Brownian Motion
Brownian motion is the ceaseless random jitter of microscopic particles suspended in a fluid — visible under a microscope as a pollen grain or smoke particle zigzagging with no apparent cause. Robert Brown observed it in 1827. For decades it was unexplained. In 1905, Einstein published a quantitative theory: the motion is caused by the continuous, randomly distributed bombardment of the particle by invisible molecules in the fluid.
The particle is large enough to see but small enough that it doesn't average out the molecular bumps. At any instant more molecules hit it from one side than the other, and it lurches in that direction. A moment later the imbalance is in the opposite direction. The path is a drunkard's walk — each step independent and random.
Why It Mattered
Before 1905, the atomic hypothesis was not universally accepted. Several prominent scientists (notably Ernst Mach) held that atoms were useful fictions — mathematical tools for organizing chemistry — but that there was no strong evidence they corresponded to anything physically real. Brownian motion changed that.
Einstein derived how far a particle should drift from its starting point over time as a function of particle size, fluid viscosity, temperature, and (crucially) Avogadro's number — the number of atoms in a mole of substance. Jean Perrin then measured Brownian motion precisely and used Einstein's formula to calculate Avogadro's number. The result matched the value derived from completely different experiments (gas laws, radioactive decay counts). That independent convergence on the same constant was the nail in the coffin: atoms are real, they have definite sizes, and their number can be measured.
The Broader Principle
Brownian motion is an example of a larger pattern: a macroscopic, directly observable effect whose quantitative behavior is predicted by an invisible microscopic mechanism. The fact that you can deduce the microscopic from the macroscopic — that a formula based on atomic theory makes the right prediction for the size of a bump you can see through an ordinary microscope — is the reason Feynman can say the atomic hypothesis is the most information-dense sentence in science.
Connections
- atomic-hypothesis — Brownian motion is the empirical confirmation
- heat-as-atomic-motion — molecular speed at a given temperature determines the magnitude of the kicks