The 1801 Experiment
At the Royal Institution in London, physician and physicist Thomas Young conducted an experiment in 1801 that changed our understanding of reality. For over a century, Isaac Newton's theory that light consists of particles, or "corpuscles," was widely accepted. Young suspected light was actually a wave. To test this, he devised a simple setup. He used sunlight that passed through a single small hole in a screen, which then fell upon a second screen with two tiny, parallel slits placed very close together.
If light were made of particles, one would expect to see two bright lines on the final screen, corresponding directly to the two slits. Instead, Young observed a pattern of multiple bright and dark bands, or "fringes." This pattern was the result of interference. As the light waves passed through the two slits, they spread out and interacted. Where the crests of two waves met, they reinforced each other to create a bright band. Where a crest met a trough, they canceled each other out, creating a dark band. This phenomenon was only possible if light behaved like a wave, similar to ripples on the surface of a pond. Young presented his findings to the Royal Society in 1803, resurrecting the wave theory of light.
The Quantum Revolution
For nearly a century, Young's experiment settled the debate: light was a wave. The early 20th century, however, brought discoveries that complicated the picture. Work by Max Planck on black-body radiation and Albert Einstein's 1905 explanation of the photoelectric effect showed that light also behaves as if it is composed of discrete packets of energy, or particles, which were later named photons. This resurrected the particle theory and established the idea of wave-particle duality: light is somehow both a wave and a particle.
The double-slit experiment took on new importance. In 1924, Louis de Broglie proposed that matter, like electrons, should also exhibit wave-like properties. Experiments soon confirmed this. When the double-slit experiment is performed with electrons, even when sending them one at a time, an interference pattern still emerges. This implies that each individual electron passes through both slits simultaneously as a wave of probability. The very act of measuring or observing which slit an electron passes through causes this wave behavior to "collapse," and the interference pattern vanishes. This experiment demonstrates of the strange principles of quantum mechanics, showing that observation affects reality.