6 August 2012. It’s a rather grey evening and I’m in the basket of a hot-air balloon, drifting over the small town of Bad Saarow–Pieskow, some 50 km south-east of Berlin. It’s a ‘first’ for me and my companions who include Bill Breisky, an American writer and former editor of the Cap Cod Times. He’s also the grandson of Victor Hess, whose balloon flight 100 years ago opened a new window on matter in the universe. On 7 August 1912, Hess had landed near Pieskow – no one now knows exactly where – but there the similarities with our small adventure end. Hess had flown for six hours, carried by a hydrogen balloon to a height of more than 5000 m. During the flight, he made measurements that showed that the natural level of radiation increases with altitude, leading him to conclude that “a radiation of very high penetrating power enters our atmosphere from above”. This was the moment that 100 years later is being celebrated as marking the discovery of ‘cosmic rays’.
Although it’s not the name that Hess gave his discovery, it’s certainly apt. We now know that cosmic rays are energetic particles from outer space. When they enter the Earth’s atmosphere, they generate showers of further particles that penetrate right down to the ground, and even below ground. As you read this, about one cosmic-ray muon, a heavier sibling of the electron, passes through your head each second, mainly from above.
Studies of cosmic rays opened the door to a world of particles beyond the confines of the atom: first, the positron (the anti-electron), then the muon, followed by the pion, the kaon and several more. Until the advent of high-energy particle accelerators in the early 1950s, this natural radiation provided the only way to investigate the growing particle ‘zoo’. Indeed, when CERN was founded in 1954, its convention included cosmic rays in the list of scientific interests. But even though accelerators came to provide the best hunting ground for new particles, cosmic rays have maintained their mystery. The record energies of the LHC are still puny compared with the highest energy cosmic rays, where a single proton entering the atmosphere can pack the punch of a tennis ball served by a top player.
Since Hess’s discovery, physicists may have answered the ‘what’ of cosmic rays – they are energetic particles – but they still haven’t answered the ‘how’ or ‘where’. Just how does nature accelerate them to such high energies? Where are the natural accelerators? These remain mysteries that continue to drive adventurous research, in places as diverse as the deep ice of the South Pole and the high plateau of central Nambia.
This brings us back to how Bill and I ended up in balloon together. Michael Walter and colleagues at the German laboratory, DESY – which has a big involvement in the IceCube experiment at the South Pole and HESS facility in Namibia – had organized a conference in Bad Saarow to celebrate the centenary. The meeting brought together historians as well as key people in the on-going study of cosmic rays. Bill was one of the invited speakers. On 7 August, he and his brother unveiled a plaque on a geological ‘erratic’ in Peiskow – a stone deposited after being carried from afar by a glacier during the last Ice Age. It was a fitting tribute to Victor Hess, who had found himself near the same place after making a long journey to study an intriguing natural phenomenon – and setting us on a road that would, among other things, lead to CERN and the LHC.