When I saw that the line up for Saturday's session on whether we have any serious chance of finding a “theory of everything” (TOE) included a mix of string theorists and a loop quantum gravity theorist (who are usually set up as being major TOE rivals), I was hoping we’d be in store for a fireworks. In the stringy corner, we had Stanford’s Renata Kallosh and Columbia’s Brian Greene, while Penn...

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When I saw that the line up for Saturday's session on whether we have any serious chance of finding a “theory of everything” (TOE) included a mix of string theorists and a loop quantum gravity theorist (who are usually set up as being major TOE rivals), I was hoping we’d be in store for a fireworks. In the stringy corner, we had Stanford’s Renata Kallosh and Columbia’s Brian Greene, while Penn State’s Abhay Ashtekar represented the loopy side.

In the event, everyone was far too genial for fisticuffs or heated words--in fact, they could have shared many of the slides for their talks, as there was much more overlap between what they had to say than disagreement. Nevertheless, it was a fun session.

Abhay Ashtekar (photo by Andrei Linde)

Ashtekar, a founder of loop quantum gravity theory, kicked things off with a jaunt through the history of the search for a theory of everything, pointing out that at the end of the 19th century, physicists thought they were pretty much done. They had Newton’s laws and Maxwell’s laws, and although there were a few loose ends (for instance, because they didn’t know about nuclear reactions in stars, the Earth seemed puzzlingly to be older than the stars), they were confident they'd soon be tied up. As it turned out, those loose ends would unravel their final theory and bring about a paradigm shift in physics.

The big challenge for TOE-ers is to bring together general relativity and quantum mechanics, given the absence of any relevant experimental evidence. On the one hand, the lack of experimental evidence could be seen as a “theorist’s heaven,” said Ashtekar, “allowing them to cook up a new theory every day.” Unfortunately, it’s also “hell” because it means a long wait to test any theories.

With loop quantum gravity (LQG), Ashtekar takes Einstein’s general-relativity view that gravity is encoded into the geometry of spacetime and runs with it: “If you want to have a quantum theory of gravity, then you have to have a quantum theory of geometry.”

Quantum mechanics taught us that matter has atomic structure, but what about geometry? Ashtekar pointed out that while the blue shirt he was wearing looks like a smooth continuous surface to the audience, homing in on it with a microscope will reveal that it’s woven out of tiny threads. LQG tells us that the fabric of space is woven by 1 dimensional “quantum threads.” (FQXi writer Anil Ananthaswamy describes more about LQG in “A Stitch in Quantum Time,” and writes about Ashtekar’s work in particular in this article in New Scientist (subscription required).)

When LQG is applied to cosmology, there are some intriguing implications, particularly when trying to understand what happened at the big bang: “The spacetime fabric is torn apart violently, quantum threads fluctuate wildly. But the quantum state of the universe has a well-defined time-evolution _across_ the big bang,” said Ashtekar.

In other words, time didn’t begin at the big bang. Rather than having a big bang, you have a “big bounce” transitioning between one universe to another--and you can ask what’s on the other side of the “quantum bridge” that crosses the bang. (You can find out more about this on Ashtekar's presentation slides.)

Brian Greene (photo by Andrei Linde)

Next up, and speaking for the string theorists, was Brian Greene. If the perceived “war” between the string theory camp and LQG camp were to be decided by the quality of animations presented during the talk, then string theory would win hands down. Though Greene has the unfair advantage of having nifty graphics courtesy of his NOVA show, “The Elegant Universe.”

Like Ashtekar, Greene described how at small scales, physicists expect that space can rip apart, becoming chaotic. Unlike Ashtekar, he takes the now familiar view that on the smallest scales, particles are made up of tiny vibrating strings and that space contains tiny wrapped-up extra-dimensions that are too small for us to traverse. As Andrei Linde described earlier in the day (and as Kallosh would describe later), Greene explained how the way that these extra dimensions are wrapped up determines the physics that we observe above around us.

Calabi-Yau space by Paul Bourke

Unfortunately, there are hundreds of thousands of shapes for these extra dimensions (which wrap up into a shape known as a Calabi-Yau space). So string theorists (or at least their grad students, as Greene joked) had to check them one by one to see if any gave us laws matching our universe. Things looked fairly bleak for a while, said Greene, as most didn’t. But it started to looked sunnier when they found one that did. However, with such a wealth of possible variants of our universe, string theory seems to have lost its hoped-for predictive power.

Will there be a way to find a unique model? Even if not, that may not be bad news, said Greene. After all, there are many different possible quantum field theories, and physicists use experiments to help find the right one to use to describe certain phenomena. Perhaps the same is true for string theory.

Greene also acknowledged the biggest criticism levelled against string theory. “From 1985-1995 there was no experimental evidence for string theory,“ he said, adding, “and from 1995 to the present there’s no experimental evidence for string theory.”

So why should we still take it seriously?

“Putting it into perspective, general relativity and quantum mechanics have both been experimentally verified, so any theory that makes them compatible--as string theory does--commands and deserves attention,” said Greene.

He also pointed out something that’s often overlooked: String theory’s successes aren’t confined to bringing general relativity and quantum mechanics together. It’s also managed to theoretically incorporate a bunch of other, unrelated but important, discoveries (familiar gauge theories, chiral family structure, symmetry breaking…).

In terms of finding experimental support for string theory in the future, Greene’s hopes lie with the LHC. If evidence of super symmetry is found, that will be a big boost for the theory.

“People sometimes think that string theorists like the fact that it’s hard to falsify string theory because they think we can keep getting funding that way,” said a frustrated Greene. “We don’t. I don’t. If I’m working on something that’s wrong, I want to know now. I want to know five years ago!”

Having asked Alan Guth earlier in the day if any observational evidence would make him give up on the idea of inflation, Greene was asked the same question regarding string theory. He gave a similar response to Guth’s. “It would be a theory showing a hole in string theory. Not an experiment.”

Renata Kallosh (photo by Andrei Linde)

Speaking last, Kallosh also addressed future experiments and observations that could provide support for string theory and help pin down a theory of everything, including the Planck mission to study the cosmic microwave background to look for gravitational waves and the start-up (fingers-crossed) of the LHC. “If supersymmetry is there, it will be the most fundamental discovery in physics after Einstein’s relativity,” she said.

She also hinted that there may soon be interesting news about the possible dark matter sightings by PAMELA and ATIC that have been in the headlines lately. (More details can be found in Kallosh’s presentation slides, here.)

Kallosh also described how some felt that the discovery of the string theory landscape, with so many possible variants of the universe allowed by string theory, was “a requiem for the dream of a final theory,” adding, “We were all spoiled by the triumph of theoretical physics, like the success of QED. Why can’t we make something so beautiful?”

But on the flip side, she admitted that the discovery of the landscape amused her friends who were biologists, who felt that physics was becoming less of an exact science and more like their own discipline. I guess, if nothing else, at least string theory is succeeding in unifying physics and biology.

The String Landscape from Kallosh's talk

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