cosmic rays

Cosmic rays are fast-moving particles raining down on the Earth from space, all day and all night, from every direction. Despite the old-fashioned name, most of them are not rays of light at all but actual chunks of matter — mostly bare protons, with a sprinkling of heavier atomic nuclei — flung at us from somewhere far out in the galaxy or beyond. A few of them are travelling so close to the speed of light that a single one packs the punch of a hard-hit tennis ball, all squeezed into one subatomic particle.

When one of these particles hits the top of the atmosphere it smashes into an air molecule and shatters it, and the debris smashes into more molecules, producing a spreading cascade of secondary particles called an air shower. By the time it reaches the ground, one original cosmic ray may have become billions of lighter particles raining over an area the size of a town. Detectors on mountaintops, on satellites, and spread across deserts catch these particles and work backwards to figure out the energy and direction of the original arrival. The energy of a cosmic ray is measured in electronvolts, and they span an astonishing range, from millions of electronvolts up to a hundred million trillion.

Cosmic rays were how human beings first met high-energy particle physics, decades before anyone built an accelerator. The positron — the first antiparticle ever seen — and the muon and several other particles were all discovered in cosmic-ray showers in the 1930s and 1940s. They still matter today: they are a free, natural beam more energetic than anything we can build, they are messengers from violent objects like exploding stars and black holes, and where they come from is still partly a mystery, because their electric charge lets the galaxy's magnetic fields bend their paths and scramble the trail back to their source.