Random Quivers
In the early nineteenth century, British botanist Robert Brown traveled around the world collecting plant specimens. He found that certain pollen grains were transparent when examined under a microscope, and he could see distinct particles inside individual grains. Under a microscope these particles appeared to be in continuous motion, zigzagging randomly about.
While examining pollen grains of the plant Clarkia pulchella (pinkfairies) under a microscope, Robert Brown observed inside the grains tiny particles in constant, jittery motion. USDA Forest Service
Brown's followup examinations of soot and other types of microscopic particles suspended in water revealed a similar quivering movement.
Scientists later suggested that the quivering is caused by the movements of the molecules making up the liquid, and they called this phenomenon "Brownian" motion.
In 1905, physicist Albert Einstein showed mathematically how tiny, randomly moving molecules can budge particles large enough to be observable under a microscope.
Molecules of a liquid are in constant motion. Each molecule travels in a straight line until it collides with a particle and bounces away, like a billiard ball. At any given moment, large numbers of molecules can strike a single particle so that each impact shoves a particle in a particular direction. The combined impacts produce more force in some directions than in others, giving the particle a net shove in one direction.
Tiny molecules moving in random directions at different speeds (pale blue dots and arrows) collide with a floating particle (shaded), causing it to move about haphazardly.
The haphazard paths of tiny particles suspended in a liquid are random, like the steps of a three-dimensional random walk (see "Wandering in Space")!
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