The Cosmic Landscape (30 page)

Anthropically, the lifetime of the proton may have to be a good deal longer than the age of the universe. To see why, let’s suppose that the proton lifetime were twenty billion years. The decay of an unstable particle is an unpredictable event that can happen any time. When we say that the proton lifetime is twenty billion years, we mean that, statistically, the average proton will last that long. Some will decay in one year, and some in forty billion years.

Your body has about 10
²⁸
protons. If the proton lifetime were twenty billion years, about 10
¹⁸
of those protons would decay every year.
⁹
This is a negligible fraction of your protons, so you don’t have to worry about disappearing. But each proton that decays in your body shoots out energetic particles: photons and positrons and pions. These particles moving through your body have the same effects as exposure to radioactivity: cell damage and cancer. If 10
¹⁸
protons decay in your body, they will kill you. So the anthropic constraints on proton decay may be stronger than what you naively think. As far as we know, a lifetime of a million times the age of the universe—10
¹⁶
years—is long enough not to jeopardize life. On anthropic grounds we can rule out all valleys of the Landscape where the average proton lifetime is less than this.

But we know that the proton lives vastly longer than 10
¹⁶
years. In a tank of water with roughly 10
³³
protons, we would expect to see one proton decay each year if the lifetime were 10
³³
years. Physicists, hoping to witness a few protons decaying, have constructed huge underground chambers filled with water and photoelectric detectors. Sophisticated modern detectors can detect the light from just a single decay. But so far, no cigar; not a single proton has ever been seen to disintegrate. Evidently the lifetime of the proton is even longer than 10
³³
years, but the reason is unknown.

To compound the problem, we also don’t know any reason why the String Theory Landscape should not have valleys in which the Laws of Physics are life-friendly but where protons live for only 10
¹⁶
or 10
¹⁷
years. Potentially the number of such valleys could vastly outweigh those with much greater lifetimes.

This is a serious concern but probably not a showstopper. Unfortunately we don’t have nearly enough information about the Landscape to know what percentage of its habitable valleys have such very long proton lifetimes. But there is some reason for optimism. The Standard Model with no modification does not permit the proton to decay at all! This has nothing to do with the Anthropic Principle; it is simply a mathematical property of the Standard Model that the proton cannot disintegrate. If the typical habitable environment requires something fairly similar to the Standard Model, then proton stability may go along for the ride.

But we know that the Standard Model is not the full story. It does not contain gravity. Even though the Standard Model may be a very good description of ordinary physics, it nonetheless must break down. This could happen many ways. Theories called Grand Unified Theories (GUTs) are, despite their awful name, very attractive. The simplest generalization of the Standard Model to a GUT brings the proton lifetime to just around 10
³³
or 10
³⁴
years.

Other extensions of the Standard Model are not so safe. One of them, based on supersymmetry, can lead to significantly shorter proton lifetimes unless it is appropriately adjusted. We need more information before we can draw far-reaching conclusions. Fortunately, particle physics experiments in the near future may bear on the validity of the Standard Model and also on the reasons for the unusual stability of the proton. Stay tuned for a few years.

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 theory
¹¹
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: