Physics

In Physics what is Meant by Brownian Motion



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Brownian Motion is the phenomenon whereby particles suspended in a fluid move in stochastic (i.e., pseudo-random) paths resulting from their bombardment by the fast moving atoms or molecules in the fluid, even if it is calm. In order for these kinetic impacts to take place, the fluid must have some temperature which would cause its molecules to become thermally excited and bump into objects around them, as well as each other. The term is named after Robert Brown, a biologist who in 1827 noticed erratic movements of particles contained in pollen grains (there is a misconception that Brown observed motion in the pollen grains themselves; however, pollen grains are too large to be moved significantly by water molecules) in calm water while looking through a microscope and was unable to determine what caused this motion. Through research and systematic investigation, Brown discovered that the particles were not propelling themselves, but were rather being moved by pseudo-random bombardments from many sides, leading to a seemingly random pattern of movement, which was determined by the number of molecules that were colliding with the particles on different sides. Since this number changed rapidly, a particle would move about in an erratic manner for what seemed to be an indefinite period of time.

Robert Brown was not the first to discover this phenomenon, however. As early as 1785, Dutch physiologist and botanist Jan Ingenhousz described the irregular motion of coal dust on the surface of alcohol, with similar observations being made by Adolphe Brongniart, a French botanist, in 1827. Robert Brown was credited with the discovery, however, due to the fact that he was the first to systematically study the phenomenon and record it, and as such it was named in his honour. It was Einstein, however, that brought popularity to Brownian Motion with his 1905 paper on the phenomenon. It is one of his most-cited papers, and a detailed summary may be read here.

Brownian Motion may also refer to the Stochastic Process, a mathematical process, modeled after the concept of Brownian Motion, named in honour of Norbert Wiener. It is very important and is used frequently in pure and applied mathematics, economics and physics. It helped develop the study of probability theory (continuous time martingales), and is vital to stochastic calculus, diffusion processes and potential theory. In applied mathematics, it can be used as a model of noise in electronics engineering, instrument error in filtering theory  and unknown forces in Control Theory. Despite being based on Brownian Motion, it can also be used to study Brownian Motion in physics, as well as other diffusion-based studies for particles suspended in fluid.

Finally, it was Jean Perrin, a French physicist who later won the Nobel prize, who finally settled the debate about the existence of atoms as theorized by John Dalton, a 19th century British chemist, using Brownian Motion. Using Einstein's work as a basis, Perrin did the necessary research to test and verify Einstein’s work on Brownian Motion, validating it and setting the record straight once and for all about the existence of atoms and molecules.

Brownian Motion may seem like a small and relatively insignificant thing as compared to more robust and complex theories formulated and defined by major scientists, such as Einstein's theory of relativity, but it is an important and vital part of modern mathematics and physics, and paved the way for many discoveries that would probably not have seen light otherwise. It proved atoms and molecules exist and it is incredibly significant to nanotechnology. It is important to the creation of aerosols and is important to even biology, as recent research suggests it may play an important role in the transport of enzymes and chemicals in the body.

Needless to say, though Brownian Motion may mean little on a physical level as you go about your daily life, those tiny, energetic molecules, and their effect on the space around them, play a significant role in science and technology, and by extension, the day to day lives of everyday citizens, as well as the future of human society.


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