Albert Einstein's Idea on Why Light Is a Particle

In 1905, Albert Einstein published a series of papers that many consider a starting point for the modern age of physics. But other than E equals mc squared, most people only know that Einstein was famous, not for what he was famous. Since this week marks the 133rd anniversary of his birth, as is customary in Western traditions, we will celebrate by taking a brief tour of his scientific publications.

Today, his paper from March, 1905 on why light is a particle. Einstein didn't just pull this idea out of thin air. He first noted that the light emitted from something hot, like a light bulb filament, actually has the same energy distribution as a gas, which is somewhat surprising if you're a 19th century physicist who thinks that light is a continuous wave, and very much not a gas composed of individual molecules.

And while the idea that light behaved kind of like a gas was already well known before Einstein, no one had taken the logical, but crazy next step, to conclude that light must, then, be made of individual particles, too. So Einstein proposed that these light quanta were, in fact, real particles that could account for a few reasons and unexplained experiments having to do with knocking electrons off of metals and gas molecules. He turned out to be right on all counts, and got a Nobel Prize for his work. But that's a story for another day.