How Science Produced the Big Bang Model
The empirical spirit on which the Western democratic societies were founded is currently under attack, and not just by such traditional adversaries as religious fundamentalists and devotees of the occult. Serious scholars claim that there is no such thing as progress and assert that science is but a collection of opinions, as socially conditioned as the weathervane world of Paris couture. Far too many students accept the easy belief that they need not bother learning—much science, since a revolution will soon disprove all that is currently accepted anyway. In such a climate it may be worth affirming that science really is progressive and cumulative, and that well-established theories, though they may turn out to be subsets of larger and farther-reaching ones—as happened when Newtonian mechanics was incorporated by Einstein into general relativity—are seldom proved wrong. As the physicist Steven Weinberg writes, "One can imagine a category of experiments that refute well-accepted theories, theories that have become part of the standard consensus of physics. Under this category I can find no examples whatever in the past one hundred years." Science is not perfect, but neither is it just one more sounding board for human folly.
Nor is science a static body of dogma, to stray from which is to risk having one's epaulets stripped off in a ceremony of banishment from the scientific community. It is a self-correcting system of inquiry, in which errors—of which there are, of course, plenty—are sooner or later detected by experiment or by more careful analysis. Science is also a "bottom-up" system, in which grand pronouncements are arrived at not in an overarching, sui generis fashion but by building up inferences from many small cases. As a result, science, while it can be exasperatingly detailed, is also pliant. Scientific findings, even the most imposing ones, customarily stumble into the world fraught with blunders that have to be worked out before they really begin to fly. They lack the satisfying, thunderclap certitude of religious and pseudoscientific dicta that admit to no error. But they are alive, and the withering of one branch of a theory does not mean that the theory as a whole is doomed.
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Cosmology today is mostly conducted within the broad framework of what is known as the "standard" or "big bang" model. It holds that the universe began in a state of high density, from which it has since expanded and cooled. For reasons I will explain, I expect this standard model to endure. This position may seem curious to readers of the many newspaper and magazine articles that have appeared during the past decade proclaiming that this or that observational finding has put the big bang theory in jeopardy. Such accounts seem to me to result from a misunderstanding of science generally and of the big bang theory in particular. My purpose in this article is to summarize the main reasons that many scientists feel confident about the standard big bang model of the universe. Admittedly, the model is far from complete. Scientists don't yet know exactly how old the universe is, how big it is, how rapidly it expands, how much matter is in it, or from where it came. (As the English astronomer royal, Martin Rees, remarks, "It's embarrassing that 90 percent of the universe is unaccounted for.") Nor is it clear how the matter we do see organized itself into stars and galaxies. There are a great many things we do not know. But it is quite possible that all these issues will be resolved, one way or another, without leaving the basic precepts of the standard model behind.
A Picture of the Universe Develops
How did science arrive at its present understanding of the age, scale, and evolution of the universe?
Here's the story so far:
The ancient Greeks thought that the earth (which they understood to be a sphere) sat immobile at the center of the universe, orbited by concentric crystalline spheres to which were attached the sun, moon, planets, and stars. This model answered well to common sense: The stars do appear to circle the earth daily, while to advocate the alternative proposal—that this effect is produced by a rotation of the earth rather than of the starry sphere—was to encounter objections that were insurmountable at the time. (If the earth is spinning, why does a man who jumps straight up land in his footprints, rather than hundreds of yards to the west?) The geocentric cosmos was also aesthetically pleasing: It portrayed our world as a sphere set at the center of a nested set of spheres, a conception that resonated with Plato's conviction that the sphere is the most perfect of all geometric shapes, since it confines the largest possible volume within a given surface area.
This model was put together by two of the keenest minds of the fourth century b.c., the philosopher Aristotle and the astronomer Eudoxus, and it won widespread acceptance. But the Greeks were not content with simply admiring its splendors. They also expected the theory to account for the data of observation—to explain motions seen in the sky in the past and to predict those coming up in the future, especially such spectacular events as eclipses of the sun and moon and conjunctions of the planets. It is for this reason more than any other that we celebrate the Greeks as the precursors of modern science. Their skepticism set in motion the questioning, subversive, and perpetually dissatisfied spirit that is characteristic of science. The ultimate failure of their model proffers a cautionary lesson as well—that in cosmology a theory can be sensible and beautiful and also quite wrong. The geocentric cosmology of Aristotle and Eudoxus did not, in the long run, generate accurate predictions of the motions of the planets.
Ptolemy and Copernicus Advance Our Thinking
Better results were obtained by the more complicated model composed in the second century a.d. by Ptolemy at Alexandria. In the Ptolemaic universe each planet orbited in an epicycle—a small circle—centered on a point in its orbit around the earth, or even on a point in another epicycle. This was clever but highly abstract; Ptolemy himself viewed his model as merely a mathematical expedient. And it was so complicated that Ptolemy's name became a lasting epithet for theories regarded as unduly elaborate or insufficiently physical. Nevertheless the Ptolemaic universe reigned in the West for fourteen hundred years, until it was challenged by Copernicus.
Schoolchildren today are still being taught that the sun-centered Copernican universe brought simplicity and light to cosmology in a single stroke. But the Copernican model in its original form was neither less complicated than Ptolemy's nor more accurate. Copernicus assumed that the orbits of the planets are circular; consequently he too had to resort to epicycles. Copernicanism was favored by some astronomers, particularly younger scholars of a radical bent, not because it solved all their problems but because, by demonstrating that a heliocentric cosmology could compete with Ptolemy's geocentric one, it opened up fresh opportunities for original thought. The prospects were enormous—literally so. The Ptolemaic universe was inherently small: The sphere of stars that enclosed it had to spin once around the earth every day, and if the starry sphere was very big it would have to rotate at such tremendous speed that it might fly apart. But if Copernicus was right, then the very fact that the stars remain in the same place in the sky while the earth moves through its orbit, thereby altering its perspective on the stars, means that the stars must be far away. In this way the Copernican proposal threw back the walls surrounding the solar system, opening up a vast universe beyond.
But the Copernican model was afflicted by two major problems. Since it portrayed the planets as orbiting the sun in perfect circles, it was driven toward complexity and error. Planetary orbits are not circular but elliptical. Trying to predicate planetary motions on circular orbits is like trying to learn how a football bounces by bouncing a basketball. And since physics had not advanced much since the time of the Greeks, advocates of any geocentric model were still stumped by the old objections: If the earth rotates, why don't jumpers land to the west of their starting point and howling easterly winds constantly rake the surface of the planet, especially at the equator, where everything is moving east at a velocity of 1,000 miles an hour?
It fell to two of the leading scholars of the Renaissance to address these problems by correcting flaws in the Copernican cosmology and marrying it to terrestrial physics. Johannes Kepler and Galileo Galilei were both talented writers whose books carried their ideas into the mainstream of intellectual discourse throughout the literate world. Otherwise they were quite different men, one theoretical and solitary, the other experimental and more gregarious.
Kepler Corrects Copernicus
To appreciate the beauty of a scientific law is for most of us an acquired taste, like drinking Scotch or enjoying the music of Alban Berg. And as few become sensitized to science during their formative years, it may well be that connoisseurs of scientific aesthetics are even scarcer than drinkers of MacCallan whisky or devotees of Wozzeck. But for those who want to learn how to value science for its beauty as well as for its accuracy, Kepler's laws are a good place to start.
The first law reveals that the orbits of the planets describe, not perfect circles, but ellipses (i.e., ovals), with the sun located at one focus of each ellipse. This masterful demonstration prompted Immanuel Kant to call Kepler the most acute thinker ever born. Like every other cosmologist up to that time, Kepler had assumed that the planetary orbits must be circular. To arrive at the elliptical hypothesis, therefore, he was required to set aside a fundamental aspect of his own intellectual architecture and that of the society to which he belonged. Upon learning of this bold step, his contemporaries reacted with dismay, criticizing not only his hypothesis but his method, which involved intensive application of more sophisticated mathematics than any astronomer of the day employed. Even his old astronomy professor disapproved. (This was Magister Michael Maestlin, who had introduced Kepler to Copernican cosmology and whom Kepler revered.) Kepler's accomplishment was all the more remarkable in that, by the time he resorted to ellipses, he had already earned an estimable reputation and was pushing 40 years of age, conditions not normally conducive to mathematical innovation. "I have spent so much pains on it that I could have died 10 times," he wrote. The effect was, he recalled, like awakening from sleep to see the light.
The elegance of the first law was not immediately apparent to casual observers, who wondered what difference it made whether planets moved in circles or in rotund ellipses that at a glance did not look all that different from circles. The aesthetic force of Kepler's discoveries emerges more clearly in the second law. It shows that the orbital velocity of each planet increases when it is near the sun and decreases when far away, at just such a rate that the area swept out within its orbit is equal during equal intervals of time. In other words, if one charts the motion of Mars over a period of one month when it is far from the sun, and draws a long, thin triangle connecting the sun with the planet's position at the beginning and end of that month, then draws a fatter triangle inscribing Mars's monthly motion when it is closer to the sun, the areas of the two triangles are equal. The same is true of any orbiting object. This subtle symmetry thrilled Kepler, who compared it to the harmony of contrapuntal music.
Kepler's third law declares that the cube of the semimajor axis (half the long axis) of each planet's orbit is proportional to the square of the planet's orbital period. The third law provided astronomers with a capable tool for mapping the solar system, since it meant that if they knew how long it took a planet to go around the sun—information already available when Kepler was alive—they could deduce the size of its orbit relative to those of the other planets. To measure the actual size of any one planetary orbit is, therefore, to have learned the actual sizes of all the other orbits. Similarly, when one examines planets like Jupiter or Saturn that have many satellites (a word coined by Kepler), measuring the size of one's satellite orbit yields the sizes of the other orbits.
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While Kepler was doing all this, Galileo was repairing some of the deficiencies in the physics of the Copernican theory. Both Kepler and Galileo were revolutionaries who had managed to shake off the ancient belief that pure thinking is superior to the awkward and often messy business of rolling balls down inclined planes, squinting through primitive telescopes, and otherwise interrogating the material world. Albert Einstein wrote encomiums in their honor, stressing their willingness to look for truth in nature, thus overcoming their culture's traditional preference for abstract thought over empirical observation. Kepler, Einstein noted, "had to recognize that even the most lucidly logical mathematical theory was of itself no guarantee of truth, becoming meaningless unless it was checked against the most exacting observations in natural science." Of Galileo he said: "Pure logical thinking cannot yield us any knowledge of the empirical world; all knowledge of reality starts from experience and ends in it. Propositions arrived at by purely logical means are completely empty as regards reality. Because Galileo saw this, and particularly because he drummed it into the scientific world, he is the father of modern physics—indeed, of modern science altogether."
Galileo Explains Why Jumpers Don't Fly Westward
Galileo's most significant contribution to the physics of cosmology came with his insight into the concept of inertia. Aristotle had assumed, and the Western world had come to believe, that the natural tendency of objects is to remain at rest. This certainly seems to accord with experience—a book or a boulder stays in one place unless one expends energy in moving it—and even today the word inertia is commonly taken to mean sluggishness or stasis. Galileo saw that this commonsense assumption was wrong. He pushed wood blocks across a tabletop, then polished the table and the blocks and pushed the blocks again, and pondered the significance of the fact that when there was less friction they traveled farther. He reasoned that if they could be polished perfectly, so that there was no friction, they would keep moving forever. Inertia, he concluded, is not just a tendency of bodies at rest to remain at rest, but also of bodies in motion to remain in motion.
Galileo's counterintuitive insight resolved the basic objection to the Copernican assertion that the earth moves. Jumpers don't fly westward nor do easterly gales constantly blow, because the jumpers and the atmosphere are already moving with the turning earth, and so tend to remain in motion. Today we have seen enough of the universe to know that motion, not rest, is the ordinary state of matter, and that to be immobile is at most a local trait, measured in terms of a local "inertial rest frame." The farther out one looks, the more one finds that everything, relative to most other things, is moving. The universe was born restless and has never since been still.
Galileo's later years were overshadowed by his futile campaign to persuade the Church to replace the Ptolemaic with the Copernican cosmology. In the end he was forced to make a humiliating recantation on his knees before the Inquisition, and he lived out the remainder of his days under house arrest. But the reason his campaign failed was not solely that the authorities in Rome were unwilling to change their ideas. It was also because Galileo, though armed with many powerful arguments from analogy, was never able to present a quantitative defense of the Copernican cosmology. That was accomplished by Isaac Newton.
Newton Provides the Equations
In The Principia, Newton presented equations that accurately predicted the motions of the planets and the rate at which objects fall on Earth, revealing both to be caused by a single force, gravity. In so doing, it vindicated the heliocentric, rotating-Earth cosmology of Copernicus, Kepler, and Galileo, while also uniting the physics of heaven and Earth. Newton's research inaugurated two scientific enterprises that have continued ever since—the progress of physics through the investigation of phenomena both on Earth and beyond, and the mapping of a universe that, though vast, is for some reason accessible to human inquiry.
Substantial progress in mapping the solar system was made during the two centuries following the publication, in 1687, of Newton's Principia. Explorers armed with telescopes and accurate clocks—marine chronometers, developed to enable navigators to determine their longitude and thus avoid blundering into coastlines at night—observed the transits of Venus across the face of the sun in 1761 and 1769 with results that yielded a fairly accurate value for the size of the earth's orbit. This in turn paved the way for measuring the distances to nearby stars by triangulation (the "parallax" method). The first accurate stellar parallax—that of the star 61 Cygni, 11 light-years from Earth—was measured in 1838.
The Rise of Astrophysics
Meanwhile, astronomy advanced from its original, taxonomic phase—in which observers classified celestial objects in something like the way naturalists collected dried plants and stuffed birds by the thousands in the days before Darwin—to mature into astrophysics, a science that not only reports extraterrestrial phenomena but offers plausible explanations for how they work. The change was rather like watching a play in a foreign language one does not speak, only to have the patterns of behavior become explicable in the second act when a translator begins to whisper explanations of what the actors are saying and how they are motivated. Through astrophysics, it became possible to go beyond describing how the sky looks and to begin learning how it got to be that way.
Essential to the rise of astrophysics was the spectroscope, which breaks down light into its constituent frequencies. The most cosmologically significant discovery to be made with the help of the spectroscope came in 1929 when American astronomer Edwin Hubble used it to confirm that most galaxies are rushing away from the Milky Way, and from one another, at rates directly proportional to their distances—the first demonstration that the universe is expanding.
The Big Bang Model is Born
The idea that cosmic space is stretching out, carrying the galaxies with it, is a 20th-century innovation—one that was unanticipated, insofar as I can find, in all the prior scientific literature. Yet curiously, the idea of cosmic expansion emerged in theoretical physics shortly before Hubble found evidence of it in the sky. The groundwork was laid in 1916 by Einstein's general theory of relativity. Researchers studying the theory found that it implied that cosmic space cannot be static but must be either expanding or contracting. Einstein at first resisted this odd idea, but soon found himself obliged to accept the validity of the mathematical reasoning involved. Then in 1929 Hubble, who was not familiar with the theory, independently discovered the expansion of the universe.
CMB Theory Develops, Predictions are Made
The so-called "big bang" model arose from thinking about what an expanding universe would have been like in its infancy. The observable universe today is roughly 15 billion light-years in radius. When its radius was much smaller—only one light-year, say—all the matter in the universe must have been packed together in a lot less space. Any given quantity of matter, compressed to a higher density, gets hotter: That's why a penny, lifted off a railroad track moments after being flattened by a passing train, is hot to the touch, and why compressing air in a bicycle pump heats the air, making the pump warm. So it seems reasonable to imagine that the early universe may have been not only dense, but also hot. Very hot: When the universe was one second old, in this scenario, every spoonful of stuff was denser than stone and hotter than the center of the sun. The expansion and resultant cooling of the universe permitted the formation of atoms, molecules, galaxies, and living creatures. What we call matter is frozen energy. It froze because the universe, owing to its expansion, cooled.
The big bang theory implied that as the young universe expanded there should have come a time, nowadays reckoned at about five hundred thousand years after the beginning, when the primordial plasma thinned out sufficiently to become transparent to light. Physicists call this event photon decoupling, meaning that photons, the particles that constitute light and other forms of electromagnetic energy, were at this point set free. Thereafter they did not often interact with one another, or with matter, but went soaring unhampered through the constantly expanding reaches of cosmic space. Hence most of them should still be around today. Cosmic expansion would have stretched them out, increasing their wavelengths from those of light to the wavelengths we call microwave radio. In microwave frequencies it is convenient to express energy in terms of temperature—as does, say, the instruction manual that accompanies a microwave oven—so another way to reason through this argument is to say that the universe, having once been hot, should remain a bit warm even today.
Physicists theorizing about the existence of this cosmic microwave background, or CMB, calculated that it should have a temperature of about three degrees above absolute zero. They also noted that it would display a "black body" spectrum, as is dictated by the relevant quantum physics equations, and that it should be isotropic, meaning that any observer, anywhere in the universe, should measure the background as having the same temperature everywhere in the sky.
One can think of the CMB as a haze of photons that has permeated space ever since the big bang. As we look far out in space—and, therefore, backward in time, to when the CMB photons were more energetic—we find the haze thickening. At the ultimate distance, where we are peering back into the first million years of time, the haze becomes opaque. Every observer using a microwave radio telescope thus sees the universe as a sphere that is almost transparent nearby but is opaque at its distant and fiery walls.
The Predictions Pan Out
When these predictions about CMB were first made, in the 1940s, they were quickly forgotten. The big bang theory was not yet taken very seriously and there was no such thing as a microwave radio receiver. Then, in 1965, two physicists working with a radio receiver built for communications satellite experiments detected the CMB. Interest mounted as scientists came to appreciate that by studying the CMB they could make direct observation of the universe as it was only half a million years after the beginning of time.
In 1989, the American space agency launched a satellite designed to study the CMB from orbit, where its detectors were free from the interference of Earth's atmosphere. Preliminary findings obtained by the COBE (Cosmic Background Explorer) satellite were announced the following year, and turned out to constitute a stunning confirmation of the big bang model. The CMB is indeed isotropic—that is, it has equal intensity all over the sky, as anything genuinely universal must. And, as expected, its temperature is about three degrees above absolute zero—2.726 degrees, to be exact. And its spectrum conforms to a black body spectrum: The fit is so precise that the researchers making the announcement had to enlarge the size of the error bars on their diagrams: Otherwise the observational data points would have disappeared into the thin, inked line describing the theoretical prediction.
A final triumph for the COBE scientists came in 1992, when an all-sky map, carefully compiled by repeated observations that pushed the sensitivity of the COBE instruments to their limits, confirmed another important prediction of the big bang theory—that matter, though generally distributed uniformly throughout the cosmos, began fairly early to clump into dense regions from which clusters of galaxies were to form. This was good news for theorists who argued that the vast clusters, superclusters, and bubbles of galaxies we see in the universe today formed by gravitational attraction from inhomogeneities in the early universe. The clumps of matter are thought to have originated as quantum fluctuations, microscopic departures from the generally homogeneous distribution of matter in the very early universe. Much remains to be studied about the spectrum and sizes of these inhomogeneities, and how, exactly, they resulted in the large-scale structures we see in the universe today. These findings led most cosmologists to agree that the universe emerged from a hot big bang state.
The Observational Evidence Accumulates
Several other sorts of evidence support the big bang theory, including these:
• The cosmic element abundance fits the predictions of the theory. Here the line of reasoning is that as the primordial fireball cooled, protons and neutrons would have joined up to form the nuclei of atoms. The calculations of the nuclear physicists—who have had a lot of experience in this sort of thing, since similar processes occur in the explosions of thermonuclear bombs—indicate that about a quarter of the atom-making stuff should have been converted into helium in the big bang, along with a bit of lithium, while the remainder survived as hydrogen (the simplest atom, whose nucleus in its rudimentary form consists of a single proton). And this is just what we do find: The universe at large is 25 percent helium and 73 percent hydrogen. The theory postulates that all the heavier elements were forged inside stars, notably in supernovae—exploding stars, which seed space with clouds of debris, enriched with the heavier elements, from which condensed latter-day stars and planets, the earth and the sun among them. If this theory is correct we should find that older stars are poorer in heavy elements than younger stars are. And this, too, turns out to be the case.
• In a big bang universe it ought to be possible to see direct evidence of cosmic evolution by looking out to great distances, since light reaching us from billions of light years away is billions of years old and so reveals what things were like billions of years ago. Such evidence has indeed been found. Much of the bright promise of deep-space astronomy comes from the prospect of directly observing cosmic evolution by using more powerful telescopes as time machines to look at the universe as it was in the distant past.
• The ages of stars fit the age of the universe deduced from its expansion rate—according to some of the data, at least. Several persuasive sets of observations suggest the universe has been expanding for approximately 15 billion years. This accords with the ages of the oldest known stars, estimated by astrophysicists at about 14 billion years. But there are other observations, which some researchers regard as persuasive, that yield a younger universe. If these prove to be correct, then something is wrong either with our understanding of the ages of the oldest stars or with some aspect of the big bang theory.
Concurrence of Other Theories
In addition to arguments based on observations, there are what might be called the theoretical proofs of the big bang scenario. It may seem perverse to speak of using one theory to prove another, since theories normally stand or fall on the verdict of observation and experiment. But facts in themselves are as disorderly as cornflakes without a bowl. In practice, science does a lot of pouring cornflakes from box to bowl—checking not just whether facts fit a given theory, but whether the theories work well together. If we ask in what regard the big bang accords with other well-established theories, we find several answers.
• General relativity has survived a great many experimental tests and seems to be perfectly accurate insofar as one is concerned with making predictions about the behavior of gravity under conditions that currently prevail throughout most of the universe. And general relativity implies that the universe must be either expanding or contracting. So the very fact that we find evidence of cosmic expansion in the sky means that a well-established theory, relativity, supports another more hypothetical one, the big bang.
• Quantum physics, too, finds a gratifying place within the big bang scheme. Using quantum mechanics, physicists are able to predict the existence and spectrum of the cosmic microwave background, calculate how much of the primordial material was turned into helium in the big bang, and estimate the ages of the oldest stars. Quantum physics makes accurate predictions about events involving three of the four fundamental forces of nature—the weak and strong nuclear forces at work in atoms, and electromagnetism, the force responsible for light and radio energy. But there is not as yet a fully accomplished quantum theory of the fourth force, gravity. This would not matter much were the province of physics limited to the contemporary universe: Gravitation is so weak that it can be disregarded when calculating the interactions of subatomic particles, which have such small mass that their gravitational pull on one another is negligible. But in the high-density early universe, subatomic particles weighed so much that their mutual gravitational influence was comparable to their interactions via the other three forces. To reconstruct events thought to have transpired during the very first fractions of a second of cosmic time will require a quantum account of gravity. Such a theory presumably would lay bare a single principle underlying both quantum mechanics and general relativity, which at our present level of understanding are based on contradictory ways of looking at the world.
• The inflationary hypothesis has generated considerable interest in cosmology. It proposes that during a dawning moment of cosmic history the expansion of the universe proceeded much faster than had been thought—indeed, at a rate far greater than the velocity of light. The inflationary hypothesis not only solves several problems that afflicted earlier versions of the big bang theory but indicates that the universe is extremely large, and flings open a door onto the startling speculation that our universe originated as a microscopic bubble arising from the space of an earlier universe, which may in turn be one among many universes strewn like stars across inaccessible infinities of random spaces and times and sets of natural laws.
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To sum up, as the 21st century opens, the big bang theory looks to be in pretty good shape. It is supported by several solid and more or less independent lines of evidence, and has at present no serious rivals. A lot of work remains to be done. Recently, for instance, astronomers have found evidence that the cosmic expansion rate is actually speeding up, rather than slowing down as had been assumed. Theorists speculate that a "dark energy" field is causing the accelerated expansion. If so, the nature of dark energy, as well as dark matter, remains to be adduced.
Nonetheless, if one were asked to make a list of the greatest scientific accomplishments of the century, somewhere on that list—along with relativity and quantum theory, the elucidation of the DNA molecule, the eradication of smallpox and the suppression of polio, the discovery of digital computation, and many other worthy attainments—there would be a place for big bang cosmology.
Timothy Ferris is author of a dozen books on astronomy and physics. For his work in increasing public appreciation of these topics, he has earned the American Institute of Physics prize in science writing and the American Association for the Advancement of Science prize. He is an emeritus professor with the Graduate School of Journalism at the University of California, Berkeley. His most recent book is Seeing in the Dark: How Backyard Stargazers Are Probing Deep Space and Guarding Earth from Interplanetary Peril, which delves into the great discoveries of amateur astronomers and offers an observers' guide. This article was adapted from The Whole Shebang by Timothy Ferris. Copyright 1997 by Timothy Ferris. Reprinted by permission of Simon & Schuster, Inc., N.Y.