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4.4. Brownian Motion

Brownian motion was first defined by the botanist Robert Brown in 1827 to describe the random motion of tiny pollen particles moving in a droplet of water under a microscope. This random motion was originally believed to be caused by some microscopic living organisms, but Einstein showed later in 1905 that Brownian motion was the result of otherwise undetectable atoms randomly interacting with the larger molecules, causing observable random motion. Although atoms were theorized at the time to be a possible physical reality, Einstein's work on Brownian motion proved their existence and gave the first method for experimentally determining their quantity and size.

In this demo, the particle simulator reproduces a similar effect. Here, a set of 50 larger brown particles interact with 1000 smaller particles, which are 4% the size/mass of the larger particles. Although the larger particles would normally travel in simple straight paths, the large quantity of smaller particles means their collisions dominate the motions of the large particles, creating haphazard meandering trajectories for the larger particles that mimic the Brownian motion described by Einstein.

And as with the Fun With Shapes demo, this simulation demonstrates the phenomenon of reaching thermal equilibrium, except in an even more exaggerated way. At the start, only the larger particles (separated to the left) have any energy, but this energy is quickly dispersed to the small particles, which are moving 25 times faster on average than the large particles once equilibrium is reached.