Trespassing on Einstein's Lawn (29 page)

The S-matrix—which Wheeler invented in 1937 and Heisenberg reinvented independently a few years later—lets you skip step two altogether. It's a probability table: plug in the initial state of the colliding particles and the S-matrix spits out probabilities for which particles will emerge at the other end.

Physicists, however, were having a hard time constructing an S-matrix that would account for the outcomes of the hadron collisions they observed in accelerators. It wasn't until 1968 that physicist Gabriele Veneziano solved the problem: he discovered an equation that governed
the S-matrix for those hadrons. But why did the equation work? No one was sure. What exactly was it describing?

After months spent holed up in his attic staring at Veneziano's equation, Susskind had an epiphany.

It was describing a vibrating string.

Susskind—and, independently, Yoichiro Nambu and Holger Bech Nielsen—proposed that instead of dimensionless point particles, hadrons had to be composed of tiny one-dimensional strings. You could think of them, Susskind had said, as two point particles, which you might call quarks, connected by a string of point particles, which you might call gluons.

It was a cool idea, but within a few years the successful development of QCD had wiped out all interest in strings. Almost. A few lonely physicists, including John Schwarz and Michael Green, refused to give up on string theory, working in virtual solitude for more than a decade.

Unfortunately, the math wasn't panning out. The string vibrations were far too energetic, which made them too small to be hadrons, and the values of their spins were off. Hadrons are matter particles—fermions—which means their spins come in half-integer values. But the strings' spins were whole integers, like force particles. Bosons. No matter how they looked at it, Schwarz and Green kept finding strings that looked like massless, spin-2 particles, which definitely weren't hadrons. They were something else entirely. They were gravitons.

Schwarz and Green realized that string theory wasn't a theory of hadrons at all. It was a quantum theory of
gravity.
Who cared that string theory didn't work out for hadrons? It was the holy grail!

The only problem was those missing fermions. The strings naturally produced bosons, but string theory could only be a theory of everything if it could account for fermions, too. If string theorists could find a way to transform bosons into fermions—whole-integer spins into half-integer spins—they'd be all set. Luckily, there was a way: supersymmetry.

In supersymmetric spaces, you can find reference frames in which a particle with whole-integer spin becomes a particle with half-integer spin. It was that observer-dependence that had inspired my father and
me to cross spin off the IHOP napkin. What one observer calls a boson, another sees as a fermion. That means that you can take the bosons that string theory produces from the start and then view them from different reference frames to get all the fermions, too. Now you've derived all the forces of nature and all of matter from just one basic ingredient: a tiny, vibrating string. Supersymmetry turns string theory into a viable theory of everything.

What made this theory of everything particularly useful was that it did away with the dangerous infinities that occur when particles collide at a single point—the ad infinitum in step two, the same kind of infinity that causes spacetime to cannibalize and dissolve into quantum foam. Strings are one-dimensional; they extend. They can't interact at singular points. Which in some sense means that spacetime doesn't
have
singular points.

“Spacetime itself may be reinterpreted as an approximate, derived concept,” the physicist Ed Witten wrote in an article entitled “Reflections on the Fate of Spacetime.” String theory's success, Witten explained, came down to this: “There is no longer an invariant notion of when and where interactions occur.”

That line kept bugging me, long after I first read it. What exactly did Witten mean? Usually physicists described string theory's taming of infinity as owing to the fact that strings stretch out over a finite distance, smearing out the otherwise singular point of collision. But Witten, the high priest of string theory, seemed to be attributing the smearing not to strings themselves but to observers. If observers can't agree that a collision occurs at a singular point—that is, if the collision's location in spacetime changes from one reference frame to another—then singular points don't exist. They're not ultimately real. Did that mean spacetime itself wasn't real? That it was observer-dependent?

Particles, the theory goes, are made of strings. But ask physicists what a string is made of, and they'll tell you it's an irrelevant question. Strings are fundamental. They're the basic building blocks. They're not made of anything other than themselves. It was hardly a satisfying answer. Was Witten offering a different one? He seemed to be saying that strings stretched across all the different points an observer might identify as the location of a collision. As if the strings themselves are maps
of our potential to observe them. As if they are made of reference frames. As if they are, in some sense, made of
us.

I couldn't help but think back to Wheeler: “One therefore suspects it is wrong to think that as one penetrates deeper and deeper into the structure of physics he will find it terminating at some nth level. One fears it is also wrong to think of the structure going on and on, layer after layer, ad infinitum. One finds himself in desperation asking if the structure, rather than terminating in some smallest object or in some most basic field, or going on and on, does not lead back in the end to the observer himself, in some kind of closed circuit of interdependences.”

Perhaps string theory could pave the way toward Wheeler's vision, but you have to be willing to make some pretty radical changes to the universe. You have to add more dimensions of space.

According to string theory, different vibrations correspond to different particles. Exactly which vibrations are possible is determined by the shape and dimensionality of the space around the strings. In a one-dimensional world things can only move back and forth; add another dimension and now they can move up and down as well. The more dimensions you have, the more ways a string can vibrate. For a string's vibrations to produce all known particles, you need nine spatial dimensions, in addition to time. The problem, obviously, is that we only see three. The extra six, physicists figured, had to be curled up and compactified—tiny, complex pieces of origami that sit at every point in our three ordinary dimensions, too small for us to see but big enough to accommodate the strings, which are less than a trillionth of a trillionth of an atom, sixteen orders of magnitude smaller than the best microscopes of the future will ever resolve.

This would have worked out perfectly had there been just one way to fold up the extra six dimensions. No such luck. In 2000, Bousso and Joe Polchinski discovered 10
⁵⁰⁰
ways, a number that's not quite as big as infinity but might as well be. There was no good reason that nature would choose to implement one folding over another, and each piece of origami created a different vacuum, which meant different particles, different constants, different physics.

The situation was a huge disappointment among string theorists,
but Susskind saw an anthropic lining. In his book, he argued that string theory's failure to deliver on the promise of a single, unique “theory of everything” was a blessing in disguise, because it allows the dreaded
A-
word to explain, among other things, that infuriatingly tiny but nonzero value of the cosmological constant, the one that required some insanely improbable mechanism to cancel the infinite vacuum energy out to 120 decimal places and then stop, leaving the perfect crumbs behind to match the observed strength of dark energy. String theory describes a vast number of universes. Eternal inflation produces a vast number of universes. Put the two together and you've got a wildly diverse multiverse in which the value of the cosmological constant changes from universe to universe—rendering ours nothing short of inevitable.

Intriguingly, Susskind's book ended with a chapter about event horizons. Susskind sympathized with the multiverse critics who claim that if other universes are walled off by our cosmic horizon, forever unknowable to us, then anthropic explanations are nothing more than empty metaphysics. But it's possible, Susskind suggested, that the Hawking radiation coming from our cosmic horizon could encode information about what's on the other side. About the multiverse. In that case, we could discuss multiverse anthropics without invoking any unmeasurable physics outside our horizon. The whole enterprise would be legit.

I knew that Susskind and Hawking had battled for decades over the possibility that information can hide in Hawking radiation, and that Susskind, famously, had won. But how did the information encode what was on the other side of the horizon? The answer was all bound up in that still mysterious AdS/CFT conjecture—
something to do with string theory … explains liquid fireball?
—and with what Susskind called “horizon complementarity,” an idea he described as
“a new and stronger relativity principle.” I wasn't sure what any of it meant, but in my gut I knew these were the clues to follow. I had to talk to Susskind.

I figured the easiest way to talk to him would be to interview him for
New Scientist
and run it as a Q&A pegged to the upcoming publication of his book. But I was curious to hear the arguments on both sides, pro- and anti-multiverse. David Gross, a fellow string theorist and Nobel Prize winner, was about as anti-multiverse as they came. Maybe
I could convince them to have a debate, I thought. We had never run anything like it in the magazine, but I figured I'd give it a shot.

Amazingly, both Susskind and Gross were enthusiastic about the idea, and Susskind agreed to fly down to Santa Barbara, where Gross was the director of the Kavli Institute for Theoretical Physics. They figured the whole thing might get too rowdy if we opened the debate up to public viewing, so they decided we ought to conduct it in private, just the three of us. Yup. Just the three of us. Just the inventor of string theory, a Nobel laureate, … and me.

Heading out to Santa Barbara, I was nervous. Not only was I going to be moderating a heated debate between two intellectual giants, but I had heard from many a fellow journalist that Susskind and Gross were notoriously intimidating. Journalists' reactions to their names alone had left me imagining an interview with either physicist to go something like this: Cowering journalist asks mildly ignorant question about string theory. Physicist stares journalist down, a ten-dimensional fire raging in his eyes. The stare pulverizes the weeping writer, leaving nothing but a puff of dust and a reporter's notebook spinning on the ground. Yeah, I was a little nervous. But when I arrived at my hotel and checked my email, I found one from Susskind. He had just arrived from Palo Alto and was wondering if I'd like to get some dinner.

We met at a quintessential Santa Barbara restaurant on the beach. In his mid-sixties, Susskind was tall and slim with a white beard and a friendly smile. I cautiously checked his eyes for any ten-dimensional flames—all clear. We sat at an outdoor table, stars shining overhead, soft surf of the Pacific sounding all around us. We chatted easily about
New Scientist
and about physics. He had this amazing voice, this thick, old-school New York accent, the kind where every syllable counts, every vowel is drawled, every consonant hammered. He had started out as a plumber in the South Bronx, and after all these years his Bronx accent hadn't faded in the California sun. Everything he said sounded tough and wise. I found myself wishing I could hire him to narrate the inside of my brain.

Susskind put me at ease, but I couldn't quite forget the fact that I was a twenty-five-year-old girl who had never taken a physics class chatting away with one of the most brilliant creative geniuses in science. I was trying my best to come across as older and professional—a plan that failed horribly the minute the waitress arrived. When Susskind ordered a bottle of wine, the waitress asked to see my ID. I blushed, then burned even redder when I realized I hadn't even thought to bring my driver's license with me. I was ready to crawl into the ocean, but Susskind just smiled. “I can vouch for her,” he offered. “I'm a physicist.”

The rest of the dinner went swimmingly. We drank our hard-won wine and talked about string theory, horizons, and Hawking radiation.

“I'm really interested to learn more about the ideas you brought up in the last chapter of your book,” I told him. “About black hole information loss and your idea of horizon complementarity.”

“That's good to hear, because I'm writing a whole book about it,” Susskind told me. “John Brockman, my agent, talked me into it.”

So that's how you get Brockman to be your agent, I thought. Just invent string theory. Or wage intellectual warfare on Stephen Hawking. And win.

I smiled. “I can't wait to read it.”

The next morning I met Susskind at the Kavli Institute on the UC Santa Barbara campus. We were early, so we hung out and drank coffee in one of the institute's common areas, which was flanked by chalkboards covered in cryptic equations. Sitting in a physics institute with Lenny Susskind was like sitting in a coffee shop with John Lennon. Passersby did a double take, then walked over to introduce themselves to him, flustered with awe and reverence. Everyone was buzzing with excitement about the debate, bummed that they wouldn't be able to listen in. It was hard to believe that I was the one with the backstage pass. I was thankful that no bouncers would be carding.

When the time arrived, we headed upstairs. Walking into David Gross's office was like walking into the captain's quarters aboard a luxury cruise ship. It was huge, with absurdly large round windows that framed the blue waters of the Pacific on all sides like portholes. I half expected a pod of dolphins to come leaping by the office. Winning a Nobel Prize did not look half bad. After being momentarily mesmerized by the view, I remembered to introduce myself; Gross acknowledged me gruffly. Something about him sent chills of terror down my spine. I checked his eyes for ten-dimensional flames. Yup. There they were.