In obituaries and biographies following Hoyle's death in 2001 at the age of eighty-six, writers seeking balance pointed out that Sir Fred Hoyle was one of the greatest astrophysicists and astronomy popularizers of the twentieth century. He captured the public's imagination in his evocative BBC radio broadcasts on astronomy and in bestselling popular-science and science-fiction books. He was the symbol of cosmic thinking for a generation, inspiring legions of budding astronomers. But Hoyle's professional career, from his very first professional presentation, was characterized first by controversy and later by outright conflict.
If you've never heard of Sir Fred, it's in large part because of what he was most famously wrong about: the origins of the universe. It was during one of his BBC broadcasts in 1949 that Britons gathered around their radios first heard of the “big bang”—a term the articulate Hoyle coined to describe the erroneous, he ventured, idea of a singular eruptive start to time and space in the distant past. Hoyle imagined, and for almost half a century championed, the dominant alternative twentieth-century story of the nature of the universe, one that envisioned an eternal cosmos. What's lost in this tale is that what Fred Hoyle was right about, and what he championed alone against great opposition, is just as central to the story of our cosmic origins. Our knowledge of the original cosmic birthplace of every atom in our bodies owes more to this straight-talking Yorkshireman than to any other person who ever lived. In fact, if there was a prophet of the Stardust Revolution, one who saw in the details a larger story of a living cosmos, it was Fred Hoyle. It is his story that carries us to the stellar origin of the elements.
From the time he entered Cambridge University as a math and physics student in 1933, Fred Hoyle—stocky, with curly black hair and a genial, forthright nature—dreamed of making a big find. He'd come from working-class origins; his father survived the trenches in World War I, but Hoyle defined himself not by where he came from but rather by his deep curiosity, tenacity, and intelligence. He wanted to discover something essential about the world, such as Newton's laws of motion or Einstein's general relativity. Upon his graduation in 1938, however, it looked like the door was slamming on Hoyle's dream before he'd even got started. He faced a problem of timing. He'd arrived at Cambridge at the tail end of the golden age of the quantum atom. A previous generation of atomic physicists, led by Niels Bohr in Copenhagen, had circumnavigated the weird and wonderful world of the quantum atom—its dual wave-particle nature, its precise energy states, and probabilistic nature.
On that very campus in 1932, Ernest Rutherford had discovered the neutron, rounding out the three-member ensemble of main atomic particles: a core of protons and neutrons surrounded by a jumping quantum field of electrons. Hoyle's thesis adviser, Paul Dirac, was part of this quantum coterie, adding key insights into the quantum nature of electrons. The positive side of this state of affairs is that Hoyle had been steeped in one of the greatest intellectual environments in Cambridge's four-hundred-year history. He'd learned quantum mechanics from Dirac and relativity from Eddington. But the bad news came from Dirac, when Hoyle sought direction about his future in science. Dirac told his student that the best berries had been picked in the field of quantum physics, and that the field would be fallow for decades to come. Another young scientist might have beetled away at secondary problems in quantum physics, content to eat the table scraps from the prior intellectual feast. Not Hoyle; if quantum physics was mined of big finds, then there were revelations to be uncovered elsewhere. He considered biology and astronomy and serendipitously landed on the latter. For in astronomy, young Hoyle's timing couldn't have been better.
During World War II, Hoyle was assigned to the intense effort to improve naval radar. But at the back of his mind were the stars. In 1944, at a conference on naval radar during his first trip to the United States, Hoyle went scientifically AWOL, taking the opportunity to visit America's leading astronomers and astrophysicists. At Princeton, he discussed the lives of stars with the preeminent American astronomer Henry Norris Russell, whose work had shown the fundamental relationship between a star's temperature and its energy output. Across the continent, at Mount Wilson, Hoyle spent hours intently listening to Walter Baade, a German refugee who'd used the wartime blackouts over Los Angeles to take the most detailed images yet of distant galaxies. Baade regaled Hoyle with his revolutionary observation of a massive star's explosive death—a supernova. Back in Cambridge at war's end, Hoyle took the stories from his travels and began to develop a radical new view of stars—the origins of not only heat and light but also of the elements. Tying together his knowledge of atomic physics and astrophysics, he began to develop a detailed view of stars as gradually evolving nuclear cauldrons, burning one nuclear fuel and then another, on a one-way path to their own deaths.
If there's a case study for great minds looking at the same information and coming to very different conclusions, it is the one concerning the cosmic origin of the elements. Fred Hoyle and George Gamow had both looked at Goldschmidt's data on the relative abundances of the elements, considered the same nuclear reactions and the particular temperatures and pressures involved, and even consulted the same scientific colleagues. While Gamow, in Washington, DC, was envisioning a singular birth for the elements, on the other side of the Atlantic, Hoyle was envisioning a radically different ongoing process of elemental creation. Within months of Gamow's 1946 paper, Hoyle published the first version of his view of the creation of the elements, a paper titled “The Synthesis of the Elements from Hydrogen.” He acknowledged that transmuting hydrogen atoms into iron required temperatures hundreds of times hotter than normal stars. But, he said, there were places in the cosmos that were just right for cooking up the elements: massive exploding stars. If stars were static, their temperature remained static. But Hoyle took to heart what astronomers were observing—stars had lives. They died. And Hoyle calculated that as a dying star collapsed and exploded as a supernova, its temperature would skyrocket, sparking a flurry of nuclear reactions.
Critically, noted Hoyle, on the basis of his quantum mechanical calculations, this chain of supernova fusion reactions would reach a final peak at iron before plummeting to lower levels of element production, just as seen in Goldschmidt's table of elemental abundances. As elements grow larger, the repulsive force between their positively charged nuclei makes it increasingly difficult to squeeze them together. Iron, element 26, is the last element of the periodic table whose creation via fusing two smaller nuclei together releases energy. Fusing two atoms of iron requires, rather than releases, energy. Thus, what has come to be known as the “iron peak” is a critical juncture in the way stars make elements. The twenty-five elements before iron on the periodic table fuel stars, whereas those after iron rely on stellar energy to forge them.
For the first three months of 1953, Hoyle returned to Caltech and Mount Wilson as a visiting astrophysics lecturer, though he was still a voice in the wilderness when it came to his view of the origin of the elements. But he'd arrived in California chewing on an idea that soon solidified into a possible solution to the 5 and 8 gap, which had tripped up Gamow's big-bang-origin-of-elements scenario. Hoyle believed he knew how atoms in stars were maneuvering around this atomic gap to make the next element along the periodic table: carbon. And he knew the one man who could prove his conjecture: William Fowler. By this time, Fowler was the acknowledged master of experimental astrophysics, of mimicking in the lab the processes going on in stars. Fowler had made his mark early by helping Hans Bethe calculate the rate at which carbon and nitrogen in stars catalyzed the fusion of hydrogen into helium. Now Hoyle needed Fowler's help. When he arrived at Fowler's Caltech office, what made Fred Hoyle's request preposterous was that he'd shown up not just with the outline of an atomic process to cook up carbon in stars, but he also came with the exact energy required to do it, 7.65 mega electron volts. Speaking in his strong Yorkshire accent, Hoyle excitedly told Fowler and his Kellogg lab colleagues that the key to the carbon conundrum, and thus the d
oorway to all the elements, lay in the final, resonant notes of something called the triple-alpha process—the stellar atomic tango of three helium nuclei, or alpha particles.
Fowler was already familiar with the basic idea. He'd demonstrated that there wasn't a stable nucleus with a mass of 8. And a year earlier, he'd worked with Hans Bethe's student, Edwin Salpeter, to help show that in stars it was possible that under just the right conditions two alpha particles could fuse, forming beryllium, and then, in the blink of an atomic eye, before the unstable beryllium lost a neutron, these two alpha particles could gobble up a third helium nucleus to form carbon. But Hoyle showed that this triple-alpha reaction as calculated resulted in a minuscule amount of carbon production, certainly not enough to explain the abundance of carbon that astronomers saw in stars.
For Hoyle, this carbon conundrum wasn't in the stars and atoms but rather was a result of our view of them. The evidence was clear—the universe is chock-full of carbon, not to mention carbon is life's backbone element. In a variation on Descartes's “I think, therefore I am,” Hoyle came to a simple yet radical conclusion: “I'm made of carbon, therefore stars must make carbon.” The question was, how? This is where the energy value 7.65 mega electron volts was significant, Hoyle told Fowler. In order for stars to cook up lots of carbon, rather than have the nuclear reactions produce nitrogen or oxygen—the neighboring elements along the periodic table—there must be a specific carbon-formation energy zone—a carbon-forming quantum sweet spot. Hoyle calculated that this occurred if the triple-alpha dance reached a quantum-resonant energy of 7.65 mega electron volts above carbon's lowest energy level, or ground state, a level where, for a nanosecond, beryllium and helium nuclei formed a resonant energetic coupling, like two notes of an octave forming a resonant pair. From this quantum harmony, carbon was born in the hearts of stars. Critically, Hoyle predicted that this carbon sweet spot took place at too low a temperature for the reaction to tip toward making oxygen.
To Fowler, Hoyle's pinpoint prediction was wild speculation. At the time, predicting a quantum-resonant state involving three particles seemed akin to predicting the winning number in a national lottery. On a more personal level, Hoyle the astrophysicist was also telling the world's best nuclear experimentalists that they'd somehow missed identifying a crucial energy state in carbon. Though he initially rebuked Hoyle, Fowler eventually gathered his Kellogg lab colleagues, and, while behind his office door he privately told them that he thought the British astrophysicist was wrong, he let Hoyle explain his carbon hypothesis. Several of the Kellogg scientists, impressed by Hoyle's public lectures at Caltech and realizing that they were perfectly set up to experimentally test Hoyle's prediction, got to work doing just that. In less than two weeks, they'd turned Hoyle's hunch into transformational insight. Hoyle was right. There, in their accelerator, were carbon-12 nuclei spitting out alpha particles that carried with them the news that the carbon-12 atom that released them had an energy of exactly 7.65 mega electron volts.
Hoyle's success was a personal epiphany, one that eventually changed him as much as it did our view of the stars. He was the first to understand that stars make carbon, the elemental core of life, and that because of a finely tuned energetic nuclear dance, the doorway to all the heavier elements starts with this life atom. For Hoyle, it was no coincidence that carbon emerged in a narrow energetic window that, pushed just a smidgen further—to 7.19 mega electron volts, where oxygen would be preferentially produced—would produce a universe largely devoid of carbon and of life as we know it. Thus, Hoyle came to believe that far from terrestrial life being a cosmic fluke, the universe is finely tuned for life. It was a revelation that deeply shaped Hoyle's future views on a living universe and the idea of an intelligent cosmic designer.
More immediately, the discovery was a huge boost for Hoyle the astrophysicist. “It was the most rapid turnaround of people's views that I’ve ever experienced,” he reflected, “my going from a voice in the wilderness to suddenly a very large number of people believing the idea.” Hoyle returned to Cambridge with not just the stellar doorway opened to carbon but also with the tantalizing glimpse of the nuclear stairway beyond. While in California in 1952, he met Mount Wilson astronomer Paul Merrill, a quarter century his senior, and Hoyle teased him for making the startling discovery of technetium in stars. He knew Merrill wasn't looking to change the world. Hoyle, on the other hand, was in the midst of doing just that. Merrill's discovery was a dazzling confirmation that, for the upstart Hoyle, he was moving in the right direction.
Back in Cambridge, this Darwin of the origin of the elements set to work calculating and imagining the intersection of the nuclear-reaction pathway and the conditions in stars that could form the most common elements from carbon to iron. The result was his 1954 paper “The Synthesis of Elements from Carbon to Nickel,” one of the landmark papers in the history of science. Here, in page after page detailing complicated series of nuclear reactions, Hoyle painted a new view of stars. Far from being static, they were gradually evolving nuclear cauldrons, burning one nuclear fuel and then another, bringing on their own deaths and seeding the universe with the elements of their creation. It was an intimate portrait of a large star's old age. Hoyle described a process in which large, aging stars, those we now know to be about eight times the bulk of the Sun and larger, gradually develop an onion-like structure of layers of different elements. For most of their lives, stars light up by burning hydrogen in their cores. The vast majority of stars we see, from our Sun to those twinkling in the night sky, are at this stage of life called the main sequence. But when much of the star's hydrogen core is fused into helium, the star gradually gravitationally contracts. This contraction boosts its core temperature, igniting the fusion of helium into carbon and oxygen. Thus begins a sequential series of midlife stages, or core evolution, in which the star burns through one nuclear fuel after another, its onion-like layers filled with the nuclear wastes from earlier stages.
Hoyle's vision extended the notion of stars as sources of energy to stars as both lighthouses and element factories. He established that these two roles were inextricably linked, that just as energy and matter are linked in E=mc2, it's impossible to talk about energy in the cosmos without its relationship to the changing nature of matter at its deepest level, and that the twinkling of stars is light from the ongoing creation of the elements. But it would take one further great effort to finally tip the scales on the origin of the elements.
THE ASTRONOMER'S PERIODIC TABLE
Meeting in the basement of the Kellogg Radiation Lab that day in 1952, Fred Hoyle and William Fowler hadn't only figured out how carbon forms in stars. In that moment, they'd forged a strong mutual respect, a common purpose, and a deep sense of awe about their pursuit of the origin of the elements.
In the fall of 1954, Fowler moved his family to Cambridge on a year-long Guggenheim Fellowship and began the focused work of solving a cosmic puzzle for which the shapes of the pieces were resolving into clearer and clearer detail. His arrival marked another serendipitous connection. In Cambridge, he attended a lecture given by Geoffrey and Margaret Burbidge, a young astronomy couple who'd been measuring the abundances of elements in various stars and had come across some in which the heavier elements were far more abundant than usual. Margaret was an acolyte of Fred Hoyle's, and both she and Geoffrey would be lifelong friends with Hoyle; Geoffrey always among his staunchest and most vocal defenders. Margaret was an astute observational astronomer who could tease a light fingerprint out of the faintest star and thus nail its elemental composition. Geoff, who resembled the actor Charles Laughton—famous for playing Quasimodo in the 1939 film version of The Hunchback of Notre Dame—was a theoretical physicist who enjoyed noting that he became an astronomer by marrying one. At the core of the team were Fred, the astrophysicist, and Willy, the nuclear physicist.
The four were gripped by the sense of being on a mission. They began by tying the quirk of elemental abundances known as Harkin's rule—that
elements with even numbers of protons are slightly more abundant than their odd-numbered neighbors—to the stars. They realized that this zigzag pattern is the result of what they later described as the shuffling and reshuffling of protons and neutrons among atoms within a stellar cauldron. In this case, first an atom in a star captures a careening neutron, getting heavier yet not changing its fundamental chemical nature. Then, however, a neutron might beta-decay—a spontaneous process in which an unstable neutron spits out an electron and morphs into a positively charged proton. With one more proton, that atom is a whole new element. Here was a process like climbing the rungs of a ladder, in which the more stable, even-numbered proton atoms represented the rungs, and the less stable, odd-numbered proton elements represented the gaps.
In the fall of 1955, their research shifted to the other side of the Atlantic, to Pasadena, where the Burbidges visited, Geoff as a Carnegie Fellow. If they were going to provide conclusive proof of elemental buildup in stars, they needed solid stellar evidence. For that, they wanted to go up the mountain to use Mount Wilson's giant telescopes. The problem was Margaret, or, more precisely, that she was a woman. Mount Wilson's common living quarters were dubbed “the Monastery” because two things weren't allowed: whiskey (in fact all alcohol) and women. There was a solid glass ceiling when it came to the stars. Before Margaret Burbidge could help change humanity's view of the universe, she needed to crack another great barrier, and indeed she went on to become both the first woman Astronomer Royal in the post's three-hundred-year history and the first woman president of the American Astronomical Society. Fowler convinced Mount Wilson's director, Ira Bowen, to allow Margaret at the observatory, with conditions: the Burbidges had to use their own transportation; reside in segregated quarters in a small cottage heated by wood in a potbelly stove; and eat on their own, bringing their own food. Given all this, Margaret opted not to share a personal fact with any of the Mount Wilson staff—she was pregnant. She was worried that she'd be barred from some of the heavy work involved in moving the observer's ladder and hid the growing evidence under her heavy observer's coat until she was six months pregnant and had collected the data she wanted.
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