It may have a boring name, but the standard model of particle physics is one of the most successful theories ever proposed. Its predictions have been tested to phenomenal accuracy; A team led by Gerald Gabrielse announced in 2008 that their measurement of the electron’s magnetic field agreed with the theory to the astonishing level of one part in a trillion. And the novel particles predicted by the theory have turned up with impressive regularity. The top quark was identified in 1995, the tau neutrino followed in 2000 and the Higgs boson completed the picture at the Large Hadron Collider in 2012.
These successes have been achieved with a theory that was never meant to be the last word. Writing to Gabrielse in 2006, Freeman Dyson looked back to the beginnings of the standard model in quantum electrodynamics (QED). “As one of the inventors, I remember that we thought of QED in 1949 as a temporary and jerry-built structure, with mathematical inconsistencies and renormalized infinities swept under the rug. We did not expect it to last more than ten years before some more solidly built theory would replace it.”
The infinities Dyson worked so hard to eliminate arise because the standard model is based on the implausible fiction that the particles the theory describes are infinitely small. As you approach an electron, the fields surrounding it increase. If the electron is taken to be an infinitesimal point, then you can get as close as you like and those fields keep getting stronger and stronger – until, that is, you wield your mathematical broom. Renormalization removes these infinities by replacing them with measured values, then allowing for the influence of the virtual particles quantum physics predicts. It won Richard Feynman a share of the Nobel prize, but he still thought renormalization was a “dippy process” that wasn’t “mathematically legitimate”.
Not only is the standard mathematical model suspected, it’s also incomplete. The “hocus-pocus” that Feynman devised for QED doesn’t work for gravity at high energies, leaving an unbridgeable gap between a successful theory of the very small – the standard model – and Albert Einstein’s elegant theory of the very large: general relativity .
String theory offers a way out of this predicament with the straightforward suggestion that the universe is constructed out of tiny vibrating strings. The menagerie of particles that physicists study in their accelerators is produced by different modes of vibration, as if the strings were playing a different note for an electron, a photon or a quark. The standard model’s infinitesimal particles are replaced by strings occupying a region of space, and gravity is baked in, emerging naturally as soon as the theory includes the constraints of quantum mechanics and special relativity.
Unfortunately, little else about string theory is at all straightforward. While the universe you and I experience has three dimensions of space and one of time, string theory only works in eleven. These extra dimensions are thought to be wound up so tightly that they are currently undetectable. The details of how these extra dimensions are wrapped determine the particles, masses, forces and symmetries that exist, with the 10500 available options describing a vast landscape of different universes. Because the strings are so small, the energies needed to probe their structure are so vast that they are far beyond the reach of any conceivable accelerator. And string theory depends on supersymmetry, which predicts that each of the particles in the standard model has a partner with a different spin, requiring a smorgasbord of new particles that have so far failed to show up in the accelerators we have built. More than thirty-five years since string theory became the dominant model in high-energy physics, there is still no prospect of a testable prediction.
The development of the theory has been equally knotty. String theory has its origins in a formula for the strong force between two particles proposed by Gabriele Veneziano in 1968. Despite hints that it might offer a route to a quantum theory of gravity, the theory’s awkwardness and the arrival in 1973 of an alternative theory ( the work of Harald Fritzsch, Murray Gell-Mann and Heinrich Leutwyler) describing the strong force made it a minority pursuit. Some theorists persisted, however, and in 1984 John Schwarz and Michael Green proved that supersymmetric string theory could avoid the anomalies that surrounded many attempts to unify the forces of nature. A revolution was launched. As Schwarz recalled in 2000, they got a phone call from Edward Witten even before they’d finished writing up their work. They Fedexed him a draft, and “the following day everyone in Princeton University and at the Institute for Advanced Study, all the theoretical physicists, and there were a large number of them, were working on this… So overnight it became a major industry, at least in Princeton – and very soon in the rest of the world.”
The backlash started in 1986, when Paul Ginsparg and Sheldon Glashow suggested that the theory “depends for its existence upon magical coincidences, miraculous cancellations and relations among seemingly unrelated (and possibly undiscovered) fields of mathematics”, whether “mathematics and aesthetics supplant and transcend mere experiment?” But for many theoretical physicists, string theory was the only game in town. Over the next ten years it became ever more complicated, with five versions competing to become the ultimate unified theory. The second string revolution came at a conference in 1995, when Witten suggested that these five string theories were all versions of a theory of surfaces, or “branes”, in eleven dimensions. This unlocked a new round of theorizing, with notable results including Juan Maldacena’s discovery that under certain constraints string theory is identical to some quantum field theories that are already well understood. String theory still dominates theoretical physics, but, despite all this progress, the criticism that it is a mathematically interesting dead end has not gone away.
As a witness to and participant in both string revolutions, the physicist Michael Dine is well placed to unpick the tangled story of this intricate theory. In his first book for the general reader, This Way to the Universe, Dine chooses instead to take a broader perspective. He begins by arguing that physicists should look to both theory and experiment, before sketching the shape of the universe according to Isaac Newton, James Clerk Maxwell and Albert Einstein. The searing temperatures of the big bang and its remnants in the cosmic microwave background open up into a discussion of quantum mechanics, where he arrives quickly at the taming of infinities in QED. Dine pursues the story of modern physics through the nuclear forces, the standard model and the imbalance between matter and antimatter in the early universe, but it takes more than half the book before he arrives at supersymmetry and string theory – the subjects in which he has made his name.
These opaque concepts are approached, as so often in This Way to the Universe, with a historical survey. Dine introduces supersymmetry via unsuccessful attempts by Paul Dirac and Leonard Susskind to explain the large disparities between some physical constants. He then compares it to isospin – an abstract property of the strong force briefly described eighty pages earlier – and introduces the raft of new particles admit the theory predicts, beforeting that the absence of these particles in recent experiments at the Large Hadron Collider is “becoming quite uncomfortable for supersymmetry proponents”. Dine takes the historical route through string theory as well, outlining how early versions of the theory with twenty-six dimensions and traveling faster than the speed of light were gradually tamed, before tackling the revolutionary contributions of Schwarz, Green and Witten. Readers meeting these ideas for the first time may wonder whether Dine delivers on his ambition to “make clear why the possibility that nature has this additional symmetry is compelling, and eventually what makes string theory so attractive”.
Dine was at Princeton when Schwarz’s Fedexed paper arrived, and was there in the room in 1995 as Witten went through his transparencies on an overhead projector, but he sometimes struggles to convey the appeal of his abstruse subject matter or the excitement it provoked in the physics community. He treads a careful line on the place of maths in physics, viewing it as “quite difficult but sometimes a helpful tool” while confessing that he has “on the occasion been seduced by beautiful mathematics”, and acknowledges both the contributions made and the difficulties faced by women such as Chien-Shiung Wu, Vera Rubin and Myriam Sarachik in a male-dominated profession.
String theory may still be our best prospect for a theory of everything, but Witten may have been right when he warned Dine that it could be several centuries before that promise was achieved. Thirty-six years after Ginsparg and Glashow suggested that “Contemplation of superstrings may evolve into an activity as remote from conventional particle physics as particle physics is from chemistry, to be conducted at schools of divinity by future equivalents of medieval theologians”, their conclusion up remarkably well: “We who are haunted by the lingering suspicion that superstrings, despite all the hoopla, may be correct are likely to remain haunted for the foreseeable future.”
Richard Le is the editor of Fictionablewhich will be launched in June
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