Imagine slicing an apple into smaller and smaller pieces. First you would reach molecules, then atoms, and eventually the subatomic particles inside them, including protons, quarks, and gluons. According to string theory, however, nature may continue even deeper. At scales far smaller than a proton, the universe could be built from tiny vibrating strings.

Originally developed in the 1960s, string theory attempts to solve one of the biggest unsolved problems in physics: combining quantum mechanics with general relativity. Quantum mechanics explains the behavior of matter and energy at extremely small scales, while general relativity describes gravity and the structure of the cosmos on the largest scales. Physicists have struggled for decades to merge the two frameworks because the mathematics tends to break down when gravity is treated quantum mechanically.

String theory offers a possible solution by replacing pointlike particles with microscopic strings. Different vibrations of these strings would produce all known particles, including the graviton, a hypothetical particle believed to carry gravity. The theory also predicts the existence of at least 10 dimensions, rather than the four dimensions humans experience in everyday life.

One of the biggest challenges is testing the theory directly. The energies needed to probe strings experimentally are so enormous that researchers would need a particle accelerator roughly the size of a galaxy.

A New Bootstrap Approach to String Theory
Unable to test string theory directly, physicists are turning to alternative methods. One increasingly popular strategy is known as the “bootstrap” approach. Instead of beginning with a complete theory, scientists start with a few broad assumptions about how nature should behave and see what mathematical structures emerge.

In a new paper called “Strings from Almost Nothing,” accepted for publication in Physical Review Letters, researchers from Caltech, New York University, and Institut de Fisica d’Altes Energies in Barcelona used this method to explore particle interactions at extremely high energies. Starting from only a small number of assumptions about scattering behavior, they unexpectedly recovered the defining features of string theory.

“The strings just fell out,” says Clifford Cheung, professor of theoretical physics and director of the Leinweber Forum for Theoretical Physics at Caltech. “We didn’t start with any assumptions about strings at all, but then the solution contained the cornerstone signatures of strings.”

Cheung explains that the work does not count as experimental proof of string theory, but the result is still significant because the assumptions could have produced many possible mathematical outcomes. Instead, the equations led to a unique structure matching string theory.