The Cosmic Landscape (56 page)

I think that the correct response to this criticism is that there is a hidden assumption that is an integral part of the Anthropic Principle, namely:
the existence of life is extremely delicate and requires very exceptional conditions.
This is not something that I can prove. It is simply part of the hypothesis that gives the Anthropic Principle its explanatory power. Perhaps we should turn the argument upside down and say that the success of Weinberg’s prediction supports the hypothesis that robust intelligent life requires galaxies, or at least stars and planets.

What are the alternatives to the populated Landscape paradigm? My own opinion is that once we eliminate supernatural agents, there is none that can explain the surprising and amazing fine-tunings of nature. The populated Landscape plays the same role for physics and cosmology as Darwinian evolution does for the life sciences. Random copying errors, together with natural selection, are the only known natural explanation of how such a finely tuned organ as an eye could form from ordinary matter. The populated Landscape, together with the rich diversity predicted by String Theory, is the only known explanation of the extraordinary special properties of our universe which allow our own existence.

This is a good place for me to pause and address a potential criticism that might be leveled against this book, namely that it lacks balance. Where are the alternative explanations of the value of the cosmological constant? Aren’t there any technical arguments against the existence of a large Landscape? What about other theories besides String Theory?

I assure you that I am not hiding the other side of the story. Throughout the years many people, including some of the most illustrious names in physics, have tried to explain why the cosmological constant is small or zero. The overwhelming consensus is that these attempts have not been successful. Nothing remains to report on that score.

As for serious mathematical attempts to debunk the Landscape, I know of only one. The author of that attempt is a good mathematical physicist, and as far as I know, he still believes his criticism of the KKLT construction (see chapter 10). The objection involves an extremely technical mathematical point about special Calabi Yau spaces. Several authors have criticized the criticism, but by now it may be irrelevant. Michael Douglas and his collaborators have found many examples that avoid the problem. Nevertheless, an honest assessment of the situation would have to include the possibility that the Landscape is a mathematical mirage.

Finally, as for alternatives to String Theory, a well-known one is called Loop Gravity. Loop Gravity is an interesting proposal, but it is not nearly as well developed as String Theory. In any case even its most famous advocate, Lee Smolin, believes that Loop Gravity is not really an alternative to String Theory but may be an alternative formulation of String Theory.

As much as I would very much like to balance things by explaining the opposing side, I simply can’t find that other side. Opposing arguments boil down to a visceral dislike of the Anthropic Principle (I hate it) or an ideological complaint against it (it’s giving up).

Two specific arguments have been the subjects of recent popular books by well-known physicists, but both have failed in my view. I’ll take a moment to explain why.

This is a favorite idea of some condensed-matter theorists who work on the properties of materials made of ordinary atoms and molecules. Its principal proponent is the Nobel Prize winner Robert Laughlin, who describes his ideas in his book
A Different Universe.
⁶
The idea at its core is the old “ether theory” that maintains that the vacuum is some special material. The ether idea was popular in the nineteenth century, when both Faraday and Maxwell tried to think of electromagnetic fields as stresses in the ether. But after Einstein the ether fell into disrepute. Laughlin would like to resurrect the old idea by picturing the universe as material with properties similar to superfluid helium. Superfluid helium is an example of a material with special “emergent” properties, properties that reveal themselves (emerge) only when huge numbers of atoms are assembled in macroscopic amounts. In the case of liquid helium, the fluid has amazing superfluid properties such as flowing without any friction. In a lot of ways, superfluids are similar to the Higgs fluid that fills space and gives particles their properties. Roughly speaking Laughlin’s view can be summarized by saying that we live in such a space-filling material. He might say it even more strongly: space
is
such an emergent material! Moreover, he believes that gravity is an emergent phenomenon.

One of the main themes of modern physics is that emergent phenomena have a kind of hierarchical structure. Little collections of molecules or atoms group together to form bigger entities. Once you know the properties of these new entities, you can forget where they came from. The new entities, in turn, combine and cluster into new groups of even larger size. Once again you can forget where they came from and group them into yet bigger groups until the whole macroscopic material is explained. One of the most interesting properties of these systems is that it doesn’t matter exactly what you begin with. The original microscopic entities don’t make any difference to the emergent behavior—the material always comes out with the same large-scale behavior—within limits.
⁷
For this reason Laughlin believes there is no point in looking for the fundamental objects of nature, since a wide variety of basic objects would lead to the same Laws of Physics—gravity, the Standard Model, and so on—in the large-scale world. In fact there are all kinds of “excitations” in materials that do resemble elementary particles but are really collective motions of the underlying atoms. Sound waves, for example, behave as though they were made of quanta called phonons. Moreover, these objects sometimes behave uncannily like photons or other particles.

There are two serious reasons to doubt that the laws of nature are similar to the laws of emergent materials. The first reason involves the special properties of gravity. To illustrate, consider the properties of superfluid helium, although any other material would do as well. All sorts of interesting things take place in superfluids. There are waves that behave similarly to scalar fields and objects called vortices that resemble tornadoes moving through the fluid. But there is no kind of isolated object that moves around in the fluid and resembles a black hole. This is not an accident. Black holes owe their existence to the gravitational force described by Einstein’s General Theory of Relativity. But no known material has the characteristics that the General Theory of Relativity ascribes to space-time. There are very good reasons for this. In chapter 10, where I dealt with black holes, we saw that the properties of a world with both quantum mechanics and gravity are radically different from anything that can be produced with ordinary matter alone. In particular, the Holographic Principle—a mainstay of current thinking—seems to require totally new kinds of behavior not seen in any known condensed-matter system. In fact Laughlin himself illustrates the point by arguing that black holes (in his theory) cannot have properties, such as Hawking radiation, that practically everyone else believes them to have.

But suppose one found an emergent system that had some of the features that we want. The properties of emergent systems are not very flexible. There may be an enormous variety of starting points for the microscopic behavior of atoms, but as I said, they tend to lead to a very small number of large-scale endpoints. For example, you can change the details of helium atoms in many ways without changing the macroscopic behavior of superfluid helium. The only important thing is that the helium atoms behave like little billiard balls that just bounce off one another. This insensitivity to the microscopic starting point is the thing that condensed-matter physicists like best about emergent systems. But the probability that out of the small number of possible fixed points (endpoints) there should be one with the incredibly fine-tuned properties of our anthropic world is negligible. In particular, there is no explanation of the most dramatic of these fine-tunings, the small but nonzero cosmological constant. A universe based on conventional condensed-matter emergence seems to me to be a dead-end idea.

Lee Smolin has attempted to explain the very special properties of the world—the Anthropic properties—by a direct analogy with Darwinian evolution—not in the general probabilistic sense that I explained earlier but much more specifically.
⁸
To his credit Smolin understood early that String Theory is capable of describing a tremendous array of possible universes, and he attempted to use that fact in an imaginative way. Although I feel that Smolin’s idea ultimately fails, it is a valiant effort that deserves serious thought. The gist of it follows:

In any universe with gravitational forces, black holes can form. Smolin speculates about what might take place inside a black hole, in particular, at its violent singularity. He believes, in my opinion with no good evidence, that instead of space collapsing to a singularity, a resurrection of the universe takes place. A new baby universe is born inside the black hole. In other words universes are
replicators
that reproduce in the interior of black holes. If this is so, Smolin argues, then by a process of repeated replication—black holes forming inside universes, which are inside black holes, which are inside universes, and so on—an evolution will take place toward maximally fit universes. By fit Smolin means having the ability to produce a large number of black holes and, therefore, a large number of offspring. Smolin then conjectures that our universe is the most fit of all—the laws of nature in our pocket are such that they produce the maximum possible number of black holes. He claims that the Anthropic Principle is totally unnecessary. The universe is not tuned for life. It is tuned to make black holes.

The idea is ingenious, but I don’t think it explains the facts. It suffers from two serious problems. The first is that Smolin’s idea of cosmic evolution is too closely patterned on Darwin’s and requires changes between generations to be small incremental changes. As I said above, the pattern suggested by the String Theory Landscape is quite the opposite. In Smolin’s defense I should point out that almost all of our knowledge of the Landscape was derived after his theory was published. At the time Smolin was formulating his ideas, the working paradigm for string theorists was the flat supersymmetric part of the Landscape, where it is indeed true that changes are incremental.

The other problem is cosmological and has little to do with String Theory. There is no reason whatever to believe that we live in a universe that is maximally efficient at producing black holes. Smolin makes a series of tortured arguments to prove that any changes in our universe would result in fewer black holes, but I find them very unconvincing. We saw in chapter 5 that it is a lucky “miracle” that the universe is
not
catastrophically filled with black holes. A relatively small increase in the early lumpiness of the universe would cause almost all matter to collapse to black holes rather than life-nurturing galaxies and stars. Also, increasing the masses of the elementary particles would cause more black holes to form since they would be more susceptible to gravitational attraction. The real question is why the universe is so lacking in black holes. The answer that seems to me to make the most sense is that many, maybe most, pockets have far more black holes than our pocket, but they are violent places in which life could not have formed.

The whole argument that we live in a world that is maximally fit to reproduce is, in my opinion, also fundamentally flawed. Space does indeed reproduce—one well-understood mechanism is Inflation—but the maximally reproducing universe is nothing like our own. The most fit universe in Smolin’s sense, the one that replicates most rapidly, would be the universe with the largest cosmological constant. But there is no overlap between the fitness to reproduce and the fitness to support intelligent life. With its ultrasmall cosmological constant and its paucity of black holes, our universe is particularly unfit to replicate.

Going back to the analogy of the tree of life, in biology there is also no overlap between reproductive fitness and intelligence. The maximally fit creatures are not humans: they are bacteria. Bacteria replicate so rapidly that a single organism can have as many as ten trillion descendants in a twenty-four-hour period! According to some estimates the earth’s population of bacteria is more than a thousand billion billion billion. Humans may be special in many ways but not in their ability to reproduce. A world that can support life is also very special—but, again, not in its ability to reproduce.

To put it another way, imagine Gregor Samsa—the hero in Franz Kafka’s
The Metamorphosis
—on that fateful day when he awoke as a giant cockroach. Might he have asked himself, while still foggy with sleep, “What kind of creature am I?” Following Smolin’s logic, the answer would be, “With overwhelming probability, I must belong to the class of creatures that are most fit to reproduce and are therefore the most populous. In short I must be a bacterium.”

But a few seconds of reflection might convince him otherwise. Misquoting Descartes, he would conclude, “I think, therefore I am not a bacterium. I am something very special—a remarkable creature with extraordinary brainpower. I am not average: I am exceedingly far from average.” We, also, should take no more than a moment to conclude that we are not average. We do not belong to the branch of the megaverse that is most fit to reproduce. We belong to the branch that can say: “I think, therefore the cosmological constant must be very small.”

My reaction to Smolin’s idea has been harsh. But the harshness is directed against particular technical points: not against Smolin’s overall philosophy. I think Smolin deserves great credit for getting the most important things right. Smolin was the first to recognize that the diversity of String Theory vacuums may play an important positive role in explaining why the world is the way it is. He was also the first to try to use this diversity in a creative way to explain our special environment. And most important—he understood that there was an urgent question to answer: “How can the deepest and most powerful ideas of modern physics provide a truly scientific explanation of the
apparent
‘intelligent design’ that we see all around us?” In all of this he was going directly against the strong prejudices of the string theorists, and I think that he was more correct then they were.