Filling the Empty Quadrants: Jim Gates on Supersymmetry and the Beauty of Broken Symmetry

Asked by Lex Fridman what supersymmetry is, Gates asks him to picture a pie with four quadrants. In one he places the matter particles, such as electrons and quarks, and in another the force carriers. The other two quadrants are empty, and Gates says humans have a deeply programmed sense of symmetry, so the picture immediately looks wrong.

One way to restore balance is to put something in those empty quadrants, and that is supersymmetry. Gates says he understood the idea as a graduate student at MIT in 1975, when its mathematics was still new. It had first been born in the Ukraine in the late 1960s, but the Iron Curtain kept Western scientists from knowing, and Bruno Zumino and Julius Wess rediscovered it independently in the early 1970s.

Gates explains that he wanted to maximize his chances of success by working in a direction few others were studying. His advisor, James Young, had taught him about electrons and the weak force, but when Gates surveyed emerging ideas, supersymmetry's mathematics bowled him over. He holds two undergraduate degrees, mathematics first and physics second, although his heart was always in physics.

Fridman asks about the most beautiful idea in mathematics and physics, and Gates answers: symmetry. He finds it striking that our innate sense of symmetry lines up with powerful mathematics. Yet he points out that perfect symmetry would mean no humans, because it is only when balances are broken that creatures, planets and stars appear.

Symmetry is therefore a scaffold for reality but not the whole of it. Gates says he is naturally drawn to areas of science where symmetry plays a dominant role, and Fridman sums up that the magic happens when you break the symmetry.

To fill the quadrants, Gates returns to his earlier image of two flashlight beams passing through each other: light is made of force carriers called photons, which carry the electromagnetic force. He calls mathematics a palette, much like a composer constructing variations on a theme, and says that Maxwell's equations for light can be varied. One variation gives a force carrier that bounces off others like an electron does, which cannot sit in the force-carrier bucket and so must go in an empty quadrant. The photon's partner is called the photino.

The same trick works in reverse. Starting from the mathematics of the electron, developed by Dirac in the late 1920s and early 1930s, one can locate the term that makes two electrons bounce off each other and modify it so the particle carries charge but passes through others. Physicists name these new objects by putting an s in front, giving selectrons and squarks, which completes the balanced pie.

Gates was amazed that nobody else at MIT found this interesting, which led him to be the first person there to study supersymmetry seriously. In 1977 he wrote the first physics PhD thesis in the department on the subject, drawn, he says, by the balance and the symmetry.

Fridman asks whether supersymmetry is fundamentally mathematical and how it stands experimentally, observing that it is closer to validation than string theory but has not yet been confirmed. Gates answers by turning to Einstein, the subject of his book Proving Einstein Right. Einstein's 1915 general relativity equations differed from Newton's and made three predictions, one of which, the orbit of Mercury, was really a postdiction because astronomers had already seen it deviate from Newton's expectation.

The second prediction was that gravity bends light, which Gates illustrates by moving a water glass between two faces so that one appears to shift. Newton also expected bending, he says, but Einstein predicted twice as much. Fridman notes how remarkable it is that a calculation plus a physicist's intuition can point to what an experiment should find.

Gates agrees that this is how fundamental modern physics works: someone writes down equations, the equations make a prediction, and experimentalists go looking. He gives the Higgs boson as another example, since in the late 1960s and early 1970s people wrote equations that predicted it, and later the search began.