This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
In 2007 Garrett Lisi was a 39-year-old physicist, unaffiliated with any institution, toiling in obscurity on what he called “An Exceptionally Simple Theory of Everything," which could account for all of nature's forces. Over the next year he became a celebrity, after The New Yorker, Outside, Discover and other publications described him as a rootless surfer and snowboarder whose unified theory intrigued big shots like Lee Smolin. I first heard about Lisi from my friend and former colleague at Stevens Institute of Technology, physicist and philosopher, James Weatherall, who helped Lisi co-write an article about his theory for Scientific American. In the fall of 2008 I met Lisi at a party thrown for him in New York City by physicist and string critic Peter Woit. Lisi was refreshingly down to earth, his ego utterly uninflated by all his fame. I was impressed not only by his theoretical ambition but also by his desire to help other researchers pursue non-traditional career paths. Lisi, who has settled down in Hawaii, agreed to answer my questions about what he's been up to. (See also Lisi's website and my "Further Reading" links at the end of this post.)
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Horgan: Do you ever regret all the attention you got back in 2008 for being the physicist "surfer dude" (as British journalist Roger Highfield put it)?
Lisi: It was very, very strange. I was pretty happy with my life before 2008, spending my time on physics and surfing. The most negative repercussion of the attention was that the physics I love got thrown into a black vs. white media machine. My emphasis on proposing a new research direction and not a completed ToE (theory of everything) was ignored, and there were premature attacks, such as "rock climber proves surfer's theory can't work," and other foolishness. The attention was fun, but bad for development of the theory, which had been building interest among physicists before the media storm got ridiculous. Fortunately, things have calmed down and I'm happily back on my island, working on physics and surfing a bit.
Horgan: Can you give a brief description for non-physicists of your "Exceptionally Simple Theory of Everything"?
Lisi: "Brief?" No, but I can give a description. The paper title was a pun based on the principal geometric object of the theory, a wonderfully intricate and beautiful 248-dimensional mathematical structure, the largest simple exceptional Lie group, named E8. The research direction proposed in that paper is called "E8 theory."
Our current best understanding of the universe consists of Einstein’s theory of gravity and the Standard Model of quantum particle physics. Matter particles, called "fermions" (electrons, neutrinos, quarks, etc.), and the Higgs particle interact via electromagnetic, weak, and strong-force particles, called "bosons." This model of spacetime and particles can be understood geometrically as different Lie groups (pretty, smooth mathematical surfaces made from joining circles and hyperbola) twisting over our four-dimensional spacetime. The fermions and Higgs also twist around the Standard Model Lie groups, with twist numbers equal to their electric, weak, strong, and gravitational charges. This very successful model, well established by experiment, was largely completed by the early 1970s. It is a wonderful geometric description of our universe—but it's a mess.
In the mid 1970s, physicists figured out that the three non-gravitational forces could be nicely combined as parts of one larger Lie group, with matter particles twisting around it, forming a Grand Unified Theory (GUT). This was a big step in explaining the Standard Model as part of something larger, but it’s not a complete picture. What I found in 2007, by extending this work, is that the gravitational Lie group, fermions, and Higgs can also be combined, forming parts of just one Lie group, E8, twisting over spacetime. All the known elementary particles, each with different charges and interactions, match parts of what many consider to be the most beautiful structure in mathematics. The fact that this unification works so well is, I think, beyond coincidence. But it does have a big problem. The parts of E8 which one might hope would correspond to the second and third generations of fermions (muons, strange and charm quarks, etc.) don't have the right charges. They're horribly wrong, even their spins. Until this gets figured out, the theory is incomplete. But so much of E8 theory works... I think it has a shot at becoming the ToE.
Horgan: Have you made any progress in the theory lately?
Lisi: Yes, I think I've got a line on it. I have two papers in preparation, but can tip my hand a little. There's an unusual description of spacetime called "Cartan geometry" that's very interesting. You start with a single ten-dimensional Lie group (a rigid geometric surface) and let it deform along four directions. The resulting structure is our four-dimensional spacetime with the six-dimensional gravitational Lie group twisting over it. It is a very efficient model. A year ago I worked out a generalization of Cartan geometry, allowing spacetime to embed in larger Lie groups. When I do this for E8, there's a symmetry called “triality” linking three different sheets of spacetime; with respect to each different sheet, each of the three different generations of fermions comes out right. If this all works, it would mean the reason we see Lie groups everywhere in physics is because we're inside of one, looking out. Our universe and everything in it might be excitations of a single Lie group.
Horgan: Edward Witten, when I asked him in a recent Q&A if string theory had any serious rivals for a unified theory, replied, "There are not any interesting competing suggestions." Comment?
Lisi: That stings a little. I don't imagine other physicists working on fundamental non-string theories appreciate it either. Ed Witten has done incredibly impressive work, opening new doors with his insights in mathematics and physics. His papers are things of beauty. He, his students, and his colleagues have dominated the high-energy theoretical physics community with string models for decades now. However, even the most enlightened foresight from the most brilliant mind can be wrong, so it would be better if he wasn't a dick about it.
And how are things going with string theory? The promises and hopes from the 1980s have not worked out. They thought they'd find the right Calabi-Yau manifold and the fermion multiplets and masses would pop out and they'd have the whole thing wrapped up before lunch. But that didn't happen. String models grew increasingly complicated. And with every fanciful step they made away from the Standard Model, the more likely they were to be wrong; they were mesmerized by their own mathematical constructions, which kept them busy but were much more complex than the Standard Model they were trying to explain. String theory became a postmodernist monstrosity, lumbering forward on self-provided momentum without ever receiving the pruning from experimental verification that physics demands. The closest thing to a physical prediction that string theory has ever produced is that there should be superparticles, but these have not shown up. String theory models lost connection to the physical world. Other physicists and mathematicians were left wondering if string theorists had joined some sort of cult. I escaped to Maui to get away from the train wreck.
There are many Loop-Quantum-Gravity researchers who have attempted to extend spin networks, spin foams, and spin connection fields in general to describe the Standard Model or parts of it. A few examples: Bilson-Thompson, He, Wan, Schiller, (with their braid models and preons), Alexander, Nesti, Percacci, (graviweak unification), and several others. There are also some outlandish non-stringy but geometric unification approaches in various stages of development, such as Weinstein and his Geometric Unity, myself and E8 Theory, etc. There's also the noncommutative geometry program, which is not inherently stringy, and several unification models based on condensed matter physics, such as spin condensates, superfluids, and even Wolfram with cellular automata. There're also people working on quantizing gravity more directly, with some promising findings for "asymptotic safety," with the Standard Model and gravity possibly consistent up to very high energies with only slight modification. And there are many researchers working even more closely to the Standard Model, without strings, trying to find geometric explanations for the structure of the fermion masses, which would certainly speak to unification. There are also fundamental issues that string theory doesn't address, such as an explanation for quantum mechanics itself, which only brave outliers such as 't Hooft have worked on. So, there are MANY interesting fundamental theories in active development, bearing on unification, that have nothing to do with strings. Of course, since string theory has become a huge toolkit and not a unified particle model, you can use it to describe all these things with enough effort, but since string models are more complicated than what they explain, there's no reason to think nature works that way.
Horgan: In 2009, you bet Frank Wilczek that super-symmetric particles would not be detected by July 2015. Are you confident you'll win this bet?
Lisi: I respect and admire Frank Wilczek a great deal. He's done brilliant work, has a wonderful sense of humor, and he's also just plain kind. While he was giving a lively conference talk, he expressed an unusual confidence in the existence of superparticles. His main reasoning was that superparticles would help the forces combine in a Grand Unified Theory, with the forces having the same strengths at tiny distances, becoming part of just one force. And he likes superparticles for other reasons, including that string theorists need them to exist. But I knew, from a review paper on renormalization, that you could get a similar unification result from having a bunch of Higgs particles and no superparticles. So, I was a bit of a punk at the end of his talk and challenged Frank to a bet on whether superparticles would be discovered. He accepted, and chose the date and amount (July 8, 2015, $1K). We both figured the Large Hadron Collider would have collected plenty of data by then. But things at the LHC didn't go entirely smoothly. We did get a good run at 7 TeV, discovering the Higgs particle (which was fantastic) and, much to the disappointment of many physicists who aren't me, no superparticles. And it now looks like our bet will come due before the LHC is able to collect much data from its run at 13 TeV in 2015. So, yes, I'm very likely to win. And if Frank would like to place another bet with a date further out, I'd be happy to do that. I don't think superparticles exist, and I hope many physicists, if they don't like losing their money and their time, will re-consider non-stringy unified theories.
Horgan: Do you ever worry that the quest for a unified theory will turn out to be a dead end?
Lisi: Einstein's description of gravity and spacetime as a curving four-dimensional geometry is so elegant and experimentally successful that it has to be essentially true. The Standard Model of particle physics is similarly successful, but not elegant, and doesn't mesh easily with Einstein's theory. But everything has to work together somehow. The universe is just one thing—it's right there in the name. And it does feel like most of the puzzle is filling in; we're getting closer. String theory may have been a wrong turn. Maybe if we try understanding physics using deforming Lie groups and representation theory, we'll have this wrapped up before lunch.
Horgan: Do you ever regret your non-traditional career path?
Lisi: I do miss universities. But I spend most of my time surfing, hiking, kitesurfing, and paragliding around Maui, working on physics and other projects when I like. Friends and students visit and talk, and I take them out to play on this beautiful island. Life isn't so bad.
Horgan: Is your passion for surfing and other sports in any way connected to your passion for physics?
Lisi: I don't know—maybe. I'm half English and half Italian, so I'm very passionate, but I suppress it.
Horgan: If young physicists ask you about the risks and benefits of a path like yours, what do you tell them?
Lisi: I'm a weird data point they should probably throw out. There was no path where I went. I do have to let students know I am not a degree-granting institution, but they're welcome to visit. In general, I advise people to do what they love, and what interests them—even if it's string theory.
Horgan: Can you describe and give an update of your proposal to create a network of science hostels?
Lisi: Twenty years ago, research scientists were anchored to academic libraries and laboratories. The internet has now set them free. Where can they go? What is an ideal theoretical research environment? I think we need something like artist retreats, but for scientists. While working on physics, I spent a decade visiting friends' vacation homes in nice locations, and group-living communities, and I think a network of such places—Science Hostels—would be a great resource for scientists and science-friendly creatives. One year ago I bought a small ranch house with a nice view here on Maui and built three guest cabins with a friend of mine (who likes beer, pizza, and nail guns a little too much). We named it the Pacific Science Institute, and over the past eight months we've had about twenty visitors come through and stay with us, for free, for a few days to a few months, to work and play. It's been great! A nice house in a beautiful location, populated by selectively social science geeks, makes for a pretty ideal living and working environment. It's the flagship Science Hostel. We also own a larger piece of land here on Maui, maybe for PSI 2.
Thank you very much for your interest. Sorry for the length. I guess I had a lot to say.
Further Reading: See my recent Q&As with physicists George Ellis, Carlo Rovelli and Edward Witten.