What We Cannot Know: Explorations at the Edge of Knowledge. Marcus Sautoy du

The particle seemed to have the same charge as the electron but was much harder to deflect.

      Now called the muon, it was one of the first new particles to be discovered in the interactions of cosmic rays with the atmosphere. The muon is unstable. It quickly falls apart into other particles, most often an electron and a couple of neutrinos. Neutrinos were another new particle that had been predicted to explain how neutrons decayed into protons. With almost no mass and no charge, it took until the 1950s before anyone actually detected these tiny particles, but they theoretically made sense of both neutron decay and the decay of this new muon. The decay rate of the muon was on average 2.2 microseconds, which is sufficiently slow that enough particles won’t have decayed by the time they reach the surface of the Earth.

      The muon helped to confirm Einstein’s prediction from special relativity that time slows down as you approach the speed of light. Given its half-life, far fewer muons should be reaching the surface of the Earth than were being detected. The fact that time slows down close to the speed of light helps explain this discrepancy. If a clock was attached to the muon, it would show that a smaller interval of time had elapsed before it hit the Earth. Thus more muons would therefore survive, as revealed by experiment. I will return to this in the Fifth Edge when I consider pushing time to the limits of knowledge.

      The muon appeared to behave remarkably like the electron but had greater mass and was more unstable. When the American physicist Isidor Rabi was told of the discovery, he quipped: ‘Who ordered that?’ It seemed strangely unnecessary for nature to reproduce a heavier, more unstable version of the electron. Little did Rabi realize how much more there was on the menu of particles.

      Having realized that cosmic ray interactions with the upper atmosphere were creating new forms of matter, physicists decided that they better not wait for the particles to reach the cloud chambers in the labs, by which time the particles might have decayed into traditional forms of matter. So the cloud chambers were moved to high-altitude locations in the hope of picking up other particles.

      The Caltech team chose the top of Mount Wilson near their home base in Pasadena. Sure enough, new tracks indicated that new particles were being picked up. Other teams placed photographic plates in observatories in the Pyrenees and the Andes to see if they could record different interactions. Teams in Bristol and Manchester also saw traces of new particles in their own photographic plates. It turned out that the muon was the least of Rabi’s worries. A whole menagerie of new particles started showing up.

      Some had masses about one-eighth that of a proton or neutron. They came in positively or negatively charged varieties and were dubbed pions. An electrically neutral version that was harder to detect was later discovered. In Manchester two photographs from their cloud chamber showed what appeared to be a neutral particle decaying into pions. The mass of these new particles was roughly half that of a proton. The cloud chamber at the top of Mount Wilson recorded more evidence to support the discovery of what would become known as kaons, four in number.

      As time went on, more and more particles were uncovered, so much so that the whole thing became totally unwieldy. As Nobel Prize winner Willis Lamb quipped in his acceptance speech of 1955: ‘The finder of a new elementary particle used to be rewarded by a Nobel Prize, but such a discovery now ought to be punished by a $10,000 fine.’ The hope was that the periodic table would be simplified once scientists had discovered how it was put together using electrons, protons and neutrons. But these three particles turned out to be just the tip of the iceberg. Now there were over a hundred particles that seemed to make up the building blocks of matter. As Enrico Fermi admitted to a student at the time: ‘Young man, if I could remember the name of these particles, I would have been a botanist.’

      Just as Mendeleev had managed to find some sort of order with which to classify and make sense of the atoms in the periodic table, the search was on for a unifying principle that would explain these new muons, pions, kaons and other particles.

      The underlying structure that finally seemed to make sense of this menagerie of particles – the map, as it were, to find your way around the zoo – ultimately came down to a piece of mathematics.

       MAPPING THE PARTICLE ZOO

      When you are trying to classify things, it helps to recognize the dominant characteristics that can gather a large mess of objects into smaller groups. In the case of animals, the idea of species creates some order in the animal kingdom. In particle physics one important invariant that helped divide the zoo into smaller groups was the idea of charge. How does the particle interact with the electromagnetic force? Electrons would bend one way, protons the other, and the neutron would be unaffected.

      As these new particles emerged from the undergrowth, they could be passed through the gateway of the electromagnetic force. Some would join the electron’s cage, others would head towards the proton, and the rest would be put together with the neutron – a first pass at imposing some order on the menagerie of particles.

      But the electromagnetic force is one of four fundamental forces that have been identified at work in bringing the universe together. The other forces are gravity, and the strong nuclear force responsible for binding protons and neutrons together at close quarters inside the nucleus, and finally the weak nuclear force that controls things like radioactive decay.

      The key was to identify other characteristics similar to the idea of charge that could distinguish the different behaviours of these particles with the other fundamental forces. For example, the mass of a particle was actually quite a good way of establishing some hierarchy in the particle zoo. It collected pions and kaons together as particles that were a factor lighter than the protons or neutrons that made up ordinary matter. A new collection of particles called Sigma, Xi and Lambda baryons had masses larger than the proton and neutron and often decayed into protons or neutrons.

      Often particles with very similar masses got the same Greek names. Indeed, the proton and neutron have such similar masses that they were believed to be intimately related, so much so that the German physicist Werner Heisenberg (whose ideas will be at the heart of the next Edge) rechristened them nucleons. But mass was a rather rough and ready way of sorting these particles. Physicists were on the lookout for something more fundamental: a pattern as effective as the one Mendeleev had discovered to order atoms.

      The key to finding patterns to make sense of the onslaught of new particles was a new property called strangeness. The name arose due to the rather strange behaviour demonstrated by some of these new particles as they decayed. Since mass is equivalent to energy via Einstein’s equation E=mc², and nature favours low-energy states, particles with larger mass often try to find ways to decay into particles with smaller mass.

      There are several mechanisms for this decay, each depending on one of the fundamental forces. Each mechanism has a characteristic signature which helps physicists to understand which fundamental force is causing the decay. Again it’s energy considerations that control which is the most likely force at work in any particle decay. The strong nuclear force is usually the first to have a go at decaying a particle, and this will generally decay the particle within 10^–24 of a second. Next in the hierarchy is the electromagnetic force, which might result in the emission of photons. The weak nuclear force is the most costly in energy terms and so takes longer. A particle that decays via the weak nuclear force is likely to take 10^–11 seconds before it decays. So by observing the time it takes to decay, scientists can get some indication of which force is at work.

      For example, a Delta baryon decays in 6 × 10^–24 seconds to a proton and a pion via the strong nuclear force, while a Sigma baryon takes 8 × 10^–11 seconds to decay to the same proton and pion. The longer time of decay indicates that it is controlled by the weak nuclear force. In the middle we have the example of a neutrally charged pion decaying via the electromagnetic force into two photons, which happens in 8.4 × 10^–17 seconds.

      Imagine a ball sitting in a valley. There is a path to the right which, with a little push, will take the ball over the hill into a lower valley. This path corresponds to the strong nuclear force. To the left is a higher hill which is also a path to a lower energy state. This