The 20th century witnessed incredible inventions and discoveries as physicists delved ever deeper inside the atom. From the apparatus used to identify the first subatomic particle to the photograph that confirmed the existence of antimatter, Collider showcases some of the most significant objects from the history of physics.
Videos | JJ Thomson and the first particle | CTR Wilson's cloud chamber | The foundations of CERN
CTR Wilson's cloud chamber
One man’s fascination with clouds was about to reveal the hidden subatomic world for the first time.
A Brocken spectre, the beautiful phenomenon that inspired Wilson's scientific career © Brocken Inaglory
Inspiration in the Highlands
Sat at the summit of Scotland’s highest mountain in 1894, CTR Wilson saw some sights that stirred his scientific imagination.
Wilson was working at a weather observatory on the top of Ben Nevis. There he witnessed 'wonderful optical phenomena' known as a Brocken spectre – an awe-inducing effect where the sun projects your shadow onto a cloud, haloed by a rainbow of colours.
'Greatly excited', Wilson wanted to replicate the beautiful effect back in Cambridge’s Cavendish laboratory. In early 1895, he built a bulb-like glass chamber to artificially create clouds.
'Almost immediately,' Wilson recalled, 'I came across something which promised to be of more interest than the optical phenomena which I had intended to study'.
In previous experiments, it seemed clouds formed when vapour condensed around dust particles. But Wilson found with an almost perfectly clean chamber that it was still possible to produce clouds – given the right level of air expansion. To find out what was causing the condensation of water vapour, he set about improving his apparatus.
Building cloud chambers was an extremely tedious process. It involved some very tricky glass blowing, which Wilson did entirely himself. 'Many hours of glass grinding had to be done', Wilson recalled, 'in the knowledge that the whole apparatus was almost certain to fly to pieces when the final glassblowing was done'.
CTR Wilson’s masterpiece, his final cloud chamber of 1911. © Cavendish Laboratory
JJ Thomson, Professor of the Cavendish at the time, revealed that the frequent breakages 'never seemed to disconcert Wilson; he would take up a fresh piece of glass, perhaps say 'Dear, dear', but never anything stronger, and begin again'.
X-rays and a foggy revelation
It was one of Thomson’s x-ray tubes that Wilson borrowed 'with excitement' to use on his second, improved cloud chamber. That year, in 1895, Wilhelm Rontgen’s revolutionary discovery of x-rays shook the scientific community and captured the public’s imagination. Photographs that revealed the bones beneath flesh caused a world-wide sensation.
At the Cavendish, Rontgen’s rays proved to be a hugely-significant catalyst for several strands of groundbreaking research. X-rays had inspired Thomson to investigate cathode-rays, which led to his identification of the first subatomic particle in 1897 – now known as the electron.
X-ray image of King George’s and Queen Mary’s hands, 1896. © Science Museum / SSPL
In bombarding his cloud chamber with x-ray radiation, Wilson observed an unprecedented thick fog. 'I still vividly remember my delight at getting these results', he wrote in 1959 at the age of 90.
Wilson soon realised why the x-rays caused more cloud to form. The radiation was stripping electrons from the air molecules in the chamber. Water droplets would then grow around the remaining electron-less debris (called ions).
One of Wilson’s fellow Cavendish students, Ernest Rutherford, showed in the following years that all radiation was composed of particles. Wilson dreamt the cloud method might one day be able to make visible the paths of these particles as they passed through the chamber.
In 1910 Wilson built his masterpiece and realised his dream: a cloud chamber that could visualise particle tracks – resembling something like the vapour trails left in the wake of an airplane. The next year he took his first photographs of tracks, exclaiming excitedly, 'they are as fine as little hairs'. For the first time, physicists could see the activity of the subatomic world.
Cloud chambers practically sparked a whole new field of physics – cosmic-ray research – as they were used as a means to detect particles that come from space. Wondering why clouds form in the absence of an obvious radiation source, Wilson even anticipated the existence of cosmic rays 11 years before they were confirmed by Victor Hess in 1911.
Cloud chamber photograph of Rochester and Butler’s discovery of the k-meson, 1946. © Science Museum / SSPL
One of the first particle detectors, the cloud chamber would be responsible for some of the great finds in 20th-century physics, including the discovery of antimatter. Rutherford described Wilson’s invention as 'the most original and wonderful instrument in scientific history'.