Ask Ethan: Are Quantum Fields Real?

An illustration of the empty space of the Universe as consisting of quantum foam, where quantum fluctuations are large, varied, and important on the smallest of scales. The quantum fields that are an intrinsic part of nature are well-defined, but do not conform to our intuitive notions of how particles or waves should behave.NASA/CXC/M.WEISS

The Universe we perceive and view, all around us, isn’t representative of what actually exists at a fundamental level. Instead of continuous, solid objects, matter is composed of indivisible quantum particles, held together through invisible forces that act across empty space. Both the particles themselves and the forces can be described by an underlying structure: quantum fields, which describe everything we know about all the particles and antiparticles of the Standard Model. But are these quantum fields real? And just what do they tell us? That’s what Patreon supporter Aaron Weiss wants to know, as he asks:

I would be very interested in a post about quantum fields. Are they generally/universally believed to be real and the most fundamental aspect of our universe or just a mathematical construct? I’ve read that there are 24 fundamental quantum fields: 12 fields for fermions and 12 for bosons. But I’ve also read about quantum fields for atoms, molecules, etc. How does that work? Does everything emerge from these 24 fields and their interactions?

Let’s start with what a quantum field actually is.

The proton’s structure, modeled along with its attendant fields, show how even though it’s made out of point-like quarks and gluons, it has a finite, substantial size which arises from the interplay of the quantum forces and fields inside it. The proton, itself, is a composite, not fundamental, quantum particle.BROOKHAVEN NATIONAL LABORATORY

In physics, a field, in general, describes what some property of the Universe is everywhere in space. It has to have a magnitude: an amount that the field is present. It may or may not have a direction associated with it; some fields do, like electric fields, some don’t, like voltage fields. When all we had were classical fields, we stated that the fields must have some kind of source, like particles, which results in the fields existing all throughout space.

In quantum physics, though, this seemingly self-evident fact is no longer true. Whereas classical physics defines quantities like position and momentum as properties of a particle, and those properties would generate a corresponding field, quantum physics treats them differently. Instead of quantities, position and momentum (among other quantities) now become operators, which allow us to derive all the quantum weirdness you’ve heard so much about.