, Clinton Davisson and Lester Germer at Bell Labs fired electrons at a nickel crystal and observed diffraction patterns — exactly as X-ray waves do. That same year, George Paget Thomson (son of J.J. Thomson who had discovered the electron as a particle!) passed electrons through thin metal films and saw concentric diffraction rings. The father proved electrons are particles. The son proved they are waves. Both won Nobel Prizes.

The clearest demonstration came in 1961, when Claus Jönsson at the University of Tübingen performed the first double-slit experiment with electrons. The electrons, one by one, hit the screen at random points — like particles. But after thousands of hits, an interference pattern gradually appeared — like waves. Each electron seemed to “know” that two slits existed, even though only one passed through at a time. In 2002, readers of Physics World voted this “the most beautiful experiment in physics.”

👁️ The paradox of observation

Here the story becomes truly strange. If we place detectors next to the slits to see which one each electron passes through, the interference pattern vanishes immediately. The electrons now form two simple bands, like classical particles. The very act of measurement changes their behavior.

This is not a matter of technology or clumsy experimentation. Richard Feynman, who devoted an entire chapter of his Lectures on Physics to the experiment, considered it foundational:

The “which-path” information destroys the coherence of the quantum wave. Without information — wave behavior. With information — particle behavior.

Even more striking: in John Archibald Wheeler's “delayed-choice” experiments, the decision to measure or not can be made after the particle has already passed through the slits — and its behavior “retroactively” changes. In “quantum eraser” experiments, if the “which-path” information is erased, the interference pattern reappears.

🧪 From molecules to fullerenes

Wave-particle duality does not concern only photons and electrons. Neutrons passed through a double slit in 1988. Helium atoms in 1991. And in 1999, Anton Zeilinger's team in Vienna achieved something remarkable: they passed entire fullerene molecules (C₆₀) — balls of 60 carbon atoms, 0.7 nanometers in diameter — through a diffraction grating. The de Broglie wavelength was just 2.5 picometers, but the interference pattern appeared. Molecules large enough to be seen under an electron microscope were behaving as waves.

In 2019, the same team broke every record: molecules weighing 25,000 atomic mass units, consisting of 2,000 atoms, exhibited wave behavior. And in February 2026, Markus Arndt's team at the University of Vienna pushed even further: sodium nanoparticle clusters weighing over 170,000 atomic mass units — heavier than some viroids and proteins — displayed quantum interference in a Talbot-Lau experiment, achieving a macroscopicity of 15.5, an order of magnitude higher than any previous measurement. Where does quantum nature end? No one knows yet. The search for the boundary between the quantum and classical worlds remains one of the greatest open questions in physics.

🌀 Quantum reality

What does all this ultimately mean? That photons and electrons are neither waves nor particles — they are something fundamentally different, something with no counterpart in the world we see. We call them “quantum objects” because no word exists for them. They behave as waves when we don't observe them and as particles when we measure them. Their wave function — Schrödinger's mathematical description — is not a wave of matter but a wave of probability: the square of its modulus gives the probability of finding the particle at a given position.

Wave-particle duality is not a strange side effect of some theory. It is the theory. Every quantum computer, every laser, every semiconductor in your phone relies precisely on this dual nature. The world at its most fundamental scale does not operate by the rules we intuit — it operates by the rules of quantum mechanics, in which one thing can truly be two at the same time.