It stands to reason that particle physics is about particles, and most people have a mental image of little billiard balls caroming around space. Yet the concept of “particle” falls apart on closer inspection. Many physicists think that particles are not things at all but excitations in a quantum field, the modern successor of classical fields such as the magnetic field. But fields, too, are paradoxical. If neither particles nor fields are fundamental, then what is? Some researchers think that the world, at root, does not consist of material things but of relations or of properties, such as mass, charge and spin.
Physicists routinely describe the universe as being made of tiny subatomic particles that push and pull on one another by means of force fields. They call their subject “particle physics” and their instruments “particle accelerators.” They hew to a Lego-like model of the world. But this view sweeps a little-known fact under the rug: the particle interpretation of quantum physics, as well as the field interpretation, stretches our conventional notions of “particle” and “field” to such an extent that ever more people think the world might be made of something else entirely.
The problem is not that physicists lack a valid theory of the subatomic realm. They do have one: it is called quantum field theory. Theorists developed it between the late 1920s and early 1950s by merging the earlier theory of quantum mechanics with Einstein's special theory of relativity. Quantum field theory provides the conceptual underpinnings of the Standard Model of particle physics, which describes the fundamental building blocks of matter and their interactions in one common framework. In terms of empirical precision, it is the most successful theory in the history of science. Physicists use it every day to calculate the aftermath of particle collisions, the synthesis of matter in the big bang, the extreme conditions inside atomic nuclei, and much besides.
So it may come as a surprise that physicists are not even sure what the theory says—what its “ontology,” or basic physical picture, is. This confusion is separate from the much discussed mysteries of quantum mechanics, such as whether a cat in a sealed box can be both alive and dead at the same time.