What is the world made of? For centuries, people have believed that matter is constructed from tiny, indivisible parts. Some of the earliest known references come from the Greek philosopher Democritus, who taught that the Universe was composed of atoms the size of dust motes floating in sunlight. Theravada Buddhism developed the concept of kalapas, indivisible bundles of properties fleeting into and out of existence. Alchemy’s description of fundamental ‘corpuscles’, expounded by Isaac Newton and others, derived from translations of Aristotle by mediaeval Islamic scholars. And Hideki Yukawa, winner of the 1949 Nobel Prize in Physics for his work developing the modern theory of elementary particles, took inspiration from a passage in the Zhuangzi, a Daoist text written during China’s warring states period, in which fast-moving entities puncture holes within formless chaos. Yukawa saw a parallel to particle collisions.
The concept of a particle, as we now refer to these indivisible parts, has therefore been repeatedly re-introduced in contradictory ways. The modern view continues this tradition. In late-19th-century physics, particles were tiny indivisible objects with well-defined positions and momenta. The advent of quantum mechanics led these clear waters to become muddied. But the basic idea persists: we are taught from a young age that matter is made of atoms, built from particles such as electrons, and electrons are not built from anything else. For this reason, these particles are sometimes said to be fundamental. But are they? Is the Universe really made from the smallest constituents, as a beach is made from sand?
The answer to this question, I will contest, is perhaps a surprising one: yes, the Universe is built from fundamental units – but fundamental need not mean smallest. This view is generally adopted by those physicists, such as myself, who work in the largest discipline within the subject: quantum matter. This is the study of quantum behaviours that manifest on everyday scales: the attraction of iron to a magnet, the flow of electricity along a wire, or the passage of sound through a crystal. In these settings, too, we find particles. But these particles are not elementary, like the electron: they are emergent.
The distinction can be pictured as follows. Imagine a lightbulb, its rays of light travelling to your eyes. We can ask what those rays are made of. Quantum mechanics has an answer: a ray of light is a stream of individual particles called photons. In turn, we can ask what the photons are made of. The answer this time is that they are not made of anything else: they are elementary. Now imagine that this lightbulb is of a vintage sort, and gives off a gentle hum. It emits waves of sound that travel to your ears. We can again ask what those waves are made of. And, once again, quantum mechanics has an answer: a wave of sound can be described by individual particles called phonons.
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