The Cosmic Landscape

by Leonard Susskind

  Philosophical Objections

  In the abstract of a paper titled “Scientific Alternatives to the Anthropic Principle,” the physicist Lee Smolin writes, “It is explained in detail why the Anthropic Principle cannot yield any falsifiable predictions, and therefore cannot be a part of science.”10

  Smolin’s paper goes on in the introduction to say:

  I have chosen a deliberatively provocative title, in order to communicate a sense of frustration I’ve felt for many years about how otherwise sensible people, some of whom are among the scientists I most respect and admire, espouse an approach to cosmological problems that is easily seen to be unscientific. I am referring of course to the anthropic principle. By calling it unscientific I mean something very specific, which is that it fails to have a necessary property to be considered a scientific hypothesis. This is that it be falsifiable. According to [the philosopher] Popper, a theory is falsifiable if one can derive from it unambiguous predictions for doable experiments such that, were contrary results seen, at least one premise of the theory would have been proven not to apply to nature.

  Richard Feynman once remarked, “Philosophers say a great deal about what is absolutely necessary for science, and it is always, so far as one can see, rather naive, and probably wrong.” Feynman was referring to Popper among others. Most physicists, like Feynman, don’t usually think much about philosophy: not unless they are trying to use it to prove someone else’s theory is unscientific.

  Frankly, I would have preferred to avoid the kind of philosophical discourse that the Anthropic Principle excites. But the pontification, by the “Popperazzi,” about what is and is not science has become so furious in news reports and Internet blogs that I feel I have to address it. My opinion about the value of rigid philosophical rules in science is the same as Feynman’s. Let me quote from a debate that appeared on the Internet site edge.org. The quote is from a short essay that I wrote in response to Smolin’s paper. Smolin’s arguments can also be found there. They are thoughtful and interesting.

  Throughout my long experience as a scientist I have heard un-falsifiability hurled at so many important ideas that I am inclined to think that no idea can have great merit unless it has drawn this criticism. I’ll give some examples:

  From psychology: You would think that everybody would agree that humans have a hidden emotional life. B. F. Skinner didn’t. He was the guru of a scientific movement called behaviorism that dismissed anything that couldn’t be directly observed as unscientific. The only valid subject for psychology according to the behaviorist is external behavior. Statements about the emotions or the state of mind of a patient were dismissed as un-falsifiable and unscientific. Most of us, today, would say that this is a foolish extreme. Psychologists today are deeply interested in emotions and how they evolved.

  From physics: In the early days of the quark theory, its many opponents dismissed it as un-falsifiable. Quarks are permanently bound together into protons, neutrons and mesons. They can never be separated and examined individually. They are, so to speak, hidden behind a different kind of veil. Most of the physicists who made these claims had their own agendas, and quarks just didn’t fit in. But by now, although no single isolated quark has ever been detected, there is no one who seriously questions the correctness of the quark theory. It is part of the bedrock foundation of modern physics.

  Another example is Alan Guth’s inflationary theory. In 1980 it seemed impossible to look back to the inflationary era and see direct evidence for the phenomenon. Another impenetrable veil called the “surface of last scattering” prevented any observation of the inflationary process. A lot of us did worry that there might be no good way to test inflation. Some—usually people with competing ideas—claimed that inflation was un-falsifiable and therefore not scientific.

  I can imagine the partisans of Lamarck criticizing Darwin, “Your theory is un-falsifiable, Charles. You can’t go backward in time, through the millions of years over which natural selection acted. All you will ever have is circumstantial evidence and an un-falsifiable hypothesis. By contrast, our Lamarckian theory is scientific because it is falsifiable. All we have to do is create a population that lifts weights in the gym every day for a few hours. After a few generations, their children’s muscles will bulge at birth.” The Lamarckists were right. The theory is easily falsified—too easily. But that didn’t make it better than Darwin’s theory.

  There are people who argue that the world was created 6000 years ago with all the geological formations, isotope abundances, dinosaur bones, in place. Almost all scientists will point the accusing finger and say “Not falsifiable!” And I would agree. But so is the opposite—that the universe was not created this way—un-falsifiable. In fact that is exactly what creationists do say. By the rigid criterion of falsifiability “creation-science” and science-science are equally unscientific. The absurdity of this position will, I hope, not be lost on the reader.

  Good scientific methodology is not an abstract set of rules dictated by philosophers. It is conditioned by, and determined by, the science itself and the scientists who create the science. What may have constituted scientific proof for a particle physicist of the 1960’s—namely the detection of an isolated particle—is inappropriate for a modern quark physicist who can never hope to remove and isolate a quark. Let’s not put the cart before the horse. Science is the horse that pulls the cart of philosophy.

  In each case that I described—quarks, inflation, Darwinian evolution—the accusers were making the mistake of underestimating human ingenuity. It only took a few years to indirectly test the quark theory with great precision. It took 20 years to do the experiments that confirmed inflation. And it took 100 years or more to decisively test Darwin (some would even say that it has yet to be tested). The powerful methods that biologists would discover a century later were unimaginable to Darwin and his contemporaries. Will it be possible to test eternal inflation and the Landscape? I certainly think so although it may be, as in the case of quarks, that the tests will be less direct, and involve more theory, than some would like.

  After this material was written, I thought of a couple of additional examples of overzealous Popperism. An obvious one is the S-matrix theory11 of the 1960s, which said that since elementary particles are so small, any theory that attempts to discuss their internal structure is unfalsifiable and, therefore, not science. Again, no one takes that seriously today.

  A famous example from the late nineteenth century involves one of Einstein’s heroes, Ernst Mach. Mach was both a physicist and a philosopher. He was an inspiration for Wittgenstein and the logical positivists. At the time when he was active, the hypothesis that matter was made of atoms was still an unproved conjecture, and it remained that way until Einstein’s famous 1905 paper on Brownian Motion unequivocally demonstrated that matter has an atomic structure.

  Even though Boltzmann had shown that the properties of gases could be explained by the atomic hypothesis, Mach insisted that it was not possible to prove the reality of atoms. He allowed that they might be a useful mnemonic, but he argued strenuously that the impossibility of falsifying them undermined their status as real science.

  Falsification, in my opinion, is a red herring, but confirmation is another story. (Perhaps this is what Smolin really meant.) By confirmation I mean direct positive evidence for a hypothesis rather than absence of negative evidence. It is true that the theory of Eternal Inflation described in chapter 9 and the existence of multiple pocket universes cannot be confirmed in the same way that the big-brained fish could confirm their version of the Ickthropic Principle. Without violating any laws of nature, the codmologists could construct a pressurized, water-filled submarine to take them to the surface and observe the existence of planets, stars, and galaxies. They could even visit these astronomical bodies and confirm for themselves the enormous diversity of environments. Unfortunately there are insurmountable (see, however, chapter 12) reasons why the analogous option is not available to us. The key
concept is the existence of cosmic horizons that separate us from other pocket universes. In chapters 11 and 12, I discuss horizons and the question of whether they are really ultimate barriers to collecting information. But certainly the critics are correct that in practice, for the foreseeable future, we are stuck in our own pocket with no possibility of directly observing other ones. Like quark theory, the confirmation will not be direct and will rely on a great deal of theory.

  As for rigid philosophical rules, it would be the height of stupidity to dismiss a possibility just because it breaks some philosopher’s dictum about falsifiability. What if it happens to be the right answer? I think the only thing to be said is that we do our best to find explanations of the regularities we see in the world. Time will shake out the good ideas from the bad, and they will become part of science. The bad get added to the junk heap. As Weinberg emphasized, we have no explanation for the cosmological constant other than some kind of anthropic reasoning. Will it be one of the good ideas that become science or one that winds up in the junk? No rigid rules of philosophers, or even scientists, can help very much. Just as generals are always fighting the last war, philosophers are always parsing the last scientific revolution.

  Before concluding this chapter I want to discuss one more favorite objection to the Anthropic Principle. This argument is that the Anthropic Principle isn’t wrong, it’s just a silly tautology: Of course the world has to be such that it supports life. Life is an observed fact. And of course it’s true that if there were no life, there would be no one to observe the universe and ask the questions we are asking. But so what? The principle says nothing beyond the fact that life formed.

  This is a kind of willful missing of the point. As usual I find it helpful to rely on an analogy. I call it the Cerebrothropic Principle. The Cerebrothropic Principle is intended to answer the question, “How did it happen that we developed such a big, powerful brain?” This is what the principle says:

  “The laws of biology require the existence of a creature with an extraordinarily unusual brain of about fourteen hundred cubic centimeters because without such a brain there would be no one to even ask what the laws of biology are.”

  That is extremely silly even though true. But the Cerebrothropic Principle is really shorthand for a longer, much more interesting, story. In fact two stories are possible. The first is creationist: God made man with some purpose that involved man’s ability to appreciate and worship God. Let’s forget that story. The whole point of science is to avoid such stories. The other story is far more complex and, I think, interesting. It involves several features. First of all it says that the Laws of Physics and chemistry allow for the possible existence of computer-like systems of neurons that can exhibit intelligence. In other words the Landscape of biological designs includes a small number of very special designs that have what we call intelligence. That’s not trivial.

  But the story requires more—a mechanism to turn these blueprint designs into actual working models. That’s where Darwin comes in. Random copying errors together with natural selection have a tendency to create a tree or bush of life whose branches fill every niche, including a niche for creatures that survive by their brainpower. Once all that is understood, the question, “Why did I wake up this morning with a big brain?” is exactly answered by the Cerebrothropic Principle. Only a big brain can ask the question.

  The Anthropic Principle can also be silly. “The Laws of Physics have to be such that they allow life because if they weren’t, there wouldn’t be anyone to ask about the Laws of Physics.” The critics are quite right—by itself, it’s silly. It simply states the obvious—we are here, so the laws of nature must permit our existence—without providing any mechanism for how our existence influenced the choice of laws. But taken as shorthand for the existence of a fantastically rich Landscape and a mechanism for populating the Landscape (chapter 11) with pocket universes, it is not at all trivial. In the next few chapters, we will see evidence that our best mathematical theory provides us with such a Landscape.

  CHAPTER SEVEN

  A Rubber Band-Powered World

  The large number of lucky accidents I’ve described so far, including the incredible fine-tuning of the cosmological constant, make a strong case for at least keeping an open mind to anthropic arguments. But these accidents alone would not have persuaded me to take a strong position on the issue. The success of Inflation (Inflation implies an enormous universe) and the discovery of a bit of vacuum energy made the Anthropic Principle appealing, but in my own mind, the “straw that broke the camel’s back” was the realization that String Theory was moving in what seemed to be a perverse direction. Instead of zeroing in on a single, unique system of physical laws, it was yielding an ever-expanding collection of Rube Goldberg concoctions. I felt that the goal of a single unique string world was an ever-receding mirage and that the theorists looking for such a unique world were on a doomed mission.

  But I also sensed an extraordinary opportunity in the approaching train wreck: String Theory might just provide the kind of technical framework in which anthropic thinking would make sense. The only problem is that String Theory, while it had a lot of possibilities, didn’t seem to have nearly enough. I kept asking my friends, “Are you sure that the number of Calabi Yau manifolds is only a few million?” Without the mathematical jargon, what I was asking them was whether they were quite certain that the number of String Theory vacuums (in other words, valleys in the Landscape) was measured in the millions. A few million possibilities when you are trying to explain the cancellation of 120 decimal places is of no real help.

  But all that changed in the year 2000. Raphael Bousso, then a young postdoc at Stanford, together with an old friend, Joe Polchinski, from the University of California at Santa Barbara, wrote a paper explaining how the number of possible vacuums could be so large that there could easily be enough to overcome the unlikelihood of tuning 120 digits. Soon after, my Stanford colleagues Shamit Kachru, Renata Kallosh, and Andrei Linde and the Indian physicist Sandip Trivedi confirmed the conclusion. That was it for me. I concluded that the only rational explanation for the fine-tunings of nature would have to involve both String Theory and some form of anthropic reasoning. I wrote a paper called “The Anthropic Landscape of String Theory,” which stirred up a hornet’s nest that is still buzzing. This is the first of three chapters (7, 8, and 10) devoted to explaining String Theory.

  Hadrons

  “Three quarks for muster mark,” said James Joyce. “Three quarks for the proton, three quarks for the neutron, and a quark-antiquark pair for the meson,” said Murray Gell-Mann. Murray, who has a fetish for words, invented a large fraction of the vocabulary of high-energy physics: quark, strangeness, Quantum Chromodynamics, current-algebra, the eight-fold way, and more. I’m not sure whether the curious word hadron (pronounced hay-dron or ha-dron) was one of Murray’s words. Hadrons were originally defined, somewhat imprecisely, as particles that shared certain properties with nucleons (protons and neutrons). Today, we have a very simple and clear definition: hadrons are the particles that are made out of quarks, antiquarks,1 and gluons. In other words they are the particles that are described by Quantum Chromodynamics (chapter 1).

  What does the word hadron mean? The prefix hadr in Greek means “strong.” It’s not the particles themselves that are strong—it’s a lot easier to break up a proton than an electron—but rather the forces between them. One of the early achievements of elementary-particle physics was to recognize that there are four distinct types of forces between elementary particles. What distinguishes these forces is their strength: how hard a pull or push they exert. Weakest of all are the gravitational interactions between particles; then come the so-called weak interactions; somewhat stronger are the familiar electromagnetic forces; and finally are the strongest of all—the nuclear, or strong, interactions. You may find it odd that the most familiar—gravity—is the weakest. But think of it for a moment: it requires the entire mass of the earth to hold us
to the surface. The force between an average person standing on the earth’s surface and the earth itself is only 150 pounds. Divide that force by the number of atoms in a human body, and it becomes apparent that the force on any atom is minute.

  But if the electric forces are so much stronger than gravity, why doesn’t the electric interaction either eject us from or crush us to the surface? The gravitational force between any two objects is always attractive (ignoring the effects of a cosmological constant). Every electron and every nucleus in our bodies gravitationally attracts every electron and every nucleus in the earth. That adds up to a lot of attraction, even though the individual forces between the microscopic particles are totally negligible. By contrast, electric forces can be either attractive or repulsive. Opposite charges—an electron and a proton, for example—attract. Two like charges, a pair of electrons or a pair of protons, repel each other. Both our own bodies and the substance of the earth have both kinds of charge—positive nuclei and negative electrons—in equal amounts. The electric forces of attraction and repulsion cancel! But suppose we could temporarily eliminate all the electrons in both ourselves and in the earth. The remaining positive charges would repel with a total force that would be incomparably stronger than the gravitational force. How many times stronger? Roughly one with forty zeros after it, 1040. You would be ejected from the earth with such force that you would be moving with practically the speed of light in no time at all. In truth this could never happen. The positive charges in your own body would repel so strongly that you would be instantaneously blown to smithereens. So would the earth.

  Electric forces are neither the strongest nor the weakest of the nongravitational forces. Most of the familiar particles interact through the so-called weak interactions. The neutrino is a good example because it has only weak forces (ignoring gravity). As I have previously explained, the weak forces are not really so weak, but they are very short range. Two neutrinos have to be incredibly close, about one one-thousandth of a proton diameter, to exert an appreciable force on each other. If they are that close, the force is about the same as the electric force between electrons, but under ordinary conditions the weak forces are only a tiny fraction of the electric.