are more readily understood if one assumes that the energy of light is discontinuously distributed in space. In accordance with the assumption to be considered here, the energy of a light ray spreading out from a point source is not continuously distributed over an increasing space but consists of a finite number of energy quanta which are localized at points in space, which move without dividing, and which can only be produced and absorbed as complete units.
Albert Einstein never won a Nobel prize for the theory of relativity—in fact, it was only through long, political jockeying within the Nobel committee that he won the prize at all. Instead, when he was given the 1921 Nobel Prize in Physics (in 1922, after a long bout of internal Nobel hand-wringing), he received it primarily for his explanation of the photoelectric effect. Extraordinarily enough, he came up with both his relativity theory, and the photoelectric effect in the same year: 1905.
At the turn of the century, physicists already knew that, in some circumstances, exposing certain materials to light could create an electric current. An American named Charles Fritts had even created a working solar cell from selenium more than two decades before, in the early 1880s.
But observing that light can create electricity is not the same as understanding why light can create electricity. That was baffling.
It was understood, at that point, that light worked as a wave. But if that was true, it didn't make any sense that light could create an electric current: A wave of light just wouldn't have enough energy to cause materials like selenium to shoot off electrons as fast as they did when exposed to light.
In 1905, Einstein was 26 and producing physics papers that would change the way we think about the world for decades to come. He wasn't quite the wild-haired celebrity yet:
But in a paper published in March 1905, Einstein suggested that, perhaps, light wasn't a wave. Phenomena like the photoelectric effect, he wrote,
In other words, light could create electricity if it behaved, sometimes, like a particle rather than a wave. (This should sound familiar to anyone who remembers physics class.)
Only one section of the paper covered the photoelectric effect, but it outlined how a light particle might deliver enough energy, all at once, to knock an electron off an atom and create an electric current. This, it turned out, was easier to show experimentally than some of the other ideas Einstein had outlined. Within a decade Robert Millikan had verified, experimentally, the equation that Einstein had used to describe the photoelectric effect.
The idea that Einstein described in 1905—that won the Nobel Prize a decade and half later—is what makes today's solar panels work at all. But it wasn't until 1954—almost 50 years later—that anyone was able to make a solar cell that created enough current to actually run electrical equipment. Just as there's a gap between observing something and knowing how it works, there's a gap between knowing how something works and being able to do anything useful with it.