Published in Annalen der Physik in 1905, this paper argued that particles small enough to be suspended in a liquid should be seen to move in an irregular, random fashion, driven by collisions with the surrounding molecules. Einstein derived the effect from the kinetic theory of heat rather than from observation, and noted that the motion he predicted might be the phenomenon already known to botanists as Brownian movement.

The central result was that the average distance a particle travels grows with the square root of the elapsed time, not in proportion to it. Einstein linked that displacement to measurable quantities including temperature, the viscosity of the liquid and the size of the particle.

This gave experimenters a way to test whether molecules exist. Jean Perrin carried out the measurements between 1908 and 1911, confirming the predictions and helping settle long-standing doubts about the existence of atoms.