Botanist Robert Brown was looking at pollen grains under a microscope when he noticed that they did not sit still but jerked around. After first pondering whether the pollen grains were alive, Brown realised that they were being knocked around by the motions of molecules within the water that he had used to coat the glass slides of the microscope. The pollen grains moved randomly, sometimes there was a little movement and occasionally a lot, and they moved across the slide in a way that could not be predicted.
Random Movement
Brownian motion occurs because the tiny pollen particles were given a little knock each time a water molecule bumped into it. The invisible water molecules are moving around and colliding all of the time and so they regularly knock into the pollen.
The pollen grain may be much bigger than a water molecule but, since the pollen is being hit at any moment by many molecules, there is usually a force imbalance which makes it move a little. This buffeting occurs again and again and so the grain particles follow a jagged path.
It was Albert Einstein who brought the mathematics behind Brownian motion to the attention of physicists in 1905, the same year that he published his theory of relativity and won the Nobel Prize for his examination of the photoelectric effect. Einstein used the theory of heat, which is also based on molecular collisions, to successfully explain the motions that Brown had observed.
After realising that Brownian motion provided evidence of the existence of molecules in fluids, physicists were compelled to also accept the theory of atoms, which was still controversial as late as the beginning of the 20th century.
Diffusion
Brownian motion may cause particles to move by quite some distance although never so far as if they were able to move in straight lines. This is due to the fact that the randomness of the movements is just as likely to send a particle backwards as it is to move it forwards. So, if a group of particles were dropped in one spot into some liquid, they would diffuse outwards even if the liquid was not stirred and there were no currents present in it.
Fractals
The path followed by a particle undergoing Brownian motion is an example of a fractal. Although each step of the particle’s path can be of any size and in any direction, an overall pattern does emerge.
The mathematics of Brownian motion can be used to generate fractal patterns that are useful in many areas of science. For example, doctors use them in medical imaging when they need to analyse the structure of complex parts of the body.
Sources:
Baker, Joanne (2007) 50 Physics Ideas You Really Need to Know (Quercus Publishing)
Holzner, Steve (2005) Physics for Dummies (John Wiley & Sons)
Kuhn, Karl (1996) Basic Physics: A Self-Teaching Guide (John Wiley & Sons)
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