What is Matter ?

A Brief History of Particle Physics

During the past two centuries, scientists have made great progress in understanding what we and the world about us are made of. First came the realisation that matter consists of basic substances, or elements, with well defined physical and chemical properties. These elements range from hydrogen, the lightest, through to uranium and beyond.

Each element consists of building blocks - atoms - unique to the element, but the different atoms can combine to form an enormous variety of compounds from simple water to complex proteins. Yet, as scientists first discovered towards the end of the 19th century, atoms are not the simplest building bricks ofmatter.

Discovering the Nucleus, Geiger and Rutherford at Manchester University.

We now know that most of the mass of an atom is concentrated in a small, dense, positively-charged nucleus. A cloud of tiny negatively-charged electrons envelopes the nucleus, but at a relatively large distance, so that much of the volume of an atom is empty space. In most atoms the nucleus contains two types of particle of almost equal mass: positively-charged protons and electrically neutral neutrons. To make the atom neutral overall, the number of protons exactly balances the number of electrons.

This picture of the atom stems largely from pioneering work at Cambridge and Manchester Universities. At Cambridge in the 1890s, two physicists began unwittingly to probe the world within the atom. One, Joseph ('J.J.') Thomson, discovered the first known subatomic particle, the electron, while one of his students, Ernest Rutherford, started to explore the new phenomenon of radioactivity, in which atoms change from one kind to another. This was to lead Rutherford eventually to the discovery of the atomic nucleus, in work with Hans Geiger (of Geiger counter fame) and Ernest Marsden at Manchester University in 1909-10. Later, Rutherford found that atoms contain positively-charged particles, identical to the nucleus of hydrogen. He called the particles protons. And at Cambridge in 1932, James Chadwick showed that the nucleus must also contain neutrons. By this time Rutherford and his colleagues had established much of the modern picture of the atom.

The first observation of a positive Kaon by Clifford Butler and
George Rochester in 1947. (The Kaon decays at 'B')