Brownian motion is the seemingly random motion undergone by particles suspended in a liquid or gas. The name comes from the fact that it was observed by Scottish botanist Robert Brown in 1827. While observing pollen grains in water under a microscope, Brown observed the motion of the particle through the liquid. For decades, the cause of the motion was unknown.
The phenomenon is also called pedesis, from the Greek for “leaping.” Indeed, the phenomena was observed in ancient Greece by the philosopher Lucretius, who used it as support of his notion of atomism.
In 1905, Albert Einstein published an explanation of Brownian motion, using the statistical methods related to diffusion that he had developed within his doctoral dissertation. His proposal was that Brownian motion was explained if you viewed the liquid as a collection of moving and vibrating particles. These particles collided with the visible particle, causing the change in movement.
The kinetic theory of gases was already well established, but most scientists of the day viewed the particles used for that statistical analysis as mathematical tools rather than as real objects. Einstein’s paper on Brownian motion was more difficult to interpret in that way. It took as an assumption that water was physically composed of particles, or molecules. The success of Einstein’s explanation in this regard helped to solidify the idea that liquids (and, indeed, everything else) was made of smaller particles. Like the earlier thinking of Lucretius, this seemed to be evidence for the modern concepts of atoms and molecules and other fundamental particles, particularly based on subsequent work by Jean Perrin (for which Perrin received the 1926 Nobel Prize in Physics).
The statistical methods applied in interpreting the random Brownian motion is useful in other areas that exhibit the “drunkard’s walk,” as it is called, such as trends within the stock market.