Trespassing on Einstein's Lawn (35 page)

That was interesting. After all, what makes quantum information different from classical information is that when you acquire a bit of quantum information you actually change it. You become entangled with it. It's not like you can look at a quantum state, get some information about it, and then walk away, leaving it undisturbed for the next
guy to look at. That was the root of the no-cloning law, which many physicists saw as the future of security and encryption. You can't eavesdrop on a quantum message, not without changing the message in the process. If I happen upon one unique quantum state—one bit of information—no other observer can ever see it the same way. It can't be invariant. Was that right? Confused, I scribbled a note for myself in my notebook:
Single, unique quantum state can't be real? Or reality is first come, first served?

“So objective reality emerges when there are enough copies for us to all agree on what's there?” I asked.

“Yes, that's precisely the idea,” Zurek said, nodding. “That's the whole point. To understand objectivity. In a quantum universe we do not measure anything directly. If I were to make a direct measurement of a system, I could disturb its state. But I never do that, because usually the environment does the measuring for me. It decides on the set of states that get found out and disseminated, and I never interact with the system directly. I just use the environment as a witness. The observer gets hold of the information that is already advertised widely all over the place.”

“Years ago, we had a conversation with Wheeler, and in it he used two rather cryptic phrases. We're hoping you can tell us what they mean. The first was that the universe is a self-excited circuit.”

“I think it's like the Taoists. It's meant to inspire you. It's not like he worked out something specific. He drew this picture of the
U
with the eye looking back at itself—I love this picture. But I don't know how to put equations on it. If you talk to people in cosmology these days, the anthropic principle is gaining more and more credence, and that goes back to some extent to Johnny Wheeler. My instinctive reaction to that is to shudder because, you know, it's like a shortcut that should somehow be illegal. But I think people are recognizing that somehow we observe the universe because it can host observers, and that if you have a huge set of possible universes with different laws then the small subset of universes that provide environments in which observers can live will be selected by the fact that observers can live there. I think that falls short of Wheeler's vision. I think he envisioned a uniqueness.”

Suddenly it occurred to me that decoherence seemed to undermine the role of the observer that Wheeler felt was so essential to reality. “He believed that it's a participatory universe because quantum mechanics requires observers to measure things and bring them into existence,” I said. “Does decoherence make that unnecessary?”

“Usually the measurement is done for you by the environment. But there are situations in which you deal with quantum systems hands-on. In that case, the choice is up to you how you want to set up your apparatus and decide what you're going to measure. Wheeler's delayed choice is a fantastic example where it's actually up to you. Generally we don't have to worry about that because of decoherence. But our universe
allows
us to do things that smack of direct intervention or something.”

“Ultimately there can't be a real distinction between the observer and the environment,” my father chimed in. “From some other perspective, the observer is the system or is part of the photons and molecules that make up the environment. So when you set up the formalism—”

“What do you mean by
ultimate
?” Zurek interrupted. “The point of view where you are outside the universe and looking at the whole wavefunction evolve? Fine. But that's God's point of view, that's not ours. We're stuck on the inside. ‘Ultimately' is a big carpet under which you can sweep a lot of important things. But I do think there are important questions along these lines. One question that ought to be looked at more carefully is, why systems? That's probably your line of thought, right? Why not the observer and the environment and the apparatus all in one? My preliminary answer is that if you don't subdivide the universe into systems, you don't have a measurement problem. So you don't have to apologize for having systems when you're trying to solve it.”

That made sense. From our point of view inside the universe, we always seem to divide it into systems using Bohr's dividing line, observer and observed, or observer, observed, and environment. As my father was pointing out, the categories themselves can't be ontologically distinct, and were, in some sense, frame-dependent. But as Zurek said, maybe the deeper question was, why divide things at all? I wondered
if Fotini Markopoulou had already given us the answer: light cones. Thanks to the finite speed of light, we're all stuck with limited perspectives, with horizons that carve the supposedly singular world into pieces. Into systems. The existence of dark energy and the de Sitter horizon it creates only made those horizons all the more permanent, a quantum universe forever divided.

“The other phrase Wheeler used was ‘The boundary of a boundary is zero,' ” I said.

“Well, that's simply true,” Zurek said. “If you put a boundary on something, you don't have to bind it anymore. It's closed. It's an observation, a regularity, that's strikingly simple. One should strive to have similarly strikingly simple explanations of other things.”

Put a boundary on something and you don't have to bind it anymore
, I wrote in my notebook.
It's that simple?

I waited for Zurek to say more, but he didn't.

“That's it?” I asked. “Wheeler seemed to think it was so profound.”

“I think he liked the idea of getting a lot from a very little. Charlie Misner, Kip Thorne, and I wrote an article about Wheeler and we included a photograph of notes he had written on a blackboard. One was a quote in Latin. We struggled with translating it because our Latin was rusty, but it turned out to be a quote from Leibniz, which he exclaimed after he realized that if you have just two elements, zero and one, you can construct complete mathematics. Adding one to zero gives you everything. I think that's the spirit.”

It reminded me of set theory, of building the whole number line from the empty set, just by putting it inside some brackets.

“Does the fact that the boundary of a boundary is zero actually give you a prescription for doing that? For getting everything from nothing?” I asked, desperate to hear something more significant. “Wheeler made it sound like a clue.”

In my mind, Zurek sighed deeply, the kind of sigh that said,
I knew one day someone would come asking for this. But I hadn't suspected that day would be today.
Then he walked over to the fireplace and pushed on a brick in the wall. The brick slid back into the hollowed-out wall, revealing a secret compartment, and in that secret compartment lay a black velvet box. Zurek carried the box back over to the couch, where
my father and I sat perched on the edge, wide-eyed. He carried it in two hands, as if the world would end should it fall. I spotted Wheeler's U-diagram embossed in gold on one side. Zurek stood before us and lifted the lid. From inside came a blinding white light. When our eyes adjusted we saw, in the tiny center, which appeared somehow infinitely far away and infinitely visible, the answer to the universe.

In reality, Zurek shrugged. “I have no idea what to say.”

I sighed and decided to let it go. “We're trying to figure out what reality is,” I said. “We've defined something as being real if it's invariant, or observer-independent. What's your view of reality?”

“We have confidence that our language is good enough to describe the world we live in, but it's not going to work,” Zurek said. “It's been developed for very specific purposes that don't have to do with analyzing fundamental physics. Philosophers try to get you to commit to a bunch of words, and all of those words dissolve when you start thinking about physics in a deep way. My view of reality has to do with what philosophers call intersubjectivity. That's what quantum Darwinism is about. Reality is what we agree on. In that sense it's what's invariant. But that invariance—and hence, quantum reality—is not fundamental, it's emergent and approximate. Big words are seductive, but if you look at them closer, you don't know what they mean.”

I nodded, though I was pretty sure I knew what reality meant.

“You and Wheeler seemed to work a good deal on the role of information in physics,” I said.

“I would very much like to understand the reason why information is around,” Zurek said. “John was much more bold in trying to use it as a foundation. The connection between information and reality—to use a word I just outlawed—is extremely interesting. In classical physics, information is completely unreal. You have objects, and information just
describes
them; you have information
about
them. It's completely subjective. In quantum mechanics, information is much more fundamental. Wheeler had that picture where the observer breaks through the boundary. Information was outside Newtonian physics, but it's part of quantum physics, it's
physical.
That's absolutely crucial. My aim is to understand how it comes out of quantum mechanics. But a connection like that is often a two-way street. You might understand how information
comes from quantum mechanics, and then you can turn around and understand how quantum mechanics comes out of a deeper understanding of information. Johnny Wheeler gave us courage to take that idea seriously.”

“Why is information more real in quantum physics?” I asked. “Because things are binary, so you can describe them in bits?”

“It's more than that,” Zurek said. “In classical physics you can find out the state of a system and then someone else can come along and find out the state of the same system and you'll both agree. In quantum mechanics, that's generally impossible. Acquiring the information does not reveal some preexisting reality. Acquiring the information somehow
defines
reality. That's closer to Wheeler's participatory universe. And yes, we don't do that normally—normally there is decoherence and information spreads by quantum Darwinism, but still, the laws of the universe allow that to happen. That's something we shouldn't ignore. That's a hint of how this universe is built.”

As we pulled away from the curb, a coyote darted across the road, pausing briefly in front of our car to stare us down. Its pelt and bone were just illusion, I told myself, information incarnate, carnivorous and evolved and only approximately objective, a product of quantum Darwinism, an endless repetition of bits emanating redundantly into the desert.

“He looked like the flutist from Jethro Tull,” my father said.

“The coyote?”

“Zurek.”

“He was really interesting,” I said. “But I still don't fully understand what Wheeler was trying to say.”

“Maybe there's not so much to it?” my father said. “Maybe it's like he said—a kind of Zen koan or something, just meant to make you think differently?”

I shrugged. “That would be depressing. What do you want to do now?”

We decided to go visit the Bradbury Science Museum, which featured exhibits about the history of Los Alamos National Lab, the Manhattan Project, and nuclear weaponry.

“When I was a kid, maybe eleven, I had this rock and mineral collection,” my dad told me as we drove into town. “And someone gave me these rocks of trinitite. I can't remember who. They were like these glass rocks that were formed in the Trinity test. The package came with this little piece of cardboard that had a picture of the explosion. I was totally fascinated by it. I think in my mind I conflated it with kryptonite.”

As we wandered around the museum, I thought about what Zurek had said. Even if decoherence does most of the participatory work for us, there's still a kind of ambiguity that requires observers to make choices, choices that bring information into the world, constructing the universe bit by bit, as Wheeler had envisioned.
That's a hint of how this universe is built.
Decoherence, as Zurek had said, effectively solves the problem of why we don't see interference effects—the trademark signs of quantum superposition—in our everyday, macroscopic lives. But a deeper question remained: why is there any interference to get rid of in the first place?

My dad talking to J. Robert Oppenheimer at the Bradbury Science Museum in Los Alamos
A. Gefter

“What Zurek said about the differences between classical and quantum information was fascinating,” my dad said, pretending to talk to a statue of Oppenheimer. The destroyer of worlds was wearing a Brockman hat. “That classical information is
about
something, but quantum information
is
the thing.”