Seeing the Invisible

When J.J. Thomson discovered the electron, he said that it was a particle "so small that its mass is an insignificant fraction of the mass of an atom of hydrogen, which itself is so small that a crowd of these atoms equal in number to the population of the whole world would be too small to have been detected by any means then known to science."

J.J. Thomson: discoverer of the electron.

Now that's small! Fundamental particles are far too small for us to see them with our eyes. To learn about them, we have to use powerful tools to extend our senses. These tools are called particle accelerators and detectors, and we'll be learning about them here. But before we do, let's have a look at the process of seeing.

Do we see or do we look?

When we observe something our eyes are entirely passive, they simply register the incoming light. Our brains interpret the information, and choose where to direct our eyes. So our brains do the looking, our eyes just see. What happens is that light is reflected from the objects we see into our eyes where it is registered and passed onto the brain for processing.

Imagine that you are entering a dark room with a torch. The torch sends a beam of light into the room, and your eyes pick up the reflected light, allowing you to see. You are using the light as a probe to study the contents of the room. This is what particle physicists do, but for them, light is not a good enough probe, as we shall see.

Astronomers use telescopes, which are very good at gathering light, to see close-ups of outer space, but what do we use to see close-ups of inner space?

Microscopes are also very good at gathering reflected light. They allow us to see small things like the faces of tiny insects.
Electron microscopes take us even further, allowing us to see down to the level of atomic structure. They use electrons as the probe.
The reason electron microscopes are able to see things so small is because the high energy electrons they use behave like waves, just like light, but their wavelength is much smaller than that of light.