Particle physics isn’t really about particles. Nor, it turns out, is it about fields—at least not in any traditional sense, argues NYU philosopher Jon Bain. Despite the name, the theories at the heart of modern physics describe a reality where the classical idea of discrete, localizable, countable particles collapses—and fields fare no better. For Bain, making sense of particle physics means abandoning these outdated intuitions and embracing a more radical view: the properties we associate with particles or fields don’t exist absolutely, but emerge only in specific regimes. What, then, underlies reality? Not objects, but structure—relations, observables, mathematical frameworks. If there’s anything fundamental, it may not be a thing at all.

To what extent is particle physics about particles? This is an important question for a naturalist who looks to science for the answers to philosophical questions about the nature of reality. When a philosopher asks “What is the nature of matter?” a naturalist will look to our current best scientific theories about matter; namely, those theories that make up the field of particle physics, and if particle physics is about particles, then the answer to the philosopher's question will be “matter consists of particles.” This seems straightforward: in order for it to be called particle physics, it must, surely, be about particles. But alas, it's not as simple as this. There is a consensus among philosophers that particle physics is not fundamentally about particles. In fact, the theories of matter that go under the rubric “particle physics” are relativistic quantum field theories (RQFTs), which are mathematical frameworks that are the result of combining Einstein's special relativity with a quantum version of classical field theory, and which, under a naive interpretation, are about quantum fields that sometimes exhibit particle-like excitations. But, as we will see, a standard argument against a particle interpretation of RQFTs also works against a field interpretation. So a naturalist seems to have her work cut out for her: why call it “particle physics” if it isn't about particles? Why can't it be about particles? If it isn't about particles, or fields, what is it about?

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1. Particles in Classical Physics

Why use the phrase “particle physics” to refer to the collection of RQFTs that purport to describe the fundamental nature of matter? I blame Isaac Newton. Newton's theory of motion (described in his 1687 Mathematical Principles of Natural Philosophy) was, in part, a critique of Descartes’ earlier theory of motion (described in his 1644 Principles of Philosophy). In a Cartesian cosmos (apart from humans and God), there are no active principles, no occult forces acting across vast distances in a vacuum; rather, there are only extended bodies acting mechanistically on each other through direct contact and ultimately individuated by their motion in a continuous plenum. In contrast, in a Newtonian cosmos, matter consists of discrete particles moving in an infinite void and governed by a single active principle (i.e. gravity). The conceptual origins of Newton's cosmos go back to ancient Greek atomism, and slightly less ancient Neoplatonism, and center stage is given to what we might call a Newtonian particle.

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It turns out that, while particle intuitions have changed since Newton, they haven't changed all that much.

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