Cosmic rays

I recently attended a fascinating history talk at the Royal Society, together with nine of my international students from CATS College, Canterbury. This report draws on the notes made by my students and myself during the lecture, ‘The Centenary of the Discovery of Cosmic Rays (1912 -2012)’. This was the first in a series of autumn Friday lunchtime events in the Library, and was given by Professor Sir Arnold Wolfendale FRS.

The room was packed to capacity as Sir Arnold, Emeritus Professor of Physics at Durham University and the former Astronomer Royal, told the story of the last 100 years of cosmic ray research and the possibilities of future research into this enigmatic matter from the most explosive events in our Universe. His first slide showed three brave balloonists ascending to 5000m with an electrometer to measure the ionization as they travelled higher and higher into the atmosphere. Until then it was noticed that an electrically charged electroscope lost its charge over time even if completely surrounded by lead. Why? There was some mysterious penetrating radiation which Victor Hess (1912) found came from outer space.

We found out that the first picture of a cosmic ray was of a cloud chamber track in 1929 which demonstrated the tremendous energy that cosmic rays have. Successive balloon flights showed how the cosmic rays were very penetrating and that the primary cosmic ray hitting nitrogen and oxygen nuclei at heights over 50km produces a tremendous number of secondary cosmic rays that can reach ground level. In 1948 Sir Arnold was hoping to do research with the famous Professor Blackett in Manchester on “oxide coated cathodes”, but was steered towards cosmic rays and has never looked back. He is still doing work researching the effect of cosmic rays on climate change.

Sir Arnold discussed some of the strange particles discovered when cosmic rays came crashing into our Earth’s atmosphere, including the discoveries of antimatter in 1933 (by Anderson) and the muon in 1936 (by Anderson and Neddermeyer). Did you know that, because of cosmic rays, five muons pass through your head every second (more if you are bigheaded)?

Sir Arnold had worked with others down deep mines like the Kolar gold mines in India and discovered cosmic ray neutrinos in 1965. The instruments (neon tubes) showed the path of ionizing particles and at one point the team thought they had evidence for the particle that mediated the weak force (the so-called intermediate vector boson).

He asked the question: where do cosmic rays come from? The obvious answer was the Sun, and he showed a slide of the Sun in X-rays showing the corona glowing at 2 million K – far hotter than the surface of the Sun at a mere 6000K. The Crab Nebula, the site of the famous supernova explosion seen by Chinese astronomers in 1054, was an example of a violent event giving rise to strong magnetic fields which accelerated the charged particles from the remnant. He also showed pictures of Eta Carinae – a star that has had several explosions in the past – and the enigmatic Centaurus A galaxy where there lurks a supermassive black hole and huge jets of particles. All these objects are probably sources of cosmic rays; unfortunately we cannot be sure of the direction of a cosmic ray, because the bending effect of our galaxy’s magnetic field means that it cannot be traced back to the source.

The most dangerous cosmic rays are the heavy nuclei – a picture of the damage that could be caused was seen in a picture of the helmet of an Apollo astronaut. One of the questions asked of Sir Arnold after his talk concerned the perils of journeying to Mars over a two-year journey in space with this cosmic ray radiation. He thought that one day we would be able to create strong magnetic fields around a spacecraft sufficient to deflect the incoming charged particles, working in a similar way to the Earth’s magnetic field.

Regarding the future of cosmic ray research, Sir Arnold pointed to four key questions:

1. Can we narrow down the exact mechanisms for producing cosmic rays which have the same energy density as starlight (though this research does not attract the same level of funding)?

2. How are the highest energy cosmic rays produced? Some have 100 million times more energy than the CERN LHC particles which helped physicists discover the particle exhibiting the properties of the Higgs Boson.

3. Was life on Earth created with the help of cosmic rays? (Urey experiments with lightning and a flask of primeval soup in the 1950s produced amino acids, the building blocks of life). Lightning requires a large electric field which can be helped by cosmic ray showers.

4.  Can cosmic rays help as a diagnostic tool in uncovering secret chambers in pyramids, and provide clues to explaining the weather on Neptune and cloud cover on the Earth?

A splendid talk was followed by numerous questions, and Sir Arnold was surrounded by many of the audience (including us) who stopped to ask more questions.

We noticed that the first slide was a historical image of a balloonist, a meteorologist and a physicist, and the last slide showed the future of cosmic ray research, including how climate may have been influenced by cosmic rays, how astronauts might journey to other planets and stars and need protection from cosmic rays, and how cosmic rays could help astronomers to understand more about dark matter/energy and the Universe.