Wave-Particle Duality

At first, physicists were reluctant to accept the dual nature of light. After all, many of us humans like to have one right answer. But Einstein paved the way in 1905 by embracing wave-particle duality. We've already discussed the photoelectric effect, which led Einstein to describe light as a photon. Later that year, however, he added a twist to the story in a paper introducing special relativity. In this paper, Einstein treated light as a continuous field of waves -- an apparent contradiction to his description of light as a stream of particles. Yet that was part of his genius. He willingly accepted the strange nature of light and chose whichever attribute best addressed the problem he was trying to solve.

Today, physicists accept the dual nature of light. In this modern view, they define light as a collection of one or more photons propagating through space as electromagnetic waves. This definition, which combines light's wave and particle nature, makes it possible to rethink Thomas Young's double-slit experiment in this way: Light travels away from a source as an electromagnetic wave. When it encounters the slits, it passes through and divides into two wave fronts. These wave fronts overlap and approach the screen. At the moment of impact, however, the entire wave field disappears and a photon appears. Quantum physicists often describe this by saying the spread-out wave "collapses" into a small point.

Similarly, photons make it possible for us to see the world around us. In total darkness, our eyes are actually able to sense single photons, but generally what we see in our daily lives comes to us in the form of zillions of photons produced by light sources and reflected off objects. If you look around you right now, there is probably a light source in the room producing photons, and objects in the room that reflect those photons. Your eyes absorb some of the photons flowing through the room, and that's how you see.

But wait. What makes a light source produce photons? We'll get to that. Next.