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The discovery of the Higgs boson completed the Standard Model’s roster of elementary particles, but it did not settle whether those particles are truly fundamental. Jay Bennett’s Scientific American feature follows physicists asking what lies beneath the smallest things currently known—and whether experiment can ever reach the scale where a final answer would appear.

A finished model with unfinished questions

In June 2012, particle physicist Sau Lan Wu and her students at CERN combined two streams of Large Hadron Collider data and found that the signal for the Higgs boson had crossed the statistical threshold required for a discovery. The Higgs, formally announced days later, brought the number of known elementary particles to 17 and filled the last vacancy in the Standard Model.

That model divides particles into fermions, the constituents of matter such as quarks, and bosons, which carry forces. It has passed extraordinarily precise tests. Yet it treats elementary particles as dimensionless points—or, in quantum-field language, as excitations in fields that fill the universe. This is an excellent approximation, but it does not prove that particles have no internal structure.

Physicists therefore pursue two related possibilities. Familiar particles may be composites made from still smaller pieces, and undiscovered particles may exist outside the Standard Model. A dark-matter particle, for example, might itself contain hypothetical “dark quarks.” Finding either kind of substructure would show that today’s elementary particles are another layer rather than nature’s final inventory.

Strings, loops and granular spacetime

String theory replaces point particles with minute vibrating strings. A particle’s identity would arise from a string’s form and behavior, much as different vibrations of a chord produce different notes. Open strings could combine and split, while closed strings could correspond to gravitons, the hypothetical carriers of gravity. To reproduce known physics, however, the strings must inhabit at least 10 dimensions, most accessible only at fantastically small scales.

The theory’s flexibility is both its attraction and its central weakness. It can accommodate established physics in roughly a quadrillion different ways. Supporters see that breadth and its ability to incorporate gravity as evidence of a deep underlying framework. Critics argue that a theory with so many possible versions cannot make a unique, testable prediction. Its mathematical beauty is not experimental proof.

Loop quantum gravity takes a different route. Instead of placing strings inside spacetime, it proposes that spacetime itself is composed of discrete quantum loops. Francesca Vidotto compares these units to photons: just as light appears continuous at ordinary scales but interacts in discrete quanta, the gravitational field might reveal grainlike units when probed deeply enough.

One speculative consequence is that quantum remnants of black holes could coexist in superposition with white holes, regions that expel matter rather than admit it. These stable black-hole–white-hole combinations might even account for dark matter. The proposal is intriguing precisely because it connects quantum gravity to something astronomers observe, but it remains theoretical.

The scale beyond reach

Both strings and quantum loops are expected to become relevant near the Planck length, approximately 1.6 × 10⁻³⁵ meter. Quantum uncertainty may make this scale a physical lower bound on meaningful measurement. James Beacham estimates that a collider able to probe it directly would need to circle the Sun and extend beyond Neptune—an experiment nowhere near practical possibility.

The Large Hadron Collider can still look for indirect cracks in the Standard Model. Upgrades expected to operate in 2030 will produce and measure far more collision debris, with special attention to the Higgs. Researchers want to know whether its decays and self-interactions differ subtly from predictions, whether multiple Higgs-like particles exist, or whether the Higgs is composite. Because it is the only known elementary particle with zero quantum spin, any anomaly could open a path into new physics.

The central difficulty is epistemic as much as technical. Scientists can keep testing particles for hidden structure, but every null result leaves open the possibility that the next layer lies just beyond the available energy. No experiment can certify that nothing smaller exists at any scale. The smallest known pieces may be fundamental, or they may merely mark the limit of present instruments.

The article’s conclusion is deliberately unresolved. Physics has repeatedly exposed deeper levels of matter, but history does not guarantee that the sequence continues—or that humans can reach its end. The productive response is neither to declare today’s model final nor to confuse elegant speculation with discovery. It is to keep asking what evidence could distinguish the possibilities, while accepting that nature may withhold a definitive bottom layer.

Source: Jay Bennett, “What Are the Smallest Pieces of the Universe?” Scientific American, October 2026, pp. 50–53.