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:

• the position and momentum of a particle,
• its energy and location in time,
• and its angular momentum in each of the three spatial dimensions that we can measure,

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.

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. Instead, the idea came about because, as it was originally understood and proposed, quantum mechanics had an inherent inconsistency built-in to it.

Think about this aspect of our quantum reality. Classically, what we think of as physical properties like “position” and “momentum” are simply quantities that are inherent properties of any particle that possesses them. Quantum mechanically, there’s a novel phenomenon there: we understand that measuring one of those properties inherently induced an uncertainty in the other. Moreover, the more precisely you measure one of these conjugate variables, the greater the uncertainty in the other conjugate variable becomes. We could no longer treat them as “properties” but rather were compelled to treat them as quantum mechanical operators, where we could only know what the probability of the set of possible outcomes could be, rather than a unique value.

For something like position and momentum, those probability distributions would have a time-dependence: the positions you’d be likely to measure or the momenta that you’d infer a particle possessed would change and evolve with time. But this suggested another problem that we couldn’t avoid once we understood Einstein’s theory of relativity: the notion of time is different for observers in different reference frames. The laws of physics must be relativistically invariant, giving the same answers regardless of where you are and how fast (and in what direction) you’re moving.

If quantum physics is time-dependent, but relativity tells you that time is relative, then how do you reconcile those two notions? You’d need for quantum physics to also incorporate Einsteinian relativity. The original problem is that old-school quantum mechanics, like that described by the Schrödinger equation, yields different predictions for observers in different reference frames: it is not, in fact, relativistically invariant! It took years of development before the first equations that described the quantum behavior of matter in a relativistically invariant manner were written down, including:

• the Klein-Gordon equation, which applied to spin-0 particles,
• the Dirac equation, which applies to spin-½ particles (like electrons),
• and the Proca equation, which applies to spin-1 particles (like photons).

This led to a scenario where you’d describe the fields (like electric and magnetic fields) that each particle generates classically, and then each quantum particle can interact with those fields.

However, that doesn’t fully resolve the problem. What do you do, for instance, when each field-generating particle has inherently uncertain properties to it, like position and momentum? How can you generate a field from a probability distribution, rather than from a point-like particle? What happens if you consider a wildly non-classical situation, like an electron that’s passing through a double slit? In that instance, you definitely can’t treat the electric field generated by this wave-like, spread-out electron as coming from a single point, and apply the classical laws of Maxwell’s equations.

Considerations such as these compelled us to advance from simple quantum mechanics to quantum field theory, which didn’t just promote certain physical properties to being quantum operators, but also promoted the fields themselves to being quantum operators.

With quantum field theory, an enormous number of already-observed phenomena finally made sense, as having field operators (in addition to “particle operators” like position and momentum) allowed us to explain several phenomena that were noted to occur, but lacked a physical explanation. These included:

• particle-antiparticle creation and annihilation,
• the radioactive decay of atomic nuclei and massive, unstable quantum particles (like the muon),
• quantum corrections to the electron’s (and muon’s) magnetic moments, including contributions from fine and hyperfine structure,

as well as many others.

But that didn’t prove that quantum fields were necessarily real. One could still ask whether these quantum fields were simply a successful mathematical description of the particles that truly made up our reality, or whether they were actually real themselves?

One way to answer this question — about whether something is “real” or not — is to ask what you can do with it. We often say that “seeing is believing” but not everything that’s real can be “seen” the way that conventional matter can be seen. For these quantum fields, we can’t directly measure the them, but if we can perform tasks with them like:

• extract energy from them,
• use them to perform “work” (i.e., to move masses a certain distance through the application of a force),
• or coax them into a configuration where they result in a definitive, observable signature that’s unique to quantum field theory,

that can prove their “realness” insofar as anything that can’t be directly measured can be said to be physically real. As of today, we already have three independent empirical, experimental proofs that quantum fields are, in fact, very real. Here’s a rundown of each one.

1.) The Casimir Effect. In theory, there are quantum fields of all types — from the electromagnetic, weak, and strong nuclear forces — permeating all of space. One way to visualize this field is to imagine a series of quantum fluctuations, or waves, of all different possible wavelengths. Normally, in empty space, these wavelengths can take on any value, and do: what we call the “zero-point energy” of space, or the “ground state” of empty space, arises from the sum of all possible contributions.

However, you can imagine setting up barriers that restrict what sorts of waves and wavelengths are possible in a given region of space. In physics, we generally call these constraints “boundary conditions,” and they enable us to control all sorts of electromagnetic phenomena, including radio and television signals.

In 1948, physicist Hendrik Casimir realized that if one were to set up a configuration where two parallel conducting plates were held very close to one another, the “allowable” wave modes from outside the plates would be infinite, while inside the plates, only a subset of modes would be allowed.

As a result, purely as an effect of the quantum fields between them, there would be a difference in the inward and outward forces acting on the plates, with the specific force dependent on the exact configuration. While it was generally accepted that the Casimir effect should exist, it turned out to be incredibly difficult to measure.

Thankfully, 49 years after Casimir proposed it, experiments finally caught up. In 1997, Steve Lamoreaux devised an experiment that leveraged a single flat plate and a section of an extremely large sphere to both calculate and measure the Casimir effect between them. Lo and behold, the experimental results agreed with the theoretical predictions to greater than 95% precision, with only a small error and uncertainty involved.

Since the dawn of the 2000s, the Casimir effect has been measured directly between parallel plates, and an integrated silicon chip has even been demonstrated to measure the Casimir force between even complex geometries. If quantum fields weren’t “real,” this very real effect would exist without a physical explanation for it.

2.) Vacuum birefringence. In regions with very strong magnetic fields, empty space itself — despite not being “made” of anything physical — should become magnetized, as the quantum fields in that region of space will feel the effect of the external field. In the real Universe, pulsars actually provide this natural laboratory: generating magnetic fields that are several billions of times greater than even the strongest electromagnets we’ve created in labs on Earth. When light passes through this highly magnetized space, that light should become polarized as a result, even if the light was completely unpolarized to begin with.

The prediction of this effect, known as vacuum birefringence, goes all the way back to Werner Heisenberg. However, it wasn’t observed until 2016, when a team looked at a remarkably “quiet” neutron star located 400 light-years away: RX J1856.5-3754. This marked the faintest object for which polarization had ever been measured, and yet the degree of linear polarization was large and significant: 16%. Without the boosting effect of vacuum birefringence in the empty space surrounding this pulsar, this polarization cannot be explained. Yet again, the effects of quantum fields show up in an unambiguous, measurable place.

3.) The Schwinger Effect. Instead of magnetic fields, imagine you’ve got an extremely strong electric field; something far stronger than you could ever make on Earth. Instead of magnetic polarization, the quantum vacuum would become electrically polarized: the same way charges migrate to opposite ends of a battery or other voltage source.

Within the depths of empty space, quantum fluctuations of all types occur, including the rare-but-important creation of pairs of particles-and-antiparticles. The lightest charged particles are the electron and its antimatter counterpart, the positron, and these are also the particles that accelerate by the greatest amounts (due to their low masses) in the presence of an electric field.

Normally, these particle-antiparticle pairs annihilate away back into “nothingness” before they can be detected. But if you turn up the strength of your electric field by a great enough amount, perhaps the electron and positron won’t be able to find one another again, because they’ll have been driven away from one another by the effects of the electrically polarized empty space that they exist in.

In theory, the very strong environments inside a neutron star should achieve these fields, and you could create new particle-antiparticle pairs out of the electric field energy via Einstein’s most famous equation: E = mc². We can’t perform experiments in that environment, however, nor could we recreate such conditions on Earth, and as a result, most researchers gave up on the idea of ever testing the Schwinger effect.

But in early 2022, a team of researchers did it anyway. By leveraging a graphene-based structure known as a superlattice — where multiple layers of materials create periodic structures — the authors of this study applied an electric field and induced the spontaneous creation of electrons and “holes,” which are the condensed matter analogue of positrons, at the cost of stealing energy from the underlying applied electric field.

The only way to explain the observed currents were with this additional process of spontaneous production of electrons and “holes,” and the details of the process agreed with Schwinger’s predictions from all the way back in 1951.

Of course, one could argue that quantum fields needed to be real from the very start: since the first observation of the Lamb shift way back in 1947. Electrons in the 2s orbital state of hydrogen occupy a very slightly different energy level than electrons in the 2p orbital state, and this is a difference that does not arise either in non-relativistic quantum mechanics or even in relativistic quantum mechanics; you need quantum field theory to account for it. A famed experiment from the early days of quantum physics, the Lamb-Retherford experiment, revealed it even before the first modern quantum field theory — quantum electrodynamics — was developed by Schwinger, Feynman, Tomonaga and others.

Still, there’s something quite special about predicting an effect before it’s observed, rather than explaining an already-observed effect after the fact, which is why the three phenomena highlighted here — the Casimir effect, vacuum birefringence, and the Schwinger effect — stand apart from any initial impetus for formulating a quantum field theory. They were predicted by the theory first, and were only predicted at all because we had the framework of quantum field theory to work within. The detection of all of them came later.

There’s even a possible connection between quantum field theory and the Universe on the largest scales of all. The observed effect of dark energy, which causes the accelerated expansion of the Universe, behaves identically to what we would expect if there were a small but positive, non-zero value to the zero-point energy of empty space. As of 2026, this potential connection is still speculative, as calculating the zero-point energy of space is beyond the present capability of physicists. Nevertheless, the fact that quantum fields carry energy and have both calculable and measurable effects on the light and matter within the Universe strongly suggests that, as much as anything can be said to be “real,” these quantum fields are indeed so. Perhaps, if nature is kind, we might someday discover an even deeper connection between quantum field theory and the cosmic Universe we all inhabit.

This article was first published in February of 2023. It was updated in October of 2026.

This article is featured on Big Think.