Brownian Motion
On the Motion of Small Particles Suspended in Liquids at Rest Required by the Molecular-Kinetic Theory of Heat (1905)
Albert Einstein's 1905 paper On the Motion of Small Particles Suspended in Liquids at Rest introduces a groundbreaking theoretical framework for understanding the seemingly random motion of particles suspended in a liquid—a phenomenon we now know as Brownian motion. While Brownian motion had been observed earlier by botanist Robert Brown in 1827, Einstein provided the first successful quantitative description of this behavior, tying it to the molecular-kinetic theory of heat.
This paper was not only pivotal in explaining Brownian motion but also instrumental in providing strong evidence for the existence of atoms and molecules at a time when their existence was still debated.
Molecular-Kinetic Theory and Brownian Motion
Einstein's core thesis is that suspended particles, such as pollen grains or dust particles, are jostled by the incessant, random collisions of the molecules in the liquid around them. According to the molecular-kinetic theory, heat is due to the motion of individual molecules. In a liquid at rest, the molecules are not at absolute rest but are constantly moving and colliding with each other.
Einstein proposed that this molecular motion should also affect larger particles suspended in the liquid, causing them to undergo an observable random motion, which we now call Brownian motion.
While scientists had noted this erratic movement before, Einstein's contribution was his quantitative prediction of how the magnitude of this motion should depend on the size of the suspended particles, the viscosity of the liquid, and the temperature. His results implied that careful observation of this motion could not only confirm the molecular theory of heat but also allow for precise measurements of the size of molecules, making it possible to compute Avogadro’s number, a critical constant in chemistry and physics.