The Perfect Theory (32 page)

Despite the euphoria and the hubris, string theory couldn't avoid an almost existential problem. There seemed to be too many versions of string theory available. And even if you stuck to a single version of string theory, there were many, many possible solutions that could correspond to the real world. A rough estimate led to the possible existence of 10
⁵⁰⁰
solutions for
each version
of string theory, a truly obscene panorama of possible universes that became known as the landscape. String theory remained unable to make unique predictions.

A number of prominent skeptics argued that string theory promised too much and delivered too little.
“I think all this superstring stuff is crazy and is in the wrong direction,” Richard Feynman said in an interview shortly before his death in 1987. “I don't like that they're not calculating anything. I don't like that they don't check their ideas. I don't like that for anything that disagrees with an experiment, they cook up an explanation. . . . It doesn't look right.”

Feynman's view was echoed by Sheldon Glashow, who, along with Steven Weinberg and Abdus Salam, had constructed the extremely successful standard model. He wrote that “superstring physicists have not yet shown that their theory really works. They cannot demonstrate that the standard theory is a logical outcome of string theory. They cannot even be sure that their formalism includes a description of such things as protons and electrons.”

Daniel Friedan, a prominent string theorist in the first string revolution of the 1980s, acknowledges string theory's shortcomings. As Friedan admits, “The long-standing crisis of string theory is its complete failure to explain or predict any large distance physics. . . . String theory cannot give any definite explanations of existing knowledge of the real world and cannot make any definite predictions. The reliability of string theory cannot be evaluated, much less established. String theory has no credibility as a candidate theory of physics.” These skeptics remained in the minority and were easily drowned out. If you were to enter the field of quantum gravity in the 1980s or 1990s, you might be forgiven if you thought that the covariant approach had won and string theory was all there was.

 

There was one thing that really riled many of the general relativists about string theory: in string theory, as in any covariant approach to quantum gravity, the geometry of spacetime, the be-all and end-all of general relativity, seemed to disappear. It was all about describing a force, like the other three forces brought together into the standard model, and how to quantize it. To a small band of relativists, the way forward was by another route, which Wheeler had embraced and DeWitt had discarded: the canonical approach. There it should be possible to cook up a quantum theory of geometry itself. In the mid-1980s, an Indian relativist named Abhay Ashtekar found a way forward.

Ashtekar was a committed relativist working at Syracuse University. He came up with an ingenious approach to untangling Einstein's field equations, rewriting them so that most of the fiendish nonlinearities disappeared and general relativity looked much, much simpler. Ashtekar's trick unlocked Einstein's equations in an unexpected way and opened the door for three young relativists to tease out their quantum nature.

Just like Bryce DeWitt, Lee Smolin fell in love with quantum gravity the moment he arrived at Harvard for graduate school in the 1970s. His adviser, Sidney Coleman, let Smolin get his hands dirty in quantum gravity by working with Stanley Deser at Brandeis. As a student, Smolin failed miserably to quantize gravity, but he remained passionate about solving the problem. It was only when he headed to Yale as an assistant professor that he realized how Ashtekar's trick made his job much easier. At Yale, Smolin teamed up with Theodore Jacobson, an ex-student of Cécile DeWitt-Morette from the Texas relativity group. Smolin and Jacobson found that instead of talking about the quantum properties of geometry at isolated points in space as they evolved over time, it was much easier to work with the geometry of a collection of points, effectively focusing on chunks of space at any given moment. In their case, the natural building blocks for the quantum theory were loops, like ribbons, in space that could be used to build solutions to the Wheeler-DeWitt equation. Things just seemed to fall into place, and a whole new way of thinking about quantum geometry emerged. The loops could link up and intertwine themselves like chain mail or an intricate fabric. As with a piece of fabric, from a distance the weaves and links disappeared and the smooth, curved spacetime of Einstein's theory would emerge. Smolin and Jacobson's approach became known as loop quantum gravity.

Smolin was joined in his quest by an iconoclastic young Italian physicist named Carlo Rovelli who had also cut his teeth working on the impossible algebra of quantum gravity. Rovelli enjoyed being a rebel. He had set up an alternative radio station during his student days in Rome, had been pursued by the Italian authorities for his political views, and had risked imprisonment for refusing conscription. Alternative views suited him. Smolin and Rovelli took the loop picture even further and looked at how the loops could be linked, braided, and knotted together. In doing so, they wandered from their starting point, the geometry of space, toward an even more broken-up and shattered view of geometry. In the mid-1990s, they stumbled upon an old idea Roger Penrose had for describing a quantum system in terms of a simple mathematical scaffolding, what Penrose called a spin network. Just like a crazy climbing frame in a children's park, the structure would be a network of links and vertices, each of which carried with it some special quantum properties. Rovelli and Smolin showed that these networks were even better solutions to the Wheeler-DeWitt equation. Yet these selfsame networks had no resemblance to the intuitive picture of space and time that any self-regarding relativist would work with.

Rovelli and Smolin's spin networks were a completely new way of looking at quantum gravity. In their model, space didn't exist at a quantum level—it was atomized or molecularized like water. Water, which looks smooth and continuous at a macroscopic level, is actually made up of molecules, little clusters of protons, electrons, and neutrons that float in empty space, loosely bound to each other through electric force. In the same way, according to Rovelli and Smolin, while space may seem smooth, it shouldn't exist if you peer at it with an extremely powerful microscope. In Rovelli and Smolin's theory, if you were able to look at distances of a trillionth of a trillionth of a centimeter, there would be no space, just the frame or network.

Loop quantum gravity was the plucky competitor to string theory in its attempts to quantize gravity. Loop quantum gravity and its progeny offered a canonical alternative to string theory's covariant approach. The devotees of loop quantum gravity made no attempt at unifying all the forces, but in taking geometry as their starting point, they tried to preserve some of the beauty of Einstein's original idea in general relativity. Ironically, in the process, they abandoned the idea of spacetime as something fundamental.

 

In a lecture Bryce DeWitt gave in 2004, shortly before his death, he marveled at how far quantum gravity had come along:
“In viewing string theory one is struck by how completely the tables have been turned in fifty years. Gravity was once viewed as a kind of innocuous background, certainly irrelevant to quantum field theory. Today gravity plays a central role. Its existence justifies string theory! There is a saying in English: ‘You can't make a silk purse out of a sow's ear.' In the early seventies string theory was a sow's ear. Nobody took it seriously as a fundamental theory. . . . In the early eighties, the picture was turned upside down. String theory suddenly needed gravity, as well as a host of other things that may or may not be there. Seen from this point of view string theory is a silk purse.”

DeWitt had never worked on string theory, but it was clear where his allegiance lay. About the canonical approach he was much less enthusiastic. Despite having created it, DeWitt hated the Wheeler-DeWitt equation. He thought it “should be confined to the dustbin of history” for, among other things, “it violates the very spirit of relativity.” In fact, according to DeWitt, “the Wheeler-DeWitt equation is wrong. . . . It is wrong to use it as a definition of quantum gravity or as a basis for refined and detailed analysis.” He acknowledged Abhay Ashtekar's work on the equation as “elegant,” but, he said, “apart from some apparently important results on so-called ‘spin foams' I tend to regard the work as misplaced.” DeWitt's antipathy reflected the popular view in the world of theoretical physics: string theory was winning.

The string theorists revel in what they perceive as their success. Mike Duff, now back in London, declares, “We have made tremendous progress with string and M-theory. . . . And it is the only attempt at unification.” Many string theorists are convinced supersymmetry and extra dimensions will soon be discovered and that string theory is the only acceptable approach. Stephen Hawking himself has said that “M-theory is the
only
candidate for a complete theory of the universe.” When asked about the rival canonical approach, seen by many as the rightful heir of Wheeler's philosophy of quantizing geometry, Duff accuses them of claiming that “quantum gravity” is synonymous
with “loop quantum gravity.” Duff is not alone. “They can't even calculate what a graviton does. How are they ever going to know that they are right
?
” argues Philip Candelas, who is firmly entrenched in the string theory camp.

In the mid-2000s, the deep-rooted antagonism between the different camps in the quest for quantum gravity came out into the open. For years, the odd op-ed articles by a few outspoken pundits had been cropping up in blogs and popular physics magazines questioning the hegemony of string theory in theoretical physics. Around 2006, two books came out claiming that string theory was, in fact, destroying the future of physics. The authors, Lee Smolin, one of the champions of loop quantum gravity, and Peter Woit, a mathematical physicist at Columbia, claimed that impressionable young physicists were being lured into working in a field that, after almost thirty years, had yet to deliver tangible hard results that would unify the forces and explain quantum gravity. According to them, academia was dominated by string theorists who hired more string theorists and kept out bright young people who didn't toe the party line. As Smolin put it in 2005, “A lot of people are frustrated that this community that styles itself as dominant—and is dominant in many places in the U.S.—is uninterested in other good work. Look, when we have quantum gravity meetings, we try to invite a representative from each of the major opposing theories, including string theory. It's not that we're so very moral; it's just what you do. But at the annual international string theory meeting, they've never done this.” The blogosphere blazed with the debate while the pro–string theory camp, flustered by the attacks, took it upon themselves to set the record straight. Statements posted on physics websites were followed by hundreds of comments, a messy mélange of technical details, punditry, and pure ignorance. Everyone had an opinion.

The hostility toward string theory was palpable in 2011 when Michael Green, who had replaced Stephen Hawking as the Lucasian Professor in Cambridge, came to give a public lecture on string theory at Oxford. Green had, with John Schwartz, kick-started string theory's growth in 1984, and I had seen him give a colloquium in London in the early 1990s to enormous acclaim. String theorists were riding high then. This time, at Oxford, the atmosphere was much cooler. While most of the questions were about the specifics of his talk, a few were needling jibes. No public string theory talk can now get by without the inevitable question: “Is this theory testable?” The question always comes from someone sympathetic to the anti-string camp.

It is too early to tell how the antagonism between the different tribes working on quantum gravity will play out. For a while those working on non-string formulations of quantum gravity found it difficult to thrive, but it now seems that string theorists working on quantum gravity are being hounded, too.

A remarkable result of the debate has been that many more people are familiar with the idea of quantum gravity than before. The war between the canonical and covariant approaches has even made network TV. On the popular show
Big Bang Theory,
two characters broke off their relationship because they couldn't agree on which approach to teach their children. As Leslie Winkle says to Leonard Hofstadter as she storms out of the room,
“It's a deal breaker.”

 

Thirty years after Stephen Hawking predicted the end of physics and then unleashed his black hole information paradox on an unsuspecting world, there isn't an agreed-upon theory of quantum gravity, let alone a complete unified theory of all the fundamental forces. Yet, despite the acrimony in the quest for quantum gravity, there is common ground. A radically new and almost
shared
view of the nature of spacetime is emerging. From string theory to loop quantum gravity to all the other niche attempts at quantizing general relativity, almost all approaches give up on spacetime as something truly fundamental. This insight can be directly related to Hawking's discovery of black hole radiation and may help resolve the problem of information loss in black holes and the end of predictability in physics. One of the key steps in resolving Hawking's paradox is to understand how black holes actually store the information that they gobble up and how they might release it to the outside world. This requires a more complicated black hole than general relativity's naive picture of a horizon and nothing else. Somewhat surprisingly, both loop quantum gravity and string theory, as well as other more esoteric and more marginalized proposals for quantum gravity, seem to shed light on this problem.