The Cosmic Landscape (27 page)

Now not all of the codmologists meant the same thing by the Ickthropic Principle. In fact it was hard to find any two who agreed. One thought that it meant that the Head Angel Fish had made the world just for the purpose of accommodating big-brained fish. Another thought that the quantum-wave function of the waterverse was a superposition of all values of T and only by observing it did some ancestral fish “collapse the wave function.”

A small number of codmologists, led by Andrei-the-Very-Big-Brained and Alexander-Who-Swims-Deep, held a very extraordinary idea. They believed that a stupendously big space existed beyond the upper water boundary. In this very big space, many other bodies similar in some ways to their water-world but different in other ways might exist. Some worlds would be unimaginably hot, so hot that the hydrogen nuclei might even fuse to form helium and then perhaps grow even hotter. Other worlds would be so cold that frozen methane would exist. Only a tiny fraction of the bodies would be at temperatures conducive to the formation of fish. Then it would be no mystery why T was fine-tuned. As every angler knows, most places are fishless, but here and there conditions are just right. And that’s where the fish are.

But the fyshicists sighed and said, “Oh Lord, there they go again with their fishy ideas. Just ignore them.” The end.

The story was a complete flop. Loud sighs and moans from the audience were audible during the seminar. Afterward people avoided me. Tini himself was less than impressed. The Anthropic Principle affects most theoretical physicists the same way that a truckload of tourists in the African bush affects an angry bull elephant.

No one, knowing what we do about astronomy, would doubt that the codmologists got it right. The story suggests that there are situations where an anthropic (or “ickthropic”) explanation makes sense. But what are the rules? When is anthropic reasoning appropriate? When is it inappropriate? We need some guiding principles.

First, there is the obvious: an anthropic explanation of proposition X can make sense only if there is a strong reason to believe that the existence of intelligent life would be impossible unless X is true. For the big-brained fish, it’s clear: too hot, and we get fish soup; too cold, and we get frozen fish. In the case of the cosmological constant, Weinberg provided the reasoning.

When you start to think about what it takes for life to be possible, the Landscape becomes a nightmarish minefield. I’ve already explained how a large cosmological constant would have been fatal, but there are many other dangers. The requirements for a universe fall into three main categories: the Laws of Physics must lead to organic chemistry; the essential chemicals must exist in sufficient abundance; and finally, the universe must evolve to create a large, smooth, long-lived, gentle environment.

Life is of course a chemical process. Something about the way atoms are constructed makes them stick together in the most bizarre combinations: the giant crazy Tinkertoy molecules of life—DNA, RNA, hundreds of proteins, and all the rest. Although chemistry is usually regarded as a separate branch of science—it has its own university departments and its own journals—it is really a branch of physics: that branch which deals with the outermost electrons in the atom. These
valence
electrons, hopping back and forth or being shared between atoms, give the atoms their amazing abilities to combine into a diverse array of molecules.

How is it that the Laws of Physics allow marvelously intricate structures like DNA that hold themselves together without collapsing, flying apart, or destructing in some other way? To some degree it is luck.

As we saw in chapter 1, the Laws of Physics begin with a list of elementary particles like electrons, quarks, photons, neutrinos, and more, each with special properties such as mass and electric charge. No one knows why the list is what it is or why the properties are exactly what they are. An infinite number of other lists is possible. But a universe filled with life is by no means what one would expect from a random choice of the list. Eliminating any of these particles (electrons, quarks, and photons) or even changing their properties a modest amount would destroy conventional chemistry. This is obviously so for electrons and quarks, which make up the atom and its nucleus, but it may be less obvious for the photon. Photons are of course the little “bullets” that make up light. True enough, without them we couldn’t see, but we could still hear, feel, and smell, so maybe the photon isn’t so important. Thinking that, however, is a big mistake: the photon happens to be the glue that holds the atom together.

What keeps the valence electrons in orbit around the central core of the atom? Why don’t they just fly off and say adios to the protons and neutrons? The answer is the electrical attraction between the oppositely charged electrons and atomic nucleus. Electrical attraction is different from the attraction between a fly and a strip of flypaper. The flypaper may be very sticky and hold on fiercely, but once you separate the fly even a little, the flypaper immediately lets go. The fly flies off, and unless it is stupid enough to come back, it is completely free. In physics jargon the flypaper force is strong but
short range
—it doesn’t reach out over large distances.

A short-range force of the flypaper type would be useless in binding electrons to the nucleus. The atom is a miniature solar system, and the all-important valence electrons are the most distant planets: the Plutos and Neptunes of the atom. Only a force that reaches out to large distances can keep them from flying off into the “outer space” beyond the boundaries of the atom.

Long-range forces,
the kind that grab from a distance, are uncommon. Of all the many different types of forces in nature, only two are long range. Both are familiar, the most familiar being gravity. When we jump off the earth, gravity pulls us back. It reaches out hundreds of millions of miles to hold the planets in their orbits around the sun and tens of thousands of light-years to keep the stars confined within galaxies. That’s the kind of force that is needed to tie the outer electrons to the central nucleus. Of course it’s not gravity that holds the atom together: it’s far too weak.

Another familiar long-range force acts between a magnet and an iron paper clip. The magnet doesn’t have to be in direct contact with the paper clip to attract it. A strong magnet tugs on the clip even from a remote distance. But more relevant for the atom, the electric cousin of magnetic force is a long-range force that acts between electrically charged particles. Much like the gravitational force, except vastly stronger, the electric force binds the valence electrons the same way that gravity ties Pluto to the sun.

As I explained in chapter 1, electric forces between charged particles are caused by photons that are exchanged between the charges.
²
The ultralight photons (remember that they have no mass) are capable of jumping long distances to create long-range forces that bind the distant valence electrons to the nucleus. Remove the photon from the list, and there would be nothing to hold the atom together.

The photon is very exceptional. It is the only elementary particle, other than the graviton, that has no mass. What if it were less exceptional and had mass? Feynman’s theory tells us how to compute the force when a hypothetical massive photon jumps between nucleus and electron. What one finds is that the heavier the photon, the less far it is able to jump. Were the photon mass even a tiny fraction of the electron mass, instead of being a long-range force, electric interactions would become short-range “flypaper forces,” totally incapable of holding on to the distant valence electrons. Atoms, molecules, and life are entirely dependent on the curious fact that the photon has no mass.

The range of the electric force is not the only feature that is essential for atoms to work properly. The strength of the force (how hard it pulls on the electrons) is critical. The force holding the electron to the nucleus is not very large by the standards of ordinary human experience. It’s measured in billionths of a pound. What is it that determines the strength of electric forces between charged particles? Again, Feynman’s theory tells us. The other ingredients in a Feynman diagram, besides particles, are vertex diagrams. Remember that every vertex diagram has a numerical value—the coupling constant—and for photon emission the coupling constant is the fine structure constant α, a number about equal to the fraction
¹
/
₁₃₇
. The smallness of a is the ultimate mathematical reason why electric forces are much weaker than their nuclear counterparts.

What if the fine structure constant were bigger, say about one? This would create several disasters, one of which would endanger the nucleus. The nuclear force that holds the nucleons (protons and neutrons) together is a flypaper force: short range and strong. The nucleus itself is like a ball of sticky flies. Each nucleon is stuck to its nearest neighbors, but if it can just separate from the others by a tiny bit, it’s free to fly off.

There is something working against the nuclear force, competing with it, to repel the protons from one another. The protons are, of course, electrically charged. They attract the negative electrons because they have the opposite charge (opposite charges attract; like charges repel). The neutrons are electrically neutral and, therefore, don’t play a role in the balance of electric forces. Protons, on the other hand, are positively charged and electrically repel one another. In fact if a nucleus has more than about one hundred protons, the repulsive long-range electric forces are enough to blow it apart.

What would happen if the electric force were as strong as the nuclear force? Then all complex nuclei would be unstable. In fact the electric force could be a good deal weaker than the nuclear force and still endanger nuclei like carbon and oxygen. Why is the fine structure constant small? No one knows, but if it were big, there would be no one to ask the question.

Protons and neutrons are no longer considered to be elementary particles. Each is composed of three quarks. As discussed in chapter 1, there are several different species of quarks labeled up, down, strange, charmed, bottom,andtop.While the names are quite meaningless, the differences between the types of quarks are important. A quick look at the list of particle masses in chapter 3 reveals that the quark masses vary over a huge range from roughly 10 electron masses for the up- and down-quarks to 344,000 electron masses for the top-quark. Physicists puzzled for some time about why the top-quark is so heavy, but recently we have come to understand that it’s not the top-quark that is abnormal: it’s the up- and down-quarks that are absurdly light. The fact that they are roughly twenty thousand times lighter than particles like the Z-boson and the W-boson is what needs an explanation. The Standard Model has not provided one.

Thus, we can ask what the world would be like if the up- and down-quarks were much heavier than they are. Once again—disaster! Protons and neutrons are made of up- and down-quarks. (Particles made of strange-, charmed-, bottom-, and top-quarks play no role in ordinary physics and chemistry. They are of interest mainly to high-energy physicists.) According to the quark theory of protons and neutrons, the nuclear force (force between nucleons) can be traced to quarks hopping back and forth between these nucleons.
³
If the quarks were much heavier, it would be much more difficult to exchange them, and the nuclear force would practically disappear. With no sticky flypaper force holding the nucleus together, there could be no chemistry. Luck is with us again.

Remember that in terms of the Landscape, our universe rests in a valley where all the fortunate coincidences are true. But in generic regions of the Landscape, things can be very different. The fine structure constant could easily be larger, the photon massive, quarks heavier, or even worse, electrons, photons, or quarks might not be on the list. Any one of these would be enough to eliminate our presence.

Even if all the standard particles existed with the right mass and the right forces, chemistry could still fail. One thing more is needed: electrons must be fermions. The fact that fermions are so exclusive—you can’t put more than one in a quantum state—is essential to chemistry. Without the Pauli exclusion principle, all electrons in an atom would sink down to the lowest atomic orbits, where they would be much more difficult to dislodge. Ordinary chemistry is completely dependent on the Pauli principle. If electrons suddenly turned into the more sociable bosons, life based on carbon chemistry would go poof. So you see that a world with ordinary chemistry is far from generic.

Physicists often use words differently from the way they are ordinarily used. When you say that something exists, you probably mean that it can be found somewhere in the universe. For example, if I were to tell you that black holes exist, you might ask me where you can find one. Black holes do exist in the ordinary sense: they are actual astronomical objects found, for example, at the centers of galaxies. But suppose I told you tiny black holes no bigger than a speck of dust exist. Again you might ask where they are found. This time I would answer that there is none; it takes a huge amount of mass to squeeze itself into a black hole. No doubt you would get annoyed and say, “Stop pulling my leg. You told me they exist!”

What physicists (especially of the theoretical variety) mean by the term
exist
is that the object in question can exist
theoretically.
In other words, the object exists as a solution to the equations of the theory. By that criterion perfectly cut diamonds a hundred miles in diameter exist. So do planets made of pure gold. They may or may not actually be found somewhere, but they are possible objects consistent with the Laws of Physics.

Long-range weak forces and short-range strong forces, acting among fermions, lead to the
existence
of complex atoms like carbon, oxygen, and iron. That’s nice, but I mean it in the theoretical sense. “What more,” you may ask, “is needed to make sure that complex atoms exist in
my
ordinary sense? What is required to actually produce those atoms and make them abundant in the universe?” The answer is not so simple. Complex atomic nuclei are not likely to result from random collisions of particles, even in the early hot universe.