Established science · Molecular motion
Brownian motion
A visible grain, an invisible crowd
In 1827 Robert Brown watched pollen grains jerk in water. The path is not alive. Einstein's 1905 account: the grain is struck unevenly by water molecules. The kicks do not cancel in any short moment, so the grain wanders.
The theory is quantitative. The mean squared displacement grows in proportion to time, and to temperature, and it shrinks as the liquid gets more viscous or the grain gets larger. That is how Perrin later counted molecules.
Watch the large grain. The little marks are the molecules. Turn them off and the wander is still there. The cause did not leave. It only got too small to draw.
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