The search for the smallest constituents of matter represents a continuous story of discovery in physics. For centuries, the atom was considered the ultimate, unchangeable building block, a notion overturned by the discovery of the electron, proton, and neutron. This pattern of finding a boundary and then pushing past it has defined the history of particle physics. Each new layer of substructure solved existing puzzles but introduced new questions about the fundamental nature of reality.
The Standard Model’s Smallest Particles
The current understanding of matter is summarized by the Standard Model of particle physics. This model describes the fundamental particles and three of the four forces governing their interactions. Matter is built from two classes of particles: quarks and leptons, which are considered truly fundamental because they possess no measurable internal structure and behave as “point-like” objects.
The quark family consists of six “flavors”: up, down, charm, strange, top, and bottom. Protons and neutrons are composite particles made of quarks; for instance, a proton contains two up quarks and one down quark. The lepton family also has six members, including the electron, muon, and tau, and their corresponding neutrinos.
High-energy collision experiments have pushed the experimental boundary for the size of these particles. Investigations have established an upper limit on the radius of a quark to be smaller than approximately \(0.43 \times 10^{-18}\) meters. When scientists probe quarks and leptons with the highest available energy, they scatter as if they have no discernible size, acting as true point particles down to a scale of about \(10^{-19}\) meters.
Hypothetical Building Blocks: Preons
The Standard Model, despite its success, contains patterns and arbitrary numbers that suggest a deeper layer of organization may exist, leading to the theoretical concept of preons. Preon theory proposes that quarks and leptons are themselves composite particles made of these even smaller, more fundamental entities. This idea is motivated by the desire to explain the three distinct generations of quarks and leptons, which appear to be identical copies but with increasing mass.
If preons exist, they would provide a more economic explanation for the multitude of particles in the Standard Model, much like the quark model reduced the large “particle zoo” of the 1950s and 1960s. For example, some models hypothesize that all quarks and leptons could be constructed from just two or three types of preons. This would simplify the overall particle landscape and potentially offer a natural explanation for the observed charge values.
These theoretical building blocks would have to be bound together by a powerful new force, operating at an energy scale far beyond what current accelerators can achieve. Experiments at the Large Hadron Collider (LHC) have searched for signs of substructure, setting lower limits on the energy scale of any possible preon binding force above 20 to 40 TeV. The absence of experimental evidence means preons remain speculative, but the search continues for deviations that would signal the composite nature of quarks and leptons.
The Ultimate Limit of Measurement
Beyond the hypothetical particle substructure, physics also considers a fundamental limit to how small any distance can possibly be. This limit is the Planck length, a theoretical distance scale of approximately \(1.6 \times 10^{-35}\) meters. This distance is vastly smaller than the current experimental limit on the quark’s size, which is around \(10^{-19}\) meters.
The Planck length is a fundamental constant derived from a combination of the speed of light, Planck’s constant, and the gravitational constant. At this scale, the effects of quantum mechanics and gravity become equally important, and the familiar concepts of space and time are expected to break down. This is the realm where a unified theory of quantum gravity is required.
String Theory, a prominent candidate for such a unified theory, posits that the truly fundamental entities are not point-like particles at all, but tiny, one-dimensional vibrating strings. The characteristic size of these strings is thought to be on the order of the Planck length. In this view, the different types of quarks, leptons, and force-carrying particles are simply different vibrational modes of a single type of string.