The Cosmic Landscape

by Leonard Susskind

  Any particular boat will be surrounded by a boundary similar to the point of no return that surrounded the drain. Imagine a dinghy hovering around its parent vessel. If by accident or design it gets beyond the point of no return, it simply cannot get back or even communicate with the parent vessel. The only difference between the black hole horizon and the cosmic horizon of inflating space is that in one case we are on the outside looking in, and in the other case, we are inside looking out. But in every other way, the black hole and cosmic horizons are the same.

  To someone outside a black hole, the events in the life of the trans-horizon explorer are behind the horizon. But those events are physics, not metaphysics. They are telegraphed to the outside in scrambled holographic code in the form of Hawking radiation. Like the prisoner’s message, it doesn’t matter if the code is lost—or even whether we ever had it. The message is in the cards.

  Are there also “cards” coming from behind the cosmic horizon with messages from billions of pocket universes? Cosmic horizons are not nearly as well understood as black holes. But if the obvious similarity between them is any guide, cosmic horizons do yield such cards, and they are very much like the photons Hawking radiation comprises. By now you may have guessed that they are the photons of the cosmic microwave background radiation that bathe us from every direction and for all time. Messengers from the cosmic horizon, they are also coded messages from the megaverse.

  George Smoot, one of the leaders in cosmic microwave detection, in an overenthusiastic moment likened a cosmic microwave map of the sky to “the face of God.” I think for inquiring minds curious about the world a scrambled hologram of an infinity of pocket universes is a far more interesting and accurate image.

  CHAPTER THIRTEEN

  Summing Up

  Slogans

  One theme has threaded its way through our long and winding tour from Feynman diagrams to bubbling universes: our own universe is an extraordinary place that appears to be fantastically well designed for our own existence. This specialness is not something that we can attribute to lucky accidents, which is far too unlikely. The apparent coincidences cry out for an explanation.

  An immensely popular story, not only among the general public but also many scientists, is that a “superarchitect” designed the universe with some benevolent purpose in mind.1 The advocates of this view, intelligent design, say that it is quite scientific and perfectly fits the facts of cosmology as well as biology. The intelligent designer not only chose excellent Laws of Physics for its purpose but also guided biological evolution through its unlikely chain, from bacteria to Homo sapiens. But this is an intellectually unsatisfying, if emotionally comforting, explanation. Left unanswered are: who designed the designer, by what mechanism the designer intervenes to guide evolution, whether the designer violates the Laws of Physics to accomplish its goals, and whether the designer is subject to the laws of quantum mechanics.

  One hundred and fifty years ago, Charles Darwin proposed an answer for the life sciences that has become the keystone of modern biology—a mechanism that needs no designer and no purpose. Random mutation, combined with competition to reproduce, explains the proliferation of species that eventually fill every niche, including creatures that survive by their wit. But physics, astronomy, and cosmology lagged behind. Darwinism may explain the human brain, but the specialness of the Laws of Physics has remained a puzzle. That puzzle may be yielding, at last, to physical theories that parallel Darwin’s biological theory.

  The physical mechanisms that I have explained in this book share two key ingredients with Darwin’s theory. The first is a huge Landscape of possibilities—an enormously rich space of possible designs.2 There are more than 10,000 species of birds, 300,000 species of beetles, and millions of species of bacteria. The total number of possible species is undoubtedly immeasurably larger.

  Is the number of biological designs as large as the number of universe designs? That depends on exactly what we mean by a biological design. One way of listing all biological possibilities is to enumerate the ways of assigning the base pairs in a large DNA molecule. A human DNA strand has about a billion base pairs, and there are four possibilities for each. The total number of possibilities is the ridiculously large number 41000000000 (or 10600000000). This is much bigger than the 10500 (similarly obtained by counting the number of ways of assigning flux integers) that string theorists guess for the number of valleys of the Landscape, but of course almost all of these do not correspond to viable life forms. On the other hand, most of the 10500 vacuums are also dead ends. In any case both numbers are so large that they are far beyond our powers of visualization.

  The second key ingredient is a superprolific mechanism to turn the blueprint designs into huge numbers of real entities. Darwin’s mechanism involved replication, competition, and lots and lots of carbon, oxygen, and hydrogen on which these mechanisms operate. Eternal Inflation also involves exponential replication—but of volumes of space.

  As I discussed in chapter 11, the process of populating the Landscape does have its similarities with biological evolution, but it also has at least two very big differences. The first was discussed in chapter 11. Biological evolution along a given line of descent is through minute, undetectable changes from generation to generation. But descent through a series of bubble nucleations involves, at each stage, large changes of vacuum energy, particle masses, and the rest of the Laws of Physics. Biologically, if only such large changes were possible, Darwinian evolution would be impossible. The mutated monsters would be at such a disadvantage relative to normal offspring that their survival in a competitive world would be impossible.

  How then does the megaverse become populated with diversity if biological evolution, under the same conditions, would stagnate? The answer lies in the second big difference between the two kinds of evolution: there is no competition for resources among pocket universes. It’s interesting to contemplate an imaginary world in which biological evolution takes place in an environment where resources are so unbounded that there is no need for competition. Would intelligent life evolve in such a world? In most descriptions of Darwinian evolution, competition is a key ingredient. What would happen without it? Let’s take a particular case, the final step in the evolution of our own species. About 100,000 years ago Cro-Magnons were in a struggle for survival with Neanderthals. Cro-Magnons won because they were smarter, bigger, stronger, or sexier. Thus, the average genetic stock of the human race was improved. But suppose resources were unbounded and that sex was unnecessary for reproduction. Would there be fewer Cro-Magnons? Not at all. Everyone who survived would survive more easily without competition. And many who did not survive would do so as well. But there would also be more Neanderthals. In fact there would be more of everyone. All populations would increase exponentially. In a world of unbounded resources, lack of competition would not have slowed the evolution of the smartest creatures, but it would have made a lot more dumb ones.

  There is a third context, after physics and biology, where the same two ingredients—a Landscape and a megaverse—are essential to our existence. Planets and other astronomical bodies come in a very large number of possible designs. Hot stars, cold asteroids, giant dust clouds are just a few. Once again the Landscape of possibilities is extremely rich. Just the variation in distance from the parent star gives great diversity to planets. As for the mechanism that turns possibilities into actualities, the Big Bang, and the subsequent clumping by means of gravity, created 1022 planets within the observable part of our universe alone.

  In each of these cases the answers to the questions of our own existence are the same. There are many creatures/ planets/ pocket universes and many possible designs. The numbers are so big that, statistically, some of them will be intelligent or conducive to intelligent life. Most creatures/ universes/astro-bodies are dead ends from this point of view. We are just the lucky few. That is the meaning of the Anthropic Principle. There is no magic, no supernatural designer: just the laws
of very large numbers.

  My friend Steve Shenker, who is one of the wisest physicists I know, likes to reduce things to slogans. He feels that unless a big important idea can be encapsulated in a short phrase or two, its essence has not really been grasped. I think he is right. Here are some examples from the past.

  From Newtonian mechanics:

  Einstein and special relativity:

  and

  Einstein and general relativity:

  Quantum mechanics:

  Cosmology:

  The best scientific slogans I know don’t come from physics or cosmology but from the theory of evolution:

  If this book were to be reduced to a single thought, it would be that the grand organizing principle of both biology and cosmology is:

  There is one frustrating difference between the biological or planetary mechanism and the Eternal Inflation that populates the Landscape. In the two former cases, we can directly observe the results of the prolific mechanism of creation. We see the diversity of bio-forms all around us. Astronomical objects are a little harder to observe, but even without telescopes we can see planets, moons, and stars. But the huge sea of pocket universes created by Eternal Inflation is hidden behind our cosmic event horizon. The problem is, of course, Einstein’s speed limit. If we could exceed the speed of light, there would be no problem traveling to distant pocket universes and back. We could navigate the entire megaverse. But, alas, punching a wormhole through space to a distant pocket universe is a fantasy that violates fundamental principles of physics. The existence of other pocket universes remains, and will remain, a conjecture, but a conjecture with explanatory power.

  Consensus?

  If the ideas that I have explained turn out to be correct, then our view of the world is about to expand far beyond the current provincial boundaries to something much grander: bigger in space, bigger in time, and bigger in possibilities. If correct, how long will it take for the paradigm to shift? Like the proverbial forest, paradigm shifts are easiest seen from a distance. While the ground is shifting, things are often too confusing, the waters too muddy, to see clearly, even a few years ahead. During those times it is almost impossible for outsiders to know whose ideas are serious and whose are fringe speculations. It’s even hard for the insiders to know. My main purpose in writing this book is not primarily to convince the reader of my own point of view; scientific arguments are best fought on the pages of technical journals and the blackboards of seminar rooms. My purpose is to explain the struggle of ideas that is about to take front-and-center place in the mainstream of science so that ordinary readers can follow the ideas as they unfold and experience the drama and excitement that I feel.

  The history of scientific ideas has always fascinated me. I am as interested in how the great masters came to their insights as I am in the ideas themselves. But the great masters are not all dead. The present—right now—is a marvelous time to watch the Weinbergs, Wittens, ’t Hoofts, Polchinskis, Maldacenas, Lindes, Vilenkins… as they struggle toward a new paradigm. As far as I can make out, here is what my most distinguished colleagues think. I will address the physicists first and then the cosmologists.

  Steven Weinberg, more than any other physicist, is responsible for the discovery of the Standard Model of particle physics. Steve is not a rash man and is likely to weigh the evidence at least as carefully as anyone. His writings and lectures clearly imply that he sees the evidence, if not as definitive, then as strongly suggesting that some version of the Anthropic Principle may play a role in determining the Laws of Physics. But his own writings express regret—regret for a “paradigm lost.” In his 1992 book, Dreams of a Final Theory, he writes:

  Thus if such a cosmological constant is confirmed by observation, it will be reasonable to infer that our own existence plays an important role in explaining why the universe is the way it is.

  For what it is worth, I hope that this is not the case. As a theoretical physicist, I would like to see us able to make precise predictions, not vague statements that certain constants have to be in a range that is more or less favorable to life. I hope that string theory really will provide a basis for a final theory and that this theory will turn out to have enough predictive power to be able to prescribe values for all the constants of nature including the cosmological constant. We shall see.

  Weinberg wrote these words during the afterglow of the discoveries of Heterotic String Theory and Calabi Yau compactification. But he now knows that String Theory will not be the hoped-for alternative to the Anthropic Principle.

  Ed Witten is one of the greatest mathematicians in the world and a Pythagorean at heart. He has built his career around the elegant and beautiful mathematics that came out of String Theory. His ability to plumb the mathematical depths of the subject is breathtaking. Not surprisingly he is one of the most reluctant of my colleagues to give up the search for a magic, mathematical silver bullet, a bullet that will pick out a unique, consistent set of physical laws for elementary particles. If such a bullet exists, Witten has the depth and power to find it. But he has been looking for a long time with no success. Although he has done more than anyone to create the tools that are needed to explore the Landscape, I don’t suppose he is at all happy about the current direction that the theory is taking.

  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 10100 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 10500 vacua, or 1010000000000? 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 certa
inty 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.