A jittering speck under a microscope finally proved atoms are real.
In 1827 the botanist Robert Brown saw tiny particles from pollen dancing restlessly in water, and nobody could explain why. In 1905 Einstein did: they are being jostled by invisible water molecules. When Jean Perrin confirmed his maths, the long argument over whether atoms exist was over.
Robert Brown was studying plant reproduction when he looked at pollen grains of Clarkia pulchella in water through a microscope. Minute particles from inside the grains, around 6 micrometres across, were moving in a continuous, jerky way. To test whether it was a sign of life, he tried particles of glass and rock dust and saw the same motion, so whatever caused it was not biological. The origin stayed a mystery for decades.
Even at the start of the 20th century, the existence of atoms and molecules was still fiercely debated. In one of his celebrated 1905 papers, Albert Einstein proposed that a suspended particle is constantly bombarded by molecules of the liquid. At any instant it is hit slightly harder on one side than another, so it lurches randomly. Crucially, Einstein linked how far such particles wander over time to measurable quantities, making it possible to calculate the size of molecules and how many are in a given amount of substance, the Avogadro number.
That turned a philosophical argument into an experiment. In 1908 and 1909 Jean Perrin and others tracked Brownian particles carefully and found they matched Einstein's predictions. The results settled the atomic debate, and Perrin received the 1926 Nobel Prize in Physics for his work on the discontinuous structure of matter.
The idea was older than Brown. Around 60 BC the Roman poet Lucretius described dust dancing in sunbeams as evidence of atoms, though that visible tumbling is mostly caused by air currents; one writer said he explained Brownian motion perfectly using the wrong example. And in 1900, Louis Bachelier had already used the same mathematics of random motion in his doctoral thesis to model stock prices, work that went largely unnoticed until the 1950s.
Source: Wikipedia — Brownian motion · Text summarised from Wikipedia (CC BY-SA 4.0)
