Quantum Legacies: Dispatches From an Uncertain World
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Gravitation narrowly escaped the pigeonhole of library-only reference work and went on to sell tens of thousands of copies. The book received extensive analysis and review in physicists’ specialist journals, even as it inspired passion—even ecstasy—among journalists and nonspecialist readers. Somehow this hulking book, stuffed to overflowing with equations so complicated they required multiple typefaces and elaborate marginal notes, exerted broad, crossover appeal. For generations, the book—like Einstein’s elegant theory at its core—has inspired alluringly large questions. Why are we here? What is our place in the cosmos?
I still cherish my own copy of the original edition of MTW, which I picked up at a used-book store during graduate school. By now the binding has all but disintegrated, the pages are a bit yellowed, and whole sections threaten to fall free. Yet I can’t bring myself to toss it, even though Princeton University Press recently published a lovely, sturdy reprint edition.30 Today the two editions sit side by side on my bookshelf, together hogging more than six inches of prime shelf space. When I want to double-check how those world-class teachers broached a particularly subtle subject, I grab the new edition. But when I want to feel—or at least imagine—some more direct connection with the generations of students and seekers who came before me, I reach for the old, with both hands.
16
The Other Evolution Wars
Will tomorrow be just like today? What about ten million tomorrows from now, or a billion yesterdays ago? Has the universe always been more or less as we observe it now, or is it cosmic destiny—as it is for people—to change and evolve over time? The question is hardly new. Plato reported that Heraclitus, before him, had observed that “all things pass and nothing stays,” a stance conveyed in the gnomic (and oddly tweetable) phrase “you cannot step twice into the same river.”1
Einstein, too, wondered about the nature of cosmic change. He focused on the question with fresh intensity soon after arriving at his general theory of relativity. With his new equations in hand, he began to wonder how his key insight about gravitation as the warping of space and time might play out not just for this or that local phenomenon—planets whirling around the Sun, the path of light rays bending as they approach a large mass—but for an entire universe at large. By design, his first models were simple, assuming, for example, that all the mass and energy within an imaginary universe were spread evenly throughout space. By 1917 he had launched a new scientific specialty, soon dubbed “relativistic cosmology”: an attempt to describe whole universes within the rubric of general relativity.2
Right away, Einstein learned—not always happily—that other colleagues held strong ideas of their own. Several put Einstein’s equations to work in ways that he found not just wrongheaded but repugnant; their cosmic visions seemed so much at odds with his own. Since that time, as younger generations have sought to sharpen and refine the earlier ideas, they, too, have found themselves sparring with each other, their debates shot through with a strange mix of mathematics, aesthetics, and religion. In our own time just as in Einstein’s, questions about cosmic evolution remain entrenched within still harder questions about how we can know—about the universe and our place within it—and who gets to say.
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One of the first scientists to seek insights into how general relativity might describe the universe as a whole was a young Russian mathematical physicist, Alexander Friedmann. Friedmann was a native of Saint Petersburg. When he first encountered Einstein’s relativity, soon after the end of the First World War, his city had been renamed Petrograd, in honor of the eighteenth-century tsar Peter the Great; by the time Friedmann produced his most ambitious cosmological work, early in 1924, the Bolsheviks had rechristened the city Leningrad. Amid such tumult, perhaps it is not surprising that Friedmann’s forays into relativistic cosmology emphasized change over time, even anticipating violent disruptions. In a series of succinct papers, Friedmann demonstrated that Einstein’s equations could describe universes that evolved, expanding from a minute speck to supergalactic scales. In some cases, Friedmann showed, a universe could halt its expansion and collapse back on itself. The determining factor—the quantity that would fix these cosmic destinies—was the average amount of matter and energy per volume. Overdense universes would bloat but then break; underdense universes would continue expanding forever.3
Einstein liked these options not one bit. He loathed the idea of cosmic expansion, or indeed of any large-scale evolution. Such changes over time lacked a certain aesthetic satisfaction, Einstein complained; he preferred the pristine symmetry of a universe that always was and always shall be. Even as Einstein was dismissing Friedmann’s solutions, however, a young Belgian mathematical physicist, Georges Lemaître, was busy reproducing them. In addition to the allure of Einstein’s equations, Lemaître was inspired by recent astronomical observations spearheaded by the American astronomer Edwin Hubble. Working in southern California with some of the world’s largest telescopes, Hubble and his colleagues had found an intriguing relationship between how far away objects seemed to be from Earth and how quickly they were moving still farther away. Lemaître drew the conclusion even before Hubble did: our universe seemed to be expanding, distant objects moving ever farther apart from other objects over time. In 1931, Lemaître pressed further, arguing that if the universe is expanding today, then it must have been smaller in the past. Some finite time ago, all the matter in the universe must have been concentrated at a single point. The universe, Lemaître concluded, must have begun in a very hot, dense state—he called it the “primeval atom”—and has been expanding ever since.4
Figure 16.1. Albert Einstein (right) meets with Georges Lemaître (center) at the California Institute of Technology in the 1930s, joined by Caltech president Robert A. Millikan (left). (Source: Bettmann, courtesy of Getty Images.)
Alongside his studies of physics and mathematics, Lemaître had pursued his other great passion: theology. He was ordained as a Catholic priest in 1923, and at least in these early days, his cosmology and theology seemed well integrated. In an early draft of his article on the primeval atom, he rhapsodized, “I think that everyone who believes in a supreme being” would be “glad” to see such congruence between science and religion. He struck out this passage just prior to publication—perhaps recognizing that articles in Nature rarely invoked God—and thereafter argued strongly against mixing theology and cosmology. He was an especially outspoken critic of biblical literalism, returning time and again to a position that Galileo had spelled out in his famous letter to Grand Duchess Christina back in 1615: the Bible teaches us how to go to Heaven, not how the heavens go.5
Lemaître’s newfound care notwithstanding, several of his colleagues continued to advise caution—not just about mixing science and religion but about believing in a universe that had a beginning and has been evolving ever since. To some, such a scenario smacked too much of the Genesis account. Arthur Eddington—devoted Quaker, giant of British astrophysics, and onetime teacher of Lemaître’s—responded that a universe that had a beginning in time might be physically possible but “philosophically it is repugnant to me.” Richard Tolman, an accomplished physical chemist at the California Institute of Technology and one of the earliest champions of relativistic cosmology within the United States, went further still. He cautioned his colleagues in 1934, “We must be especially careful to keep our judgments unaffected by the demands of theology and unswerved by human hopes and fears.”6
Many of these early cosmologists became best-selling authors. In their popular books, they freely debated each other’s conclusions—scientific, aesthetic, religious, and otherwise. Yet their discussions received little pushback from nonscientists at the time. Just when “evolution” of a different sort was on many people’s minds—with the Scopes trial of 1925 inspiring sensational coverage of Darwin’s theory of biological evolution and forcing hard questions about whether such ideas could be taught in Tennessee’s public schools—the cosmologists’ evolutionar
y musings inspired more laughter than fear or anger.7 Consider these snippets of advice from the New York Times: “Einsteinism: just ignore it as of no concern to us” (1923); readers should file modern physics under “things you needn’t worry about just yet” (1928); modern cosmologists were just as quaint as medieval theologians counting the number of angels who can sit on the head of a pin (1931); and modern physics failed to answer life’s most important questions (1939). While Einstein and his colleagues battled over the idea of an evolving cosmos, few outside their circle felt compelled to weigh in.8
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Soon after the Second World War, a trio of physicists working in the United States returned to Lemaître’s ideas, now armed with new ideas and data about nuclear physics—data obtained from the wartime Manhattan Project, from the postwar efforts to design hydrogen bombs, and from ongoing work with nuclear reactors. The moving force behind the new work was Russian émigré George Gamow, who had first learned Einstein’s general relativity from Alexander Friedmann in the 1920s. Joining Gamow were Robert Herman and Ralph Alpher, two young physicists at the Johns Hopkins Applied Physics Laboratory. The laboratory had been established in 1942 to work on military projects, such as the proximity fuze. Most of the projects after the war were missile projects for the Navy.9 Immersed in details about defense systems, Alpher and Herman managed to reserve a portion of their time to think with Gamow about the cosmic and otherworldly. In particular, they worked hard to flesh out Lemaître’s picture of a universe beginning in time and evolving through various stages. Their main question: where did the elements come from?10
Figure 16.2. In this composite image, George Gamow’s face emerges from a bottle of “Ylem,” an ancient Greek word meaning “substance.” Gamow and his colleagues Robert Herman (left) and Ralph Alpher (right) used the word to refer to the primordial mixture of protons and neutrons from which heavier nuclei might have formed soon after the big bang. (Source: Public domain.)
Their answer, which formed the basis of Alpher’s dissertation under Gamow’s direction in 1948, came to be known as “nucleosynthesis.” Gamow and company calculated that in the earliest moments after the beginning of the universe, ambient temperatures would have been unimaginably high. Energetic photons (individual particles of light) from the hot surroundings would each carry so much energy that they would blast apart nuclear particles, such as neutrons and protons, when those particles began to stick together. That is, the photons’ energy would overwhelm the binding energy of the strong nuclear force, which would otherwise make the nuclear particles form atomic nuclei. As the universe expanded, however, it would cool, just like the gas inside an expanding balloon. At a calculable moment—roughly one second after the beginning—the force of nuclear attraction would begin to win out over the less-energetic photons, and neutrons and protons would begin to stick together in stable deuterium nuclei. As the universe continued to expand and cool, additional nuclear particles would glom on to these light nuclei, building up the heavier elements.
Never one to be shy about his findings, Gamow trumpeted his group’s work in playful terms. He wrote to Einstein about this new account of “the Days of Creation” and titled one of his popular books The Creation of the Universe (1952), echoing the biblical term. Late in 1951, Pope Pius XII delivered a lecture before the Pontifical Academy of Sciences. Impressed by the easy fit between Gamow’s developing model and scriptural accounts, the pope declared that the physicists’ work “invokes no new ideas even for the simplest of the faithful. It introduces nothing different from the opening words of Genesis, ‘In the beginning God created heaven and earth. . . .’” To Gamow—an inveterate jokester—the pope’s offering seemed too good to be true. Three months later he submitted a brief article to the Physical Review in which he quoted extensively from the pope’s lecture, citing it as an authority for his latest research.11
Fred Hoyle, a British astrophysicist based at Cambridge University, did not find Gamow’s pranks amusing. Hoyle had learned his general relativity during the 1930s from Arthur Eddington, and he, too, sought to build a coherent cosmological model soon after the war. Together with the Austrian transplants Hermann Bondi and Thomas Gold—each of whom had left the Continent to study at Cambridge before the Nazis overran Austria—Hoyle developed a rival cosmology to Gamow’s. Hoyle, Bondi, and Gold argued that all astronomical observations to date could be accounted for by a steady-state universe. In their model, the universe had no beginning in time, and it has always been expanding. That second part—an eternal, nonstop expansion—could be possible, they reasoned, if a trace amount of new matter was constantly being created throughout space, far less than could be detected experimentally. If a tiny little bit of new stuff was constantly being made, then on average the universe would look the same at any given moment; there would be no evolution. In place of early-universe nucleosynthesis, Hoyle and company hypothesized that all atomic nuclei were cooked inside stars and then dispersed throughout the cosmos when heavy stars blew up in supernova explosions.12
More than physics seemed to be at stake. Hoyle spoke out vigorously against any theological incursions into physics. In his 1950 popular book, The Nature of the Universe, based on a series of radio lectures for the British Broadcasting Company, Hoyle charged that the very notion of a universe beginning in time was “quite characteristic of the outlook of primitive peoples,” who turn to gods to explain physical phenomena. Ironically, Hoyle himself coined the term “big bang” to describe Gamow’s program during these radio lectures; it was meant to sound childish and dismissive. Moreover, Hoyle and his colleagues insisted, physicists had little warrant to trust physical laws like general relativity and the behavior of nuclear forces when extrapolated so far from the regimes in which they had been tested. Big-bang advocates who stuck doggedly to such calculations, Hoyle charged, behaved just like Catholics and Communists: both, he said, were blind believers, too easily swayed by dogma.13
Although most physicists ignored Gamow’s and Hoyle’s colorful exchanges at the time, popular news media did cover the debate. Where once the New York Times had playfully chided physicists and cosmologists for their utter irrelevance, few drew the same conclusions after the Second World War. In the wake of wartime projects like radar and the atomic bomb, physics filled a special—indeed unprecedented—cultural niche, especially in the United States. In fact, the overwhelming majority of times that the phrase “big bang” appeared in major newspapers during the 1940s and 1950s, it referred not to Gamow’s cosmology but to nuclear weapons testing and Cold War brinksmanship with the Soviet Union.
Perhaps the intertwining of cosmology, nuclear physics, and geopolitics explains another curious reaction to the postwar research. Though the postwar decades saw a resurgence in the United States of strident opposition to Darwinian evolution by evangelical Christians, few but the most hard-core biblical literalists rose to challenge the physicists’ ideas about the big bang. Even the most influential advocates of “creation science” after the war drew distinctions between the age of the Earth—which they took to be roughly six thousand years old, as required, in their view, by the book of Genesis—and the age of the universe at large. John Whitcomb and Henry Morris’s runaway best seller The Genesis Flood (1961), for example, argued that the biblical account of creation applied to the solar system but not to the entire cosmos.14 Even as they argued passionately against standard geology and biology, these prominent creationists issued physics and cosmology a free pass. In the nuclear age, physics seemed untouchable.
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Scientific consensus in favor of the big bang coalesced during the mid-1960s. Several new astrophysical observations lent strong support to the model but seemed irreconcilable with a steady-state universe.15 Yet few in the field had time to take a victory lap. Before long, a whole new set of challenges emerged. A small group of physicists began to wonder if Einstein’s framework for all these cosmological discussions—general relativity—was itself only an approximation. What if the
universe was built, not out of point-like particles dancing on a curved spacetime, but out of tiny, one-dimensional “strings”? In that case, general relativity would be no more fundamental than Newton’s physics: each would be a useful approximation, accurate for certain types of applications but not truly a law of nature. And if that was the case, then physicists would need to rethink relativistic cosmology.
String theory remained a marginal curiosity in the field for several years before bursting onto center stage in the mid-1980s. In a swift series of developments, later dubbed the “first string revolution,” two groups of mathematical physicists demonstrated that string theory might offer a way to make gravity compatible with quantum theory, while avoiding many of the traps that had marred earlier efforts to combine the two. Such unification had long been the holy grail of theoretical physics: only a quantum theory of gravity could possibly be merged with the other known forces of nature, each of which is clearly quantum-mechanical in origin. String theory’s champions began to proclaim it a “theory of everything.”16
Today, string theory is undeniably at the forefront of high-energy physics. Since the early 2000s, physicists have routinely published two thousand articles or more on the topic each year. Yet as physicist Lee Smolin has argued, string theory is something of a “package deal”: it combines many features that physicists want with others that are far less desirable. (Smolin compares the situation to the challenge of buying a new car: the sunroof you like comes packaged with a fancy stereo system you don’t really need.) For starters, string theory requires an as-yet-undetected symmetry among the known particles, known as “supersymmetry.” More troubling, the theory can only be formulated in ten spacetime dimensions, rather than the four in which we seem to live: one dimension of time, plus the three spatial dimensions of length, breadth, and height. Worst of all, at least according to some critics, string theory leads to a huge number of possible universes, with (as yet) no way to choose between them. It has gone, in effect, from a “theory of everything” to a “theory of anything.”17