Trespassing on Einstein's Lawn (12 page)

After introducing myself, I asked him whether physicists would abandon inflation should the low quadrupole turn out to be real. Apparently, asking Andrei Linde to give up inflation was like asking the Pope to spit on a Bible.

“No one should abandon inflation!” he yelled in a thick Russian accent. I cowered and frantically looked around, expecting everyone to have stopped what they were doing or bolted in fear, only no one seemed fazed. “If you have a model that shows why the universe is isotropic and why you get these density fluctuations, then you don't abandon that theory unless you have another theory that can account for those things. Inflation can suppress power on large angular scales; it just requires fine-tuning and it's ugly. But the universe is ugly—the standard model is ugly, the cosmological constant is ugly, dark matter, dark energy, ninety percent of the universe, what the hell is it? It's ugly. But that doesn't mean you abandon inflation.”

Leonard Susskind
,
Alan Guth
,
and Andrei Linde enjoying the California sunshine
A. Gefter

* * *

Alan Guth, the man of the hour, seemed oddly approachable for a guy who had been tapped to win a Nobel Prize. He was in his fifties but exuded a cartoon-like youthfulness with his mop of brown hair and giant yellow backpack. He was famous for sleeping through every lecture, then waking up just in time to ask a bizarrely insightful question—a phenomenon I had already witnessed more than once. I asked him if he had any spare time to talk with me, and he graciously agreed. So in between lectures, wide awake, we sat outside in the sunshine.

“Inflation tells us what might have happened in the first fraction of a second after the universe was born,” I said, “but what do we know about the actual birth?”

“We clearly don't have a definitive theory of how the universe originated,” Guth said, “but the kinds of speculations people have, which I think are vague enough to be true even though we don't really know what we're talking about, is that the universe began as some sort of quantum event.”

Understanding that event, he explained, would require a theory of quantum gravity.

“The general idea would be to have complete quantum descriptions of spacetime geometry. Then we would want to have some notion of what it means to have nothing, and nothing would have to be one of the quantum states. A state that describes no space, no time, no matter, no energy, nothing. But it would still be a state, a possible state of existence. That's the key feature. I'm assuming, without necessarily any right to, but I'm assuming that the laws of physics are somehow in place even before the universe. If we don't assume that, we can't get anywhere.”

“Making that assumption means the origin could be knowable?”

“That's right. The origin could be knowable within the framework of the laws of physics. Right now I have no idea where we should look to uncover the origin of the laws, but we should worry about that later. So in this system that describes the ultimate laws of physics, hopefully there will be some quantum state of existence which will correspond to nothing. We know that quantum systems can undergo spontaneous transitions from one quantum state to another—atoms do this all the
time when they decay. In a quantum system, any state can make random transitions into any other state, so you could start there, at nothing, and transition to a small universe, then inflation could take over and turn that small universe into a big universe. In vague terms, I think that's a plausible picture of how the universe could have begun.”

“In that sense, it's possible to get something from nothing?” I asked.

“Our thinking about that question has changed rather dramatically since I was in graduate school,” Guth said. “At that time everyone believed that the universe had many conserved quantities that had large values, and that the only way to produce the universe was to start out with that much stuff. But those conservation laws have all more or less disappeared. Today we think that the universe has zero values for all conserved quantities.”

Conserved quantities are the features of nature that can never change, enshrined as they are in inviolable laws—laws such as the conservation of energy, which says that whatever happens, the amount of energy that comes out of an interaction had better equal the amount that went in. Energy can neither be created nor destroyed, only redistributed. Conservation laws are what keep the universe running smoothly from moment to moment. Without them, atomic bombs could appear in your bathtub, or your dog could suddenly blink out of existence. Physics would be impossible. Its equations would fall apart before you reached the other side of the equals sign.

But now Guth was saying that all conserved quantities are zero. That was rather shocking. You'd think the laws of physics are here to conserve
something
—like the “something” that came into existence 13.7 billion years ago. But if all conserved quantities are zero, it's as if the laws of physics are here to conserve
nothing.

“Quantities like energy?” I asked.

“Energy was the most problematic because if you count up the mass in the universe and use E = mc
²
, there seems to be a huge amount of energy. But the key realization was that gravity has a negative contribution to the total energy. It's not hard to prove that, but a crude way of thinking about it is to compare gravity to Coulomb's law of electrostatics. In electrostatics, if you want to push two positive charges together,
they'll repel each other, so to build up a large charge you have to put a lot of work in to push together a large number of charges. It costs energy. For gravity it's exactly the opposite. Mass only has one kind of charge: positive. It always attracts. You can build up a large mass by pushing lots of mass together. It costs energy to pull them apart. So gravity's contribution to the total energy of the universe cancels out the positive energy of all the mass.

“The other important quantity in the history of this was baryon number,” Guth continued, referring to the number of protons and neutrons that constitute every atom. “When I was in graduate school everyone thought that baryon number was conserved, and that the observed universe had a very large baryon number—that is, a large number of protons and neutrons and, as far as we could tell, very few antiprotons or antineutrons. Some people thought that maybe there was some large amount of antimatter out there somewhere that we hadn't found yet, but that idea never worked. With the development of grand unified theories in the 1970s, physicists realized that we didn't really know that baryon number was conserved. Later it was discovered that even in the so-called Standard Model of particle physics, where everybody thought baryon number was exactly conserved, it actually wasn't because of peculiar quantum effects. The evidence today seems overwhelming that baryon number is not a conserved quantity.”

So energy was conserved but it didn't matter, because gravity always canceled it out, and the number of baryons wasn't conserved. If it was, then the total number of protons and neutrons in the universe today would have to be the same number that you started with at the beginning of the universe, and there would be no way to explain how all those protons and neutrons got there in the first place.

“Does that mean matter can spontaneously appear from nothing?”

“Yes.” Guth nodded. “In the early days of inflation I made the statement that the universe could be the ultimate free lunch. Since then the idea of inflation in our visible universe has been elevated into a whole multiverse that just keeps growing and growing. If that picture is right, it's abundantly clear that you're getting something for nothing, and you just keep getting it. And it's all based on the idea that the universe does not have any nonzero conserved quantities.”

“Gravity cancels out positive energy throughout the whole multiverse?”

“That's right,” he said.

“What about things that
are
supposedly conserved, like, say, angular momentum?”

“We believe that angular momentum is conserved, but as far as we can tell the total angular momentum of the universe is zero. If you add up the spins of all the galaxies spinning in different directions, as far as the astronomers can tell it really is zero. Electric charge is another quantity that we believe is absolutely conserved, but the universe, as far as we can tell, is electrically neutral.”

“So if we observationally discovered that there was some conserved quantity with a nonzero value, that would mean it's impossible to get something from nothing?”

“That's right. That would change everything. The idea of eternal inflation would not be conceivable anymore. If our universe really needed a nonzero conserved value in order to make it something we would call a universe, then you could not make more and more of them without violating the conservation law.”

“But as long as the only conserved quantities have zero values, you can get something from nothing.”

“Maybe a better way of saying it is that something
is
nothing,” Guth said. “Everything we see is in some sense nothing.”

When it came time for Hawking to deliver his talk, I could barely contain my excitement. Hawking was notoriously stubborn, mischievous, and iconoclastic. He was a world-class troublemaker, and I couldn't wait to see what kind of trouble he was going to make today.

He was wheeled out to the center of the stage. “Can you hear me?” his computer politely inquired.

“Yes,” the audience replied.

“In this talk, I want to put forward a different approach to cosmology that can address its central question: why is the universe the way it is?”

A different approach to cosmology? This was going to be good.

How can we ever figure out how the universe began? Hawking asked. “Some, generally those brought up in the particle physics tradition, just ignore the problem. They feel the task of physics is to predict what happens in the lab.… It amazes me that people can have such blinkered vision, that they can concentrate just on the final state of the universe and not ask how and why it got there.”

Those who do attempt to explain the origin, he said, use a bottom-up approach, starting from some initial state and then evolving it forward to see if it develops into something that remotely resembles our universe. Inflation is just such an approach, he said, but even for a bottom-up theory, it doesn't make any sense.

This was just getting better. Here everyone was, celebrating the great successes of inflation, and now Hawking gets up and says it never made any sense to begin with.

Inflation, Hawking explained, lacked general covariance, the key ingredient of Einstein's theory, which ensured that every reference frame contains an equally valid description of the universe. Rather than working with the fully unified four-dimensional spacetime, inflation required spacetime to be broken apart into three dimensions of space and one of time. But whose space? Whose time? Breaking spacetime apart amounted to choosing a preferred reference frame—the ultimate crime against relativity. Worse, he said, if you choose certain coordinates to play the role of time, the inflaton field no longer expands. In other words, the theory only works in certain reference frames to begin with.

This was fascinating, but the talk was slow going. Minutes passed between sentences, eternal minutes during which the audience did their best to stay respectfully silent, the shifting of weight in seats and the clearing of throats resounding against the painful silence.

Suddenly his right leg began shaking violently, causing the computer mounted to his wheelchair to vibrate. His aide rushed over and knelt on the floor, holding Hawking's foot down as he continued with his talk.

Beyond the problems with inflation, Hawking said, there's a fundamental problem with the bottom-up approach as a whole. “The bottom-up approach to cosmology is basically classical, because it
assumes that the universe began in a way that was well defined and unique. But one of the first acts of my research career was to show with Roger Penrose that any reasonable classical cosmological solution has a singularity in the past. This implies that the origin of the universe was a quantum event,” he said.

Quantum events are described not by unique states but by superpositions of every possible state. It's not simply that we can't know which of those states the universe was actually in—it's that the universe wasn't actually in any of them. For that reason, Hawking said, we need to work from the top down, from the present to the past. By looking at the features our universe has today we can figure out all the possible histories that could have led to such a universe. Somehow, in doing so, we
create
the history of the universe. “This means that the histories of the universe depend on what is being measured, contrary to the usual idea that the universe has an objective, observer-independent history,” he said.