The Universe is unlikely. Very unlikely. Deeply,shockingly unlikely.
"It's quite fantastic," says Martin Rees, Britain's Astronomer Royal, waving a hand through the steam rising from his salmon-and-potato casserole.
A casual observer might think the gesture encompasses just this room, the dining hall at King's College in Cambridge, England, where scholars have traded erudite quips for nearly two centuries. Rees digs into his lunch, just as he has dispatched meals here since 1973. In such a clubby, comfortable place, pronouncements about the origin of the cosmos seem a bit overreaching. But Rees's wrist flick takes in the whole universe, this universe, the one that gave rise to Earth and supports life, from the bristle worms on the ocean's floor to the swallows soaring over the college spires to human beings— including astronomers royal.
In his newest book, Just Six Numbers, Rees argues that six numbers underlie the fundamental physical properties of the universe, and that each is the precise value needed to permit life to flourish. In laying out this premise, he joins a long, intellectually daring line of cosmologists and astrophysicists (not to mention philosophers, theologians, and logicians) stretching all the way back to Galileo, who presume to ask: Why are we here? As Rees puts it, "These six numbers constitute a recipe for the universe." He adds that if any one of the numbers were different "even to the tiniest degree, there would be no stars, no complex elements, no life."
The six numbers lurk in the universe's smallest and largest structures. To select one from the small end: The nucleus of a helium atom weighs 99.3 percent as much as the two protons and the two neutrons that fuse to make it. The remaining .7 percent is released mainly as heat. So the fuel that powers the sun— the hydrogen gas at its core— converts .007 of its mass into energy when it fuses into helium. That number is a function of the strength of the force that "glues" together the parts of an atomic nucleus.
So what? Consider this: If the number were only a mite smaller— .006 instead of .007— a proton could not bond to a neutron, and the universe would consist only of hydrogen. No chemistry, no life. And if it were slightly larger, just .008, fusion would be so ready and rapid that no hydrogen would have survived from the Big Bang. No solar systems, no life. The requisite number perches, precariously, preciously, between .006 and .008. And that's just one of Rees's six numbers. If you toss in the other five, life and the structure of the universe as we know it become unlikely to an absurd degree. Astronomer Hugh Ross has compared the state of affairs to "the possibility of a Boeing 747 aircraft being completely assembled as a result of a tornado striking a junkyard."
Faced with such overwhelming improbability, cosmologists have offered up several possible explanations. The simplest is the so-called brute fact argument. "A person can just say: 'That's the way the numbers are. If they were not that way, we would not be here to wonder about it,' " says Rees. "Many scientists are satisfied with that." Typical of this breed is Theodore Drange, a professor of philosophy at the University of West Virginia, who claims it is nonsensical to get worked up about the idea that our life-friendly universe is "one of a kind." As Drange puts it, "Whatever combination of physical constants may exist, it would be one of a kind."
Rees objects, drawing from an analogy given by philosopher John Leslie. "Suppose you are in front of a firing squad, and they all miss. You could say, 'Well, if they hadn't all missed, I wouldn't be here to worry about it.' But it is still something surprising, something that can't be easily explained. I think there is something there that needs explaining."
Meanwhile, the numbers' uncanny precision has driven some scientists, humbled, into the arms of the theologians. "The exquisite order displayed by our scientific understanding of the physical world calls for the divine," contends Vera Kistiakowsky, a physicist at the Massachusetts Institute of Technology. But Rees offers yet another explanation, one that smacks of neither resignation nor theology. Drawing on recent cosmology— especially the research of Stanford University physicist Andrei Linde and his own theories about the nature of the six numbers— Rees proposes that our universe is a tiny, isolated corner of what he terms the multiverse.
The idea is that a possibly infinite array of separate big bangs erupted from a primordial dense-matter state. As extravagant as the notion seems, it has nonetheless attracted a wide following among cosmologists. Rees stands today as its champion. "The analogy here is of a ready-made clothes shop," says Rees, peeling his dessert, a banana. "If there is a large stock of clothing, you're not surprised to find a suit that fits. If there are many universes, each governed by a differing set of numbers, there will be one where there is a particular set of numbers suitable to life. We are in that one."
Rees thinks about big ideas in a roughly 10-by-12-foot office in the Institute of Astronomy, which is housed in a one-story redbrick building on the green, serene bucolic edge of Cambridge University. Horses, experimental subjects of the agriculture department, munch clover in the field outside his window. Every morning at 11, ladies in pink aprons serve tea to the faculty and students. It's a humble but civilized setting for the Astronomer Royal, a title granted Rees by Queen Elizabeth in 1995. King Charles invented the job in 1675, when he began paying John Flamsteed 100 pounds annually to solve navigation problems.
Today the designation is honorary, pays nothing, and seems to slightly embarrass Rees, who is also a Royal Society Research Professor at Cambridge. Calling Martin Rees "Sir Martin," he emphasizes, is not at all necessary. Such humility befits the man's appearance and manner: slight, soft-spoken, and unfailingly solicitous.
But Rees is as intellectually brave as he is otherwise self-effacing. "The trend in astronomy today is hyperspecialization, but he is a cosmologist in the largest sense of the word," says Priya Natarajan, a research associate at the Institute of Astronomy and a former student of Rees's. "He was one of the first people who had the idea of black holes at the center of galaxies, that almost every galaxy probably ought to have one. Only recently, with the Hubble Space Telescope, was there a survey of about 40 galaxies, and every one had a black hole." Natarajan is in awe of Rees's prescience. "He sees the connections that a lot of people don't see, partly because he is so smart, and partly because he is so versatile."
Phillip James Peebles, a physics professor emeritus at Princeton who has known Rees for more than 30 years, agrees. "With Martin," he says, "there is never any danger of missing the big picture."
Indeed, recognizing the improbable connections that hold together the universe as we know it requires flinging the widest of intellectual nets, encompassing everything from quantum weirdness to biological imperatives to galactic clumping. Of Rees's six numbers, two relate to basic forces, two determine the size and large-scale texture of the universe, and two fix the properties of space itself. Rees's six numbers are:
, the .007 figure, which describes the strength of the force that binds atomic nuclei together and determines how all atoms on Earth are made.
N, equal to 1,000,000,000,000,000,000,000,000,000,000,000,000. The number measures the strength of the forces that hold atoms together divided by the force of gravity between them. It means that gravity is vastly weaker than intra-atomic attraction. If the number were smaller than this vast amount, "only a short-lived, miniature universe could exist," says Rees.
, which measures the density of material in the universe— including galaxies, diffuse gas, and dark matter. The number reveals the relative importance of gravity in an expanding universe. If gravity were too strong, the universe would have collapsed long before life could have evolved. Had it been too weak, no galaxies or stars could have formed.
, the newest addition to the list, discovered in 1998. It describes the strength of a previously unsuspected force, a kind of cosmic antigravity, that controls the expansion of the universe. Fortunately, it is very small, with no discernable effect on cosmic structures that are smaller than a billion light-years across. If the force were stronger, it would have stopped stars and galaxies— and life— from forming.
Q, which represents the amplitude of complex irregularities or ripples in the expanding universe that seed the growth of such structures as planets and galaxies. It is a ratio equal to 1/100,000. If the ratio were smaller, the universe would be a lifeless cloud of cold gas. If it were larger, "great gobs of matter would have condensed into huge black holes," says Rees. Such a universe would be so violent that no stars or solar systems could survive.
D, the number of spatial dimensions in our universe— that is, three. "Life could not exist if it were two or four," contends Rees.If each of the six numbers Rees has identified were dependent upon the others— in the same sense that, say, the number of arms and fingers in a family depends upon the number of family members— the fact that they allow for the existence of life would seem less of a shock. "At the moment, however," says Rees, "we cannot predict any of them from the value of the others." So unless theoreticians discover some unifying theory, each number compounds the unlikeliness of each of the other numbers.
Of the many possible explanations for these life-affirming values, Rees favors the multiverse theory because it has at least the potential to be tested and scientifically confirmed. Labeling any theory "metaphysics," he contends, "is a damning put-down from a physicist's point of view," because metaphysical notions cannot be proved or disproved. The multiverse, on the other hand, "genuinely lies within the province of science," says Rees, although he concedes that the concept remains speculative.
The multiverse idea is, in fact, far from new. In the late 1700s, philosopher David Hume mused that other universes might have been "botched and bungled, throughout eternity, ere this system." The problem then, as now, is that most theories say the universes must remain forever inaccessible to one another even in principle, which makes the multiverse seem little more compelling than the conjured-by-God hypothesis. Rees admits that, at present, the premises upon which many multiverse calculations rest are "highly arbitrary," but he is confident that they need not remain so. "Within the next 20 years," he says, "we may be able to put the multiverse on a firm scientific footing or rule it out."
Current multiverse theorizing is the latest wrinkle in the Big Bang theory of the universe's origin. Springing from Edwin Hubble's 1929 observation that every galaxy appeared to be racing away from every other galaxy, the Big Bang theory today has decades of evidence on its side. For example, in 1965 Arno Penzias and Robert Wilson discovered faint microwave radiation coming from all directions in the sky and found it corresponds to theoretical predictions of the Big Bang's explosive residue. The theory also neatly explains the universe's relative proportions of various elements, such as the abundance of hydrogen and helium.
But from the start, the theory had serious shortcomings. Among other mysteries, astronomers were stumped as to how the microwave background radiation could be so smooth but still permit matter to "clump" into stars and galaxies. Alan Guth of MIT solved this and other technical inconsistencies with his inflation model, published in 1981. Guth proposed that in the first tiny fraction of a second after the Big Bang— a period of just 1/100,000,000,000,000,000,000,000,000,000,000,000 of a second— the universe grew much more quickly than it did later. Inflation, according to Guth's theory, created stretched quantum waves of vacuum, leading to nonhomogeneous regions, leading to density variations, leading to galaxies. Inflation theory is now wedded to Big Bang theory; together, they come as close to dogma as anything can in the contentious realm of cosmology.
But for various reasons, including questions raised by the oddly life-hospitable numbers cited by Rees and others, Guth's inflationary model is giving way to what Andrei Linde at Stanford calls the "self-reproducing inflationary universe." Linde's model, based on advanced principles of quantum physics, defies easy visualization. Quite simplified, it suggests quantum fluctuations in the universe's inflationary expansion have a wavelike character. Linde theorizes that these waves can "freeze" atop one another, thus magnifying their effects. The stacked-up quantum waves in turn can create such intense disruptions in scalar fields— the underlying fields that determine the behavior of elementary particles— that they exceed a sort of cosmic critical mass and start birthing new inflationary domains. The multiverse, Linde contends, is like a growing fractal, sprouting inflationary domains that sprout more inflationary domains, with each domain spreading and cooling into a new universe.
If Linde is correct, our universe is just one of the sprouts. The theory neatly straddles two ancient ideas about the origin of our universe: that it had a definite beginning, and that it has existed forever. In Linde's view, each particular part of the multiverse, including our part, began from a singularity somewhere in the past, but that singularity was just one of an endless series that was spawned before it and will continue after it.
Digging up experimental evidence for Linde's theory will be a challenge, as the model specifies that each universe in the multiverse is a separate, closed volume of space and time. "The other universes are unavailable to us, just as the interior of a black hole is unavailable," says Rees, adding that we cannot even know if the universes are finite or infinite in number. But he emphasizes that proof of some kind is at least theoretically possible. "Some details of the fluctuations of ripples in background radiation may help us determine the truth," says Rees. "Until then, the theory hangs on assumptions we must make about the physics of very dense states of matter."
What intrigues Rees is that Linde's theory permits differing fundamental constants and differing numbers of dimensions in this ever-blooming collection of universes. Universe A could feature six dimensions, universe B could sport ultraweak gravity. The possibilities are literally endless. The multiverse could, indeed, be an off-the-rack store. Most of the universes it spawns, Rees believes, are inhospitable to life, but a precious few, including ours, happen to fit the requirements of life as we know it through sheer force of numbers, in the same sense that snapping up all of the lottery tickets guarantees buying the winner.
Rees is also tantalized by the fact that our universe displays a certain "ugliness and complexity" that goes along with the idea that it is a subset of a larger series. Consider: Earth orbits in an elliptical path, not a circle. If its orbit were a circle— which would permit life but is not required by life— this would raise suspicions that either God or chance had fixed its course; we would have to accept that such fine-tuning was due to either brute fact or providence. But an elliptical orbit, and similar less-than-elegant aspects of the universe as we find it, such as the fact that l is just a smidgen above zero, suggests that, as Rees says, "our universe may be just one of an ensemble of all possible universes" that allow our emergence. In other words, this universe looks more like a narrow-subset dweller than an amazing one-of-a-kind case. As Rees says, the numbers are "no more special than our presence requires."
The totality of the mystery, he emphasizes, will most likely never ultimately yield to the prying of cosmologists. "Why are we here?" is a big question, but Rees concedes that a bigger mystery probably resides outside the grasp of science altogether. "The fundamental question of 'Why is there something rather than nothing?' remains the province of philosophers," he concedes. "And even they may be wiser to respond, with Ludwig Wittgenstein, that 'whereof one cannot speak, thereof one must be silent.' "
"The Self-Reproducing Inflationary Universe," Andre Linde, Scientific American, Special Issue: The Magnificent Cosmos, March 1998, pp. 98-104. Also available at: www.sciam.com/specialissues/0398cosmos/0398linde.html.
"The Fine-Tuning Argument," Theodore M. Drange, 1998, can be found at: www.infidel.org/library/modern/theodore_drange/tuning.html.
To search for information about life in the universe, see www.seti.org. To participate in the search using your PC, go to the Seti@Home site: setiathome.ssl.berkeley.edu.