But, Fowler continued, for the sake of the Kellogg lab (which, though he didn't mention it, was financially in dire straits) and his colleagues there, he couldn't turn down the world's highest scientific honor. As if in a Shakespearean tragedy, one of the greatest scientific mysteries of all time ended with star-crossed friends facing choices that could never be fully satisfied. Together they'd shared a cosmic quest for the origin of the elements. It was an intellectual journey that would take them from the beginning of time and into the hearts of stars. And it would reveal that the three greatest secrets of the universe are in fact strands in a single cosmic cord: what makes stars shine, the origin of the elements, and the birth of all we see. These were cosmic riddles that for three hundred years perplexed and at times tortured the greatest minds in science. They began with the alchemist's dream to turn base metals into gold and thus to know the mind of God.
THE ALCHEMIST'S DREAM
For centuries, the origin of the elements was the elephant in the room of scientific questions: Where does all this stuff ultimately come from, me, you, the metals that make your watch, the gold in your wedding ring, the air we breathe? And, just as importantly, can one element be changed into another? This last question was the basis of the alchemists' Philosopher's Stone—the knowledge of the power or substance that could transform base matter into precious gold. Medieval and Renaissance alchemists pursued an ancient belief that there existed some primordial matter or spiritual essence that directed matter to be one element or another and that, with enough work and purity of spirit, it would be possible to grasp this timeless, all-powerful knowledge—to wield the Philosopher's Stone.
As such, modern science has its roots in the quest for the origin of the elements. You probably know Sir Isaac Newton from high school for his laws of motion and optics, but more often than not his prodigious mind raced with images of lead, gold, suns, moons, serpents, and dragon's wings. At heart, Newton was an alchemist, probably the greatest ever. The father of modern science spent more time in his private shed attached to Cambridge University's Trinity College trying to secretly transmute elements than in working on his monumental theories of gravity or light. His alchemical experiments lasted twenty-five years, during which he carefully amassed notes and reflections that could have formed the equivalent of a five-hundred-thousand-page book on the topic. This painstaking effort wasn't in quest of riches but was in search of a far greater jewel: to share in the act of creation, to know the mind of God. “Just as the world was created from dark Chaos,” Newton reflected in a note written in the 1680s, “through the bringing forth of the light and through the separation of the fiery firmament and of the waters from the earth, so our work brings forth the beginning out of black chaos and its first matter through the separation of the elements and the illumination of matter.”
Yet, after a quarter century of secretive effort, a despairing Newton shelved his crucibles and vials of mercury and abandoned his search for the origin of the elements. The world's greatest scientist was stumped. Little did he imagine that he'd paved the way: his understanding of gravity and light would eventually illuminate the way to understanding the origin of the elements. In the realm of scientific timing, Newton was three hundred years too early.
By the late 1800s, talk in the physical sciences about the origin of the elements was considered to be in bad taste. Although biology had entered the new world of Darwinism and evolutionary change, astronomy, physics, and chemistry were still largely grounded in a visceral and intellectual sense of permanency. Just as nature abhors a vacuum, science abhors an untestable hypothesis, and the origin of the elements appeared to live squarely in the land of speculation. However, the Victorian-era British polyvalent Sir William Crookes wasn't a man to shy away from an unpleasant topic. He'd been knighted by Queen Victoria for his contributions to solving one of Britain's most pungent and deadly problems of the era, urban sewage disposal. Crookes had penned titles such as A Solution of the Sewage Question and its follow-up, The Profitable Disposal of Sewage. He'd survived the discovery of what would become a favorite assassin's poison: a leading chemist, he'd discovered the toxic element thallium and named it from the Greek thallos, for “green shoot” or “twig,” after its telltale bright-green Bunsenian light fingerprint. For Crookes, identifying an element from the hurly-burly of all other elements on Earth only further piqued his rich curiosity and brought him to the nagging question of what natural process produced the element. That was the topic of his presentation on the evening of February 18, 1887, when London's scientific elite arrived by carriage and foot at the British Association for the Advancement of Science's Royal Institution to hear his lecture, “Genesis of the Elements.”
“In the very words selected to denote the subject I have the honour of bringing before you, I have raised a question which may be regarded as heretical,” Crookes began, warming up his audience.
At the time when our modern conception of chemistry first dawned upon the scientific mind, the average chemist…regarded his elements as absolutely simple, incapable of transmutation or decomposition, each a kind of barrier behind which we could not penetrate. If closely pressed he said they were self-existent from all eternity, or that they had been individually created just as we now find them at the present day. Or he might argue that the origin of the elements did not in the least concern us, and was, indeed, a question lying outside the boundaries of science.
In mentioning eternal elements, Crookes was referring to the work, some decades earlier, of his countryman, the Manchester schoolteacher and chemist John Dalton. In 1805, Dalton had published a remarkable list in the Memoirs of the Literary and Philosophical Society of Manchester, but there was nothing literary about the list: the story Dalton told was all about numbers and the elements. He'd resurrected the ancient Greek idea of the atom—that all matter was composed of tiny, indestructible particles. Distancing himself from the alchemists, Dalton argued that each element had its own unchanging atom, thereby barring any alchemical transmutation. The evidence for this, he said, was in the way elements combine to form compounds. Experimenting with combining different elements, he'd discovered that they always combined in fixed ratios and that compounds contain specific amounts of each constituent element. The most well-known modern combination is water, H2O, in which every molecule of water is a marriage of two hydrogen atoms and one oxygen atom. On the basis of these fixed mixing ratios, Dalton could identify the intrinsic atomic weight of the then-known elements; that is, how much each element weighed in relation to any other. He assigned hydrogen, the lightest element, the number 1. Using Dalton's system, carbon had atomic weight of about 4, while sulfur weighed in at about 14.4. Thus he showed that the elements didn't just have different chemical properties but, based on their mass, appeared to be completely different atoms.
Crookes knew there was another way of interpreting Dalton's observations. Not surprisingly, while Dalton insisted that his atomic-weight tables showed that the elements weren't transmutable, others saw in Dalton's table just the opposite story. One of them was the English physician William Prout in the early 1800s. When he wasn't lancing boils, treating gout, or setting broken bones, Prout, like Dalton, was fascinated by the burgeoning new field of chemistry. When he looked at Dalton's atomic-weight tables, he saw a pattern of wonderful Pythagorean beauty: each of the elements had a weight that was essentially a multiple of 1, or hydrogen. Perhaps, Prout conjectured, in what's become known as Prout's hypothesis, all the elements somehow begin with hydrogen. “If the views we have ventured to advance be correct, we may also consider that protyle of the ancients to be realized in hydrogen.” The “protyle” he referred to was from Greek philosophers' proto-hyle, or “first stuff,” which some thought was the essence of all matter.
Over the next half century, Prout's hypothesis fell in and out of fashion. The biggest knock against it was that not all the atomic weights were exact multiples of hydrogen, and some were quite far off. No one at the time knew of isotopes�
�varieties of an element with the same number of protons and electrons but with different numbers of neutrons, giving them all similar chemical characteristics but different atomic weights. For some theorists, Prout's hypothesis was boosted by Mendeleev's periodic table. The genius of the Russian chemist's ordering system is that it is primarily based on atomic weights. Hydrogen, the lightest element, is number 1 and is located in the upper-left-hand corner of the periodic table, while much heavier lead is located down toward the bottom-right corner. Some chemists looked at this sequential buildup of atomic weights and saw deeper meaning. The seemingly intractable alchemist's dilemma remained: If hydrogen was the alpha element, how did it turn into all the others?
The Victorian-era man of science William Crookes had an answer. “In these our times of restless inquiry,” he reflected, “we cannot help asking what are these elements, whence do they come, what is their signification?…I venture to say that our commonly received elements are not simple and primordial, that they have not arisen by chance or have not been created in a desultory and mechanical manner but have evolved from simple matters—or indeed from one sole kind of matter.” What he offered next really helped him to make his mark. Crookes made the first systematic proposal that the origin of the elements had not been found because researchers had been looking in the wrong place. The origin of the elements isn't the Earth; the elements, he offered, are forged in stars.
Minutes later, he held up a small glass jar to demonstrate the technical prowess that undergirded his science. It was an early vacuum tube—a glass tube from which air had been mechanically removed. “I have in this glass tube,” he continued, “perhaps the nearest approach to perfect emptiness yet artificially obtained.” He explained that his little jar, with a volume of five cubic centimeters, had been exhausted to just one part in fifty million of the air in that room. He described, to his by-now rapt audience, how his experiments involved zapping small samples of metals placed in similar vacuum tubes with high-voltage electrical discharges and studying the spectrum of the light they emitted.
Crookes reminded his audience that it was part of the chemist's creed that each element has a distinctive spectral fingerprint, such as thallium's green emission line and sodium's bright yellow band. Yet when he examined the spectra of the substances in his electrical discharge tubes, he found that the spectra varied. The reason, he surmised, was that the intense energy of the discharge tube rendered the seemingly immutable elements into even smaller bits. (Crookes was right: he'd created plasmas, atoms stripped of electrons. But the electron itself wouldn't be discovered until 1897, a decade later, by J. J. Thomson and his colleagues at Cambridge University's Cavendish Laboratory.) Crookes argued that in the intense heat and electrical forces present in the Sun, matter was reduced to Prout's protyle, or first stuff. He explained that as this plasma cooled, the various elements were formed or frozen out—the lightest first, followed by those with heavier atomic weight. In one of the most wonderfully coincidental intersections in science, Crookes described his stellar ladder of the elements as a double helix, with hydrogen at the top of one strand, moving down to uranium. There was, he concluded, “a genetic relation among the elements.”
A RECIPE FOR SUNSHINE
There was a singular, aggravating problem with Crookes's proposal that the elements were formed in the Sun and other stars. No one really knew what the Sun was. More precisely, no one knew what made it shine. The search for the origin of the elements had run full-on into one of the other great scientific mysteries of the day. For all of human history, the Sun had risen in the east and set in the west, and on each daily journey it glared like a burning question mark in the sky. The search for the ingredients of sunshine stretched over almost a century and involved many of the greatest scientists of the period.
As a scientific problem, the Sun presented two main challenges: the enormous energy it produces and its age. For more than a century, physicists speculated about what substance could produce enough heat to keep sunbathers tanned for millennia. The first candidate was coal. But in 1848, the German physician Julius Robert Mayer calculated that a Sun-sized chunk of coal, a sphere about a million miles across, would burn for about five thousand years. Even by biblical standards, that wasn't long enough to have warmed Adam and Eve and not extinguished in a sooty poof by the start of Queen Victoria's reign.
Here was the second part of the Sun enigma. The Sun didn't just pump out vast amounts of heat and light; it had been doing so for a long time. Mayer and others realized that the Sun must be powered by something other than chemical energy, and the next contender was something Newton had also thought about—gravity. The champion of a Sun heated by gravity was Lord Kelvin, now perhaps best known for giving the world the concept of “absolute zero” on his eponymous Kelvin temperature scale. Lord Kelvin and his German colleague Hermann von Helmholtz proposed that the Sun's energy came from the gradual gravitational contraction of gas. On one level, this makes perfect sense: if stellar matter was continually squished against itself, as it surely was, this friction would generate an enormous amount of heat. If the Sun was formed from a cloud of continually contracting cosmic gas, Kelvin estimated its age at about thirty million years.
Kelvin and von Helmholtz's model, however, still ran into the age problem. Nineteenth-century geologists had realized that the Earth is hundreds of millions, if not billions, of years old. Here was the rub: it didn't make sense that the Earth was older than the Sun, as calculated from Kelvin's gravitational-heating model. Kelvin, a fundamentalist Christian who never accepted Darwinian evolution, stuck to his gravitational-heating model, preferring his faith to geological evidence. As a result, he concluded, humanity's future looked dim. If the Sun shone by gravitational contraction alone, sooner or later it would shrink to such an extent that the Earth would face the ultimate energy crisis: its solar light and heat would shut off. “We may say, with certainty,” Kelvin said in 1862, “that the inhabitants of the Earth cannot enjoy the light and heat essential to their life for many millions of years longer, unless sources now unknown to us are prepared in the great storehouse of creation.”
Fortunately for all involved, as Kelvin argued for a gravity-heated Sun, a previously unimaginably powerful source of energy came to light: radioactivity. The French physicist Pierre Curie showed that the radioactive element radium possessed energy like no other known substance. Pound for pound, radium generated a million times as much energy as dynamite, and not only was radioactivity powerful, it was long-lived. At Cambridge, the atomic physicist Ernest Rutherford realized that radioactive substances have clear ages, measured in half-lives—they emit energetic particles, or decay, in predictable intervals. Radon, the first element Rutherford examined, has a half-life of just less than four days for its most stable isotope. But uranium, he determined, has a half-life of 4.5 billion years. Here, finally, for all intents and purposes, was an eternal and enormously powerful energy source.
But again there was a problem. It was clear from spectroscopic analysis that there wasn't enough radium, uranium, or any other radioactive substance in the Sun to turn it into a radioactive celestial torch. What radioactivity did do, though, was open the door to the possibility of obtaining energy from the atom. Not chemical energy from the bonds between atoms, but the incredible energy from an atom's nucleus. Rutherford figured out that radioactive substances were emitting an alpha particle—a helium atom stripped of its electrons—that was the atomic equivalent of a runaway freight train, with all its massive force and potential for damage. What would happen if alpha particles were fired at atoms? To find out, while the Great War raged across the English Channel, Rutherford fired alpha particles at nitrogen gas. The result changed the world and revealed a new kind of nuclear reaction: the alpha particles split the nitrogen atoms and, in doing so, transformed them. When the helium nucleus, with atomic weight 4, slammed into the nitrogen nucleus, with atomic weight 14, it fused with the nitrogen nucleus, spitting out a hydrogen atom (atomic weight 1
) in the process. That hydrogen nuclei could be emitted as part of radioactive decay was amazing enough, but even more amazing was what happened to the nitrogen. It no longer had an atomic weight of 14 but rather one of 17. It was no longer nitrogen. It was oxygen. On realizing this, Rutherford is claimed to have bellowed at his lab assistant, “For God's sake don't use the word transmutation! They'll take us for alchemists.” Whatever word he used, Rutherford had hinted at the deepest link between atoms and the stars.
The news of Rutherford's splitting the atom made headlines worldwide, but it didn't surprise his Cambridge colleague, the astronomer Arthur Eddington. For Eddington, it was the confirmation of something he already suspected: the Sun and the stars were powered by the transmutation of atoms. In person, Eddington was shy, and his Cambridge lectures were notoriously diffuse. “The problem,” one student of his recalled, “was that Eddington had no proper connection between his brain and his mouth. As far as I could tell, he began in midsentence and stopped at the end of the hour, without any full stops between.” For all his verbal maladroitness, given a pen and paper, Eddington had the voice of an angel. He was the greatest astronomy popularizer of the early twentieth century. His bestselling books, including Stars and Atoms, were wrapped and given as Christmas presents across the British Empire. Eddington, like many great physicists and astrophysicists, didn't just work through equations as equations. He had an intuitive, visceral sense of how mathematics applied to real-world objects—atoms, stars, and us.
When Eddington considered Kelvin's gravitational contraction model, he saw dim-witted thinking: “If the [gravitational] contraction theory were proposed today as a novel hypothesis,” he told an audience in 1920, “I do not think it would stand the smallest chance of acceptance…. Only the inertia of tradition keeps the contraction hypothesis alive—or rather, not alive, but an unburied corpse.” What was remarkable about stars, Eddington told the crowd, is not that they gravitationally contract, but that they don't contract more. Furthermore, astronomers had recently found stars that weren't getting smaller, but bigger. Recent observations of red giant stars showed that these stars were producing vast amounts of energy that puffed them out into distended versions of their earlier selves. These puffed-out stars didn't make any sense if a star's energy resulted only from gravitational collapse, in which case, a star's life was a one-way track of shrinking. “If we decide to inter the corpse,” Eddington continued, drawing out his line of reasoning, “let us frankly recognize the position in which we are left. A star is drawing on some vast reservoir of energy unknown to us. This reservoir can scarcely be other than the sub-atomic energy.”
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