Probing the Structure of Matter
Particle physics is the study of matter and its interactions at the smallest length scales - the "pieces" you find when you keep pulling matter apart:
- cells (or fibres, or crystals, or whatever) into molecules
- molecules into atoms
- atoms into electrons and nuclei
- nuclei into protons and neutrons
- protons and neutrons into quarks
and the way they behave. We call it "particle" physics because we often imagine the individual pieces as hard, isolated "particles" - like the small spheres drawn in the picture below - but this shouldn't be taken too seriously. A proton is a lot more complicated than the "bag" containing three quarks suggested by the picture.
... and waves
For small objects such as the organs of insects, the edges of crystals and so on, we get around this problem by using beams of electrons in place of light. Electrons, like all "particles", have a wavelength, given by
where p is the momentum of the particle (for slow particles, this is just the mass times the velocity) and h is a constant. The value of h is very small, which is one of the reasons why the "wave nature" of particles wasn't noticed until this century.
So the larger the momentum of the "particle", the shorter the wavelength. An electron travelling at 730 kilometres per second - which is pretty slow for an electron - has a wavelength short enough to notice a molecule, say 1 nanometre, a billionth of a metre. It's impractical to use light to make a picture of a molecule because "light" with a wavelength this short is actually X-radiation - and if you ever work out how to make lenses to focus X-rays properly, the US military, and a lot of other people, would like to hear from you. (In fact you can use x-rays to get a lot of information about large, regular arrays of atoms - such as crystals, or repeating structures like DNA - but that's not quite the same as taking a picture.)
... and energy
Finally, to resolve objects the size of a proton requires another factor of ten in both momentum and energy, to one billion electron volts. Under the right conditions, one billion electron volts is enough energy to make a proton from scratch: therefore you cannot "see" a proton in the way you can "see" a car or an atom, since an electron of the wavelength required to "resolve" a proton will typically destroy it, making a big mess. It's a bit like shining a light on a mouse, where every single "piece" of light has the energy of a nuclear bomb.
... and beyond?
People used to say that you could never see an atom, which was wrong: nowadays this is fairly routine. But for protons and smaller objects there doesn't seem to be any way around the energy problem: we'll never be able to take their picture. We can still see where they've been, and experiments that analyse the "mess" left behind when we hit them with electrons or other particles have told us a great deal about them, but in these experiments we're stuck with graphs and equations at the other end. And what do we find when we "look inside" a proton? Well, three quarks, for starters, themselves at least one hundred times smaller than a proton, if they have a size at all. As we said, it's actually much more complicated than that ... but that's another story.
NEXT: The fundamental particles ...
About this document ...
The main figure and the outline of this material were prepared by Paul Soler for a presentation at the International Science School for gifted secondary school students, held at the University of Sydney in July 1995.