A Faustian Pact and the Chamber of Secrets

by Nicholas Mee on October 28, 2012

To Brocken’s tip the witches stream,
The stubble’s yellow, the seed is green.
There the crowd of us will meet.
Lord Urian has the highest seat.

In 1894, the physicist C.T.R. Wilson spent a few weeks away from the Cavendish laboratory in Cambridge working as a meteorological observer on the summit of Scotland’s highest mountain, Ben Nevis. For much of his time there, mist and cloud shrouded the mountain tops around the research station. One day, while Wilson was making observations up in the mist, he was confronted by a huge ghostly figure surrounded by a multicoloured aura. This was the Brocken spectre, the giant who would direct Wilson towards his future research.

The Brocken Spectre

If weather conditions are just right, hikers in the mountains can witness the phenomenon of the Brocken spectre. When the Sun hangs low in the sky and shines through the misty gloom, it may cast a shadow onto a bank of fog. Hikers facing away from the Sun will sometimes see a giant shadow of themselves enveloped in an aetherial multicoloured halo. The coloured rings forming the halo are optical effects known as glories. They are produced by sunlight that is refracted and scattered back by uniformly sized water droplets and appear as circular rainbows. Any movement of the mist enhances the eerie appearance of the giant figure by making the looming spectre seem to hover in the air.

The Witches’ Sabbath

The Brocken spectre takes its name from the highest peak of Germany’s Harz Mountains, a peak that often projects through the cloud layer and provides a good display of the giant. This was the ominous site chosen by Goethe for the scene of the Witches’ Sabbath on Walpurgis Night in his epic drama, Faust. Goethe refers to the giant as Herr Urian – Lord Urian in the above translation. Wilson may not have sold his soul, as Faust did, but his experience of the spectre on the summit of Ben Nevis would lead him to the highest accolade for a physicist – a Nobel Prize.

Head in the Clouds

Wilson was intrigued by the spectacular optical phenomena he had witnessed in the mountains. When he returned to Cambridge early in 1895, he set about reproducing the optical effects he had seen in Scotland. To do this he needed a reliable laboratory method of creating miniature clouds that resembled the real thing, so he built a sealed chamber containing air that was saturated with water vapour. He then attached a piston to rapidly expand and cool the air inside the chamber. As humid air cools, its ability to retain water vapour is reduced, and so the water vapour is brought to the brink of condensation. But water droplets can form only if there are dust particles or tiny ice crystals present, around which the water can condense. Wilson soon discovered, however, that even when the vapour was completely dust free, a few small water droplets would always form in his chamber.

X-ray Vision

Carl Anderson published this photograph in 1932. It shows the curved path taken by a positron in his cloud chamber. (The positron is the antiparticle of the electron.) The discovery of antimatter in this photograph was one of the most important discoveries of the 20th century.

After a couple of months of studying cloud formation in his device, Wilson speculated that these droplets might be forming around charged ions in the vapour. The newly discovered X-rays were known to ionise air, so he tested his idea by firing X-rays into the cloud chamber. His suspicions were confirmed. The path of an X-ray through the chamber was made visible as a vapour trail of tiny water droplets. What was happening was that when a high energy X-ray photon or a charged particle passed through the cloud chamber, it collided with electrons in atoms in the vapour. The electrons were knocked out of the atoms, leaving charged ions behind. Water vapour then condenses around the ions, and the path of the particle becomes visible as a vapour trail. Wilson realised that he had developed the perfect instrument for detecting the presence of any electrically charged fundamental particles.

Chamber of Secrets

Over the course of many years, Wilson perfected his cloud chamber. By placing the chamber in a magnetic field he was able to determine the electric charge of a particle passing through it. In a magnetic field, charged particles follow curved trajectories, with positive and negative particles curving in opposite directions. The cloud chamber was instrumental in the first discovery of a subatomic particle – the electron. In a series of methodical experiments in the late 1890s, Wilson’s boss J.J. Thomson, the director of the Cavendish Laboratory in Cambridge, gradually elucidated the properties of the electron. The decisive factor in gaining Thomson the credit for the discovery of the electron was his access to a crucial piece of equipment, Wilson’s cloud chamber, which he used to measure the electric charge of the electron. Thomson was awarded the 1906 Nobel Prize in Physics for this discovery. Wilson would receive the same prize twenty-one years later for his invention of the cloud chamber, one of the most important tools of twentieth-century physics.

This article is a short excerpt from my book Higgs Force. You can find out more about the book here: http://quantumwavepublishing.co.uk/higgs-force/

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