Brownian motion
the random motion of particles suspended in a fluid resulting from their collision with the quick atoms or molecules in the gas or liquid / From Wikipedia, the free encyclopedia
Brownian motion is the random motion of particles suspended in a medium (a liquid or a gas). The motion is caused by fast-moving atoms or molecules that hit the particles. Brownian Motion was discovered in 1827 by the botanist Robert Brown. In 1827, while looking through a microscope at particles trapped in cavities inside pollen grains in water, he noted that the particles moved through the water; but he was not able to find out what was causing this motion.
Atoms and molecules had long been theorised as the main parts of matter. Albert Einstein published a paper in 1905 that explained in precise detail how the motion that Brown had observed was a result of the pollen being moved by individual water molecules. This was one of his first big contributions to science, and convinced many scientists that atoms and molecules exist. It was further verified experimentally by Jean Perrin in 1908. The direction of the force of atomic bombardment is constantly changing, and at different times the particle is hit more on one side than another, leading to the seemingly random nature of the motion.
There are too many molecular impacts making the Brownian pattern, so no scientific model can account for all of them. That is why only probabilistic models of molecular populations can be used to describe it. Two such models of the statistical mechanics were made by Einstein and Smoluchowski. Another, pure probabilistic kind of models are stochastic process models. There exist both simpler and more complicated stochastic processes which in extreme ("taken to the limit") may describe the Brownian Motion (see random walk and Donsker's theorem).
Norbert Wiener also studied Brownian Movement, with greater mathematical precision.