The tools of the trade 1

Introduction to accelerators

When we want to look into Outer Space we use telescopes but what do we use to look into Inner Space?

Microscopes 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.

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. The smaller the wavelength, the smaller the objects that can be resolved.

To understand why, imagine a duck on a pond. Long wavelength waves gently lift him up and down but continue undisturbed by his presence. Another duck, flying over the other side of the pond and detecting the waves, would not be able to infer from them that there was a duck in the way. In other words, long wavelength waves do not "see" the duck.

When the wavelength gets smaller, the duck is tossed about more violently because shorter wavelength waves carry more energy. The waves themselves are modified by the presence of the duck and this time a distant observer would be able to tell that there was a duck in the way.

It's just the same when it comes to tiny things. The smaller the object we want to study, the shorter the wavelength and the higher the energy of the probe we need to use. Electrons in an electron microscope have shorter wavelengths than visible light which is why they can see smaller things.

To study things smaller than atoms, physicists take particles like electrons and boost them up to very high energies in machines called particle accelerators.

Throughout the 1990s CERN physicists used a particle accelerator called the Large Electron Positron collider, or LEP for short. LEP is a giant underground machine. It is circular with a circumference of 27 kilometres and, as its name suggests, it accelerates electrons and their antimatter counterparts, positrons, and then collides them head on. This technique is an alternative to "shining" a beam of particles on a sample to be studied, though that is done too at CERN. Colliding particles maximises the energy available. The resolution achieved with the LEP accelerator is a thousand times smaller than the size of a proton.

However, apart from seeking the smallest constituents of matter there is another argument behind striving for more powerful accelerators. Assuming that the whole Universe was born in a Big Bang the temperature must have been enormous during the first few seconds, meaning that the Universe behaved very differently from what it does today. In order to understand the laws that determined the birth and the initial evolution of the Universe we must be able to recreate the conditions of the Big Bang.

What happens when an electron and a positron collide in LEP is that they annihilate with each other giving rise to a very high density of energy, or in other words high temperature. From this mini-Big Bang a whole set of newly created particles emerges, some of which don't even exist naturally in the Universe today but were only found in the very early stages of the Universe. Under the extreme conditions of high temperature generated at LEP the particles show us how they and the forces behaved shortly after the Big Bang.

In fact, the energy density attained at the LEP collider corresponds to a temperature of one trillion degrees, that is a 100 million times the temperature in the centre of the Sun! With existing models it is possible to guess how long after the Big Bang the Universe had "cooled" to this temperature. The answer is only a tenth of a billionth of a second! In other words, what the world of particles reveal to us in the LEP collisions is the way the Universe looked only a fraction of a second after the Big Bang.

Thus by looking into inner space we also learn about outer space.

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Accelerators in the laboratory Accelerators in the Universe


Particle Physics Education CD-ROM 1999 CERN