Introduction

Humanity has always been driven to understand what the world is made of. As far back as antiquity, Greek philosophers asked: what is the smallest building block of matter? Thinkers such as Democritus arrived at remarkably modern intuitions. He introduced the concept of atomos – Greek for 'indivisible' – and proposed that all matter is composed of small, immutable particles with properties such as size and shape. It was a brilliant idea for its time, though still far from the reality we understand today. Many of his contemporaries believed that everything was composed of just four elements: earth, water, air, and fire.

More than two thousand years passed before science revisited this idea and began to test it experimentally. In the early nineteenth century, John Dalton proposed that each chemical element consists of its own kind of atoms. Water, for instance, is built from hydrogen and oxygen. About ninety elements occur naturally, though their abundance and distribution vary enormously. Dmitri Mendeleev arranged those elements into his periodic table – and remarkably, a clear structure emerged within it. But why?

The answer began to unfold at the start of the twentieth century. In 1904, J.J. Thomson demonstrated with a cathode ray tube that atoms are not solid and indivisible: they contain negatively charged particles with a mass far smaller than that of a hydrogen atom. The electron had been discovered. A few years later, Ernest Rutherford identified the positively charged atomic nucleus, and it eventually became clear that the nucleus contains protons – the particles that determine the atomic number. Yet atomic mass increased faster than the atomic number alone could account for, so something else had to be present. In 1932, the neutron was discovered: an electrically neutral particle that, together with the proton, builds and stabilises the nucleus.

An atom turns out to be a remarkably sparse structure. The nucleus is extremely small compared to the whole atom, while the surrounding electrons are, as far as we know today, point-like. The reason matter feels solid to us is not that atoms are massive little spheres, but because electromagnetic interactions and quantum-mechanical effects prevent atoms from simply collapsing into one another. Yet what holds the positively charged nucleus together, despite the mutual electrical repulsion between protons? A far stronger interaction is responsible: the strong nuclear force, which binds the building blocks of the nucleus together at very short range. The energy associated with this force underlies both the fusion reactions in the Sun and the nuclear reactors that generate electricity.

But the story does not end there. Protons and neutrons turn out not to be fundamental particles at all, but composite objects with an internal structure, built from even smaller constituents: quarks. The electron, by contrast, appears – with today's knowledge – to be truly fundamental.

How do we study particles far smaller than the wavelength of visible light? An ordinary microscope is not up to the task. Scientists therefore use a very different kind of instrument: the particle accelerator. Such a device brings charged particles to enormous speeds and lets them collide at very high energies. In such collisions, new, heavier particles can briefly come into existence that would not be observable under ordinary conditions. It is precisely this technique that, over the past decades, has led to the discovery of a whole series of fundamental particles and to a far deeper understanding of the forces through which they interact.

All of these insights have been brought together in the Standard Model: one of the most successful theories in the history of physics. It describes the fundamental particles, their properties, and three of the four known fundamental interactions. In the chapters that follow, we explore this model step by step.