The Stardust Revolution

by Jacob Berkowitz

  In hindsight, with our homes powered by nuclear fission and our thoughts on the atom turned to avoiding the nuclear obliteration of human civilization, Eddington's claim seems pretty blasé. To most in his audience, however, it was wild speculation, and Eddington knew it. Yet, among astronomers at the time, he was geographically and intellectually closest to the seat of an emerging new world order. He told his audience about the work going on across the Cambridge commons at the Cavendish Laboratory. There, the physicist Francis William Aston had built one of the first mass spectrometers, the equivalent of a bathroom scale for atoms. For several years, and intensely since the end of the Great War, Aston had been extending Dalton's work of a century earlier by carefully weighing different elements. Aston's greater level of accuracy led to two new dramatic insights. First, not all atoms of an element are equal. Aston demonstrated the existence of hundreds of isotopes, atoms of an element with one or more additional neutrons. This in itself opened a crack in the idea of the perfect, immutable atom. But the accuracy of Aston's measurements added a kicker. Using Einstein's revolutionary insight into the equivalency of mass and energy—contained in the iconic formula E=mc2, Aston's measurements enabled physicists to calculate each atom's equivalency in energy. For other scientists, the physical reality of this equation might have seemed more fantasy than fact, but Eddington was one of a handful of scientists in the world who'd seen the power of Einstein's relativistic predictions with his own eyes. In May 1919, he'd led an expedition to the island of Principe, off the coast of West Africa, and during a solar eclipse had measured the gravitational bending of starlight by the Sun, the first observational confirmation of Einstein's general theory of relativity.

  What was remarkable, Eddington told his audience, was that a helium atom, the second atom in the march of the periodic table, weighed just a little less than the cumulative mass of four hydrogen atoms. Aston had calculated the atomic weight of hydrogen as 1.0008, and that of helium as 4. If four hydrogen atoms were being fused together to form helium, what happened to the seemingly negligible 0.0032 atomic weight difference? It was raining down on them as light and heat from the Sun and stars, said Eddington. It might seem like a tiny amount of mass for such a huge job, but a small amount of mass is converted into an enormous amount of energy. Eddington calculated that if just 5 percent of a star is initially hydrogen, its gradual fusion into helium would give stars the power to shine for fifteen billion years. There was no doubt, he concluded, “that all the elements are constituted out of hydrogen.” For the doubters in the audience, Eddington referred to his Cambridge colleague Rutherford and his transmutation of atoms: “What is possible in the Cavendish Laboratory may not be too difficult in the Sun.”

  Two decades passed before other scientists turned Eddington's wild speculation into conclusive mathematical description. By that time, atomic physicists had calculated hydrogen's and helium's precise quantum energy states and behaviors. With this in his metaphorical pocket, the German American physicist Hans Bethe—dubbed “the great problem solver” by his nuclear physics colleagues—solved a century-old riddle while waiting for dinner in a train car. His back-of-the-napkin calculations would lead to a 1939 paper published in the Physical Review, “Energy Production in Stars,” the basis for his 1967 Nobel Prize. This was one of the last papers this refugee from Nazi anti-Semitism wrote before leading the theoretical division of the Manhattan Project, where he helped to build the world's first atomic bombs. In his paper, Bethe showed how stars get their energy: not by splitting atoms but by fusing them. A star is a balancing act between the gravitational forces exerted on matter, heating it so intensely that it fuses atoms, and the massive energy this process releases, pushing the ball of atoms outward. George Ellery Hale's dream had come true—atomic physics and the stars were as one. Bethe had convinced both physicists and astronomers that nuclear reactions fuel stars. His calculations showed that the Sun's luminosity was just as could be expected from the nuclear reactions at the estimated temperature at the Sun's core.

  There was a critical wrinkle in Bethe's conclusion about stellar fusion. According to his calculations, stars quickly reached an elemental dead end. Bethe concluded that no elements heavier than helium could be formed in ordinary stars. The stars and the elements were joined, but only by the thinnest of threads—from hydrogen to helium. It appeared that stars could only account for the simplest of alchemical transformations, those from element 1 to 2. Bethe, “the great problem solver,” had solved one line of the cosmic crossword, only to find that it intersected with long rows of other unknown letters. Now the hunt was really on for the origin of the elements.

  BIG-BANG ATOMS

  With the eruption of World War II, the world's leading physicists turned their knowledge of Nature's ways to aid the frenzied war effort, from fine-tuning the nascent science of radar to the elaborate mathematical cracking of secret codes and, in the United States, the atomic-bomb-building Manhattan Project. Yet, for many of them, the scintillating, unsolved mystery of the origin of the elements burned in the back of their minds. At the war's end, the question reemerged in a dramatic new light, now with three solid clues to guide the quest—clues that for the first time set firm, measurable limits on where to look, and a way to confirm or reject any potential answers. From these clues, two epic opposing views emerged in a clash that pitted the big bang against the stars.

  The first clue was the question of temperature. Bethe had concluded that stars weren't hot enough to forge elements heavier than helium. He'd calculated that the temperatures needed to forge the elements heavier than helium required temperatures in the range of billions of degrees Fahrenheit, far hotter a temperature than was thought to occur in any star. When astronomers scoured the heavens, it appeared there was nowhere hot enough to cook up the elements—a cauldron hundreds of times hotter than the core of the Sun at twenty-eight million degrees Fahrenheit.

  The second clue lay in a new, nuanced understanding of atoms and how they change, which had been provided by atomic weapons. The world's first nuclear bomb blast, on July 16, 1945, in the New Mexico desert, followed by the devastating nuclear attacks on Hiroshima and Nagasaki, Japan, were for astrophysicists terrible beacons pointing the way in the quest for the origin of the elements. As scientists shielded their eyes from a blast brighter than a thousand Suns, they were viscerally aware that they'd stepped into the new realm of the atom, one they could explore and test here on Earth. For the new breed of experimental astrophysicists, such as Caltech's William Fowler, the combination of Bethe's 1939 paper and his bomb unleashed a new view of what nuclear physicists were really doing. They now had their hands on cosmic powers in their labs.

  The first computer models of stars were built on computer simulations of the atmospheric detonation of nuclear weapons—the closest events on Earth to what happens in stars—developed at the US Department of Energy's Lawrence Livermore National Laboratory. Experimental astrophysicists witnessed the transmutation of elements on a global scale: nuclear fallout. The same intense Cold War nuclear weapons testing that began in the mid-1940s and led to radioactive residues in breast milk around the globe eventually produced a slew of declassified data that, by the early 1950s, gave nuclear physicists the results of the elements forged in these atomic blasts. Nuclear scientists could also use the earliest particle accelerators, such as those at Caltech's Kellogg Radiation Lab, to test the burning question of how protons and neutrons got into and out of a nucleus and resulted in the transmutation of atoms.

  While the temperature and details of nuclear cookery were important in unraveling the origin of the elements, the most important key to unlocking this secret lay in a third, jagged clue: a new graph not of types of elements but of their relative abundances. Judging by the equally apportioned Scrabble®-tile-like boxes that make up the periodic table, there is no hint that huge differences exist in the relative abundances of the elements. In the periodic table, each element gets the same-sized square, but not so in Nature, where
there are enormous disparities. Until the early twentieth century, little scientific attention was paid to the relative abundances of the elements. The ancients knew that iron is much more abundant than gold, but not until the late 1930s—just as Hans Bethe was figuring out what makes stars shine—did the aptly named German geochemist Victor Moritz Goldschmidt reach for a cosmic perspective on the abundance of the elements—their universal abundances. To do this, Goldschmidt combined elemental data from terrestrial rocks and added data from extraterrestrial rocks—meteorites—which he believed might represent the average composition of cosmic matter. The resulting graph, the first detailed estimate of the cosmic abundance of the elements, struck all who viewed it as a cosmic riddle.

  Unlike the even pattern of the periodic table, Goldschmidt's graph of cosmic elemental abundances is a saw-toothed jumble that gives the same impression of looking into the jagged interior of the Himalayas. Arranged by atomic number, from hydrogen on the left to uranium on the right, what's immediately clear is a steeply cascading saw-toothed pattern, in which hydrogen is an Everest, with a secondary helium peak, towering over a zigzagging flatland of the majority of the heaviest elements, from germanium to sparse uranium, far below. Between these two extremes lies a striking intermediate range of elemental peaks. After plummeting down to lithium, beryllium, and boron, the graph of abundances shoots skyward to carbon, climbs to peak at neighboring oxygen, before starting a zigzag descent, interrupted by a spiking peak at iron.

  We now know that, taken together, all the elements that we think of as “stuff,” from carbon to cobalt and potassium to plutonium, make up less than a measly 2 percent by mass of all the atoms in the universe—hydrogen accounts for about 75 percent, and helium for 23 percent. On an exam, it probably wouldn't make that much difference whether your mark is 84 or 86 percent. But in our cosmos, at least from our perspective, it's this 2 percent that makes all the difference. And among this 2 percent, there are vast differences in abundance. For example, in our Solar System, for every trillion atoms of hydrogen there are about 600 million atoms of oxygen and 33 million of iron; but there are only about 8 atoms of gold and 19 of silver, giving a cosmic context to bullion's enduring value.

  Any theory of cosmic-element formation had to fit Goldschmidt's rudimentary map of the cosmic elemental abundances. There was a great cosmic story hidden in its atomic valleys and summits, but what was it?

  In 1946, the war over, Russian émigré George Gamow thought he'd finally grasped the location of the Philosopher's Stone. To know the origin of the elements, researchers had to go back to another beginning: the origin of time. One of the first protocosmologists, the stocky and jocular Gamow, who'd landed a position at George Washington University in the American capital, reasoned that astronomers couldn't spot the celestial furnace that fit the bill for element forging because it no longer existed. The origin of the elements wasn't in the stars but in the fiery birth of the universe. The universe and the elements were born together.

  George Gamow believed that all the evidence pointed to a massive primeval explosion—the then-controversial and unproven idea of the eruptive birth of the universe, or the big bang. Gamow's scientific upbringing had perfectly positioned him to make this conjecture. He'd earned his PhD in Leningrad under the influential mathematician Alexander Friedmann, who'd independently discovered that when Einstein's equations of general relativity were crunched, the results predicted that the universe had expanded from a much smaller beginning. Einstein initially balked at the idea, preferring the aesthetics of a static cosmos. But this prediction of general relativity was corroborated by Edwin Hubble's 1929 discovery that the galaxies are indeed racing away from one another; the ones farthest from us are moving at greater speed than those closest.

  Gamow later forged his own reputation when he showed how the new theory of quantum mechanics could describe radioactive alpha decay—the phenomenon that occurs when an atom spontaneously spits out a helium nucleus. What was even more impressive was that he predicted that this quantum decay process also worked in reverse: that with enough energy, an alpha particle could burrow into a nucleus in a process called quantum tunneling, triggering artificial nuclear decay. Gamow had considered the nature of matter from the atom to the cosmos, and now he envisioned its emergence from a single creative event, meaning that all the phenomena we see and experience—time, space, and the varieties of matter—were created together in little more than the blink of an eye. It was an idea of enormous mythic resonance, a single act of creation that would later get an enthusiastic nod from Pope Pius XII as evidence of biblical veracity.

  For Gamow, the origin of the elements was most important not in itself but for understanding the origin of the universe. The distribution of the elements was the remnant signal that indicated the universe's temperature, density, and rate of expansion at the beginning of time and space. What had it been like? The answer lay in the nature of the elements from hydrogen to gold and uranium. In September 1946, Gamow published an article titled “Expanding Universe and the Origin of Elements,” which laid out what could be called his “Big Build-Up” hypothesis. Gamow argued that the key pieces of the origin-of-the-elements puzzle—temperature, nuclear transformations, and, above all, Goldschmidt's graph of cosmic elemental abundances—all made sense in the context of a super-hot (more than 180 billion degrees Fahrenheit), rapidly expanding moment of cosmic birth, during which the universe cooked up the contents of the periodic table in the first hour of creation.

  The key to this, Gamow said, was a continuous building-up process arrested by the rapid expansion and cooling of the primordial matter. He painted a picture of the cosmos' first seconds as a scorching, dense soup of (free) neutrons glomming onto one another, building up larger and larger nuclei in not much more than a second, after which the soup was too cool and diffuse to squeeze together neutrons. After this period, there was a settling out as neutrons decayed into protons, releasing electrons and eventually forming the abundance distribution we see today. Therefore, there was little time for the heaviest elements to form—thus their low cosmic abundances as determined by Goldschmidt—and the distribution of the lighter elements was in part determined by a process of nuclear decay and settling.

  The key nuclear physics was the gradual fattening-up of atoms from hydrogen to carbon and onward up the periodic table by the addition of progressively more neutrons. As neutral particles (those without a negative or positive charge), neutrons have a much easier time entering a positively charged nucleus full of protons than does another proton, which is forcefully repelled—as are the positively charged ends of two magnets—by what physicists call the Coulomb barrier. Gamow imagined the newborn universe awash in a tsunami of neutrons transmutating every atom in their path.

  But as with so many contenders before Gamow, there was a problem, and this time it lay with atomic masses 5 and 8. And Gamow and his PhD student Ralph Alpher, who led much of the number crunching, knew it. A quick look at the periodic table reveals that there's no element with an atomic weight of 5 or 8. Atomic physicists also knew that no element had an isotope (a variant with more neutrons) that was stable for any length of time at this atomic weight. If the elements were built in a singular stepwise process up the ladder of atomic weights, a deadly gap existed at 5 and then again at 8. However, like many great scientists, Gamow didn't let these details destroy the central storyline of what looked like an elegant theory. Gamow and Alpher knew they had a core problem, but in the muddle-through approach that scientists often take, they chose to ignore it in public, in the hope that they'd resolve it in private in time to provide a convincing public explanation.

  When the criticisms started, Gamow sidestepped them with his characteristic mirth, illustrating articles with images of himself energetically jumping ditches at atomic weights 5 and 8. But there was another problem. If the elements were created simultaneously, they ought to be evenly distributed throughout the cosmos. Yet astronomers were finding stars with widely varying
amounts of heavier elements, up to a hundredfold difference from one star to another.

  Gamow had time to look for an explanation. Most astrophysicists were taken with the notion that time and all matter had started as one in a single conflagration. But there was a man who thought otherwise. He didn't like the idea of this singular cosmic birth—dismissively calling it a “big bang”—and he hadn't given up on the stars as the birthplace of the elements. The stars didn't lack creativity; it was we who lacked it in our imagination of the stars. The man was Fred Hoyle, the scorned prophet of the Stardust Revolution.

  LET THERE BE HOYLE

  In his later life, it would be fair to say that a lot of fellow scientists thought that Cambridge astronomer Sir Fred Hoyle was a great mind gone far, far astray. Those less familiar with his enormous lifelong creativity and insight—or those just less forgiving of human foibles—simply called him a nut. Perusing Hoyle's prodigious and diverse body of work accomplished in the two decades before his death, it's easy to see why. The topics that filled his mind scream out like the garish cover of a supermarket tabloid. The astrophysicist with no fossil training took on the world of paleontology, claiming that the famous bird-dinosaur fossil Archaeopteryx was a Piltdown Man–like fake. He argued vociferously that disease outbreaks were caused by viruses from outer space. And his cosmological journey led him to believe that the only rational conclusion is that the universe was created through intelligent design.