Trespassing on Einstein's Lawn (47 page)

Susskind had suggested we meet him at a little café in Palo Alto. My father and I arrived early and snagged a corner table on the outdoor patio. Soon, Susskind came strolling down the street looking casual and cool.

He greeted me warmly and in turn I introduced him to my father. As I watched them shake hands I caught a glimpse of something I had rarely seen in my life: my father was nervous.

Susskind and I headed inside to order some beverages while my father held the table. When we emerged with coffees and teas in hand, I looked over at my father and knew exactly what he was thinking: Holy shit. Lenny Susskind is carrying my coffee.

As we set the drinks down, the table wobbled. “Every time I'm with another physicist and we wind up with a wobbly table, we always end up trying to figure out the laws of table mechanics,” Susskind said. “Nobody's had a good idea!”

With Leonard Susskind in Palo Alto
W. Gefter

“Maybe you need eleven dimensions?” I offered, cringing at my own stupid joke.

“Well, in a one-dimensional world, this wouldn't be a problem, would it?” Susskind said. “It gets worse as the number of dimensions goes up.” He laughed. “That's the kind of brilliant insight I'm capable of coming up with.”

“It seems like there's a huge change under way in cosmology,” I said when the conversation grew serious. “Maybe even a paradigm shift, from a God's-eye view to the perspective of a single observer. Do you think that's happening?”

“Yeah, I do,” Susskind said. “I do think that idea is starting to take off. But at the same time there are times when it's useful to think about the global perspective. We do attribute a degree of reality to what's out there beyond the horizon whenever we invoke things like anthropic reasoning. We do. On the other hand, we should be able to formulate complete theories of observation and experiment without invoking
anything beyond the horizon. So there's a tension there. There's a real tension there. And I think that tension is going to come to a head. And hopefully when it comes to a head we'll find a clearer picture of this relationship between the local and the global—it's something I've been thinking about for years.”

The tension, I knew, was mounting in his own work. On one hand, Susskind had argued in Santa Barbara that he believed strongly in the explanatory power of the anthropic principle, power made possible by the intriguing convergence of the string theory landscape and the infinite bubble universes produced by eternal inflation. On the other hand, his own discovery of horizon complementarity gave cosmology no choice but to toss aside the unphysical God's-eye view and describe the universe in terms of what a single observer can see.

“We've been thinking a lot about the nature of reality,” I said, “and defining what's real as what's invariant. But when we look at the ingredients we think constitute reality, we keep discovering that none of them is invariant. So what's ultimately real?”

Susskind shook his head. “My only guess is that there will be a big surprise and everything will be turned on its head.”

“Do you suspect that string theory has the answer?” my father asked.

“No, I don't,” Susskind said. “String theory is an incredible edifice that has a startling degree of internal consistency, and it does contain quantum mechanics and gravity of some kind, but it doesn't describe the universe. It doesn't describe any known cosmology except empty space.”

By “empty” Susskind meant a space with no dark energy lurking in its depths—a space, that is, with no event horizons. Strings are described by the S-matrix, which computes the probability of what will come out of a string interaction given what went in, ignoring all the convoluted crap in the middle. In such a space, a string actually means something. But as Hawking said, “We live in the middle of this particular experiment.” We don't know and can't measure what went in or what comes out; all we'll ever know is the middle. Here in the middle, the S-matrix is useless—and so are strings.

It suddenly occurred to me why: it's because the S-matrix loses its
invariance. Strings interacting in the middle of the universe can be viewed from countless perspectives, from different directions in space and time, from reference frames in various states of motion. What looks to one observer like a string vibrating at one particular frequency and producing its associated particle will look to another like a string vibrating at a different frequency and producing a different particle. Observers can't agree on what's what, and, as Einstein emphasized, no observer's view should be more valid than another's.

It reminded me of particle handedness. For all observers to agree on the handedness of a particle, the particle has to be moving at light speed so that no observer can outrun it and see it spinning the other way. Similarly, by defining strings at future infinity, the S-matrix ensures that every observer will see them the same way. In a universe free from dark energy, every observer's light cone, given infinite time, will grow large enough to encompass the entire universe and overlap completely with everyone else's, so all observers will share a single reference frame. Strings will be observer-independent. They'll be
real.

But in a universe with dark energy—a universe like ours—no one can access future infinity, trapped, as we are, behind observer-dependent event horizons. In our de Sitter universe, we are all Screwed, adrift in an ever-accelerating, ever-emptying space, each of us surrounded by our own steadfast horizon, forever doomed to provinciality and finitude. Our light cones will never overlap completely, no matter how long we wait. No two observers will ever see the same universe. In de Sitter space, all hope of invariance is lost. The S-matrix doesn't mean anything, and neither does string theory.

“Is there any hope of doing cosmology in de Sitter space?” I asked.

“There are three ideas that I know of,” Susskind said.

The first, he explained, was to look to the boundary of our universe at future infinity and find the field theory that encodes our higher-dimensional de Sitter space as a hologram. If it worked, we'd have a dS/CFT.

Susskind, however, wasn't convinced that it was such a good idea. “That may be misguided, because it's global,” he said. It encompasses every observer's light cone—Safe's, Screwed's, and infinite others.

Having a hologram at future infinity wouldn't do anyone much
good, because whose world would it be describing? No observer can access the whole space; only a God's-eye view could make sense of the hologram, which would have to simultaneously describe the insides and outsides of infinite regions separated by event horizons. According to horizon complementarity, the hologram wouldn't make any sense—it would overcount elephants and violate the very laws of quantum mechanics that made the world holographic in the first place. “We learned from horizon complementarity that quantum mechanics will only apply within a single causal patch,” Susskind said.

So much for dS/CFT.

“The second idea,” Susskind said, “is to formulate physics in a single causal patch in de Sitter space.”

That made more sense. Just place the hologram on the event horizon of a single observer's patch. If each of us is stuck with a permanent horizon, why not make use of it and find the dual physics that lives there? Of course, that horizon is observer-dependent. Which would make the cosmic hologram observer-dependent. Which would make the
universe
observer-dependent.

“That probably can't be right,” Susskind said, “because the de Sitter space can decay.”

As Bousso had explained, in a universe ruled by eternal inflation, every vacuum with a positive cosmological constant is unstable and bound to decay. Our vacuum, with its trace amount of dark energy, is a false vacuum, one that's temporarily stable—if “temporarily” means billions of years—but will eventually drop to a lower energy state. It's happened before. It was called the big bang.

“According to our understanding of eternal inflation,” Susskind said, “the endpoint of evolution along any trajectory is an open, FRW universe.” When our space decays, it will drop to a state with a smaller cosmological constant; in turn, that vacuum will most likely decay to an even lower energy state, and so on and so forth, rolling down the hills of string theory's landscape until it bottoms out in the lowest possible valley, the one with a cosmological constant of zero—an ordinary, expanding, flat space, also known as Friedmann-Robertson-Walker, or FRW, space.

“It's not clear that that helps,” Susskind said, “but it might.”

Well, sure, I thought. An ordinary, expanding, flat space is exactly the kind of space you need to have an invariant S-matrix, to make sense of string theory. Reality holds up there, because all observers can see the same thing. “It helps because observers will no longer be stuck in finite patches?” I ventured.

He nodded. “An open FRW universe has an infinite number of particles in it, and a late-time observer can look back and as time goes on see more and more of it. Eventually he can see an arbitrarily large amount of it. There's always more out there, so you'll never see the whole thing, but take any portion of it and you will eventually see it. That situation doesn't have the same problems that we face in de Sitter space.”

That brought Susskind to the third idea for dealing with the problem of cosmology. “The idea is to take the point of view of that late-time observer,” he said—the one who lives in FRW space. “I call him the Census Taker.” The plan, he explained, was to construct a holographic duality between the Census Taker's patch of flat space and a quantum conformal field theory living on the patch's two-dimensional boundary: FRW/CFT.

I understood the appeal. Because it has room for invariance, FRW/CFT would allow cosmologists to save not only string theory but reality as well. At the same time, by restricting the theory to a single observer's causal patch, it would satisfy the demands of horizon complementarity and avoid any breakdowns of physics. They could even keep eternal inflation and the string landscape in play, allowing for an anthropic explanation of the value of dark energy or anything else in need of an explanation.

Then again, satisfying the demands of horizon complementarity in flat space was pretty trivial. In flat space, causal patches don't mean very much, because at the end of the day—or at the end of infinite time—every observer shares the same patch. At that point, you might as well just call it the universe. And sure, FRW/CFT respects the holographic principle, but how hard is that? The area of the observer's boundary becomes infinite. Respecting the holographic principle in flat space requires about as much constraint as a driver needs to obey an infinite speed limit.

Besides, I wasn't sure how the Census Taker's cosmology was supposed to help us here and now. Don't we want to account for
our
universe? So what if our de Sitter space decays to a flat space billions of years from now? Is that really such a comfort? Is it even fair to consider the FRW universe a continuation of our own? If our universe decays, it will be destroyed in the process, the same way that whatever universe lived uphill from us was destroyed in our big bang.
This universe isn't real now, but someday it will be.
I just wasn't sure if that was enough.

“Maybe one of these ideas will be right, maybe all of them, maybe none of them,” Susskind said. “But it seems to me important to delineate what the right questions are as clearly as possible. Understanding the relationship between the local and the global views of physics, I think that's
the
really big question. There's going to be a lot of crap published about it. That's obvious. But it's a real question. It's all connected with questions of horizons, the holographic principle … all these things are interconnected and they haven't come together into a single, comprehensive view. I hope that will happen. This question about cosmology, about how to think about it, I'm obsessed by it. But I'm seventy-one years old. The prospects of me solving it are very small. So I am sort of involved in proselytizing, not for any viewpoint but for the questions. I'm telling people, I think these are important questions, maybe the most important questions.”

His urgency reminded me of Wheeler. On first glance, they couldn't be more different. Where Wheeler was sweet and soft-spoken, Susskind was brash and tough; where Wheeler was open to wild speculation, Susskind was cautious and skeptical. But both physicists were prone to bold ideas, to great leaps of intuition, and both were ahead of their time.

“Can I ask you for one last thing?” my dad said, reaching into his briefcase.

Oh, God, I thought. What is he doing?

He pulled out Susskind's book
The Holographic Universe.
“Could you sign this?”

I blushed, embarrassed. Here I was trying to act cool and professional,
and he was acting like a star-struck fan. But as Susskind scribbled in his book, I smiled. All those years ago in the Chinese restaurant, my father had chosen to confide his idea about nothing, about the H-state and the universe, in me. Now, sixteen years later, I was overwhelmed by the feeling that I was finally beginning to return the favor.