Learn about Albert Einstein's theory of Brownian motion and how he derived the size of atoms based on how much the Brownian particles move

One thing that's astounding about Einstein's publications in 1905 is that they spanned such a large range of physics. After illuminating the quantum nature of light by explaining the photoelectric effect in March, April saw Einstein turn to something apparently more mundane-- particles suspended in fluids. In particular, if you look at tiny particles in water or dust motes in the air, you'll see that they jitter about in a very odd and random way.

That is, it appears odd and random unless you believe that the air or water itself is made up of even smaller particles called atoms or molecules, which just bounce off each other according to very simple rules. This is called Brownian motion, despite the fact that botanist Brown wasn't the first to discover it. And similarly, Einstein wasn't the first to describe it mathematically.

But he did draw the conclusion that the mathematical description of Brownian motion is evidence for the existence of atoms, even if you can't see them directly. And then he cleverly derived how big atoms should be based on how much the Brownian particles move. That's like measuring the size of a penguin just by looking at how icebergs jiggle. Talk about cold and calculating.