Brownian motion is the random movement of small particles in a liquid due to collisions with the liquid’s molecules. It was first observed by botanist Robert Brown in 1827 and later studied by Albert Einstein, who used it to prove the existence of atoms and molecules. Mathematical models of Brownian motion are used as approximations of other models of stochastic motion.
Brownian motion is a phenomenon whereby small particles suspended in a liquid tend to move in pseudo-random or stochastic paths through the liquid, even if the liquid is still. It is the result of the asymmetry in the kinetic impacts of the molecules that make up the liquid. The liquid phase, by definition, must have a certain temperature, which means that its molecules or atoms must be thermally excited, colliding with each other and the objects suspended within them. To imagine this phenomenon, a person can imagine golf balls moving across a table filled with thousands of ball bearings moving in fast trajectories.
The phrase Brownian motion can also refer to mathematical models used to describe the phenomenon, which have considerable detail and are used as approximations of other models of stochastic motion. Mathematical motion is related to, but more structured than, the random walk, in which the motion of a particle is entirely randomized. The phenomenon has the Markov property, a probability theory term that means that the future state of the particle is determined entirely by its current state, not by any past state. Used in this sense, the mathematical concept is slightly different from, but very similar to, physical Brownian motion.
The scientist who made Brownian motion famous is Albert Einstein, who brought the phenomenon to the attention of the wider physics community by publishing an article on it in 1905, his personal annus mirabilis or “wonderful year.” The phenomenon was observed as early as 1765, but not described or studied in detail until the research of botanist Robert Brown in 1827, and is named in honor of his work. As a botanist, Brown first observed the effect in pollen floating in water, where it is visible to the naked eye. Through experimentation, Brown determined that the pollen grains weren’t propelling themselves independently, but rather that their movement was pseudo-random.
Jean Perrin, a French physicist who later won the Nobel Prize, was the springboard for Einstein’s work. Using Brownian motion as proof, in 1911 he proved once and for all that matter is made up of atoms and molecules. Although atomic theory is originally attributed to John Dalton, the 18th and 19th century British physicist, it has been the subject of controversy for over a century, and it was Perrin’s work that led to its universal acceptance.
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