Paper Recovers String Theory Amplitudes From Four Assumptions Using Bootstrapping

A paper by Clifford Cheung and colleagues was accepted to Physical Review Letters after the researchers used a strategy known as bootstrapping to show that string theory stands alone among possible theories of everything. Given just four basic assumptions, the only scattering amplitudes that emerge are the Veneziano amplitude and the Virasoro-Shapiro amplitude, which are precisely those predicted by string theory.

Clifford Cheung, a theoretical physicist at Caltech, and his collaborators focused on scattering amplitudes, mathematical expressions that determine the probability that two particles will interact in a particular way.

Without assumptions, a vast range of such amplitudes are possible. By imposing four conditions, the team bootstrapped its way to the specific amplitudes demanded by string theory without starting from the premise that subatomic particles are made of vibrating strings of energy.

The four assumptions were unitarity, Lorentz invariance, physics is well-behaved at extremely high energies, and minimal zeroes.

Unitarity comes from quantum mechanics and means that the probabilities of all possible options must add up to 100 percent. Lorentz invariance is a principle of Einstein’s special theory of relativity. The high-energy assumption addresses a realm where general relativity stops working.

The minimal zeroes assumption selects the simplest possible version of the scattering amplitude. Based on those constraints the researchers landed on the Veneziano amplitude and the Virasoro-Shapiro amplitude.

The result suggests that, given tenets of quantum mechanics and special relativity among other theoretical ideas, string theory follows uniquely. ” Usually physicists write down a beautiful theory based on a deep idea and then calculate something measurable.

The bootstrapping approach works in the opposite direction. Andrea Guerrieri, a theoretical physicist at City St George’s, University of London, who was not involved with the research, said the work is part of a recent wave of interest in understanding what is special about string theory.

Yu-tin Huang, a theoretical physicist at National Taiwan University in Taipei, called the derivation surprising.

“It’s very surprising that they could derive this in such a clean fashion,” Huang said. ” The result does not mean string theory is correct. One of the assumptions could be wrong. Playing with such assumptions remains one way of exploring potential theories, Guerrieri said.

Science News reported that string theory has garnered skepticism, most famously centering on the issue that its predictions thus far have not been testable and may never be. Cheung said researchers can ask the next best question short of experimental proof. The bootstrapping method flips the script for theoretical physics by beginning from basic assumptions rather than a proposed theory.

The paper demonstrates that the Veneziano and Virasoro-Shapiro amplitudes emerge cleanly once the four conditions are applied.