The question of what lies smaller than subatomic particles pushes the boundaries of human understanding, moving from the experimentally confirmed world to the realm of theoretical physics. Atoms were once considered the smallest units of matter, but the discovery of internal components like the electron, proton, and neutron shifted that perspective. This search for the ultimate, indivisible building block has uncovered layers of structure and led to concepts that redefine what “small” means.
The Standard Model Baseline
The current accepted framework for describing the smallest known particles and the forces that govern them is the Standard Model of particle physics. This model establishes a set of particles that are currently considered fundamental, meaning they have no known internal structure and cannot be broken down further. These fundamental particles fall into two main categories: matter particles, known as fermions, and force-carrying particles, known as bosons.
The fermions that constitute matter include two groups: quarks and leptons. Quarks, of which there are six “flavors” (up, down, charm, strange, top, and bottom), combine to form composite particles like protons and neutrons. Leptons, which include the familiar electron, the muon, the tau, and their associated neutrinos, are currently thought to be genuinely point-like.
The Standard Model views all these particles—quarks, leptons, and force carriers like the photon and the Higgs boson—as zero-dimensional, point-like entities. High-energy scattering experiments have successfully probed the internal structure of protons and neutrons, confirming their quark components. However, these experiments have been unable to find any evidence of substructure within the quarks and leptons themselves, setting a boundary for their potential size at less than \(10^{-18}\) meters. This establishes the particles of the Standard Model as the baseline for what is currently considered fundamental.
Hypothetical Sub-Quark Particles
Despite the experimental success of the Standard Model, some theories propose that quarks and leptons are not truly fundamental but are composed of even smaller entities. The most prominent of these proposals is the concept of Preons, hypothetical point-like particles that would act as the constituents of both quarks and leptons. Preon models, first introduced in the 1970s, aimed to simplify the “particle zoo” by reducing the elementary fermions in the Standard Model to combinations of just a few preon types.
Physicists were motivated to explore preons because they offered a potential explanation for the patterns observed in particle families and generations. By postulating a small number of preons, the theory aimed to unify the diverse charges and properties of the quarks and leptons.
While theoretically appealing, preon models have largely fallen out of favor due to the lack of experimental evidence. High-energy colliders have placed increasingly strict lower limits on the size of quarks and leptons, suggesting that the energy required to observe their constituents is far higher than what is currently accessible. Furthermore, the confirmation of the Higgs boson contradicted the predictions of many preon theories. The experimental constraints and the predictive power of the Standard Model have reduced the scientific motivation to pursue composite models for quarks and leptons.
Particles as Vibrating Strings
A dramatically different answer to the question of what is smaller than subatomic particles comes from String Theory, which completely redefines the nature of a particle. In this theoretical framework, the zero-dimensional, point-like particles of the Standard Model are replaced by one-dimensional, minuscule filaments of energy known as “strings.” These strings are the fundamental building blocks of the universe.
The size of these strings is theorized to be incredibly small, on the order of the Planck Length, which is about \(10^{-35}\) meters. This size is many orders of magnitude smaller than the current experimental limit on the size of a quark or lepton. In String Theory, the different properties of observed particles, such as their mass, charge, and spin, are determined by the way the string vibrates.
Just as different vibrational patterns on a violin string produce different musical notes, different modes of vibration in a fundamental string correspond to different particles. One vibrational pattern might result in an electron, another in a quark, and a specific vibrational state corresponds to the graviton. String Theory’s premise is that all matter and all forces arise from the oscillations of these identical, ultra-tiny strings, providing a unified description that inherently includes gravity.
The Smallest Unit of Space
Beyond the composition of particles, another way to address the concept of “smaller” is by considering the ultimate limit to measurable distance itself. This limit is defined by a theoretical boundary known as the Planck Length (\(\ell_P\)), which is approximately \(1.6 \times 10^{-35}\) meters. The Planck Length is a unit of length derived from a combination of three fundamental constants of nature: the speed of light, the gravitational constant, and the Planck constant.
This length scale represents the point at which the laws of quantum mechanics and general relativity are thought to collide and break down. Trying to measure a distance smaller than the Planck Length would require so much energy, concentrated in such a small space, that it is theorized to immediately create a black hole. Since this black hole would swallow the measurement, it is impossible to physically probe a distance shorter than this.
The Planck Length is often considered the absolute theoretical minimum size for any physically meaningful measurement in our universe. It represents the scale at which the very fabric of space is expected to become a turbulent, fluctuating “quantum foam,” making any smaller distance unobservable under current physical laws.