Scientists and mathematicians #3…
Thursday, April 14th, 2016Some theoretical physicists play loose with mathematics. Feynman, for example, never worried about the convergence of the series needed to calculate things in QED (that’s quantum electrodynamics); in fact, he designed a technique, now called Feynman diagrams, to allow him to throw away infinities easily and just hoped things would work out. That’s called renormalization. The renormalization of quantum field theories became a big deal. The electro-weak theory of Weinberg, Glashow, and Salam, designed to explain and combine the electromagnetic and the weak forces, for example, was never fully accepted until ‘t Hooft proved it could be “renormalized.”
Dirac’s bras and kets were just vectors and linear functionals on a Hilbert space, but I doubt he worried too much about functional analysis, the study of infinite-dimensional vector spaces and their linear operators. Even Maxwell’s brilliant synthesis of all classical electromagnetic phenomena—gamma rays, x-rays, visible light, electricity, and magnetism—had to wait years until vector calculus was invented by Gibbs before its true beauty could be seen. A classical vector field is determined by its curl and divergence, and that’s exactly what Maxwell’s equations say about the electric and magnetic fields.
Sometimes physics gets ahead of mathematics. Sometimes it’s the reverse. The key to quantum chromodynamics isn’t Gell-Mann’s Eightfold Way. The representations for the special unitary group SU(3) he used to organize hadrons into composites of quarks already existed. His contribution was to recognize that the representations could organize the hadronic particle zoo. Similarly, I always thought that algebraic topology was an esoteric branch of mathematics, and yet it has found multiple uses in particle physics.
On the flip side, Ed Witten’s treatment of string theory (part of quantum field theory) has led into many breakthroughs in the theory of knots, an unusually esoteric mathematical subject, so much so that Witten received the Fields Medal, the prestigious mathematics equivalent of a Nobel prize (the story about why Nobel didn’t want to give a prize to mathematicians seems apocryphal). Much earlier, Einstein’s General Theory of Relativity wouldn’t have gone anywhere without tensor calculus—his friend Grossman, a mathematician, even helped him with the math.