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Cosmic Rays
(by Alison Wright)

Cosmic rays from outer space were the first high energy particles ever studied. They gave a tantalizing glimpse of the subatomic world before accelerators were invented. A few cosmic rays pass through your body every second of every day, no matter where you are.

It is difficult to work out the exact origin of cosmic rays because they are arriving from all directions. Many were probably thrown into space by supernovae, the huge explosions of dying stars.

Cosmic rays hitting the outer atmosphere are mainly fast-moving, high-energy protons. As they hurtle towards the Earth, they collide with atoms in the air. Some of the collision energy reappears as the mass of new pairs of particles and antiparticles, following Einstein's famous equation E=mc2.

Cosmic rays are thus a natural source of antiparticles - and in 1932 Carl Anderson's studies of cosmic rays revealed the first antiparticle ever seen, the antielectron, or "positron".