The Cosmic Landscape (55 page)

If Witten is the driving force behind the mathematical tools of String Theory, Joe Polchinski has been the primary source of “parts” for the great machine. Joe, together with the brilliant young Stanford physicist Raphael Bousso,
³
made the first use of these parts to construct a model of the Landscape with a huge “discretuum” of vacuums. In many conversations Joe has expressed a belief that there is no alternative to the populated Landscape viewpoint.

My old comrade in arms Gerard ’t Hooft has always been skeptical of String Theory’s claim of approaching a Theory of Everything and recently elaborated in an e-mail message:

Nobody could really explain to me what it means that string theory has 10
¹⁰⁰
vacuum states. Before you can say such a thing you must first give a rigorous definition of what string theory is, and we haven’t got such a definition. Or was it 10
⁵⁰⁰
vacua, or 10
^10000000000
? As long as such “details” are still up in the air, I feel extremely uncomfortable with the anthropic argument.

However, some form of anthropic principle I cannot rule out. After all, we live on Earth, not on Mars, Venus or Jupiter, for anthropic reasons. This, however, makes me distinguish the Discrete from the Continuous Anth Principle. Discrete means something like: the fine-structure constant is an inverse integer, happens to be
¹
/
₁₃₇
, that gets higher order corrections. Continuous means this constant is
¹
/
_{137.018945693459823497634978634913498724082734}
and so on, all of these decimals being determined by the anthr. princ. That I find unacceptable. String theory seems to be saying that the first 500 decimals are anthropic, the rest mathematic. I think it is far too early to make such speculations.

Roughly translated, what ’t Hooft means by the Discrete Anthropic Principle is that the Landscape should not contain so many vacuums that every value of the constants of nature can be found. In other words, he would be less unhappy with anthropic reasoning if the number of distinct possibilities were finite as opposed to infinite.

I think it is noteworthy that, skeptical or not, Gerard neither rules out anthropic explanations nor offers an alternative explanation for the incredible fine-tuning of the cosmological constant. But about his skeptical attitude toward a final Theory of Everything, I think he is probably right.

Tom Banks is another skeptic. Tom is one of the deepest thinkers in physics and one of the most open-minded. His skepticism, like ’t Hooft’s, is not so much about anthropic reasoning but rather about String Theory’s determination of the Landscape. Tom himself has made numerous important contributions to String Theory. But his own view is that the Landscape of metastable vacuums may just be illusory. He argues that String Theory and Eternal Inflation are simply not well enough understood to be certain that the Landscape is a mathematical reality. If certainty is the criterion, then I agree with him. But Banks feels the mathematics is not only incomplete but may actually be wrong. So far his arguments have not been persuasive, but they do raise serious concerns.

What do today’s younger physicists think? By and large they are open-minded. Juan Maldacena, who is in his early thirties, has had the biggest impact on theoretical physics of anyone of his generation. It was largely his work that turned the Holographic Principle into useful science. Like Witten, he has contributed important new mathematical insight, and like Polchinski, he has had a deep impact on the physical interpretation of the mathematics. Of the Landscape he remarked, “I hope it isn’t true.” He, like Witten, had hoped for uniqueness, both in the Laws of Physics and in the history of the universe. Nevertheless, when I asked him if he saw any hope that the Landscape might not exist, he answered, “No, I’m afraid I don’t.”

At Stanford University—my home—there is pretty near unanimity on the issue, at least among the theoretical physicists: the Landscape exists. We need to become explorers and learn to navigate and to map it. Shamit Kachru and Eva Silverstein, both in their early thirties, are two of the world’s young leaders. Both are busy constructing the Landscape’s mountains, valleys, and ledges. Indeed, if I were to attribute to anybody the title of the Modern Rube Goldberg, it would be to Shamit. Don’t get me wrong; I don’t mean to say that he makes bad machines. On the contrary—Shamit has brilliantly used the complicated machine parts of String Theory better than anyone to design models of the Landscape. And the Anthropic Principle? It goes with the territory. It’s part of the working assumption of all my close colleagues at Stanford, young and old.

At the other end of the country, New Jersey is the home of two of the world’s great centers for theoretical physics. Princeton, with its university Physics Department and the Institute for Advanced Study, is first and foremost, but twenty miles north, in New Brunswick, is another powerhouse—Rutgers University. Michael Douglas is one of Rutgers’s star attractions. Like Witten, he is both a brilliant physicist and a serious mathematician. But more important for this tale, he is a bold explorer of the Landscape. Douglas has set himself the task of studying the statistics of the Landscape rather than the detailed properties of individual valleys. He uses the laws of large numbers—statistics—to estimate which properties are most common, what percentage of valleys lies at different altitudes, and what the likelihood is that a valley that can support life exhibits approximate supersymmetry. While he prefers to use the term
statistical approach
instead of Anthropic Principle, it’s probably fair to say that Douglas is on the anthropic side of the divide.

Cosmologists are equally split on the issue. Jim Peebles of Princeton University is the “grand old man” of American cosmology. Peebles has been a pioneer in every aspect of the subject. In fact in the late 1980s he was one of the very first people to suspect that cosmological data indicated the existence of something like a cosmological constant. In discussing the problems of cosmology with him, I was struck by his rather automatic acceptance that many features of the universe could be explained only by some kind of anthropic reasoning.

Sir Martin Rees, the British Astronomer Royal, is an all-out enthusiast for the Landscape, the megaverse, and the Anthropic Principle. Martin is Europe’s leading cosmologist and astrophysicist. Many detailed arguments that I have used to motivate the Anthropic Principle I learned from him and from the American cosmologist Max Tegmark.

Andrei Linde and Alexander Vilenkin you have already met. Like Rees and Tegmark, they are firmly in the anthropic Landscape camp. Linde has expressed his opinion: “Those who dislike anthropic principle are simply in denial. This principle is not a universal weapon, but a useful tool, which allows us to concentrate on the fundamental problems of physics by separating them from the purely environmental problems, which may have an anthropic solution. One may hate the Anthropic Principle or love it, but I bet that eventually everyone is going to use it.”

Stephen Hawking is Martin Rees’s colleague at Cambridge University, but I have no doubt that his views are very much his own. Here is a quote from a lecture Stephen gave in 1999: “I will describe what I see as the framework for quantum cosmology, on the basis of M theory. I shall adopt the no boundary proposal, and shall argue that the Anthropic Principle is essential, if one is to pick out a solution to represent our universe, from the whole zoo of solutions allowed by M theory.”

So it seems that Stephen and I finally agree on something.

But not all cosmologists agree. Among the best-known Americans in the field, Paul Steinhardt and David Spergel are vehement foes of anything that smells vaguely anthropic. Steinhardt, whose feelings are more or less representative, says he hates the Landscape and hopes it will go away. But like Maldacena he can find no way to get rid of it. From Steinhardt’s writings (in “The
Edge
Annual Question—2005,” at www. edge.org): “Decades from now, I hope that physicists will be pursu-ing once again their dreams of a truly scientific ‘final theory’ and will look back at the current anthropic craze as millennial madness.”

Alan Guth, the father of Inflation, is a fence sitter. Alan is a thorough believer in the populated Landscape. Indeed, it was he who coined the term
pocket universe.
But not being a string theorist, he takes a wait-and-see attitude toward the discretuum—in other words, he is less committed to the proposition that the number of possible vacuum environments is exponentially large. As for the Anthropic Principle, I suspect Alan is a closet believer. Whenever I see him I say, “Well Alan, have you ‘come out’ yet?” He invariably answers, “Not yet.”
⁴

I’ve saved for last my old friend David Gross. David and I have been good friends for forty years. During that time we have fought and argued incessantly, sometimes fiercely, but always with great respect for each other’s opinions. My guess is that we will become two crusty old curmudgeons, battling to the very end. Maybe we already are.

David is, without doubt, one of the world’s greatest living physicists. He is best known as one of the principal architects of Quantum Chromodynamics, i.e., the dynamics of hadrons.
⁵
But more important for this story, he has long been one of the most senior generals in the army of string theorists. In the mid-1980s, while a professor at Princeton, David and his collaborators Jeff Harvey, Emil Martinec, and Ryan Rohm created a sensation when they discovered Heterotic String Theory. This new version of String Theory looked much more like the real world of elementary particles than any previous version. Moreover, at about the same time Ed Witten (also at Princeton) was busy with his collaborators—Andy Strominger, Gary Horowitz, and Philip Candelas—inventing Calabi Yau compactification. When the two came together, the world of physics gasped—the results looked so realistic that it seemed only a matter of months until a definitive, final, unique theory of elementary particles would be in hand. The world held its breath—and held its breath and turned blue.

Fate was not kind. The more time that passes, the more it becomes clear that the Princeton enthusiasm was, at best, premature. But David has never given up the hope that the silver bullet will turn up and make the earlier enthusiasm justified. Myself? I suspect that, in the end, the Heterotic theory will turn out to be a very important component of Rube’s great machine. Its resemblance to the Standard Model is impressive. But I would also guess that it is not the only component. Fluxes, branes, singularities, and other features may expand the Heterotic Landscape far beyond what the authors of the theory originally imagined.

Gross, as I said, is an extremely formidable intellectual opponent, and he is very opposed to the Anthropic Principle. Although his reasons are more ideological than scientific, they are important to discuss. What bothers him is an analogy with religion. Who knows? Maybe God
did
make the world. But scientists—real scientists—resist the temptation to explain natural phenomena, including creation itself, by divine intervention. Why? Because as scientists we understand that there is a compelling human need to believe—the need to be comforted—that easily clouds people’s judgment. It’s all too easy to fall into the seductive trap of a comforting fairy tale. So we resist, to the death, all explanations of the world based on anything but the Laws of Physics, mathematics, and probability.

David, along with many others, expresses the fear that the Anthropic Principle is like religion: too comforting, too easy. He fears that if we begin to open the door, even a crack, the Anthropic Principle will seduce us into a false belief and stop future young physicists from searching for the silver bullet. David eloquently quotes Winston Churchill’s 1941 address to students at his own school: “Never, ever, ever, ever, ever, ever, ever, give up. Never give up. Never give up. Never give up.” But the field of physics is littered with the corpses of stubborn old men who didn’t know when to give up.

David’s concern is very real, and I don’t mean to minimize it, but I also think it’s not as bad as he says. I don’t for a moment worry about the younger generation lacking the moral fiber to avoid the trap. If the populated Landscape is the wrong idea, we (or perhaps I should say, they) will find it out. If the arguments that indicate the existence of 10
⁵⁰⁰
vacuums are wrong, string theorists and mathematicians will discover it. If String Theory itself is wrong, perhaps because it is mathematically inconsistent, it will fall by the wayside and, with it, the String Theory Landscape. But if that does happen, then as things stand now, we would be left with no other rational explanation for the illusion of a designed universe.

On the other hand, if String Theory and the Landscape are right, with new and improved tools we may locate our valley. We may learn about the features in neighboring locales—including the inflationary ledge and steep downhill approach. And, finally, we may confirm that the rigorous use of mathematics leads to many other valleys, in no special way distinguished from ours except by their inhospitable environment. David has honest concerns, but to shun a possible answer because it runs counter to our earlier hopes is itself a kind of religion.

Gross has another argument. He asks, “Isn’t it incredibly arrogant of us to presume that all life must be just like us—carbon based, needful of water” and so on. “How do we know that life can’t exist in radically different environments?” Suppose, for example, some strange forms of life could evolve in the interior of stars, in cold dust clouds of interstellar space, and in the noxious gases that surround giant gas planets like Jupiter. In that case, the Ickthropic Principle of the codmologists would lose its explanatory power. The argument that life’s need for liquid water explains the fine-tuning of the temperature would lose its force. In a similar vein, if life can form without galaxies, then Weinberg’s explanation of the smallness of the cosmological constant also loses its force.