Particles are nature’s smallest constituents, but that doesn’t mean they’re fundamental. So what is the Universe made of?

by Felix Flicker  BIO

Interactive illustration (use click and cursor) by Richard Wilkinson

is a theoretical physicist and senior lecturer in physics at Bristol University in the UK. He is the author of The Magick of Matter: Crystals, Chaos and the Wizardry of Physics (2023).

Edited byRichard Fisher

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. Now, if you are familiar with the Standard Model of particle physics, you will know that it contains photons but not phonons. The reason is that phonons are not elementary. If you ask what a phonon is made of, there is an answer: it is a pattern of vibrations of the atoms in the air. In the study of quantum matter, however, we say it is an emergent particle.

So what are emergent particles? Are they as real as elementary particles? And, perhaps most importantly, can they tell us anything new about the nature of reality?

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To answer these questions, we first need to agree on the meanings of the terms emergent, fundamental, and particle. Naturally, this being a philosophical discussion, each term has been widely debated, with textbooks devoted to each. I will give a short and hopefully not too offensive summary.

A phenomenon is emergent if it is built from parts but cannot be reduced to them without losing some key aspect of the description. One of the pioneers of quantum matter, Philip Anderson, conceived of the subject with an update to Aristotle’s adage: the whole is not only more than the sum of its parts – it can also be fundamentally different from it. Take ice, for example. Ice has emergent properties not present in any of its constituent water molecules. It is cold, say. Cold is not a property an individual molecule can possess. Ice is also rigid: push one edge of an ice cube and the other edge moves. This is neither a property of individual molecules nor of their sum, since those same molecules can also form liquid water, which does not possess such rigidity. Ice is purely emergent.

Entities are fundamental if they are independent of one another and form a basis for understanding all other phenomena. The atoms in the periodic table were once thought to play this role: a basis for understanding molecules, then living beings, and so on. But we later discovered that atoms can themselves be described in terms of elementary particles such as electrons, protons and neutrons. (Hence, we jumped the gun on calling them atoms!) Realising this, there are two possible conclusions to draw. One is to say that elementary particles are fundamental, not atoms. The other approach is to say that things can be fundamental at one level of description, while acknowledging that other things might separately be fundamental at other levels. Atoms are fundamental for chemistry, but not for nuclear physics. The latter is the approach typically taken by scientists and philosophers. This makes sense, since we cannot ever be sure if we have reached the bottom of the pile. For instance, protons are made from quarks, and various hypotheses such as string theory attempt to comment on some lower level of description still. Importantly, science can never tell us if ours is the lowest possible level of description: it can only posit this as a hypothesis that could be falsified by further data. So if we restrict ‘fundamental’ to mean ‘most fundamental’, or ‘fundamental at every possible level’, the concept would fall outside the realm of science.

I am struck with an idea – Eureka! The hole is like a particle, and the circling water is like its field

A particle was originally a pretty straightforward concept: it was something with a well-defined position (requiring it to be incredibly small) and a well-defined momentum. All of Newtonian mechanics can be written in terms of those two quantities, position and momentum,........

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