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.

In Our Time: Exoplanets

The first planet orbiting a star other than the Sun wasn’t discovered until 1992 and since then the subject of exoplanets has gone from being something you argue about the existence of to a rapidly expanding field with new discoveries all the time. The experts who discussed exoplanets on In Our Time were Carolin Crawford (University of Cambridge), Don Pollacco (University of Warwick) and Suzanne Aigrain (University of Oxford).

One of the reasons it took so long to discover any extra-solar planets, despite people speculating about their existence for centuries, is that they are very hard to directly see. In fact I think they were saying that none of the known ones have actually been seen. Instead a variety of more indirect techniques are used to detect them, and these required both sophisticated technology & sophisticated knowledge of physics before they could be used. The technology needed to develop to a point where small differences in stars could be measured accurately and consistently over time. And the physics is required to both predict how a star without planets would behave and then to figure out what the differences from this prediction mean.

In the programme they ran through a variety of techniques used to detect planets. One of these is to look at the colour of the star’s light and see if it’s changing between blue-shifted & red-shifted over time. If the star has no planets then you won’t detect that. When there’s a planet orbiting the star it’s not quite as straightforward as the planet circling the star, actually the star and the planet are both circling a point between them (that’s a lot lot closer to the centre of the star than it is to the planet). So the star will seem to move back & forth relative to us observing it. This is biased towards detecting more massive planets, as they’ll move the centre of gravity from the centre of the star more – so-called “hot jupiters” for instance, which are planets the mass of Jupiter that orbit close to their star.

Another method is to look for the changes in the star’s light caused by the transiting of a planet across the face of the star. Obviously this is only possible to detect if the planet is orbiting in the right plane for us to see it. But if you have one transiting where we can detect it then you can detect the existence of other planets in that system by looking at the perturbations of the orbit of the one that transits. You can also detect things about a planet’s atmosphere with this method. The changes in the light of the star can be used to tell you something about the size of the planet (in terms of diameter), and if you look at different wavelengths of light then you’ll see varying diameters. This tells you when the atmosphere of the planet is thin enough to be transparent to that wavelength, and different gases absorb different wavelengths differently so you can figure out the gases that are present. Apparently you can even detect the presence of clouds using this technique.

Another method uses the phenomenon of gravitational lensing. If the light from a distant star passes by a closer to us star on it’s way to the telescope then it will be bent by the gravity of the middle star. A planet orbiting that middle star will affect the lensing effect, and you can figure out things about the size & distance from the star by exactly how the lensing is affected.

If you use the first two methods together you can tell things about the density of the planet. Is it small & heavy? Is it big & fluffy? Or even small & fluffy? There seem to be a wide variety of planet types out there, not all of which are represented in our own solar system. There are also a wide variety of types of solar system out there – Pollaco pointed out that one reason there was argument about the reality of the first exoplanet discovered was because people were assuming that our own solar system was a good model for “all systems everywhere”. It turns out it’s not. The example they used in the programme was systems that have hot jupiters – the first exoplanet was one of these, and the very idea of a Jupiter type planet orbiting with a periodicity of only 4 days was almost unthinkable. They also talked about planetary systems detected around brown dwarves – stars which weren’t quite massive enough to ignite at the end of the formation process. And planets around pulsars (again like the first ones detected) – and one of the experts (I think it was Crawford?) made a throwaway remark about how these are probably not the first planetary system for the star in question. Before a star becomes a pulsar it goes through a supernova explosion, which would probably destroy any original planets – the ones orbiting afterwards are probably secondary captures.

They also discussed looking for planets which might be habitable. Bragg asked if we are thinking about life like ourselves, or germs. The answer was (paraphrasing) “yes”. At the moment no-one knows enough to know what we’re looking for in terms of life on other planets, and at first we’re obviously limited to things we know about life on Earth as a starting point for what to look for. So looking for rocky planets which are neither too big nor too small, that are in the right zone for liquid water. And other things about our own solar system might’ve been necessary – like the presence of Jupiter which draws away some of the comets that could bombarded Earth & wipe out all life. I think it was Aigrain who talked about other ways of detecting life – looking at what we can tell about the atmospheres of the planets. If there are very reactive gases present then they must be being made constantly – some of these we only know of biological processes that make them. So if one could detect such gases that’d be a sign of life.

It was a little bit of an odd In Our Time episode, because there was less of a sense of a narrative than they normally have. It felt like this is because the study of exoplanets is in its infancy – we’re at a point where most of the work is data gathering. I mean in the sense that a lot of planets are being discovered and categorised, but as yet they’re not classified and grouped into types. Nor are there overall theories about how solar systems in general work or were formed – it’s now clear that the one we know isn’t the only sort there can be, nor is it particularly typical of what we’re detecting now.