This is an updated post – I’ve deleted the original one.
Nothing’s new under the sun…we are told. In about 60 BCE, the poet Lucretius wrote…
“Observe what happens when sunbeams are admitted into a building and shed light on its shadowy places. You will see a multitude of tiny particles mingling in a multitude of ways… their dancing is an actual indication of underlying movements of matter that are hidden from our sight… It originates with the atoms which move of themselves.”
Robert Brown, a doctor and a botanist, was extremely ticked off. No matter how hard he tried to twiddle the focus knob on his fancy new microscope – they weren’t very good in 1827 – the pollen grains he was trying to look at wouldn’t STAY IN FOCUS! They seemed to have a mind of their own, jiggling about as if being pushed around by even tinier little particles. At first, he thought the pollen grains were alive. In fact, they were being pushed around by air molecules. Because they were so small, the colliding air molecules could actually move the pollen grain.
Imagine it like this.. a really big balloon, 10m in diameter in a football stadium. The balloon is so large that it sits on top of lots of fans in the crowd at the same time. Because they are excited, these fans hit the balloon at different times and in different directions with the motions being completely random. In the end, the balloon is pushed in random directions, so it should just be jiggled about. Consider now the force exerted at a certain time. We might have 20 supporters pushing to the right, and 21 other supporters pushing left, where each supporter is pushing with the same force. In this case, the forces exerted from the left side and the right side are unbalanced in favour of the left side so the balloon will move slightly to the left. A bit later on, the number of pushers and their direction might randomly change so causing random motion of the balloon. If we imagine an observer in a helicopter looking down at this situation from far above, he can’t see the supporters, all he sees is the big balloon jiggling randomly about.
It was dear old Uncle Albert who finally came up with the best solution to this problem – here he is again, riding a bike
Brownian motion is the most important piece of evidence for the random movement of gas (and liquid) molecules
This little Java applet shows how it works