When old age this generation waste,
Thou shalt remain, in midst of other woe
Than ours, a friend to man, to whom thou say’st,
“Beauty is truth, truth beauty,”—that is all
Ye know on earth, and all ye need to know. — John Keats, “Ode on a Grecian Urn”

What is the role of aesthetics in science? Or, to lower the question to something on my pay scale, how do we judge the “beauty” of a physical theory? I was prompted to think about this question by several things: the ongoing conversation about MOND, the proposed alternative to standard gravitational theory, and another conversation on Facebook about supersymmetry (SUSY). I also just started Lee Smolin’s book The Trouble With Physics, which came out a few years ago but I’m only getting around to reading now (and which you should expect to hear more about soon!). As I teach modern physics and a course specifically on quantum mechanics, the question of “beauty” in a theory is a legitimate topic to discuss in the classroom.

Relativity (both the special and the general theory) are often described as beautiful; quantum mechanics is less often called beautiful because of the difficulty of interpretation, but it still possesses a great deal of elegance. The properties that make a theory “beautiful” seem to be a certain mathematical elegance and conceptual simplicity. (The reverse need not be true! Both quantum mechanics and relativity can prove mathematically challenging to non-specialists.) Despite how weird quantum theory is, it is at its heart a simple set of ideas that is incredibly good at describing the microscopic world. By contrast, MOND isn’t particularly beautiful, and it’s relativistic version (called TeVeS) is downright ugly.

With the advent of the Large Hadron Collider (LHC) on the border of France and Switzerland, a lot of attention is being paid to a theory that melds quantum mechanics with the special theory of relativity: the Standard Model of particles and interactions and particularly the Higgs boson, a particle predicted by the model that hasn’t been detected yet. I would venture to say that nobody really believes the Standard Model is anything like a final theory of particle physics: it doesn’t include gravity, and nothing in it corresponds to dark matter (assuming modified gravity won’t solve that particular problem). So there are a number of extensions and modifications to the Standard Model.

One of these modifications is supersymmetry (SUSY), which I mentioned above. SUSY predicts that for each particle in the standard model, there is a supersymmetric partner with a name like something from Lewis Carroll: electrons are partnered with selectrons, leptons gain brother sleptons, quarks (already literally Joycean in name) garner sister squarks. It’s a beautiful theory in the sense above: it postulates a complete symmetry of nature between all types of particles, and could help solve the problem of dark matter, since it provides a lot of potential candidates to make up the missing mass we see in galaxies and galaxy clusters.

However, beauty isn’t enough to make something correct. SUSY has a lot of flexibility as a physical model, so it’s hard to rule it out definitively in a simple set of experiments, but the LHC can push the bounds of what is allowed—and so far, things aren’t looking so good for SUSY. I have no stake in the theory one way or the other, but I admit that I’m kind of rooting for SUSY to fall simply because that means more work for us theorists, which keeps us off the streets and out of trouble. (Nobody wants to be harassed by gangs of roving physicists, after all.) In terms of dark matter, the ball has been in our court for decades, and we’ve failed to make much progress on it; without SUSY to fall back on, we really have to face up to the challenge. Not that we haven’t been trying, of course, but as long as we could say that the LHC will find the correct SUSY particle that solves all our problems, we’ve been able to dilute responsibility.

If I may be allowed to quote myself (from the exchange on Facebook), Einstein’s genius lay in constructing realistic theories from intuitive leaps; his weakness lay in thinking that was always possible. Something beautiful need not be something true, and although correct theories do seem beautiful in some way—simply because they are correct—that alone isn’t a criterion we can use as scientists.