Brownian motion

Brownian motion has been observed since the invention of the microscope (circa 1650). The biologist Robert Brown (1773–1858) published systematic experimental studies of the erratic motion of pollen and of other microscopically visible grains swimming in drops of water. He called them primitive molecules, and it was unclear what made them move. In 1905, Albert Einstein predicted a diffusive motion of mesoscopic particles (i.e., macroscopically very small, but still visible through a microscope) immersed in a fluid, by adopting Boltzmann’s view of atomism. When the molecules in the fluid undergo heat motion and hit the mesoscopic particle in a random manner, they force the particle to move around erratically. He suggested that this might be the already known Brownian motion.