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What is Brownian motion?

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Quick answer

Brownian motion is the random, jittery movement of tiny particles suspended in a fluid, caused by countless molecules of that fluid colliding with them. It is visible proof that matter is made of fast-moving, invisible atoms and molecules. Pollen grains or dust specks in water jiggle erratically with no outside push.

Key things to understand

  1. Pollen grains or dust specks in water jiggle erratically with no outside push.
  2. The cause is constant, uneven bombardment by the fluid's moving molecules.
  3. Einstein explained it mathematically in 1905, giving strong evidence atoms are real.
  4. The smaller the particle, the more visible the jiggling.
  5. It's an everyday example of statistical, random ('stochastic') motion.

People also ask

What causes Brownian motion?
Invisible fluid molecules in constant motion strike the particle from all sides; the imbalance of these hits at any instant nudges it randomly.
Why is Brownian motion important?
Einstein's 1905 analysis of it gave some of the first hard evidence that atoms and molecules actually exist.
Where do you see Brownian motion?
Dust dancing in a sunbeam, ink spreading in still water, and smoke particles drifting all show it.