Brownian Motion

The discovery. In 1827, the Scottish botanist Robert Brown reported that tiny grains of pollen suspended in water moved about randomly, in an unpredictable zig-zag path, with no apparent cause -- a phenomenon now called Brownian motion. The same random jittering can be seen with fine dust particles suspended in water. For decades no explanation seemed adequate, and many scientists were reluctant to accept that all matter was actually built from tiny discrete atoms or molecules.

Einstein's explanation. In 1905, Albert Einstein gave the first rigorous, systematic explanation of Brownian motion, based directly on kinetic theory: he proposed that a particle suspended in a liquid or gas is continuously bombarded, from every direction at once, by the surrounding fluid's own molecules. Because these molecular collisions arrive from random directions with random strengths at every instant, the tiny suspended particle is repeatedly kicked this way and that, producing the observed random, zig-zag path (Figure 9.9). Because each individual molecular impact is far too small to notice on its own, this random walk is only visible for particles small enough (like pollen grains or dust) that the countless tiny impacts do not simply average out to zero net motion -- the mean free path effectively vanishes at this scale, since the particle is bombarded from all sides essentially continuously.

Why your hand feels nothing in water. Placing your hand in water produces no random jittering at all -- not because the same molecular bombardment isn't happening, but because a hand's mass is so enormous compared with a water molecule that the tiny, randomly-directed momentum transferred by each collision is nowhere near enough to produce any noticeable motion of the hand. …