What Is Smaller Than Quarks? The Search for Substructure

No experiment has ever detected anything inside a quark. As far as the most powerful particle colliders and precision measurements can determine, quarks behave as point-like objects with no measurable internal structure. Yet the history of physics is a story of peeling back layers, and several serious theoretical frameworks propose that quarks might not be the end of the line. The search for sub-quark structure sits at the frontier where experiment and speculation meet, and the answer turns out to depend on what you even mean by “smaller.”

Why Physicists Suspect There Might Be Another Layer

Every time physicists have declared something “fundamental,” a deeper layer eventually showed up. Atoms were supposed to be indivisible until electrons, protons, and neutrons were found inside them. Protons and neutrons were supposed to be elementary until the 1960s, when experiments revealed they were made of quarks and gluons. Each discovery followed roughly the same script: unexplained patterns in the “fundamental” particles hinted that they were composites, and then higher-energy experiments confirmed the substructure.

The Standard Model of particle physics contains six types of quarks, six types of leptons (the electron being the most familiar), and a set of force-carrying particles. That is a lot of supposedly fundamental objects, and they come in three nearly identical “generations” that differ mainly in mass. To some physicists, this pattern looks suspiciously like it did before quarks were discovered: too many particles, arranged too neatly, to really be the bottom layer. That aesthetic unease has driven theoretical work on quark and lepton substructure for decades.¹

What Experiments Actually Show

The most direct way to look for substructure is to smash particles together at high energy and see whether they scatter in a way that suggests internal parts. This is exactly how quarks inside protons were discovered in the late 1960s. Modern colliders have repeated this kind of experiment at much higher energies, probing quarks and leptons down to scales far smaller than a proton. Every time, the particles scatter as if they are structureless points.

If quarks or leptons did have internal components, those components could potentially be excited to higher energy states, the way an atom’s electrons can be kicked into higher orbits. Early theoretical work explored what excited quarks with masses around 60 to 80 billion electron volts might look like in collider data.² The CMS experiment at the Large Hadron Collider searched for excited leptons and found nothing, ruling out excited electrons and muons with masses below about 1.9 trillion electron volts.³ That is a staggeringly high energy threshold, and the absence of excited states pushes any possible substructure to extremely tiny scales.

Precision measurements offer a complementary approach. The electron’s electric dipole moment, for instance, is a quantity that would be nudged away from zero if the electron had internal structure or if undiscovered heavy particles existed. The most precise measurement of this quantity, using electrons trapped inside molecular ions, found a result consistent with zero and set constraints on new physics at energy scales beyond what current or near-future colliders can reach directly.⁴ Every null result like this tightens the noose on substructure theories, forcing any proposed internal components to be bound together at extraordinarily high energies.

Preons and the Idea of Sub-Quark Particles

The most direct answer to “what is smaller than quarks” would be preons: hypothetical particles that make up quarks and leptons the way quarks make up protons. The idea has been around since the late 1970s and early 1980s, when several groups independently proposed that a small number of more fundamental building blocks could combine to produce the observed zoo of Standard Model particles. In these models, the interactions binding preons together would be mediated by a new confining force, sometimes called “hypercolor,” operating at a scale far above anything accessible to current experiments.⁵

One appealing feature of preon models is that they can explain the three generations of quarks and leptons. If quarks are composites of two or three preons, the different generations could correspond to different internal arrangements or excitations. The Harari-Shupe model, one of the earliest and simplest proposals, used just two types of preons to build all the known quarks and leptons. More recent theoretical work has connected this model to the mathematics of braids, showing a correspondence between preon arrangements and the observed properties of Standard Model particles.⁶

Deep inelastic scattering experiments, which probe the internal structure of particles, have been analyzed for signs of compositeness. The theoretical framework treats leptons and quarks as composites of more fundamental constituents confined within a radius inversely proportional to some very large energy scale.⁷ So far, no deviations from the point-like prediction have appeared, meaning that if preons exist, their confinement scale is extremely high.

The Mass Paradox

Preon models run into a serious problem that has haunted them for decades. In quantum mechanics, confining a particle to a very small region forces it to carry a large amount of energy, which translates to a large mass. This is a direct consequence of the uncertainty principle. If preons are confined to a region small enough to explain why quarks and leptons look point-like, the resulting composite particle should be extremely heavy. But quarks and especially leptons are very light. The electron, for instance, has a mass about a hundred thousand times smaller than a proton. How can something made of tightly bound, high-energy components weigh almost nothing?

This “mass paradox” is the single biggest obstacle to preon theories. Various solutions have been proposed over the years, typically involving extreme cancellations between the binding energy and the kinetic energy of the preons. Recent theoretical work has explored whether diffusion-based quantum mechanical effects could provide a microscopic mechanism for circumventing the uncertainty-relation constraints that create the paradox.⁸ Whether any of these mechanisms actually work in a fully consistent theory remains an open question. This unsolved problem is a major reason preon models have not gained mainstream acceptance despite being around for over forty years.

String Theory and the Replacement of “Smaller”

String theory takes a completely different approach to the question. Rather than proposing smaller particles inside quarks, it proposes that quarks themselves, along with every other particle, are different vibrational modes of one-dimensional objects called strings. In this picture, there is nothing “inside” a quark in the traditional sense. Instead, what distinguishes a quark from an electron or a photon is the pattern of vibration of the underlying string.⁹

The strings in string theory are fantastically small, typically imagined to be around the Planck length, which is about ten trillion trillion times smaller than a proton. At that scale, you are not really talking about one particle being “inside” another in any spatial sense. The concept of physical space itself starts to break down. String theory also requires extra spatial dimensions beyond the three we experience, which would be curled up at scales too small to detect. The framework is mathematically rich and has produced important insights in pure mathematics and theoretical physics, but it has not yet made a testable prediction that would distinguish it from the Standard Model at accessible energies.

The Planck Scale and the End of “Smaller”

Whether you are talking about preons, strings, or any other sub-quark proposal, you eventually run into a hard wall: the Planck length, roughly 1.6 × 10⁻³⁵ meters. This is not just an arbitrary small number. Theoretical arguments show that the Planck length represents a lower bound on physically meaningful length measurements. Below this scale, the energy required to probe a region becomes so large that it would create a black hole, making the measurement self-defeating.¹⁰

Vacuum fluctuations of the gravitational field set an irreducible noise floor on distance measurements, and this noise is characterized by the Planck length.¹¹ The implication is striking: there may be a smallest meaningful distance in nature. If so, the question “what is smaller than quarks?” might not have an infinitely recursive answer. At some point, the very notion of spatial extent loses its meaning, and “smaller than” stops being a coherent question.

Discrete Spacetime and Loop Quantum Gravity

Loop quantum gravity offers a concrete picture of what happens at the Planck scale. In this approach to quantum gravity, space itself is not a smooth continuum but has a granular microstructure. The theory predicts that quantities like area and volume come in discrete chunks, analogous to how energy levels in an atom are quantized rather than continuous. The spectrum of area and volume eigenvalues has been computed explicitly, with the smallest possible area being on the order of the Planck length squared.¹²

In this framework, the “atoms of space” are not particles inside quarks but rather the fabric that quarks move through. The microstructure of quantum spacetime is characterized by Planck-scale discreteness, where the geometry itself is built from discrete quantum states.¹³ This is a different kind of answer to “what is smaller than quarks.” It suggests that at the deepest level, the interesting structure is not inside the particles but in the space they inhabit. Quarks would still be point-like, but the points themselves would be nodes in a discrete quantum geometry rather than locations in a smooth background.

Testing this picture is extremely difficult, but not entirely hopeless. Ultra-high-energy cosmic rays travel across billions of light-years, and if spacetime has a granular structure, those rays might experience tiny violations of the symmetry that says physics looks the same regardless of your speed. Observations of cosmic ray spectra have been used to search for these violations, setting tight limits. One analysis found a best-fit violation consistent with zero, constraining any Planck-scale effects to an almost negligibly small level.¹⁴ The granularity of spacetime, if it exists, is subtle enough to hide from even these sensitive probes.

What “Size” Means for a Quark

Part of what makes this question so slippery is that “size” does not mean the same thing for a quark as it does for, say, a marble. In quantum field theory, every particle is surrounded by a cloud of virtual particles that constantly pop in and out of existence. These virtual particles affect measurable properties like mass and charge. What you actually detect in an experiment is always the “dressed” particle, the bare entity plus its cloud, and it is only these dressed properties that can be measured.¹⁵

When physicists say a quark is “point-like,” they mean that the hard scattering core has no detectable spatial extent. But the virtual cloud around it extends outward and gives the quark an effective presence that depends on the energy scale at which you probe it. At low energies, you see a fuzzy, spread-out object. At higher energies, you resolve a tighter core. This scale dependence is not a sign of substructure in the traditional sense, and it does not mean there is a little ball hiding at the center. It is a genuinely different way of existing in space from anything in everyday experience.

Why the Layers Might Stop

There is a powerful theoretical reason to think the layers might actually stop, even if the historical pattern of finding deeper ones is suggestive. The framework of effective field theories shows that physics at one energy scale can be largely independent of whatever is happening at much higher scales. The Standard Model itself is treated by many physicists as an effective field theory: a description that works perfectly well at accessible energies, regardless of what the deeper theory turns out to be.¹⁶

This “decoupling” of scales means that even if there is sub-quark structure, it might be permanently irrelevant to any experiment we could build. The physics at the energy scales of daily life, of chemistry, of nuclear reactors, and even of the Large Hadron Collider would be the same regardless of whether quarks are truly fundamental or are composites bound at energies a trillion times higher than anything we can produce. In one sense, this is reassuring: the Standard Model works. In another sense, it is deeply frustrating, because it means nature might be hiding its deepest secrets behind an energy barrier we have no practical way to cross.

Composite Dark Matter

An interesting twist on the substructure question comes from dark matter, which makes up about a quarter of the energy content of the universe but has never been directly detected. One class of theories proposes that dark matter is itself a composite particle, built from “dark quarks” bound together by a dark version of the strong force. In these models, the dark sector has its own internal dynamics, with dark fermions forming bound states whose lightest member is stable and serves as the dark matter candidate.¹⁷

This line of thinking does not answer whether ordinary quarks have substructure, but it raises a broader point: compositeness might be a more general feature of nature than the Standard Model suggests. If dark matter turns out to be a baryon-like composite of dark quarks, that would mean the universe contains at least two separate layers of composite structure, one visible and one dark, each with its own binding dynamics. The search for sub-quark structure, in other words, is not just about ordinary matter. It is about whether the principle of ever-deeper composition is a fundamental architectural feature of reality or just an accident of the particular energy scales humans happen to be able to probe.

Emergent Structure and Holographic Ideas

Some of the most adventurous recent work does not ask what is “inside” a quark at all, but whether particles and even spacetime might emerge from more abstract underlying structures. Holographic duality, an idea that grew out of string theory, suggests that a theory in a higher-dimensional space can be fully equivalent to a different theory living on its boundary. In this framework, the “deeper” layer is not smaller in any spatial sense. It is a different mathematical description of the same physics.

Recent work has explored how composite higher-spin fields can emerge from fundamental fermions, building infinite towers of composite particles whose dynamics admit dual holographic descriptions.¹⁸ These are toy models, not descriptions of real quarks, but they illustrate a possibility that challenges the whole premise of the question: maybe the relationship between “fundamental” and “composite” is not a one-way street from big to small but a duality, where either description is equally valid depending on what you want to calculate.

If something like this turns out to be true for the real world, the search for what is smaller than quarks might end not with a definitive smallest object but with the realization that “smaller” was never quite the right question to ask. The deepest description of nature might not involve spatial size at all, but rather mathematical structures from which both particles and the space they inhabit emerge together.