The Cosmic Landscape (31 page)

“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.

T
he 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.

“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, 10
⁴⁰
. 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.

Finally we come to the strongest of all forces, those that hold the atomic nucleus together. A nucleus is composed of electrically neutral neutrons and positively charged protons. No negative charges are found in a nucleus. Why doesn’t it blow up? Because the individual protons and neutrons attract with a nonelectric force about fifty times stronger than the electrical repulsion. The quarks that make up a single proton have even stronger forces binding them together. How is it that our protons and neutrons aren’t attracted to the protons and neutrons of the earth by such powerful forces? The answer is that although the nuclear force is powerful, it is also very short range. It is easily strong enough to overcome the electric repulsion of protons, but only when particles are close together. Once they separate by more than a couple of proton diameters, the force becomes negligible. Underlying the strong inter-actions are the powerful forces between quarks, the elementary particles that make up hadrons.

I often feel a discomfort, a kind of embarrassment, when I explain elementary-particle physics to laypeople. It all seems so arbitrary—the ridiculous collection of fundamental particles, the lack of pattern to their masses, and especially the four forces, so different from each other, with no apparent rhyme or reason. Is the universe “elegant” as Brian Greene tells us? Not as far as I can tell, not the usual laws of particle physics anyway. But in the context of a megaverse of wild diversity, there is a pattern. All of the forces and most of the elementary particles are absolutely essential. Change any of it more than a bit, and life as we know it becomes impossible.

A peculiar ideology insinuated itself into the high-energy theoretical physics of the 1960s. It paralleled almost exactly a fad that had taken hold in psychology. B. F. Skinner was the guru of the
behaviorists,
who insisted that only the external behavior of a human being was the proper material of mind science. According to Skinner, psychologists had no business inquiring into the inner mental states of their subjects. He even went so far as to declare that no such thing existed. The business of psychology was to watch, measure, and record the external behavior of subjects without ever inquiring about internal feelings, thoughts, or emotions. To the behaviorists a human was a black box that converted sensory input into behavioral output. While it is probably true that Freudians went too far in the other direction, the behaviorists carried their ideology to extremes.

The behaviorism of physics was called
S-matrix theory.
Sometime in the early sixties, while I was a graduate student, some very influential theoretical physicists, centered in Berkeley, decided that physicists had no business trying to explain the inner workings of hadrons. Instead, they should think of the Laws of Physics as a black box—a black box called the Scattering Matrix, or S-matrix for short. Like the behaviorists the S-matrix advocates wanted theoretical physics to stay close to experimental data and not wander off into speculation about unobservable events taking place inside the (what was then considered) absurdly small dimensions characteristic of particles like the proton.

The input to the black box is some specified set of particles coming toward one another, about to collide. They could be protons, neutrons, mesons, or even nuclei of atoms. Each particle has a specified momentum as well as a host of other properties like spin, electric charge, and so on. Into the metaphorical black box they disappear. And what comes out of the black box is also a group of particles—the products of collision, again with specified properties. The Berkeley dogma forbade looking into the box to unravel the underlying mechanisms. The initial and final particles are everything. This is very close to what experimental physicists do with accelerators to produce the incoming particles and with detectors to detect what emerges from the collision.

The S-matrix is basically a table of quantum-mechanical probabilities. You plug in the input, and the S-matrix tells you the probability for a given output. The table of probabilities depends on the direction and energy of both the incoming and outgoing particles, and according to the prevailing ideology of the mid-1960s, the theory of elementary particles should be confined to studying the way the S-matrix depends on these variables. Everything else was forbidden. The ideologues had decided that they knew what constituted good science and became the guardians of scientific purity. S-matrix theory was a healthy reminder that physics is an empirical subject, but like behaviorism, the S-matrix philosophy went too far. For me it turned all of the wonder of the world into the gray sterility of an accountant’s actuarial tables. I was a rebel, but a rebel without a theory.

In 1968 Gabriele Veneziano was a young Italian physicist living and working at the Weizmann Institute, in Israel. He was not especially ideological about S-matrix theory, but the mathematical challenge of figuring out the S-matrix appealed to him. The S-matrix was supposed to satisfy certain technical requirements, but no one at that time could point to a specific mathematical expression that satisfied the rules. So Veneziano tried to find one. The attack was brilliant. The result, famous today as the “Veneziano amplitude,” was extremely neat. But it was not a picture of what particles were made of or of how the processes of collision could be visualized. The Veneziano amplitude was an elegant mathematical expression—an elegant mathematical table of probabilities.