Einstein's 1905 Explanation of Brownian Motion Finally Proved Atoms Exist
In 1827, botanist Robert Brown saw pollen grains jittering in water, a mystery that stood for decades. In 1905, Einstein proposed that invisible molecules were bombarding the particles, and Smoluchowski independently derived the same theory. Perrin's experiments confirmed it, helping to establish the reality of atoms. This post explains Brownian motion, from random walks and binomial trees to the normal distribution, showing how a completely random process yields precise probabilistic predictions.
At first, he thought this meant that the pollen particles were alive, but he tested and then rejected this hypothesis by observing the same effect with particles that he was convinced were inanimate, such as glass powder, minerals, and even pulverized fragments of the Egyptian Sphinx.