The capacity for narrative is, as far as we know, unique to our species, because our species is, as far as we know, the only one that possesses self-consciousness. We see ourselves. Not only do we exist, but we think about our existence. We envision ourselves occupying a context—or, in storytelling terms, a setting: a place and a time. To see yourself as existing in a specific place and at a particular time is to suggest that you have existed and that you will exist in other places and at other times. You know you were born. You wonder what happens when you die.
But it's not just you that you wonder about. You take a walk and look at the stars, and because you know you are taking a walk and looking at the stars, you understand that you are joining a story already in progress. You ask yourself how it all got here. The answer you invent might involve light and dark, water and fire, semen and egg, gods or God, turtles, trees, trout. And when you have fashioned a sufficiently satisfying answer, you ask yourself, naturally, where it all—and you with it—will end. Bang? Whimper? Heaven? Nothing?
These questions might seem to lie outside the realm of physics, and before 1965 most scientists considered cosmology to be mostly that: metaphysics. Cosmology was where old astronomers went to die. It was more philosophy than physics, more speculation than investigation. The fourth member of the Princeton team—the one who didn't make the trip to Bell Labs—would have included himself in the category of cosmology skeptic.
Phillip James Edwin Peebles—Jim to everyone—was all angles.
Tall and trim, he explained himself to the world through his elbows and knees. He would throw his arms wide, as if to embrace every possibility, then wrap them around his legs, as if to consolidate energy and focus—mannerisms not inconsistent with a man of conflicting sensibilities, which was how Jim Peebles saw himself. Politically he called himself a "bleeding-heart liberal," yet scientifically he identified himself as "very conservative," even "reactionary." He had learned from his mentor, Bob Dicke, that a theory can be as speculative as you like, but if it doesn't lead to an experiment in the near future, why bother? On one occasion (before he knew better), Peebles had mentioned that he might try to reconcile the two great physics theories of the twentieth century, general relativity and quantum mechanics. "Go find your Nobel Prize," Dicke answered, "and then come back and do some real physics."
Cosmology, to Peebles, was not real physics. It was a reversion to how scientists did science in the two millennia before there were scientists and science as we know them. Ancient astronomers called their method "saving the appearances"; modern scientists might call it "doing the best you could under impossible circumstances." When Plato challenged his students, in the fourth century B.C., to describe the motions of the celestial bodies through geometry, he didn't expect the answers on paper to represent what was actually happening in the heavens. That knowledge was unknowable because it was unattainable; you couldn't go into the sky and examine it for yourself. What Plato wanted instead was an approximation of the knowledge. He wanted his students to try to find the math to match not the facts but the appearances.
One student, Eudoxus, arrived at an answer that, in one form or another, would survive for two thousand years. For mathematical purposes he imagined the heavens as a series of nesting, concentric, transparent spheres. Some of these spheres carried the heavenly bodies. Others interacted with those spheres to retard or accelerate their motions, in order to account for the appearance that the heavenly bodies all slow down or speed up throughout their orbits. Eudoxus assigned the Sun and the Moon three spheres each. To each of the five planets (Mercury, Venus, Mars, Saturn, Jupiter) he assigned an extra sphere to accommodate the appearance that they sometimes briefly reverse their motions against the backdrop of stars, moving west to east from night to night rather than east to west.* And then he added a sphere for the realm of the stars. In the end his system consisted of twenty-seven spheres.
Another student of Plato's, Aristotle, amended this system. He assumed the spheres were not just mathematical constructs but physical realities; to accommodate the mechanics of an interlocking system, he added counterturning spheres. His total: fifty-six. Around A.D. 150, Ptolemy of Alexandria assumed the task of compiling the existing astronomical wisdom and simplifying it, and he succeeded: His night sky was overrun with only forty spheres. The math still didn't match the appearances exactly, but close enough was good enough—as good as it was ever going to get.
Today, the 1543 publication of De revolutionibus orbium coelestium ("On the Revolutions of the Heavenly Spheres"), by the Polish astronomer Nicolaus Copernicus, is synonymous with the invention of a new universe: the Copernican Revolution. It has become a symbol of defiance against the Church's teachings. But it was the Church itself that had invited Copernicus to come up with a new math for the motions in the heavens, and it had done so for a sensible reason: The appearances once again needed saving.
Over the centuries the slight inconsistencies in the Ptolemaic version—the areas where the math departed from the motions—had led to a gradual drift in the calendar, until seasons diverged from their traditional dates by weeks. Copernicus's work allowed the Church to reform the calendar in 1582, incorporating his math while dispensing with the notion of a Sun-centered universe. Like the ancients, Copernicus wasn't proposing a new universe, either physically or philosophically. Instead, he was formulating a new way to "save the appearances" of the existing universe. The true motions of that universe, however, were out of reach, had always been out of reach, and would always be out of reach.
And then, they weren't. In 1609, the Italian mathematician Galileo Galilei found new information about the universe at his fingertips—literally, thanks to the invention of a primitive telescope. Look, he said, leading the elders of Venice up the steps of the Campanile in the Piazza San Marco in August 1609 to demonstrate the benefit of fitting a tube with lenses: seeing farther. Look, he said barely six months later, in his pamphlet SidereusNuncius ("Starry Messenger"), heralding a new lesson: Seeing farther means seeing not just more of the same—a fleet of rival merchants or the sails of an enemy navy—but seeing more, period. That autumn, Galileo had trained his tube of longseeing on the night sky and had begun a lengthy program of discovering celestial objects that no other person had ever seen: mountains on the Moon, hundreds of stars, spots on the Sun, satellites of Jupiter, the phases of Venus. The invention of the telescope—the first instrument in history to extend one of the human senses—changed not only how far we could see into space, or how well. It changed our knowledge of what was out there. It changed the appearances.
Here was evidence that corroborated the central tenets of Copernicus's math—that Earth was a planet, and that it and all the other planets orbited the Sun. But just as important, here was evidence —the tool of the scientific method. Seeing farther didn't have to mean seeing more. The night sky might not have held more objects than met the naked eye. And we still couldn't go into the sky and see for ourselves how its motions worked. But we could examine the heavens closely enough to find not only the appearances but the facts.* And facts needed not saving but explaining.
In 1687 the English mathematician Isaac Newton provided two of those explanations in Philosophic Naturalis Principia Mathematica ("Mathematical Principles of Natural Philosophy"). He reasoned that if Earth is a planet, then the formulae that apply in the terrestrial realm must apply in the celestial as well. Building on the mathematical work of Johannes Kepler and the observations of Galileo and his successors in astronomy, he concluded that the motions of the heavens require not dozens of spheres but a single law: gravitation. In 1705 his friend and sponsor Edmond Halley applied Newton's law to past observations of comets that had appeared in 1531, 1607, and 1682 to make the claim that they were one comet and that it* would return in 1758, long after his own death. It did. No longer would the math have to accommodate the motions of the heavens. Now the heavens had to accommodate the math. Take Newton's law of universal gravitation, apply it to the increasingly pr
ecise observations you could make through a telescope, and you had a universe that was orderly and predictable and, on the whole, unchanging—a cosmos that ran, as the most common metaphor went, like clockwork.
In the more than three and a half centuries between Galileo's climb up the Campanile and the phone call from Crawford Hill, the catalogue of the universe's contents seemed to grow with every improvement of the telescope: more moons around planets; more planets around the Sun; more stars. By the early twentieth century, astronomers had determined that all the stars we see at night, whether with our naked eyes or through telescopes, are part of one vast collection of stars, numbering in the tens of billions, that we long ago named the Milky Way because it seems to spill across the night sky. Did other vast collections of stars, each numbering in the tens of billions, exist beyond the Milky Way? A simple extrapolation from the earlier pattern of discovery raised the possibility. And astronomers even had a candidate, a class of celestial objects that might qualify as "island universes" all their own.
In 1781, the French astronomer Charles Messier had published a catalogue of 103 celestial smudges—blurry objects that he feared would distract astronomers looking for comets. Astronomers could see that several of those 103 smudges were bunches of stars. As for the others, they remained mysteries, even as the quality of telescopes improved. Were these nebulous objects clouds of gas in the process of coalescing into yet more stars within our system? Or were the nebulae vast collections of tens of billions of stars separate from but equal in magnitude to our own vast collection? The astronomy community split on the question, and in 1920 two prominent astronomers conducted a so-called Great Debate at the National Museum of Natural History in Washington, D.C., to present the pros and cons of each argument.
Three years later, the American astronomer Edwin Hubble did what debate alone couldn't do: resolve the question through empirical evidence. On October 4, 1923, using the largest telescope in the world—the new 100-inch* on Mount Wilson, in the hills outside Pasadena—he took a photograph of the Great Andromeda Nebula, or M31 in the Messier catalogue. He thought he noted a "nova," or new star, so he returned to M31 the following night and took another photograph of the same spiral arm. When he got back to his office, he began comparing the new plates with other photographs of the nebula on a number of different dates and found that the nova was actually a variable, a kind of star that, as its name suggests, varies: It pulsates, brightening and dimming with regularity. More important, it was a Cepheid variable, the kind that brightens and dims at regular time intervals. That pattern, Hubble knew, could resolve the debate.
In 1908, the Harvard astronomer Henrietta Swan Leavitt had discovered a proportional relationship between the pulsation period of a Cepheid variable and its absolute brightness: the longer the period, the brighter the variable. Astronomers could then take that measure of luminosity and match it with another quantifiable relationship, the one between luminosity and distance: A source of light that's twice as distant as another source of light with the same luminosity appears to be one-fourth as bright; a source of light three times as distant appears to be one-ninth as bright; a source of light four times as distant would be one-sixteenth as bright; and so on. If you know how often a variable pulsates, then you know how bright it is relative to other variables; if you know how bright it is relative to other variables, then you know how distant it is relative to other variables. When Hubble compared the pulsation period of the Cepheid variable he'd found in M31 with the pulsation periods of other Cepheid variables, he concluded that the variable was at sufficient distance that it (and therefore its host nebula, M31) lay beyond the "island universe"—or, as we would now have to think of it, our island universe.
Hubble went back to H335H, the photographic plate he made on October 5, and in posterity-radiant red he marked the variable star with an arrow, along with a celebratory "VAR!" He declared M31 an island universe all its own, and in so doing, he added to the cosmic canon one more more: galaxies.
Newton's clockwork universe began to come apart in 1929. After his "VAR!" breakthrough, Hubble had continued to investigate "island universes," especially some inexplicable measurements of them that astronomers had been making for more than a decade. In 1912 the American Vesto Slipher began examining the nebulae with a spectrograph, an instrument that registers the wavelengths from a source of light. Much like the sound waves of a train whistle as the train approaches or departs from a station, light waves are compressed or stretched—they bunch up or elongate—depending on whether the source of the light is moving toward you or away from you. The speed of the light waves doesn't change; it remains 186,282 miles (or 299,792 kilometers) per second. What changes is the length of the waves. And because the length of the light waves determines the colors that our eyes perceive, the color of the source of light also seems to change. If the source of light is moving toward you, the waves bunch up, and the spectrometer will show a shift toward the blue end of the spectrum. If the source of light is moving away from you, the waves relax, and the spectrometer will show a shift toward the red end of the spectrum. And as the velocity of the source of light as it moves toward you or away from you increases, so does the blueshift or redshift—the greater the velocity, the greater the shift. Slipher and other astronomers had shown that some of the nebulae were registering significant redshifts, suggesting that they were moving away from us at great velocities. Now that Hubble knew these nebulae were galaxies, he wondered what these motions might mean. He found out when he compared the velocities of eighteen of these nebulae with their distances: The two measurements seemed to be directly proportional to each other—the farther the galaxy was, the faster it appeared to be receding. In other words, the universe might seem to be expanding.
Suddenly the universe had a story to tell. Instead of a still life, it was a movie. And like any narrative, the story of the universe now had not only a middle—the present, swarming with galaxies fleeing one another—but the suggestion of a beginning.
Precisely—precisely —at this point, at least from the perspective of a philosophically cautious sort like Jim Peebles, cosmology departed from science, passing from math to myth. You couldn't know how the universe began because the evidence was out of reach, just as it had been for Aristotle, Ptolemy, and Copernicus. They couldn't go across space to retrieve it; you couldn't go into the past. All you could do was observe the present phenomena—these redshifted galaxies—and try to find the math to accommodate their motions. All you could do was try to save the appearances, if that was your idea of science.
Hubble himself, as an observer, hoarding evidence and leaving the theorizing to the theorists, preferred to remain agnostic as to whether the universe really was expanding or whether another interpretation might explain the apparent correlation. But some theorists couldn't resist the challenge of rewinding the film. The Belgian priest Georges Lemaitre, a physicist and astronomer, imagined the expansion unreeling in reverse, the size of the universe shrinking, smaller and smaller, the galaxies rushing back together, faster and faster, until the infalling matter would reach a state that he called the "primeval atom" and that other astronomers would come to call a "singularity": an abyss of infinite density and incalculable mass and energy.
But words such as "infinite" and "incalculable" aren't of much use to mathematicians, physicists, or other scientists. "The unrestricted repeatability of all experiments is the fundamental axiom of physical science," Hermann Bondi and Thomas Gold, two Austrian expatriates living in Britain, wrote in the first line of a paper they submitted in July 1948 that outlined an alternative to Lemaître's theory. The following month, their friend Fred Hoyle, a British astronomer, submitted his own variation on this theme. Rather than a big bang—the term Hoyle applied, during a BBC radio broadcast in March 1949, to the idea of a universe expanding* from, as he wrote in his paper, "causes unknown to science"—they postulated a steady state. Through "continuous creation of matter," Hoyle wrote, "it might be possible to obtain a
n expanding universe in which the proper density of matter remained constant." Over the course of cosmic history, the creation of even infinitesimal amounts of matter could become cumulatively significant. Such a universe wouldn't have a beginning or an end; it would just be.
For many astronomers, however, "continuous creation" was no more appealing than a "singularity." Both the Big Bang and Steady State theories seemed to require a leap of faith, and faith not being part of the scientific method, there they let the matter rest.
But what if there was evidence for one theory or the other?
Bob Dicke asked Jim Peebles this question one sweltering evening in 1964. Peebles had arrived at Princeton as a graduate student six years earlier. At the University of Manitoba he had been the top student in physics, winning academic honor after honor. At Princeton he was shocked at how much physics he didn't know. He spent his first year trying to catch up, and then one day some friends invited him to a get-together that Dicke ran most Friday evenings in the attic of Palmer Physical Laboratory. The Gravity Group was an informal gathering of a dozen or so undergraduates, graduates, postdocs, and senior faculty—"Dicke birds," they called themselves. Peebles went, and then he went back. He began to understand that here, in a sometimes-stifling setting at an inconvenient hour, he could get an education: eating pizza, drinking beer, and trying to figure out how to rehabilitate general relativity.
General relativity had been around for nearly half a century; Einstein had arrived at the equations in late 1915. Whereas Newton imagined gravity as a force that acts across space, Einstein's equations cast gravity as a property that belongs to space. In Newton's physics, space was passive, a vessel for a mysterious force between masses. In Einstein's physics, space was active, collaborating with matter to produce what we perceive as gravity's effects. The Princeton physicist John Archibald Wheeler offered possibly the pithiest description of this co-dependence: "Matter tells space how to curve. Space tells matter how to move." Einstein in effect reinvented physics. Yet by 1940 Dicke could ask a professor of his at the University of Rochester why the graduate physics curriculum didn't include general relativity, and the answer was that the two had nothing to do with each other.
The 4-Percent Universe Page 2
