Forces of Nature. Andrew Cohen

with equal amounts of matter and anti-matter, there is matter left over in the Universe today to make stars and people. But that’s not an answer to the ‘Why?’ question, and it would be nice to know if the existence of planets, stars and galaxies is down to more than blind luck.

      With these extra families, there are twelve fundamental particles of matter, four different sorts of force-carrying particle and the Higgs particle. That’s it, as far as we know – although I wouldn’t be surprised if some more pop up at the Large Hadron Collider over the next few years. This is fuelled by the fact that we already have good evidence from many independent astronomical observations that there is another form of matter in the Universe known as dark matter. There is five times more dark matter than ‘normal’ matter in the Universe by mass, and the dark matter cannot be made up out of the twelve particles that we’ve seen in experiments at particle accelerators such as HERA or the LHC. The collection of fundamental building blocks, circa 2015, is shown in the illustration below.

      

      The fundamental building blocks of the natural world, and three of the four fundamental forces of Nature: the strong nuclear force, carried by gluons; the weak nuclear force, carried by W and Z bosons; and the electromagnetic force, carried by photons.

      This isn’t intended to be a complete course on particle physics, much as I’d like to deliver that; rather, it is a chapter about shapes and patterns in Nature and what they reveal about the way in which the Universe works. Having said that, if you’ll allow me one last foray into particle physics, the story of the discovery of the quarks inside the proton and neutron is a very beautiful example of the way physicists notice patterns and attempt to explain them. The remarkable thing is that quarks were predicted before they were discovered experimentally.

      The theoretical prediction that building blocks exist beneath the level of protons and neutrons was made by Murray Gell-Mann and George Zweig in 1964. It was based on a pattern in the subatomic particles known at the time. By the early 1960s, an inelegant, profligate and seemingly ever-expanding list of subatomic building blocks had been discovered. The proton and neutron are part of a whole family of particles known as baryons; there are Lambdas, Sigmas, Deltas, Cascades and a host of others. There is also a family of particles known as mesons: Pions, Kaons, Rho and so on. There are thirteen different types of Lambda particle alone, nine Sigmas and eight Kaons. Particle physics was looking increasingly like a subatomic branch of botany. Then Gell-Mann and Zweig noticed a beautiful pattern. The particles could be arranged according to their observed properties in geometrical patterns. One such pattern is shown in the illustration here. Today, these are known as ‘super-multiplets’.

      As Kepler suspected when he considered the six-fold symmetry of snowflakes, patterns in Nature are often a clue that there is a deeper underlying structure. The patterns may or may not be easy to recognise – Gell-Mann received the Nobel Prize in Physics in 1969 for noticing the pattern amongst the particles – but they are the Rosetta Stone that allows Nature’s language to be deciphered. In this case, the pattern in the particles suggested to Gell-Mann and Zweig that the baryons are all constructed out of three smaller building blocks, that Gell-Mann called quarks. When they first recognised the pattern, they included three quarks in their scheme: up, down and strange. The different baryons on the lower planes of the super-multiplets are the possible three-fold combinations of the three building blocks. Adding a fourth quark – charm – constructs the higher layers. The quark constituents of the particles are shown in the illustration opposite: for example the ∆⁺⁺ contains three up quarks.

      A baryon ‘super-multiplet’ showing the quark content of each baryon.

      The particle on the base of the pyramid in the illustration, known as the Omega-minus, is of particular historical interest because its existence was predicted by Gell-Mann at a meeting at CERN in 1962, based solely on the pattern of the base of the pyramid. It was subsequently discovered at the Brookhaven National Laboratory in the United States in 1964. When a theory predicts the existence of something new that is subsequently discovered, we can have particular confidence that we are on the right track.

      We’ve met three of the four fundamental forces of Nature; the strong and weak nuclear forces and electromagnetism, and the twelve building blocks of Nature. We will now turn to the final, weakest and most familiar force – gravity – and investigate it by thinking about the size and shape of the objects it sculpts. These are not tiny things like subatomic particles, or small things like snowflakes, but very much larger structures: planets, stars and galaxies.

Why is the Earth a sphere?

      There is a photograph of our planet known as the Blue Marble. It was taken on 7 December 1972 by the crew of Apollo 17 during their journey to the Moon. Close to the winter solstice, Antarctica is a continent of permanent light, and Madagascar, the island of lemurs, takes centre stage. Ochre deserts set against blue oceans, green hues hinting at life.

      On 5 December 2012, NASA released the Black Marble, an image of the Americas at night. Now we see a civilisation on the planet; the lights herald the dawn of the Anthropocene – the age of human dominance. What do we see in these images? What is the most basic property of Earth? Alexei Leonov, on completing the first human spacewalk on 18 March 1965, had an answer.

       ‘I never knew what the word round meant until I saw Earth from space.’

      Alexei Leonov, Voskhod 2, Soyuz 19/ASTP

      Seen from space, the Earth is a near-perfect sphere. All the planets in the Solar System, all the large moons and the Sun itself share this property, as does every star in the Universe. Why? If lots of different objects share a common feature, there must be an explanation. To make progress, let’s think about what could affect the shape of a planet, moon or star. It can’t be much to do with their composition because planets are made of different stuff to stars. The Earth is made up of heavy chemical elements such as iron, oxygen, silicon and carbon. The Sun, on the other hand, is primarily hydrogen and helium; it’s a giant ball of plasma with no solid surface. Giant planets such as Jupiter have more in common with stars than with Earth, at least in terms of their composition. They too are primarily composed of hydrogen and helium. Stars and planets are united, however, by the force that formed them and holds them together – gravity. So to understand why they are all spherical, we should explore the nature of the gravitational force further.

      Defying gravity

      For most of the time Tarragona is a quiet Mediterranean port on the northeastern coast of Spain, but each September it explodes into vivid, violent colour as teams compete against gravity in the Tarragona Castells competition. Castells are human towers, reaching ten people high and involving an intricate mix of strength, balance, strategy and teamwork to be built up to the top. Each team begins by forming the foundations of the tower, with up to two hundred people creating the pinya. Once the foundation is in place, a variety of human geometries are used to build as high as possible, with each level taking shape before the next is added. The most successful team is the Castellers de Vilafranca, having won the Tarragona competition eight times since 1972. A mass of green shirts acting in unison flows from one level to the next, with higher levels consisting of fewer people, until two children form a final stable platform for the enxaneta – the casteller who ascends daringly to the summit; since low mass, agility – and perhaps a lack of fear – are called for, the enxaneta will be as young as 6 or 7 years old. This is what the crowds have come to see. Towers give way, human buildings come tumbling down, falls softened by the elbows, knees, heads and shoulders, colliding and crashing, usually delivering only bruises, bumps and the occasional lost tooth. Serious injuries are very rare.

      It is obvious why people fall to