Brownian Motion
The random, erratic wandering movement of microscopic particles suspended in a fluid caused by relentless collisions with fast-moving fluid molecules.
Governing formula
⟨x²⟩ = 2 D t, D = kB T / (6 π η r)
SI unit
m²
In depth
Einstein's 1905 mathematical explanation of Brownian motion provided definitive, undeniable proof for the physical existence of atoms and molecules.
Examples in the real world
Pollen grains or smoke particles under a microscope jiggling continuously in water or air.