Brownian Motion

Brownian Motion

If we cannot see atoms, how do we know they exist? This is a problem that has been debated for hundreds of years. A lot of evidence have been accumulated. Today, we can even "see" it directly using a special microscope called the scanning tunnelling microscope (STM).

In the early days, there are other ways. One of them is to look at smoke particles under a microscope. The smoke particles are inside a small glass box, so there is no wind. Even so, the particles are observed to move slightly all the time, in a random way.

This random motion is called Brownian motion. Why is there any motion at all?

One possible explanation is that the air is made up of tiny atoms we cannot see. These atoms move around and hit the smoke particle at random. As a result, the smoke particle moves randomly.

Of course, we could imagine other explanations that are even more fantastic. All we can say of Brownian motion, is that it provides some evidence that air is made up of atoms. Many other types of observations (e.g. diffusion, crystallisation, chemical reactions) must be made before scientists accept that atoms do actually exist.

The Physics Notes