New Discoveries Using Bootstrap

Tuesday, June 20, 2017

In the 1960s, the charismatic physicist Geoffrey Chew espoused a radical vision of the universe, and with it, a new way of doing physics. Theorists of the era were struggling to find order in an unruly zoo of newfound particles. They wanted to know which ones were the fundamental building blocks of nature and which were composites. But Chew, a professor at the University of California, Berkeley, argued against such a distinction. “Nature is as it is because this is the only possible nature consistent with itself,” he wrote at the time. He believed he could deduce nature’s laws solely from the demand that they be self-consistent.

Scientists since Democritus had taken a reductionist approach to understanding the universe, viewing everything in it as being built from some kind of fundamental stuff that cannot be further explained. But Chew’s vision of a self-determining universe required that all particles be equally composite and fundamental. He conjectured that each particle is composed of other particles, and those others are held together by exchanging the first particle in a process that conveys a force. Thus, particles’ properties are generated by self-consistent feedback loops. Particles, Chew said, “pull themselves up by their own bootstraps.”

Chew’s approach, known as the bootstrap philosophy, the bootstrap method, or simply “the bootstrap,” came without an operating manual. The point was to apply whatever general principles and consistency conditions were at hand to infer what the properties of particles (and therefore all of nature) simply had to be. An early triumph in which Chew’s students used the bootstrap to predict the mass of the rho meson — a particle made of pions that are held together by exchanging rho mesons — won many converts.

But the rho meson turned out to be something of a special case, and the bootstrap method soon lost momentum. A competing theory cast particles such as protons and neutrons as composites of fundamental particles called quarks. This theory of quark interactions, called quantum chromodynamics, better matched experimental data and soon became one of the three pillars of the reigning Standard Model of particle physics.

But the properties of individual quarks seemed arbitrary, and in another universe they might have been different. Physicists were forced to recognize that the set of particles that happen to populate the universe do not reflect the only possible consistent theory of nature. Rather, an endless variety of possible particles can be imagined interacting in any number of spatial dimensions, each situation described by its own “quantum field theory.”

The bootstrap languished for decades at the bottom of the physics toolkit. But recently the field has been re-energized as physicists have discovered novel bootstrap techniques that appear to solve many problems. While consistency conditions still aren’t much help for sorting out complicated nuclear particle dynamics, the bootstrap is proving to be a powerful tool for understanding more symmetric, perfect theories that, according to experts, serve as “signposts” or “building blocks” in the space of all possible quantum field theories.

Read entire article in Quanta Magazine

Natalie Wolchover