Quantum fields carry energy. Does that prove that they’re real?

One of the biggest questions that appears right at the intersection of physics and philosophy is as simple as it is puzzling: what is real? Is reality simply described by the particles that exist — that make us up, that we interact with, and that we can observe and measure — atop the background of spacetime that’s described by General Relativity? Is it fundamentally wrong to describe these entities as particles, and must we consider them as some sort of hybrid wave/particle/probability function: a more complete description of each “quantum” that exists within our reality?

Or can we go even deeper than this description of reality, noting that there are fields, fundamentally, that underpin all of existence, where the “quanta” that we typically interact with are simply examples of (at least temporarily) stable excitations of those fields? When quantum mechanics arrived on the scene, it brought with it the realization that quantities that were previously thought to be well-defined, such as: could no longer be assigned well-defined values in an objective fashion. Instead, quantum physics brings along properties like an inherent uncertainty to these things that we think of as measurables, where we can only describe them by a probability distribution for the possible values they could take on at any moment.

This weirdness, on its own, brought about many arguments over the nature of reality at the start of the 20th century, and quite of few of those arguments still persist, even today. However, things would soon get even weirder with the introduction of the concept of quantum fields. For generations, physicists argued whether those quantum fields were actually real, or whether they were simply calculational tools: useful for modeling the mathematics of the reality that we can measure, but not reflective of reality itself.

Nearly a full century later, we’re pretty much certain that quantum fields are indeed real for one unambiguous reason: they carry energy. Energy is something that we think of very much as being real, and if energy is real, then so are quantum fields. Here are the three experiments that proved that quantum fields do indeed carry energy, and hence, are themselves real.

This diagram illustrates the inherent uncertainty relation between position and momentum. When one is known more accurately, the other is inherently less able to be known accurately. Both position and momentum are better described by a probabilistic wavefunction than by a single value.

Other pairs of conjugate variables, including energy and time, spin in two perpendicular directions, or angular position and angular momentum, also exhibit this same uncertainty relation. Quantum field theory has an interesting history. It didn’t come about because people thought, “well, we had classical particles and classical fields like gravity and electromagnetism, and so if the particles are quantum, the fields are too.” That idea might have been floating around, but it wasn’t the motivation for the origin of quantum fields.