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Q & A: A microscope for subatomic particles

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Is the a microscope that enables us to see sub-atomic particles? If not, why?
- Muhannad (age 18)
University of Ontario Institute of Technology, Oshawa, Ontar
Sure! They are called particle accelerators.
(Of course, what they do is provide information on the structure of the sub-atomic particles, a sort of 'seeing', but not quite direct ordinary seeing.)

Subatomic particles are so small they always have to be treated quantum-mechanically, which means that they occupy states that may extend over larger amounts of space than their natural radius, just as electrons occupy cloudlike orbital states around the centers of atoms. Protons occupy similar cloudlike states inside of atomic nuclei, and so do neutrons. Protons themselves are made up of smaller pieces, called quarks and gluons. The proton itself is the collection of cloudlike states of the constituent parts.

The reason we know all of this is that we've been able to fire projectiles very fast at these objects. Light works great in an ordinary microscope, but the wavelength of visible light limits our ability to see small stuff. To 'see' small stuff, you need light with short wavelengths. Pretty soon you need x-rays and higher-energy photons (with shorter wavelengths). But x-rays are hard to focus and hard to measure precisely from where they come. So instead a favorite projectile is the electron. Electron microscopes work by throwing energetic electrons at a target specimen and looking at how they bounce off. The higher the energy of the electrons, the shorter their quantum-mechanical wavelength, and the smaller the features can be resolved. Electron microscopes are very useful, but cannot see inside the nucleus of atoms unless the energy is very very high.

At the Stanford Linear Accelerator Center is a 2-mile-long electron accelerator, with a peak energy of 45 billion electron volts per electron it accelerates. These can be trained on a target of protons or neutrons and the scattered electrons detected in elaborate experimental apparatus. It is experiments done there in the 1960's which proved the existence of quarks inside the proton, acting just like an electron microscope on a big scale.

Even bigger particle accelerators have been built, and they have been used to produce new, exciting particles no one dreamed existed. Now we have a good model which explains what we see, but it has been an exciting ride, and there are promises to find more stuff as we look at higher energy scales, which corresponds to smaller and smaller distance scales.


(republished on 07/20/06)

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