Everything acts like a wave while it propagates, but behaves like a particle whenever it interacts. The origins of this duality go way back.
One of the most powerful, yet counterintuitive, ideas in all of physics is wave-particle duality. It states that whenever a quantum propagates through space freely, without being observed-and-measured, it exhibits wave-like behavior, doing things like diffracting and interfering not only with other quanta, but with itself. However, whenever that very same quantum is observed-and-measured, or compelled to interact with another quantum in a fashion that reveals its quantum state, it loses its wave-like characteristics and instead behaves like a particle. First discovered in the early 20th century in experiments involving light, it’s now known to apply to all quanta, including electrons and even composite particles such as atomic nuclei.
But the story of how we discovered wave-particle duality doesn’t begin and end in the early 20th century, but rather goes back hundreds of years: to the time of Isaac Newton. It all began with an argument over the nature of light, one that went unresolved (despite both sides declaring “victory” at various times) until we came to understand the bizarre quantum nature of reality. While wave-particle duality owes its origin to the quantum nature of the Universe, the human story of how we revealed it was full of important steps and missteps, driven at all times by the only source of information that matters: experiments and direct observations. Here’s how we finally arrived at our modern picture of reality.
Huygens: light is a wave
Picture a wave propagating through water, such as in the ocean: it appears to move linearly, at a particular speed and with a particular height, only to change and crash against the shore as the water’s depth lessens. Back in 1678, Dutch scientist Christiaan Huygens recognized that these waves could be treated — rather than as linear, coherent entities — as a sum of an infinite number of spherical waves, where each…
