Trespassing on Einstein's Lawn (30 page)

See those round windows? That's David Gross's office—at the Kavli Institute for Theoretical Physics
A. Gefter

Gross had decided that each physicist should begin by trying to present the opponent's view, a strategy I found strange and essentially unusable for the magazine, but I was curious to see it play out and too terrified of him to argue.

Gross went first, presenting the usual arguments in favor of anthropic reasoning: string theory has 10
⁵⁰⁰
vacua, inflation grants each one of them physical existence, and values of things like the cosmological constant vary from universe to universe—therefore things that
we once hoped to deduce as unique answers derived from simple principles can be explained away as completely random. He tried to present the case fairly, but eventually he got fed up. “That's it! I can't do any more before I go crazy.”

Now it was Susskind's turn to present the anti-landscape view. “What are the objections to it? Jesus, it beats me!” he began. He noted emotional objections (“It's like fingernails on a blackboard”) and philosophical objections (the falsifiability requirement demanded by the “Popperazzi”). He mentioned historical objections (“There are lots of examples of strange, mysterious numbers—the period of the Moon, the height of the tides—that were subject to ignorant, superstitious explanations before science explained them.”). He agreed with the objection that anthropic arguments inherently assume that our kind of life—low-entropy, carbon-based, water-dependent—is the only kind possible.

But there was only one objection that Susskind found worrisome. Because eternal inflation and the string landscape produce an infinite number of universes, everything that can happen does happen—an infinite number of times. In such a world, probability loses all meaning. If I wanted to calculate the probability that John Brockman would be our agent, I'd have to divide the number of universes in which he becomes our agent by the total number of universes. In other words, I'd have to divide infinity by infinity, which yields a depressing “undefined.”

You'd think you could get around the problem by taking a finite slice of the multiverse at some particular time, counting up the number of Brockman-is-our-agent universes and the total number of universes in the slice, and then taking the limit of that ratio as the size of the slice goes to infinity. But thanks to Einstein, that won't work. There's no global meaning to the phrase “at some particular time,” no clock sitting outside the multiverse and defining a singular time that's the same for everyone. Time is observer-dependent. Once you've sliced the multiverse into coordinates of space and time you've broken general covariance and selected a preferred frame. Worse, the outcomes you'll get for various probabilities change drastically from one time slice to the next, yet no slice is “truer” than any other. What you need
is a probability measure that tells you how much weight to assign to each kind of universe. Without a measure, there's no way to say things like “The observed value of dark energy in our universe is probable in the multiverse,” which, of course, was the whole point of the thing.

The infinity crisis was known among cosmologists as “the measure problem”—a weirdly innocuous name for something that threatened to take down the whole multiverse and the standard model of cosmology with it. The reason cosmologists believed in inflation and its subsequent multiverse in the first place was because the theory was able to predict things such as the smoothness of the CMB and the flat geometry of the space within our horizon. But under the weight of the measure problem, inflation was beginning to implode. The theory was forced to retract all of its successful predictions. It
didn't
predict a smooth CMB or a flat geometry—it predicted
every
CMB and
every
geometry, and offered no way for us to predict which ones we were most likely to observe.

“Eternal inflation is full of infinities, infinities over infinities, and it's possible that once we learn to deal with these infinities the whole idea of eternal inflation will collapse,” Susskind said. “To me, this is the real Achilles' heel. It's the one I worry about most.”

Finally Susskind offered up a mock rendition of Gross's famous objection. “There's a cultural danger and a danger to science itself that doesn't have to do with the truth or falsity of the proposition: if we don't resist the temptation of the anthropic principle, young people will give up looking for the mathematical, rational reasons for things. The danger that the real explanation will be overlooked is so strong and the anthropic principle is so seductive that we had better discourage thinking about it,” he said, his voice dripping with sarcasm.

“What is at stake here?” I asked.

“Everything,” Gross said. “There's a recent article by [Steven] Weinberg in which he claims that this is one of the great sea changes in fundamental science since Einstein. That may be carried away, but this is a very radical change in what I regard as the direction of science.… And my general feeling is that the arguments aren't by any means strong enough yet; they have holes everywhere you look.… Enormously powerful statements are made about the inevitability of
this conclusion, the lack of principles that distinguish the state of the universe in string theory, but everyone admits that we don't know what string theory is.… We don't have the equations, the principles, the theory yet, and to base very far-reaching conclusions on a theory which everyone admits doesn't yet exist seems dangerous to me.”

String theory doesn't yet exist? But Gross is a string theorist.

“And the thing that bothers me most,” Gross continued, “is the absence of any consistent cosmology. Since Einstein, it has become the goal of fundamental physics to predict not just the present state, but rather the whole spacetime manifold. The business of physics, we've learned from general relativity, is the whole damn thing! The present is an illusion anyway. One of the disappointing aspects of string theory, which doesn't yet exist as a theory, is that so far we've made no insight into constructing consistent cosmologies.… To imagine this totally radical change in the scope of physics based on ignoring the initial conditions, the big bang, the construction of a consistent cosmology … okay, we have 10
⁵⁰⁰
possible metastable states in the landscape, and on the other hand we have zero cosmologies.”

I scribbled a reminder to myself in my notebook:
Why can't string theory deal with cosmology?

“Normally in science, a scientific principle grows more powerful the more we know,” Gross said. “But the anthropic arguments are the opposite—the more we know, the less force the anthropic principle has. It thrives on ignorance. And I don't think that's the way science should be done. I think the whole thing is premature and slightly dangerous. Why do I think it's dangerous? Because it's giving up the traditional route, and it can divert people from what might be the right directions.… In the face of all these issues, I draw a different conclusion. We're missing an important principle. We're missing the principles that allow us to construct a cosmology, to discuss eternal inflation. I would take all of the arguments for the present situation not as arguments for being stuck with anthropic reasoning but as suggestions that something fundamental is missing.”

“David, you've reminded me of a few sayings,” Susskind said. “ ‘Old men are doomed to forever relive their past.' And, ‘The less you have to say, the longer it takes.' As far as being dangerous, I think the shoe can
be put on the other side of the foot, if that's the expression. David is very beloved in physics, he's extremely admired, and he's feared.”

You think? He's freaking terrifying.

“By whom?” Gross demanded.

“Oh, by a young man we had an occasion to talk to earlier today. But also I've had many occasions to talk to young men—women seem immune to this—who when I discussed with them the possibility that the world might be anthropic were very reluctant to discuss it; they became embarrassed. Young people are being intimidated by a very powerful hostility toward these ideas.”

“That people like myself have strong opinions? I'm not worried about that. What worries me is when these young people get up and talk with utter authority as if this set of ideas is based on firm knowledge.”

“Boy, is the pot calling the kettle black! Wow! David, David, David. Do you remember the days shortly after heterotic string theory?”

Oh, snap!

“So? Fine. It's a little different. When these young people talk I have to remind the audience that here speaketh someone who is talking as if we know what the hell we're talking about. String theory is not something we know what we're talking about, right?”

“Yes, I completely agree.”

He agrees? But he invented it.…

Gross continued. “So when someone gets up and says, ‘String theory says …,' it is part of the reaction you have aroused with your exuberance for this idea. And I can easily understand about my excitement about the heterotic string revolution of 1984—I can understand and see it happening again now with some of my colleagues, the allure of the random universe. I see it happening with poor Steve Weinberg. Steve is a man who is driven by atheism. And he is exuberant in the end of his paper because the Catholic Church has come out against the landscape idea.”

“We all like that. We all find it amusing.”

I chuckled.

Gross was referring to Weinberg's paper “Living in the Multiverse.” In it the Nobelist wrote,
“Just as Darwin and Wallace explained how
the wonderful adaptations of living forms could arise without supernatural intervention, so the string landscape may explain how the constants of nature that we observe can take values suitable for life without being fine-tuned by a benevolent creator.”

Weinberg went on to quote a
New York Times
op-ed by Christoph Schönborn, a cardinal and the archbishop of Vienna, who wrote,
“Faced with scientific claims like neo-Darwinism and the multiverse hypothesis in cosmology invented to avoid the overwhelming evidence for purpose and design in modern science, the Catholic Church will again defend human nature by proclaiming that the immanent design evident in nature is real.” Weinberg then noted, rather exuberantly,
“Martin Rees said that he was sufficiently confident about the multiverse to bet his dog's life on it, while Andrei Linde said he would bet his own life. As for me, I have just enough confidence about the multiverse to bet the lives of both Andrei Linde
and
Martin Rees's dog.”

“What I'm saying,” Gross continued, “is that some of the reaction is exactly like the reaction I got for exuberance in 1984, when we believed the answer was around the corner and we got carried away with that position. And, Lenny, you are carried away with this position. The stakes are damn big. So you are open to severe criticism.”

“But make it scientific criticism,” Susskind insisted. “Scientific criticism would be a theorem that says you can't have metastable de Sitter space in string theory, or a demonstration that eternal inflation is internally inconsistent. That is science, David.”

“It is incumbent upon those who build on these shaky things to make them less shaky,” Gross retorted. “I don't have to prove that something that's ill defined doesn't make sense.”

“David, do you see a way out of the landscape?” I asked. “Do you see any particular route?”

“Do I see a particular route? Obviously not. If I did I would be in a much better position. The anthropic line of reasoning can only be killed by science increasing its power. It's very unlikely that over the age of sixty I'm going to come up with the necessary insight. But where is it going to lie? It's going to lie in the question ‘What is string theory?' It's going to lie in the question ‘How does one construct one consistent
cosmology? One universe that makes sense?' We don't have one universe that makes sense!”

I turned to Susskind. “Even if you accept the landscape and the multiverse, and even if you accept that certain local physical laws are anthropically determined, don't you still need a metatheory? Don't you need something unique? Doesn't it just push the question back?”

“Yes. That's certainly true. Absolutely. The bottom line is that we need to describe the whole thing, the whole universe or multiverse. And it's a scientific question, not an ideological one.”

The debate lasted a couple of hours, and afterward the three of us went to a beachside restaurant for a seafood lunch, where we chatted happily about physics. I felt at ease, as if we had bonded somehow, like soldiers returned from battle. Having heard extensive arguments on both sides, I wasn't totally persuaded by either one. Gross's view was more appealing: I, too, would breathe easier knowing that physics was determined by unique, necessary, elegant equations, and not the luck of the draw. On the other hand, I wasn't sure the universe cared much about what helped me breathe easy. And the confluence of the string landscape with eternal inflation's physical multiverse did seem to point in one obvious direction. Still, it was too soon to say anything for sure. There were too many questions hanging everywhere.