It has been observed that cosmologists are often wrong but seldom uncertain, and the authors of this long, fascinating, exasperating book are no exceptions. They are John Barrow, astronomer at the University of Sussex, and Frank Tipler, Tulane University mathematical physicist. Physicist John Wheeler provides an enthusiastic foreword. No one can plow through this well-written, painstakingly researched tome without absorbing vast chunks of information about QM (quantum mechanics), the latest cosmic models, and the history of philosophical views that bear on the book’s main arguments.

Just what is this “anthropic principle” that has become so fashionable among a minority of cosmologists, and is arousing such passionate controversy? As the authors make clear in their introduction, there is not one principle but four. Each is more speculative than the previous one, with the fourth blasting the authors out of science altogether into clouds of metaphysics and fantasy.

The simplest of the four is called (the authors are fond of acronyms) WAP, or the Weak Anthropic Principle. Although it goes back to Protagoras’s famous declaration that “man is the measure of all things,” its modern cosmological form seems first to have been stated by the physicist Robert Dicke in the late 1950s. As Barrow and Tipler readily admit, it is a trivial tautology, totally noncontroversial. It merely proclaims that because we exist the universe must be so constructed as to allow us to have evolved. The laws of nature clearly must be such as to permit, if not actually force, the formation of CHON (carbon, hydrogen, oxygen, and nitrogen), the four elements essential to life as we know it.

Does this mean that all life must be carbon based? Although the authors believe this, it does not follow from WAP. Even if there is noncarbon life elsewhere in the universe, the fact that we are carbon imposes a variety of tight restraints on the universe and its past. For example, the cosmos has to be about 15 billion years old. Why? Because, the authors argue, elements necessary to organic molecules are cooked inside stars. If the universe were much younger, those elements would not be available and we wouldn’t be here. If the universe were much older, all the suns would have burned out, and we wouldn’t be here either.

WAP was invoked over and over again in earlier centuries by proponents of the design argument for God. It was WAPish to point out that if the earth were slightly closer to the sun, like Venus, water would boil away and carbon life would be impossible. If the earth were slightly farther from the sun, water would freeze and Earth would have the barren deserts of Mars. Theists liked to note that when water freezes it expands and floats on water, otherwise lakes and rivers would freeze to the bottom in winter and all their life be destroyed. If Earth did not have an ozone atmosphere, animals could not survive ultraviolet radiation. And so on. Hundreds of similar arguments, most of them analyzed by Barrow and Tipler, seem to show that our universe, and especially our planet, were carefully designed to permit us to exist.

The close ties between WAP and the creation hypothesis impel the authors to write almost one hundred pages on traditional proofs of God from design. It is an excellent history, followed by almost as long a section on more recent teleological arguments. There are informative discussions of such post-Darwinian “process” thinkers as Henri Bergson, Samuel Alexander, Alfred Whitehead, and Charles Hartshorne, who see the universe as rolling toward a predetermined goal, as well as “process theologians” who anchor the goal in God.

If WAP were all there is to the anthropic principle, the book would not have been worth writing. The authors continually stress the triviality of asserting no more than that the universe has a structure that makes carbon life possible. It is easy to caricature such retrograde reasoning. Instead of saying I am here because my parents met, I say that because I am here I know my parents met. How lucky for vacationers that sandy beaches are so near the sea! From the fact that I wear spectacles I can deduce the positions of my ears and nose. If a chess game ends with no queens on the board, I can infer with iron logic that both queens were captured. From the present state of the world one can obviously make all sorts of highly probable, sometimes certain, conjectures about its distant past.

But there is more to the anthropic principle than WAP. The next step is SAP, the Strong Anthropic Principle. Proposed in 1974 by the British cosmologist Brandon Carter, it maintains that life of any sort is impossible unless the basic laws of nature are exactly what they are. Consider gravity. If it were slightly stronger, the cosmos would long ago have stopped expanding, gone the other way, and collapsed into a black hole before galaxies could form. If gravity were slightly weaker, it would have expanded too rapidly to allow matter to clump into stars. In either case, you and I wouldn’t be here.


The strength of gravity is one of a dozen or more constants called dimensionless because they are independent of any measuring system. If one banana is twice as long as another, the number two is the same whether you measure the banana in inches or centimeters. It turns out that these fundamental constants are so finely tuned that if they varied ever so slightly, there couldn’t be any carbon atoms and we wouldn’t be here. Instead of saying we are here because the constants are precisely what they are, SAP turns it around. We are here, therefore the constants had to be what they are.

In long chapters on physics, astrophysics, and biochemistry, often dense with technical details and mathematical formulas, Barrow and Tipler defend this reverse way of reasoning. A recurring theme is that SAP puts such narrow constraints on the constants and natural laws that it can lead to falsifiable predictions. Opponents of SAP take a dim view of this claim. The physicist Heinz Pagels, in a slashing attack on anthropic arguments in his article “A Cozy Cosmology,”1 dismisses WAP and SAP as pure flimflam. Although they may occasionally suggest testable conjectures, they do so in such obvious ways that nothing is gained by elevating them into new principles.

According to Barrow and Tipler, the first successful anthropic prediction was made by the University of Chicago geologist Thomas Chamberlain. Geological evidence indicates a great age for the solar system. If the sun did not feed on atomic energy, Chamberlain guessed, it would have long ago burned out and we wouldn’t be here. Chamberlain guessed right, but did he do anything except apply ordinary reasoning?

Writing just before the Barrow and Tipler book was published, Pagels cites a more recent example. England’s Stephen Hawking and Barry Collins once invoked the anthropic principle to explain why the universe is so isotropic—the same in all directions. If it were less so, matter would not condense into galaxies and we wouldn’t be here. This, says Pagels, explains nothing. By contrast, the new inflationary models of the big-bang hypothesis of the origins of the universe actually do provide a plausible mechanism for isotropy. In old big-bang models the initial explosion would have produced permanent irregularities. In the inflationary models, immediately, after the bang the universe jumps from a trillionth the size of a proton to about the size of a softball. This sudden inflation smoothes out all irregularities, leaving an isotropic cosmos expanding at its present slow rate. In light of such speculations, the anthropic principle seems irrelevant. Surprisingly, Barrow and Tipler agree. They are strongly critical of Hawking and Collins for what they see as a misuse of the principle.

Similar efforts to use SAP as a tool for investigating the constants have been equally feeble, Pagels continues. Meanwhile, the new unified-field theories really are providing significant explanations of why the constants are what they are. WAP and SAP are so needless that they raise a new mystery. “How can such a sterile idea,” Pagels asks, “reproduce itself so prolifically?” He suspects it may be because scientists are reluctant to make a leap of faith and say: “The reason the universe seems tailor-made for our existence is that it was tailor-made…. Faced with questions that do not neatly fit into the framework of science, they are loath to resort to religious explanations; yet their curiosity will not let them leave matters unaddressed. Hence, the anthropic principle. It is the closest that some atheists can get to God.”

Leaving aside the hypothesis of a transcendent Creator, or a Mind that either is the universe or permeates the universe, what alternatives are left? Barrow and Tipler consider several possibilities.

One is the startling view that only one kind of universe is possible—the one we know. This was skillfully defended by the Harvard chemist Lawrence Henderson in two books that were largely ignored until recently: Fitness of the Environment (1913, reprinted in 1970 by Harvard University Press) and The Order of Nature (1917). Leibniz argued exactly the opposite. He believed an infinity of universes are logically possible, and God selected the one he liked best.

The authors discuss several variations of the many-possible-worlds view. Other universes could have the same laws as ours, but entirely different histories depending on different initial conditions before the big bang. Parallel worlds could flourish side by side in our familiar three-dimensional space, or in higher spaces, but because of limitations on the speed of light, no contacts between them are possible even if they are all in our space. We need not, however, assume infinite space. Alternate worlds could follow one another in some sort of supertime. Each explodes into existence, expands, contracts, and vanishes in the Big Crunch to be followed (whatever that means) by another fireball.


John Wheeler has a stupendous vision in which an infinity of universes pop in and out of existence, each with a randomly determined set of laws. Every logically possible universe appears an infinite number of times. (If an infinity of bridge hands are dealt, every possible distribution of the cards will be dealt an infinite number of times.) Of course only a tiny subset of these possible worlds will have forces and particles that permit life. This naturally emasculates any argument from design to God. It is not surprising we are in a universe that allowed us to evolve. How could it be otherwise?

The Wildest of all variants of the infinite-universe theme, designed to counter the standard Copenhagen interpretation of QM (named for the city where Niels Bohr worked), is the many-worlds interpretation. In the Copenhagen view, the central mystery is what happens when a quantum system is measured. Take the case of a single particle. Every particle has associated with it a set of probability waves in an artificially constructed multidimensional space. A single expression, called the wave function, gives the probabilities of a particle assuming each of its possible states when it is measured. Before measurement, all possible states of the particle are said to be mixed in some sort of weird potential sense. Not until the particle is measured does nature “decide,” by pure chance, what value to give a variable. At that instant the wave function is said to “collapse” from an indefinite to a definite state.

This notion of wave-function collapse leads to all sorts of paradoxes of which the EPR paradox (after the initials of Einstein and two associates) has recently become the most notorious. A particle and its antiparticle can be simultaneously created by an interaction that sends them in opposite directions. Regardless of how far apart they get, perhaps light years in distance, they remain “correlated.” If, for instance, one particle is measured for the direction of its spin, the wave function for the pair instantly collapses and the other particle acquires an opposite spin. Since neither particle has a definite spin until one is measured, and since there is no known causal connection between the pair, how does the other particle “know” what spin to acquire?

Einstein was deeply troubled by what he called the spooky telepathic aspect of this famous thought experiment, which he believed showed that QM is not a complete theory. Niels Bohr strongly disagreed. Most quantum experts still side with Bohr, though a growing number are beginning to suspect that the old maestro may have been right after all. The EPR paradox has recently been confirmed by several laboratory tests that could not have been made in Einstein’s day. Perhaps this has awakened physicists to a fuller awareness of the paradox’s deep implications.

The many-worlds interpretation dissolves the mystery of the EPR and similar paradoxes by denying that wave functions ever collapse. For this simplification, however, a horrendous price is paid. At every instant when a collapse seems to occur, the entire universe is said to split into parallel worlds, each containing one of the possible outcomes of measurement. At every instant billions upon billions of such splits take place. There is no communication between these worlds. We cannot tell that we are constantly splitting into duplicate selves because our consciousness rides smoothly along only one path in the endlessly forking chains. This splitting process is completely deterministic, perhaps guided by one monstrous wave function that keeps expanding but never collapses unless there is a God outside the universe to observe it.

The many-worlds interpretation has been called a beautiful theory that nobody can believe. Nevertheless, a number of eminent physicists, including Wheeler and Hawking, have taken it seriously, at least as a way of interpreting QM that removes its thorniest difficulties. Although Wheeler has withdrawn his support of the theory, Barrow and Tipler are defenders. “The wave function collapse postulated by the Copenhagen Interpretation is dynamically ridiculous,” they write, “and this interpretation is difficult if not impossible to apply in quantum cosmology. We suggest that the Many-Worlds Interpretation may well eventually replace the Statistical and Copenhagen Interpretations just as the Copernican system replaced the Ptolemaic…. Physicists who think in terms of the Copenhagen Interpretation may become handicapped in thinking about quantum cosmology.” Poor old Bohr! Too bad he didn’t think of how to solve the problems of measurement by letting the universe copy him billions of times!

We come now to Wheeler’s radical version of SAP, which the authors call PAP, or the Participatory Anthropic Principle. No universe can exist in a strong sense, Wheeler maintains, unless it contains conscious observers. This view rests on the fact that when a wave function collapses, the measuring instrument (a device or a person) becomes part of a larger system. All the potential states remain mixed as before until the larger system is measured.

Erwin Schrödinger, who disliked QM even though he helped get it started, invented a famous cat paradox to ridicule wave-function collapse. The cat is in a closed box with a mechanism that will kill the cat when it is triggered by a quantum event such as a click in a Geiger counter. The click has an equal probability of occurring or not within, say, an hour. At the end of the hour QM seems to say that until the cat is “measured” by someone looking into the box, the cat is neither alive nor dead. The two states remain mixed until an observer collapses the cat’s wave function.

The paradox gets worse when you realize that even if an observer looks into the box he at once becomes part of a still larger system in which the cat’s two states continue to be mixed until someone observes the observer. This is called the “paradox of Wigner’s friend” after Eugene Wigner, a physicist who is troubled by it. It obviously leads to an infinite regress of observers. Wigner avoids the regress by cutting it whenever a chain of events is registered in a conscious mind. This raises more difficulties. Is the cat conscious enough to end the regress if it isn’t killed? If the mechanism merely chops off a leg? Although the chain ends for Wigner’s friend when he opens the box, it doesn’t end for Wigner until he observes his friend, so the regress doesn’t really go away.

These solipsistic speculations have led Wheeler to the view that our universe is a participatory one in which reality is a collusion between minds and whatever is out there, perhaps only a bare mathematical field. For Wheeler the universe does not exist except in a pale mathematical sense unless it contains conscious observers. Here again the continuum of minds in the animal world raises disturbing problems. Einstein said he couldn’t believe the moon’s reality depends on being observed by a mouse. If a mouse will do, why not a bee?

Wheeler’s view seems to be that a universe becomes real only when it is structured at the big bang so that it eventually can observe itself through conscious minds. Or perhaps reality can be thought of as a spectrum. The universe grows more real as life evolves to higher forms. In either case, Wheeler’s vision sees conscious life as essential if a universe is to be more than a mathematical abstraction. His vision is close to Bishop Berkeley’s “to be is to be perceived,” except that Wheeler, unlike the Irish cleric, does not restore the external world by having it observed by God. Not for a moment did Berkeley, as sometimes said, doubt the external world’s full reality. He only denied it was material. Indeed, he argued anthropically. Because we and the external world surely exist, there must be a God.

Barrow and Tipler move on from PAP to what they call FAP, the Final Anthropic Principle, but not before a long attempt to show that ETIs (Extraterrestrial Intelligences) do not exist. For many years Tipler has been arguing strenuously in both technical and popular articles that there are compelling grounds to assume that life on a level above micro-organisms exists nowhere else in our galaxy, perhaps nowhere else in the universe. This has understandably brought him into sharp conflict with Frank Drake, Philip Morrison, Carl Sagan, and other scientists who strongly support SETI (the Search for Extraterrestrial Intelligence). Tipler is convinced that this search is a foolish waste of money.

Those who agree usually base their reasoning on the fact that long sequences of improbable events appear to be required for life even to get started. First there must be a sun with a planet on which conditions are extremely close to those on Earth. Assuming such planets exist, the probability that a self-replicating molecule will arise on one of them by chance may be vanishingly small. Finally, even if such a molecule does arise spontaneously, another sequence of improbable events must occur if it is to evolve into anything as intelligent as a fish or a bird.

In Science Year (1973) Wheeler had a science-fiction story called “Beyond the Black Hole.” In it a character called Audrey, with whose views I assume Wheeler identifies, comes to this conclusion:

“Let’s carry what you are saying to the logical extreme, Fred. It takes a very narrow squeak for a cycle of the universe to permit life at all, even at one place. If life had originated in more than one place, that would have meant that the universe was larger and longer-lived than necessary. The creation of life would be more ‘expensive’ than it needed to be. So the chances are overwhelming that Earth is the sole outpost of life in the universe, and we had no right to expect to find life on Zeta Zeta. Am I wrong in my reasoning?”

To such reasoning Tipler has added a curious new argument that goes like this. There are planets in the cosmos millions of years older than Earth. If there is intelligent life on any of them, its technology would be far more advanced than ours. We know from experience that there is an overwhelming desire to explore the universe, and that this is possible. It can best be done, Tipler claims, by what he calls von Neumann machines after the mathematician John von Neumann, who first proved that self-replicating robots can be constructed. Superbeings on other planets would surely build such robots and give them an intelligence equal to or surpassing their own. These robots would multiply at an explosive rate. In a short time they would be poking their spaceships into every corner of the galaxy. Because we see no signs of them (the authors have no interest in UFOs), ETIs don’t exist.

Sagan is understandably infuriated by this reasoning. “Absence of evidence is not evidence of absence,” he and William Newman said in their paper on “The Solipsist Approach to Extraterrestrial Intelligence.”2 In any case, the only way we can know is by searching. “We have an alternative denied to the medieval scholastics; we are able to experiment.”

Why has the notion that we are alone in the galaxy been gaining ground? Partly, I suspect, because of the shock of finding no traces of life on Mars, partly because of a revival of theism that seems to be taking place among intellectuals. Whatever the reasons, those who have shared Tipler’s unbelief in ETI include many distinguished evolutionists (Alfred Russel Wallace wrote an entire book about it), and such top physicists as Enrico Fermi and Freeman Dyson. “I find that the universe in some sense,” wrote Dyson in his autobiography, “must have known that we were coming.”

In his latest collection of essays, The Flamingo’s Smile, Stephen Jay Gould attacks what he calls the “moth-eaten” arguments of the anthropicists. He accuses Tipler of misinterpreting what evolutionists mean when they speak of improbable events. They mean only that it is highly improbable evolution would take precisely the paths it has taken on Earth. They do not mean that once life starts the steps would not lead to intelligence. No one expects to find animals on another planet that duplicate beasts on Earth, but there are no good reasons for assuming that evolution could not take many paths to other forms of intelligence.

The assumed absence of ETI leads Barrow and Tipler to FAP, their Final Anthropic Principle. Although life probably exists only on Earth, now that it has begun FAP says it will be impossible to destroy. Otherwise, the universe would lose all its observers, and by PAP would have demolished itself! In the author’s FAP fantasy, life is now taking its first faltering steps toward colonizing the universe. This is likely to be completed by intelligent von Neumann machines. The authors expect about half the universe to be colonized by the time our universe reaches the limit of its expansion and starts the other way. The red shift of stars turns to blue. Colonization goes on until the entire cosmos teems with computer life.

If there is an infinity of other universes, presumably these events will take place in all of them that permit life. Borrowing from Teilhard de Chardin, the Catholic paleontologist, the authors posit an Omega Point that will be the end of Everything. Here are the book’s final sentences:

At the instant the Omega Point is reached, life will have gained control of all matter and forces not only in a single universe, but in all universes whose existence is logically possible; life will have spread into all spatial regions in all universes which could logically exist, and will have stored an infinite amount of information, including all bits of knowledge which it is logically possible to know.123 And this is the end.

Footnote 123 “A modern-day theologian might wish to say that the totality of life at the Omega Point is omnipotent, omnipresent, and omniscient!” The God of Moses, Jesus, and Mohammed will finally have come into being. Instead of creating all the universes, however, it’s the other way around. The universes got together and created the Almighty. This places the authors within the tradition of Samuel Alexander who, in his masterwork Space, Time, and Deity (1920), put forth the notion of a finite God who is slowly developing and growing in perfection as the universe evolves. Their eschatology is even closer to several famous science-fiction yarns. In Isaac Asimov’s “The Last Question” a supercomputer evolves in hyperspace into a deity who creates a new universe to replace the old one that wore out and died the “heat death” dictated by thermodynamics.

What should one make of this quartet of WAP, SAP, PAP, and FAP? In my not so humble opinion I think the last principle is best called CRAP, the Completely Ridiculous Anthropic Principle.

This Issue

May 8, 1986