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The first attempt seems to have come from ancient Greece where the four elements of earth, water, air and fire were recognized.
This scheme is quite attractive at a trivial level and if you get bored during the rest of the talk you could amuse yourselves constructing some of the objects around us from these four elements.
However during the last few hundred years a more useful idea of elements developed. Substances such as gold, sulphur, lead, nitrogen were recognized as elemental in the sense that they were contained in different substances in different mixtures but could not themselves be broken down further.
By the beginning of the last century Dalton had identified twenty elements and listed them together with their relative weights.
 
However the definitive word was given by Mendeleev in 1869 when he arranged the known elements in a beautiful pattern. Moreover his pattern was predictive in that some elements were missing with properties that could be predicted. All were later discovered.
Thus by the end of the last century the concept of elements was well developed. The smallest piece of an element was known as an atom with atoms imagined as small spheres.
However 100 years ago the electron was discovered and it became clear that it not only played an important role in electricity but was also contained inside atoms, i.e. atoms have sub-structure.
However it was in 1912 that the New Zealander Ernest Rutherford gave us our modern view of the atom when he showed that atoms have a positive nucleus surrounded at a relatively large distance by the electrons, a picture that everyone today recognizes as the symbol for the atom.
The photograph shows Rutherford in his lab. The sign says ‘Talk softly please’ supposedly put there because the detectors were sensitive to noise. However the more likely explanation is that it was aimed at Rutherford by his colleagues, since he was renowned for his booming voice.
Now it became clear what differentiates the elements - the number of electrons and the charge on the nucleus - for example hydrogen has one electron, helium has two, carbon six, lead eighty-two etc.
However the story doesn’t stop there.
By 1936 Anderson had made yet another discovery in his cloud chamber - a completely new particle, seemingly identical to the electron but 200 times as heavy. This particle is produced in the cosmic radiation that continually bombards the earth but lives for only about a millionth of a second, decaying by a radioactive decay process. It seems to have no use at all in the Universe - an idea beautifully expressed by Isidore Rabi with the question ‘Who ordered that?’
This is still a challenging question as I hope will become apparent later.
The first of these is antimatter, predicted by the British theoretical physicist Dirac in 1928 when he was developing an equation to describe the behaviour of the electron.
Dirac was a very shy man; you see it in his demeanour as he gives a lecture. However the respect in which he is held is shown by his plaque in Westminster Abbey, placed there in 1995, which is reputedly the only equation in the Abbey.
For Dirac the equation was obvious; however he could only find a solution to it describing the behaviour of the electron if there was also another solution which seemed to describe something with negative energy. This solution he eventually ascribed to the positron, the antiparticle of the electron.
The first of these is antimatter, predicted by the British theoretical physicist Dirac in 1928 when he was developing an equation to describe the behaviour of the electron. Dirac was a very shy man; you see it in his demeanour as he gives a lecture. However the respect in which he is held is shown by his plaque in Westminster Abbey, placed there in 1995, which is reputedly the only equation in the Abbey. For Dirac the equation was obvious; however he could only find a solution to it describing the behaviour of the electron if there was also another solution which seemed to describe something with negative energy. This solution he eventually ascribed to the positron, the antiparticle of the electron.
The simplest way to describe antimatter is in terms of its behaviour - when particle and antiparticle meet they annihilate and their mass is turned into energy in the form of g-rays, also called photons.
This process is governed by the second and last equation you will see this evening, and surely known to the whole audience - Einstein’s famous equation E=mc2.
Not only is this true for electrons and positrons, it also occurs for quarks and anti-quarks and more complicated objects like protons and anti-protons. You would also be in trouble if you met your anti-you.
However this possibility is still someway off; it was not until 1996 that anti-hydrogen, the simplest anti-atom was produced.
The inverse to the annihilation process also occurs and g-rays with sufficient energy can convert to matter and antimatter. We can sketch it using a simple line drawing again.
Evidence that it occurs is found on another of Anderson’s beautiful cloud chamber pictures where we see three g -rays have converted to electron-positron pairs just outside the cloud chamber.
Four years later the positron was discovered by the American Carl Anderson in one of his pictures of the tracks of particles in a detector known as a cloud chamber.
This device was in a magnetic field produced by the coils of cable you see in the photograph. The magnetic field has the effect of bending charged particles. From the direction of bend it is deduced that the particle has positive charge. From the amount of energy it loses as it passes through the plate in the middle of the chamber it is possible to deduce its mass.
Note the size of his detector. Later we will see others, somewhat larger, but with similar features.
We now know that all particles have antiparticles and I will now take a minute or two to discuss antimatter since it is not just the stuff of Star Trek science fiction, it is also science fact.
What happened in that era is therefore debated amongst physicists with great fervour.
Nevertheless there are some general principles that we understand.