In Our Time: Cosmic Rays

We’re back to listening to episodes of In Our Time on Sunday mornings. The one we listened to this week was about that staple of 1930s science fiction – cosmic rays. The three experts who were talking about the reality of this phenomenon were Carolin Crawford (University of Cambridge), Alan Watson (University of Leeds) and Tim Greenshaw (University of Liverpool).

Cosmic rays were discovered about a century ago. The first indications that they existed came from detection of increasing radiation levels as you go higher up in the Earth’s atmosphere. At first they were assumed to be photons and the name “cosmic rays” was coined. This turns out to be a misnomer, they are in fact charged particles – bits of atoms. Some of them are atomic nuclei, some are electrons and some are more exotic things like positrons. They travel at a variety of speeds, from a variety of sources. Crawford told us that they are categorised into three broad groups. The first of these are relatively slow-moving particles that come from relatively local sources – the sun for instance – and are very common. These are the particles that are involved in creating the Aurora Borealis. The next group are moving more quickly, and come from further away – generally these are thought to be generated as as side-effect of supernova explosions. And the last group are the fastest moving and are thought to be from outside our galaxy, these are the rarest type of particle.

The particles aren’t detected directly (on Earth) instead what we detect is the side-effects of these particles hitting the Earth’s atmosphere. As the particles collide with atoms in the upper atmosphere they generate a shower of secondary particles and it’s these that are detected. The types and numbers of these particles can be used to work out what hit the atmosphere, how fast it was going and the direction it was travelling. We know they are charged particles (and which charge) because of the effects of the Earth’s magnetic field – the number of particles hitting the atmosphere varies with latitude with most of them at the poles. This is also why the Aurora Borealis are mostly at the poles. That phenomenon is formed by the particles exciting the electrons in atmospheric atoms, when the electrons return to their original energy states they emit light. They went off into a slight digression on the programme when talking about this – predicting the Aurora Borealis requires prediction of solar weather and that’s being worked on because particularly bad solar weather can lead to EMPs that can affect satellites.

All three experts agreed that the fastest moving group are the most interesting – in part because we still don’t know much about where they come from or how they’re generated. They’re pretty rare, so a normal sized detector (I don’t think they said how big) would only detect about one a century – so Watson was talking about a project he helped set up that built a detector the size of Luxembourg and this detects 3 or 4 of these rare particles a year. One theory of where they come from is that they are generated in galaxies with super massive black holes at the centre. Another is that they have something to do with dark matter.

Particle physics as a discipline grew out of the study of cosmic rays. The Large Hadron Collider does under controlled conditions what cosmic ray particles do when they hit the atmosphere. This is another reason why the fastest particles are the most interesting – they travel at a much higher speed than the LHC can achieve. The fastest moving particles travel faster than the speed of light in air, generating Cherenkov radiation. Again the programme took a little digression to explain this. Light travels at different speeds in different media – and so these particles aren’t travelling faster than light does in a vacuum (like the space the particle was just travelling through), it’s just that they don’t slow down when they enter the atmosphere. So the radiation that is released in front of the particle is moving slower than the particle and so can’t move away from the particle. It’s effectively being pushed along in front of the particle and that’s what we detect as Cherenkov radiation. It’s a bit like the sonic boom you get when something breaks the speed of sound.

As an aside – something I didn’t know before was that 14C dating is a direct result of cosmic rays. The 14C in the atmosphere is generated by cosmic ray particles hitting nitrogen atoms, if cosmic rays didn’t exist we’d not have such a good way of dating organic material (like bones).

Future work on cosmic rays is quite concentrated around figuring out what the fast ones are. There is also data being gathered more directly on the particles involved. The ISS currently has a cosmic ray detector fitted to the side of it, which has been gathering data since 2011 and is planned to continue for ten years.