Cosmic rays

In part two of our series, Govert Schilling looks at cosmic rays, the high-energy particles that bombard Earth from space

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Like a metal roof being struck during a heavy hailstorm, Earth is under constant bombardment by energetic subatomic particles from outer space. They travel at almost the speed of light, pack more energy than terrestrial particle laboratories can ever achieve, and their origin is largely unknown. Studying these cosmic rays, as they are called, is an active field of research and a prime example of the meeting-point between astronomy and particle physics.

Cosmic rays were discovered way back in 1912. Austrian physicist Victor Hess wanted to know why air molecules become ionised - that is, how over time some oxygen and nitrogen atoms lose one or more of their electrons, leaving them with an electrical charge. At first, scientists thought natural radioactivity from terrestrial rocks was the culprit, but balloon experiments by Hess revealed that the ionisation source had to be extraterrestrial.

It took more than two decades for scientists to understand the true nature of these cosmic rays. Famed American physicist Robert Millikan, who coined the term, believed them to be highenergy photons, a form of light. But experiments by his younger colleague Arthur Compton and other researchers revealed that the 'rays' are influenced by Earth's magnetic field, indicating that they consist of electrically charged particles.

Where are they from?