Thursday, March 10, 2005

On the origins of Brownian motion

Brown knew particles were smaller than pollen:
Sir – In your Year of Physics supplement (Nature 433, 213–257; 2005), several authors repeat the mistaken idea that the botanist Robert Brown observed the motion that now carries his name while watching the irregular motion of pollen grains in water. The microscopic particles involved in the characteristic jiggling dance Brown described were much smaller clay particles. I have regularly studied pollen grains in water suspension under a microscope without ever observing brownian motion.

From the title of Brown's 1828 paper “A Brief Account of Microscopical Observations ... on the Particles contained in the Pollen of Plants...”, it is clear that he knew he was looking at smaller particles (which he estimated at about 1/500 of an inch in diameter) than the pollen grains.

Having observed 'vivid motion' in these particles, he next wondered if they were alive, as they had come from a living plant. So he looked at particles from pollen collected from old herbarium sheets (and so presumably dead) but also found the motion. He then looked at powdered fossil plant material and finally inanimate material, which all showed similar motion.

Brown's observations convinced him that life was not necessary for the movement of these microscopic particles. Brown was not the first to observe the motion that now carries his name and that Einstein famously explained. However, he was convinced his was the first really detailed study of the phenomenon and he clearly hoped for priority for his description.

David M. Wilkinson
Biological and Earth Sciences, Liverpool John Moores University, Byrom Street, Liverpool L3 3AF, UK
The experiments described above are intriguing, because Brown was working at a time when it wasn't clear what made life live. How small could you grind a living thing up and still have it act as if it were alive? What about fossilized things that might have had all their organic material replaced with non-living minerals?

In the end, he found that the motion (random movements caused by the stochastic collisions of extremely small particles) was not the result of some mystic life force, but a pure consequence of random chance.

At first, he thought he recognized something like design in the motion of the tiny particles. He thought that some force (either a vitalistic power or tiny flagellae) were moving the particles. It turned out that what appeared to be directed action was a result of purely random processes.

There's a lesson there somewhere. A common Creationist Canard is that it's possible to recognize supernatural forces through trivial inspection. Whenever people try to show it, they wind up either waving their hands around, or being wrong.

But, to play our little game, the pollen gave us Brownian motion, so pollen is part of any Brownian motion. Therefore, Einstein must be a plant.