This was only the beginning. The electron, proton and neutron proved to be the first members of a rich pageant of subatomic particles. During the 1930s and 40s, many physicists studied cosmic rays, the steady rain of high energy subatomic particles that originate in outer space.
The collisions of high energy cosmic rays with atoms in the atmosphere prised open the nucleus to reveal new kinds of short-lived particles that could be seen only through the tracks left behind in sensitive detectors. There were particles such as the muon, which behaves like an electron, but is 210 times heavier; the pion, which is just a little heavier than the muon; the kaon at little more than half the proton's mass; and the lambda, which is about 20 percent heavier than the proton.
One particularly intriguing particle, discovered in 1932 by Carl Anderson atthe California Institute of Technology, is the positron - as light as an electron, but with positive charge. Its existence, at first a puzzle, was soon explained by Paul Dirac, a theoretical physicist at Cambridge University.
According to Dirac's theory, the positron is a particle with exactly opposite properties to an electron - an anti-electron. The theory showed how an electron and a positron can emerge together from pure energy, provided the energy is sufficient to supply the total mass of the two particles, in accordance with Einstein's equation, E=mc2.
If they collide, the particle and antiparticle disappear to leave only energy - an act of mutual destruction called annihilation. Experiments have since demonstrated that most other particles, protons, neutrons, muons and so on - have antiparticles.
By the early 1950s, the study of these particles had become a branch of physics in its own right - particle physics had come of age. To aid them, the physicists had machines that could accelerate protons and electrons to high energies, mimicking the cosmic rays but in more controlled conditions.
Work in the early 1930s by John Cockcroft and Ernest Walton at Cambridge, and by Ernest Lawrence and Stanley Livingston at Berkeley in California, had provided the first artificially accelerated protons. Their pioneering ideas gave birth in the 1950s and 60s to large machines capable of producing millions of protons, electrons, pions or kaons each second. With the invention of more sophisticated detectors to complement the accelerators, physicists now had the tools to study the many varieties of particle in detail.