Probing the Structure of Matter

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Particles

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.

Scales.75.gif

... and waves

As we pull matter apart into smaller and smaller pieces, it becomes more and more difficult to see the pieces. Normally one "sees" an object by "shining a light" on it and watching how the light is reflected or blocked by the object. This works fine with people, cars and tables, and even objects as small as (large) cells, like the one in the picture above - all you have to do is magnify the image (with lenses, say) if the object is small. But ordinary "visible" light has a wavelength a bit less than a millionth of a metre, so you can only use it to "see" objects a millionth of a metre in size, or larger - objects big enough to "disturb" the wave, as in the upper diagram to the right.

Objects much smaller than the wavelength are practically invisible - the light just washes past the object, more-or-less undisturbed, like a waves on water washing past a small object, as in the lower diagram to the right. Moving to shorter-wavelength light - ultraviolet light or X-rays, or even gamma rays - is not much help since we can't build lenses etc to focus them.

Diffraction.50.gif

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

Debroglie.gif

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

Our 730 km/s electron has a kinetic energy - what you describe as the "kick" or "punch" of a flying object when it hits you - of one-and-a-half electron-volts, which is not much kick at all. With two pieces of metal and a one-and-a-half volt battery, you could give an electron that much energy (hence the name "electron-volt" for the unit of energy - see, it does make sense after all!).

(We said that you can't focus X-rays. In fact, it's pretty hard to focus electrons, too. Electron microscopes need to use electrons of much shorter wavelength than 1 nanometre to get a decent image - and the shorter the wavelength, the larger the momentum, and the larger the kinetic energy. Energies as high as one hundred thousand electron volts are fairly common in electron microscopes - requiring 100,000 volts to bring the electrons up to speed, i.e. well beyond the range of a pocket D-cell battery, and more than enough to be dangerous. This is one of the reasons that electron microscopes are expensive.)

graphic: atom, nucleus and proton, with length and energy scales Molecules are made up of atoms. If we only had "lenses" that were good enough, we could see atoms using beams of electrons with an energy of only 10 electron-volts, which would be easy to produce. But the nucleus of an atom - so-called because it forms a small, hard centre to an object which is otherwise rather "spread out" - is very small indeed, ten-thousand times smaller than an atom. A typical nucleus is about one hundred-million-millionth of a metre across, and electrons with this wavelength have an energy of 100 million electron volts.

You cannot give electrons that much "kick" using bench-top equipment. Large particle accelerators are required, which is why particle physics is now almost exclusively carried out at specialised facilities by international teams of scientists. The large energies to which the probe particles - electrons, in our example - must be raised, give the discipline its other name: high energy physics.

Scales.anp.75.gif

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.