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Tech & Science
Researchers uncovered key string theory signatures through a bootstrap approach, starting from minimal assumptions about particle scattering.
Physicists have identified fundamental characteristics of string theory by applying a bootstrap technique that begins with only a few assumptions about particle interactions at high energies. This approach revealed the hallmark features of string theory without initially assuming its framework.
The research team from Caltech, New York University, and Institut de Fisica d’Altes Energies in Barcelona published a paper titled “Strings from Almost Nothing” in Physical Review Letters. They explored scattering amplitudes at extreme energies using minimal assumptions, such as ultrasoft behavior and “minimal zeros” in scattering probabilities. Unexpectedly, the mathematical solutions they derived matched the defining elements of string theory.
Clifford Cheung, a theoretical physics professor at Caltech, described the findings: “The strings just fell out. We didn’t start with any assumptions about strings at all, but then the solution contained the cornerstone signatures of strings.” Although this does not constitute experimental proof, the uniqueness of the solution under the given assumptions is significant.
A key outcome of the study was the emergence of the string spectrum, an infinite sequence of particles with increasing masses and spins. This concept dates back to the late 1960s when Gabriele Veneziano formulated a function describing particle patterns observed in collider experiments. The spectrum resembles harmonic vibrations similar to those of a musical string, where different vibrational modes correspond to different particles.
Caltech physicist John Schwarz and Joël Scherk later recognized that string theory naturally incorporates gravity, a feature that distinguishes it from general relativity at high energies. Schwarz recalled that string theories behave well at very high energies, unlike Einstein’s theory, which acts as a low-energy approximation.
At energies near the Planck scale, general relativity’s predictions break down due to infinities in scattering calculations. String theory avoids this issue through ultrasoftness, where interaction probabilities decrease sharply at high energies, preventing divergences. The researchers used this ultrasoftness as a foundational assumption in their bootstrap approach.
Cheung explained, “In a string theory framework, as you increase the energy transfer between particles, you will see a swift fall off in the probability that the particles will scatter. The scattering amplitudes don’t go to infinity. It’s better behaved.” The assumption of “minimal zeros” further constrained the mathematical solutions, requiring the fewest possible points where scattering probabilities vanish.
The bootstrap method, an approach dating back to the 1960s, involves deducing physical theories from basic consistency conditions rather than starting with a complete model. Cheung likened it to solving a sudoku puzzle, where a limited set of rules leads to a unique solution. Earlier pioneers like Steven Frautschi and Geoffrey Chew applied bootstrap ideas to particle physics, uncovering early signs of the infinite particle spectrum later associated with string theory.
Hirosi Ooguri of Caltech noted that modern tools and improved understanding have revitalized the bootstrap approach, enabling researchers to translate fundamental assumptions into precise properties of scattering amplitudes and other observables.
The study received funding from the US Department of Energy, the Walter Burke Institute for Theoretical Physics, the Leinweber Forum for Theoretical Physics, the James Arthur Postdoctoral Fellowship at New York University, and the Next Generation EU. In addition to Cheung and Remmen, co-authors include Francesco Sciotti from Institut de Fisica d’Altes Energies and Michele Tarquini, a Caltech graduate student.
