About 100 years ago scientists were starting to realise that there were several different types of elementary particles, rather than the single, indivisible atom favoured by the ancient Greeks.

This realisation came from the development of quantum mechanics, one of the successes of which was a novel way to view the way electrons behave when trapped in an atom: they are forced into certain behaviours with respect to the nucleus and the other electrons, if present.

Electron

Theoretical physicist Paul Dirac got his head around both the brand new quantum theory, which explained slow or trapped electrons, and the pretty new special relativity, which described what happens when a particle such as an electron is free to move at high speed. Dirac had a brain the size of a planet, evidenced by his ability to sit down and write an entire book on the subject without a single mistake or revision.

The Dirac equation described the behaviour of a relativistic electron. A strange and beautiful outcome of this equation was that there were two ‘electron’ solutions, meaning that if Dirac were right, there should be another kind of electron with positive charge, and exactly the same mass and spin as the electron.

Positron

A few years later, experimental physicists observed tracks in a cloud chamber that were mirror-images of one another. The cloud chamber was inside a strong magnetic field, such that particles with positive charge would be deflected and curved in one direction, and particles with negative charge would be deflected and curved in the opposite direction. The negative particle was identified as the electron, and thus the first anti-particle was observed: the anti-electron, or `positron’.

A particle and its anti-particle are the same in every way except that they have opposite charge. The positron is like the `doppelganger’ of the electron! When a positron and an electron collide with one another, they annihilate into pure energy in the form of photons.

Positron emission tomography

The annihilation behaviour of positrons and electrons can be, and is, exploited to help humans. The atoms in our bodies all contain electrons: we, and everything around us, are made of matter rather than anti-matter. Positrons can be produced in the radioactive decays of certain elements. By using small quantities of these elements (known as radionuclides) in a dye that is injected into the bloodstream, they are transmitted through every part of the body that has a blood supply. The positrons emitted by the radionuclides collide with the body’s own electrons and produce high energy photons (known as gamma rays) that fly out of the body and are detected by the Positron Emission Tomography (PET) scanner. This enables medical doctors to identify areas of the body that may not be getting a good supply of blood, as these will show up as darker areas on the scan.