First of two parts
If Shakespeare were alive today, he’d write a tragedy about physics. I think he’d call it Romeo and SUSY.
It would start out with Romeo, a kind of avatar for all theoretical physicists, falling deeply in love with SUSY, a very beautiful but also very shy embodiment of the deepest insights into reality. Romeo had never actually met SUSY, though. She was kind of a magical, mythical creature, maybe a little like Hermione Granger. Romeo worshiped her nevertheless, entranced by the reports of her great beauty and intellectual sophistication.
For three decades Romeo searched far and wide for SUSY, eventually reaching her last conceivable hiding place, in a tunnel near Geneva. He got ever so close, but then a magnet blew up and he had to wait another few years. Finally he made it through the tunnel and didn’t find SUSY there. And so Romeo, in despair, declared his life a meaningless failure and had to decide between killing himself or writing a blog.
To hear many physicists talk these days, Romeo ought to go ahead and opt for worms as chambermaids and die with a kiss. On the other hand, there are a few who say there’s still some crimson in SUSY’s cheeks and lips. Or at least maybe she has an even hotter sister who is still alive.
OK, today’s real-life scenario isn’t so dire that SUSY fans should be poisoning themselves. But physicists do seem to be in a tizzy these days over the experimental findings, or lack thereof, at the Large Hadron Collider, the world’s most powerful atom smasher. (It occupies the tunnel outside Geneva.) Everyone exulted last year when the LHC found the Higgs boson. But upon reflection, many were saddened that the Higgs was the only big thing the LHC found. No SUSY!
SUSY is shorthand for supersymmetry, the odds-on favorite to solve many of the mysteries about the physical world that have stumped theorists for decades. Supposedly the LHC should produce actual evidence for SUSY, but it hasn’t. And so some physicists have begun to declare SUSY dead, or at least on life-support. Consequently, they say, physics now faces one of the greatest crises in its history. A typical lament, from one recent paper:
“After three years of very successful experimental work at Large Hadron Collider (LHC) at CERN, theoretical physics is apparently in the greatest crisis in its history.” The experimental findings “have nearly eliminated supersymmetry as a possible physical theory. It seems inevitable that we have to face the Nightmare Scenario (i.e. no signs of new physics at LHC) and the unprecedented collapse of decades of speculative work.”
In September, Neil Turok, director of the Perimeter Institute in Canada, squirted lighter fluid into the flames during a lecture to students. “Theoretical physics is at a crossroads right now. In a sense we’ve entered a very deep crisis,” he said. SUSY (and other theories) predicted that the LHC would find new particles. “And they’re not there,” Turok declared. “And so to a large extent, the theories have failed.”
Well, maybe. But it might not really be so bad. Perhaps a little context is in order, starting with why physicists loved SUSY so much in the first place and then investigating the reasons for concern a little more deeply. There may turn out to be various ways the play could end without everybody having to die.
First of all, supersymmetry seems like such a good idea to physicists because symmetry even without the super was so successful. (After all, isn’t a supernova better than a nova? Supermodel better than a model? Superman better than Clark Kent?) Symmetry showed its power in Einstein’s theory of relativity, for example, and later on in the development of the modern theory of particles and forces, the standard model (or as Nobel laureate Frank Wilczek likes to call it, the Theory of Matter).
In this context, symmetry is a mathematical concept, but it has its nonmathematical illustrations. At its most basic, symmetry means that changing one thing doesn’t change everything, or to sloganize it, symmetry is change without change. (Physicists prefer to say invariance under transformation, but it’s the same idea). If you look at a circle in a mirror, you’ve flipped left and right, but the circle looks exactly the same. It possesses mirror image symmetry. If you rotate a snowflake by 60 degrees, it looks just like it did before you rotated it. Rotational symmetry.
All these and other symmetries can be described with equations that seem to be very helpful in making sense of the universe. Einstein was particularly skillful when it came to symmetry. His theory of relativity embodies the principle that the laws of physics stay the same no matter how you move. In the 1920s, Hungarian physicist Eugene Wigner pioneered the application of symmetry math to particle physics, describing particle properties using “quantum numbers” that emerge from equations describing symmetries. Symmetry equations led Murray Gell-Mann to propose the existence of quarks in 1964.
Further pursuit guided by symmetry math led to the creation, in the 1970s, of the standard model, which provides a precise description of all of nature’s basic particles and forces. Some of those particles had not been discovered at the time, but they all have since, including the Higgs boson just last year. Symmetry has a better track record than even Bill Belichick, and is much more polite.
For a long time, though, it has been clear that the standard model symmetries do not solve all the problems that nature poses. During the 1970s, just as the standard model was taking shape, various physicists began to explore additional symmetries. In particular, several investigators discerned a symmetry (eventually called supersymmetry) relating the matter particles (technically, fermions) with the force particles (bosons). In 1981 Savas Dimopoulos and Howard Georgi produced the math describing the complete supersymmetric version of the standard model. For each fermion in the standard model, there should exist a supersymmetric partner boson. Each standard boson should have a superpartner fermion. So nature ought to possess twice the number of known particles.
Dimopoulos once told me that doubling the number of particles in nature wasn’t as radical as it might have seemed. He pointed out that similar considerations had multiplied the number of particles in nature before. In the late 1920s, Wolfgang Pauli figured out that electrons must have a two-valued property later known as spin. That was really the same thing as doubling the number of electrons, Dimopoulos said. And a few years later Paul Dirac proposed the existence of antimatter — a new “antiparticle” for every known particle. Dimopoulos and Georgi were simply doing the same thing again.
There was one tiny (actually, big) problem: no evidence for any such particles. Superparticles therefore must be much more massive than their standard partners — otherwise they would already have been noticed. Hence the need for powerful atom smashers, capable of reaching energies high enough to make such particles. By most calculations, the LHC should have had enough energy to produce the lightest of the superparticles. But even though the LHC succeeded in finding the Higgs, it failed to find any signs of SUSY. To hear some physicists tell it, there never was a story of more woe.
But let’s remember that Shakespeare isn’t writing this story. It may be fair to say that the simplest version of SUSY (known as the minimal supersymmetric standard model) seems a little shaky. But that’s not the end of the play.
Many variations on SUSY have been proposed — in other words, SUSY has siblings and cousins, perhaps not as beautiful as SUSY, but still possibly able to help solve some of the outstanding problems facing physics today. SUSY’s no-show at the LHC may not imply supersymmetry’s absence in nature, but merely that superpartner particles in the correct version of SUSY are too heavy for the LHC to see.
Heavier SUSY particles do pose a problem, though. They seem to complicate the theory more than necessary, and may defy a criterion that physicists call “naturalness,” as Nathaniel Craig of the Institute for Advanced Study in Princeton pointed out in recent lectures, published at arXiv.org.
“The march of null results suggests that we were mostly wrong about precisely how supersymmetry would appear at the LHC,” Craig writes.
It turns out that whether SUSY lives or dies may hinge on how physicists decide to define what is natural, and whether they should insist that a theory be as simple as possible, or parsimonious. And that turns out to be a rather complicated task. To be continued.
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