Celebrating 100 years of cosmic ray science

Back in 1901, some English scientists noticed a puzzling thing. They were experimenting with the radioactive element radium (radioactivity itself had only been discovered five years earlier). They were measuring its radioactivity using a gold leaf electroscope, which used an electric field to hold two strips of gold leaf apart – when “radium rays” entered the devise, they ionised the air around the gold leaf, which allowed electrical charge to escape – the more radiation was present, the less charge the gold leaf held and the closer the two strips would move together.

The scientists noticed that, even when the electroscope was removed from the radium, it still lost electrical charge. Somehow radiation was coming somewhere else.

Nor was it an isolated event. Laboratories all over the world were reporting the same phenomenon.

Most scientists of the time believed that the radiation must have been coming from minerals in the ground. But, in 1910, a German physicist, Theodore Wulf, took an electroscope to the Eiffel Tower and tested ionisation levels at ground level and at the top of the tower. He found that the effect was actually stronger at altitude – the radiation wasn’t coming from the ground.

It was mystery worthy of Indiana Jones (if Indiana Jones was a detector-brandishing physicist and not a whip-wielding archaeologist) – enter our hero: Austrian physicist, Victor Hess.

Hess had the idea that the ionising radiation was coming from the sky rather than from the ground. He build brand new measuring devises that could survive the temperature and pressure changes that occur at altitude – then he took to his balloon.

His initial trips in 1911 and 1912 were promising. He found that, although levels of ionising radiation initially fell off, the higher he travelled, the higher the levels became and, at a height of several kilometres, ionisation was many times greater than at ground level. Hess concluded that “a radiation of very high penetrating power enters our atmosphere from above.”

But where was it coming from? One obvious candidate was that great nuclear fusion factory in the sky – the Sun.

On April 12, 1912, Hess took to his balloon once again, but this time he made his trip during a total eclipse of the Sun. If the Sun was the source of the mysterious emissions, the levels should drop right off when the Moon passed across and blocked the radiation.

But the levels measured by Hess didn’t decrease at all. He concluded that the radiation wasn’t coming from the Sun and must be coming further out in space.

Hess’s finding were confirmed in 1925 by American physicist, Robert Milikan, who dubbed the mysterious radiation “cosmic rays”. In 1936, Hess and Milikan shared the Nobel Prize in Physics for the discovery. They shared the prize with American physicist, Carl D Anderson, for his discovery of the positron (the antimatter version of an electron) – a discovery that stemmed from cosmic ray research.

After one hundred years of research in cosmic radiation, you’d think we’d have it pretty much sussed out, but, even today, there is a lot we still don’t understand – such as exactly where it comes from.

What ever the source turns out be (supernovae, black holes and starburst galaxies are hot favourites) we do know that it makes the Large Hadron Collider look like a particle pea-shooter – pumping out particles with 1,000 times more energy.