Forces of Nature. Andrew Cohen

of our language makes it easier for us to have foolish thoughts.’ ⁵ Physics is about precision of thought, which is aided and evidenced by precision of language.

      Here is the meaning of the caption. Neutral current means that the electron bounces off the proton by exchanging an electrically neutral object with it – in this case, a photon; a particle of light. The photon is shown in the diagram as the wavy line, labelled by the Greek letter γ. DIS stands for ‘Deep Inelastic Scattering’, which means that the photon is hitting something deep inside the proton, resulting in the proton being broken into pieces. This is how a modern particle physicist would describe the interaction between any two particles; interactions involve the ‘exchange’ of some other particle that carries the force. In this case, the force is electromagnetism and the force-carrying particle is a photon. The most fundamental description of the mechanism by which water molecules stick together to form ice is that photons are being emitted and absorbed by electrons in the water molecules, with the net result that water molecules stick together.

      There is another way of thinking about this electron–proton collision. You can imagine the photon emitted from the electron smashing into the proton and revealing its inner structure. That structure is shown in the second figure from my thesis, shown opposite.

      Allow me a single paragraph of postgraduate-level physics. I want to take this liberty for two reasons. The first is that there is great joy to be had in understanding a complex idea, and in doing so glimpsing the underlying simplicity and beauty of Nature. The biologist Edward O. Wilson coined the term ‘Ionian Enchantment’ for this feeling, named after Thales of Miletus, credited by Aristotle as laying the foundations for the physical sciences in 600 BC on the Greek island of Ionia. The feeling is one of elation when something about Nature is understood, and seen to be elegant. The second reason is to revisit and enhance an idea we’ve been developing. Science is all about making careful observations and trying to explain what you see. That might be the hexagonal structure of a beehive, the jagged symmetry of a snowflake, or the details of how electrons bounce off protons. Careful observations lead to Ionian Enchantment.

      At HERA, we measured the angle and energy of the electrons after they hit the protons. This is a simple thing to do, and it allowed us to build up a picture of what the electron ‘bounced off’ – the fizzing heart of matter. Two different ways of visualising the inside of a proton are shown in the figure. The thing called F₂ (x,Q²) is known as the proton structure function. Now for the precise bit of observation that requires thought. Have a look at illustration (a) here and focus on the bottom line of the graph labelled x = 0.13. The points along this line tell you the probability that an electron will bounce off something inside the proton that is carrying 13 per cent of the proton’s momentum – this is what x = 0.13 means. The quantity Q² is known as the virtuality of the photon that smashes into the proton. One way to think about this quantity is as the resolving power of the photon. High Q² corresponds to short wavelength, which means that high Q² photons can see smaller details. The x = 0.13 line is pretty flat, which means that whatever the photon is bouncing off, it behaves as if it has no discernable size. This is because what we see does not change as we crank up the resolving power of the microscope (which corresponds to going to higher Q²), and this is what would happen if the photon were scattering off tiny dots of matter inside the proton. The dot is known as a quark, and as far as we can tell, it is one of the fundamental building blocks of the Universe. Together, these two plots describe in detail the innards of the proton as revealed by years of experimental study by many hundreds of scientists at the HERA accelerator.

      The proton is a seething, shifting mass of dot-like constituents, continually evolving around scaffolding. The scaffolding consists of three quarks; two ‘up’ quarks and one ‘down’ quark. The quarks are bound together by the strong nuclear force, which is carried by particles called gluons in much the same way that the electromagnetic force is carried by photons. Unlike photons, however, the gluons can interact with each other through the exchange of more gluons, and that results in the proton having an increasingly complex structure as we dial up the resolving power. Illustration (b) shows this behaviour; the rising curves towards smaller x are telling us that there is a proliferation of gluons, each carrying very small fractions of the proton’s momentum. Illustration (a) also shows this. The lines are not flat at smaller x. In the jargon, this behaviour is known as ‘scaling violation’, which means that as we dial up the resolving power the dot-like constituents appear to be increasingly numerous. In other words, at low resolving power we tend to resolve only the scaffolding, i.e. the three quarks, while at high resolving power the full glory of the proton’s gluonic structure is revealed to us. Roughly speaking, gluons carry around half of the momentum of a proton, because there are so many of them buzzing around between the quarks. The lines on these graphs, which go pretty much through the data points, are calculated using our best theory of the strong nuclear force: Quantum Chromodynamics, or QCD. QCD is a set of rules that specifies the probability that a quark will emit a gluon, and also how gluons interact with other quarks and gluons. It’s a quantum theory – the same basic framework we referred to when we discussed the structure of the water molecule. When we are dealing with electric charges – for example, the interactions between electrons and the atomic nucleus – we use our quantum theory of electromagnetism called Quantum Electrodynamics, or QED.

      I remember writing computer programs to skim through vast amounts of data about individual electron-proton collisions and make figures like the one above. On the computers we had in the 1990s these programs took days to run. Even now, looking at these plots, I find it exhilarating to consider that I’m looking at the structure of an object a thousand million millionths of a metre in size, measured using a machine 6.7 kilometres in circumference beneath the city of Hamburg, and that we have a theory that allows us to understand and describe what we see. Industrial engineering and subatomic beauty in concert. The Ionian Enchantment.

      On the next page you will find a snapshot of the deep structure of ordinary matter. You are this, at the level of accuracy we can measure today. Two sorts of quarks, stuck together by gluons, to make protons and neutrons that are stuck together by more gluons to make atomic nuclei. Electrons are stuck in orbit around the nuclei by photons to make atoms and atoms stick together by exchanging photons between their electrons to make molecules. And so it goes! This simple picture is the result of a hundred years of experimental and theoretical investigation. The structure of everything can be explained using a set of building blocks and some rules. We’ve met three of the building blocks; up quarks, down quarks and electrons. We’ve also met two forces; the strong nuclear force and the electromagnetic force. There is another force called the weak nuclear force that can convert up quarks into down quarks, with the simultaneous emission of another sort of particle called the electron-neutrino. In total that makes four matter particles. The weak force is carried by particles known as the W and Z bosons. There is also the Higgs boson, discovered in 2012 at the Large Hadron Collider (LHC) at CERN, in Geneva, which gives the building blocks their mass.

      The fourth and final fundamental force is the most familiar – gravity. It is so weak that its effects on the subatomic world are invisible even in our most high-precision experiments, like those at HERA. If this statement seems a little mystifying, particularly if you’ve ever fallen off a ladder, then park it in your memory for a while; we’ll get back to gravity later when we discuss the shape of planets and galaxies.

      These four particles, four forces and the Higgs boson appear to be all that is needed to make a water molecule, a honeybee, a human being, or planet Earth. This is a dazzlingly elegant and simple structure. For some reason, Nature didn’t adopt this economical scheme but instead made two further copies of the family of up quarks, down quarks, electrons and electron neutrinos. These two extra families are identical to the first family in every way except that they are more massive, possibly because they interact with Higgs particles in a different way. The existence of the three families of particle is another of the great mysteries, and discovering why Nature appears to have been unduly profligate is one of the most important goals of twenty-first-century particle physics. She won’t have been unduly profligate, of course! We know that three families is the minimum number to accommodate a process known as CP