Double-slit experiment

Thomas Young's double-slit experiment did something that seemed impossible in 1801: it challenged Isaac Newton. For over a century, Newton's corpuscular theory of light—the idea that light consisted of tiny particles—had dominated physics. No one dared contradict Newton. But Young, a polymath who had already helped decipher Egyptian hieroglyphics, designed an experiment so elegant that the light itself would reveal its true nature.

The adjacent possible for the experiment required both the conceptual framework to ask the right question and the technical precision to observe the answer. Young understood wave phenomena from studying sound. He knew that when two waves overlapped, they could reinforce each other (constructive interference) or cancel each other out (destructive interference). If light were a wave, it should exhibit the same behavior.

The experimental setup was deceptively simple. Young allowed sunlight to pass through a small pinhole, creating a point source. This light then passed through two closely spaced slits onto a screen. If light were particles, the screen should show two bright bands—one behind each slit. If light were a wave, something stranger should happen.

What appeared on the screen was an interference pattern: alternating bands of bright and dark. Where waves from the two slits arrived in phase, they reinforced each other, creating brightness. Where they arrived out of phase, they canceled, creating darkness. The pattern was unmistakable evidence that light behaved as a wave, spreading out from each slit and interfering with itself.

Young's contemporaries were not impressed. In England, attacking Newton's theory was nearly heretical. The scientific establishment dismissed Young's work, and acceptance came only after French physicists Augustin Fresnel and François Arago independently confirmed and extended his findings decades later. Scientific revolutions sometimes require generational change.

The double-slit experiment would return to haunt physics in the 20th century. Einstein's 1905 explanation of the photoelectric effect showed that light also behaved as particles (photons). Quantum mechanics emerged from the realization that light—and eventually matter—exhibited both wave and particle properties depending on how it was observed. The double-slit experiment, performed with individual electrons and even molecules, became the defining demonstration of wave-particle duality: send particles through one at a time, and they still form an interference pattern, as if each particle interfered with itself.

Young's 1801 experiment remains one of the most elegant in physics. With nothing more than sunlight and two slits, it revealed a fundamental truth about the nature of reality—one that continues to challenge our intuitions about how the universe works.