Sun in a Bottle: The Strange History of Fusion and the Science of Wishful Thinking - Charles Seife (2008)
Chapter 2. THE VALLEY OF IRON
... materials dark and crude,
Of spirituous and fiery spume, till, touched
With Heaven’s ray, and tempered, they shoot forth
So beauteous, opening to the ambient light?
These in their dark nativity the Deep
Shall yield us, pregnant with infernal flame
—JOHN MILTON, PARADISE LOST, VI, 478-83
Nearly a century before the quest for superweapons split the scientific community, fusion was at the center of another debate; physicists just didn’t realize it at the time. Decades before fusion was discovered, it was at the heart of an argument between physics and biology, between those who studied the fundamental laws that govern the universe and those who observed the processes of life on Earth. It was a battle between two of the leading scientific lights of the day: William Thomson (also known as Lord Kelvin), and Charles Darwin.
In 1862, Thomson, one of the brightest—and most famous—physicists in Britain, was absolutely certain: Darwin was wrong. The theory of evolution could not possibly be correct. He had ironclad proof. According to Thomson’s calculations, it was not possible for species to change form over millions and millions of years because the sun could not have been around that long. It was a physical fact, Thomson thought, and it would destroy the biologist’s pretty theory once and for all.
Thomson’s argument was far more ambitious than a mere attack on Darwin’s proposal. In fact, the physicist was trying to answer some of the biggest scientific questions of the day. Astronomers were just beginning to understand what the sun was made of, and they were suddenly faced with a new set of questions that once seemed unanswerable. Where did the sun come from? How old was it, really? Where did it get its energy?
When Thomson estimated the sun’s age, he calculated that it could only be a few tens of millions of years old, far fewer years than Darwin’s natural selection would take to generate the amazing diversity on Earth. It was a major puzzle; two branches of science were giving mutually contradictory answers. It would take decades before scientists uncovered the truth. The secret was fusion. Only when physicists could understand fusion could they understand the nature of the sun, much less create one for their own use.
Thomson’s calculations were extremely bad news for Darwin. The physicist argued that there was a fundamental problem with the theory of evolution, a problem that seemed to contradict a law of thermodynamics. If true, it would devastate Darwin’s theory. The laws of thermodynamics are among the most fundamental and sacrosanct laws of physics, and they brook no contradiction.
The field of thermodynamics studies the relationships among heat, work, and energy. Its first law has to do with energy: energy cannot be created out of nothing. Energy can be transferred from place to place. It can change form. For example, the energy of water spinning a wheel can light a lightbulb: the energy of motion is converted into electric energy then into light energy. However, energy always has to come from somewhere; it can’t be created or destroyed. Nature has a fixed amount of energy, and it is impossible to make more. This law is central to physics, yet Thomson believed Darwin’s theory fell afoul of it.
He based his argument on the energy flowing from the sun. When you go out on a bright summer day, you feel the warmth of the sun on your skin. The sun, shining continuously in the sky, transfers some energy to you in the form of light. As you soak up the rays, that energy warms your skin. You are absorbing the sun’s energy. Since energy cannot be created or destroyed, that solar energy has to come from somewhere; the sun can’t just create energy out of nothing. The sun must be using up some sort of energy reserves. As our star shines, it constantly emits more than 1026 watts of power, roughly equivalent to one hundred billion Ivy Mike bombs exploding every second, every day, every year. All that energy is radiated into space.
This baffled Thomson. The sun was emitting energy, and things that emit energy tend to cool down over time. So how can the sun stay so hot? Perhaps it was able to replenish its energy reserves by burning fuel—but what sort of fuel could it be using? It couldn’t be burning like a giant charcoal; burning is merely a chemical reaction, and no known chemical reaction could release the sorts of energy that the sun was radiating. No fire could generate that much heat. Could the sun be getting energy from another source—by gravity, perhaps? Meteors occasionally strike the sun, adding energy to the solar furnace, but there aren’t that many meteors around, and their energy is just a tiny drop in the tsunami of power coming from the sun. Thomson knew of no possible way to generate the quantity of energy that was escaping the sun every second, yet the first law of thermodynamics dictates that the energy has to come from somewhere.
It was clear to the physicist: no mechanism—chemical, gravitational, electrical, or whatever—existed that could generate the amount of energy the sun was emitting every moment. Since energy can’t be created out of nothing, the sun must be depleting its energy reserves at an enormous rate. And that meant that the sun must be slowly getting colder and colder. Unable to replenish its reserves through any means known to man, the sun, presumed to be a gigantic ball of hot liquid, must slowly be cooling down.
Once Thomson reached that conclusion, he wondered: If the sun is merely a huge molten sphere of liquid, where did it get its energy in the first place? The only answer he could think of was the energy due to gravitation. Imagine that the sun came from an enormous cloud of tiny rocks. Those rocks are attracted to one another by the force of gravity. Under their mutual attraction, they begin falling toward one another. As they fall inward toward the center of the cloud, they move ever faster. The cloud of rocks begins to collapse. The individual rocks speed inward quicker and quicker, because their gravitational energy is being converted into kinetic energy—the energy of their motion. As the fast-moving rocks stream toward the center of the cloud and collide, their kinetic energy gets converted into heat energy: the cloud heats up. Eventually, it gets so hot it glows.
Thomson calculated how hot such a protosun could have been. Then he calculated how long it would have taken to cool to its present temperature. Not long. The sun wasn’t more than a few tens of millions of years old, not long enough for the long, slow process of evolution Darwin proposed.
In fact, Darwin was deeply shaken by the calculations. He considered Thomson’s challenge to evolution “one of the gravest” that the theory had to face, and he could do little to counter it other than argue that scientists did not have a perfect understanding of the nature of the universe.
It was an impasse. The laws of physics seemed to say one thing, while the observations of biologists seemed to tell another.
Physicists would have to follow a tortuous path before they could resolve the contradiction—a path that led, first, to understanding the mystery of matter.
By the end of the nineteenth century, physicists and chemists had unraveled many of the mysteries of the universe. Isaac Newton had divined the physical laws that govern how objects move and how gravity works. James Clerk Maxwell had figured out the subtle interrelationships between electric and magnetic forces. Thermodynamicists had codified the laws of energy and heat. At the same time, though, scientists did not know much about matter; they had little idea what sort of stuff made up stars and planets and people. That was soon to change, as they came rapidly to the conclusion that matter was composed of tiny building blocks known as atoms.
Atomic theory, in its most primitive form, goes back to the ancient Greeks. In the fifth century BCE, the philosopher Democritus held that all matter was created out of little indivisible particles. These particles, far too tiny to see, were considered to be uncuttable. Democritus’s idea was just one of a huge number of competing theories about the universe. Some philosophers argued that everything was made of fire; others thought that objects were made from a mixture of earth, air, fire, and water. Some argued that matter was infinitely divisible; others, like Democritus, argued that there was a limit to how finely you could slice an object. Though we now know that Democritus’s idea was closest to the truth, for millennia it had no special status.
More than two thousand years later, a steady march of experimentation and observation led scientists to the conclusion that Democritus was essentially correct: matter is made up of tiny atoms. Chemists had led the way; the work of chemists such as the Briton John Dalton, the Italian Amedeo Avogadro, and the Russian Dmitri Mendeleev began to produce a picture in which all matter consisted of a collection of invisible “elemental” particles. Water, for example, was made up of two particles of hydrogen and one of oxygen; alcohol had two of carbon, six of hydrogen, and one of oxygen.
There was only a handful of known elements, and they each had different properties. For example, the atoms of some elements, such as hydrogen, oxygen, and carbon, were very light. Other elements’ atoms, like those of mercury, lead, and uranium, were very heavy. And these particles—these atoms—were fixed in their properties; it was impossible to transmute an atom of hydrogen, say, into an atom of lead.
This picture explained the nature of matter extremely well. Within a century, atomic theory changed the subject of chemistry from a quasi-mystical hodgepodge of contradictory ideas into a real science. Physicists soon joined the chemists in their support of atomic theory; they began to provide evidence for the existence of tiny atomic particles. Theorists like Ludwig Boltzmann realized that you could explain the properties of gases simply by imagining matter as a collection of atoms madly bouncing around. Observers even saw the random motion of atoms indirectly: the jostling of water molecules makes a tiny pollen grain swim erratically about. (Albert Einstein helped explain this phenomenon—Brownian motion—in 1905.) Though a few stubborn holdouts absolutely refused to believe in atomic theory,14 by the beginning of the twentieth century the scientific community was convinced. Matter was made of invisible atoms of various kinds: hydrogen atoms, oxygen atoms, carbon atoms, iron atoms, gold atoms, uranium atoms, and a few dozen others. But, as scientists were soon to find out, atoms are not quite as uncuttable as the ancient Greeks thought. Indeed, to figure out why different elements have different properties, physicists had to slice the atom into pieces.
The first piece came off in 1898. The Cambridge physicist J. J. Thomson was studying a mysterious phenomenon known as cathode rays. He used electric and magnetic fields to deflect the rays and came to the correct conclusion that the rays were made up of negatively charged particles that had been stripped away from atoms. These very, very light particles came to be known as electrons.
Since an atom is, on balance, neither positively nor negatively charged, the positive and negative charges in the atom must be equal and opposite; the charges in the atom have to cancel each other out. This means that for every electron in an atom, there has to be something else in the atom that carries the equivalent positive charge. About a decade after the discovery of the electron, the physicist Ernest Rutherford found out where that equal and opposite charge sits. It resides in tiny, but extremely solid, nucleus at the very center of the atom. This nucleus is quite heavy, thousands of times heavier than an electron, so the nucleus of an atom had to be made of stuff very different from electrons. Rutherford soon figured out what that positively charged stuff was: he realized that the positive charge is cloistered inside a heavy particle known as a proton.
For every electron zipping around in the outer regions of the atom, a proton had to be sitting in the nucleus. Since positively charged objects attract negatively charged ones, the nucleus attracts the electrons through electrical forces, in roughly the same way that the sun attracts its planets with gravitational forces. Rutherford took this analogy fairly literally; he imagined the atom to be like a miniature solar system. At the center is a heavy, dense, positively charged nucleus. Quite a distance away, lighter, quick-moving, negatively charged electrons are in “orbit” around it.15 In between, there is empty space—lots of it.
When physicists discovered the proton and electron, they sparked a revolution in the scientific understanding of matter. Two subatomic particles suddenly explained the properties of the elements. No longer were atoms of different elements considered to be fundamentally different objects; an atom of gold need not be thought of as a different sort of creature compared with an atom of lead. Gold and lead were essentially the same kind of object: bundles of protons surrounded by bundles of electrons. Gold has properties different from those of lead—and they both have properties different from the other elements—because they have different numbers of protons in their nuclei (and, hence, different numbers of electrons). A hydrogen atom has one proton per nucleus, helium has two. Oxygen has eight; gold, forty-seven; lead, eighty-two; uranium, ninety-two. In each case, the number of protons in a nucleus—known as an atom’s atomic number—determines how the atom behaves chemically. It tells you which atoms it will react with and which it won’t; it tells you whether a collection of atoms is likely to be a gas or a metal, whether it will burn in oxygen or explode in water or refuse to react with anything at all. This theory was a tremendous success for science. The uncuttable atom had been dissected into its component parts. But one piece was still missing.
The discovery of the electron had come from Thomson’s investigations into cathode rays. Cathode rays come from a fairly simple piece of laboratory equipment: put a couple of pieces of metal in a vacuum tube, hook them to a battery, and radiation streams from one end to the other.
The concept of radiation was a new phenomenon at the turn of the twentieth century. Scientists knew little about it, but they were beginning to detect it everywhere. Marie Curie’s radium emitted a substance—particles or rays or something as yet unknown—that carried energy; something was fogging a photographic plate. That was one kind of radiation. The German scientist William Roentgen discovered another kind in 1895. When he sent electrical current through an evacuated tube, he noticed it would generate invisible rays that could make fluorescent screens glow. Like the rays coming from radioactive elements like radium and uranium, Roentgen’s x-rays could expose a photographic plate. X-ray radiation, too, carries energy. (It turned out that x-rays are beams of light so energetic that they pass right through flesh.) Then there were the mysterious rays coming from Thomson’s cathode. By the turn of the century, scientists across the world were finding all sorts of rays in strange places. The scientific world was going radiation crazy.
We now know that these “radiations” are not all the same thing. Some, like x-rays, are varieties of light. (Gamma rays, too, are light beams even more energetic, and more penetrating, than x-rays.) These high-energy light rays penetrate matter relatively easily. Not all the radiations had this property. Thomson’s cathode rays couldn’t penetrate very far into an object before being absorbed. Neither could beta radiation, another type of emanation that streams from certain kinds of unstable atoms. Alpha radiation, which comes from yet other varieties of unstable atoms, penetrates even less than beta rays. It turns out that cathode rays, beta rays, and alpha rays are all subatomic fragments. Cathode rays and beta rays are both made up of electrons; alpha rays are made up of heavier, positively charged pieces of large atoms.16
Not surprisingly, researchers were so excited about finding new kinds of radiation that some of their discoveries were entirely fictional. In 1903, the French physicist René Blondlot thought he had discovered a new type, which he dubbed the “N-ray.” But Blondlot had deceived himself; his desire to believe in N-rays made him ignore the evidence against them. When a skeptical researcher removed a crucial component of the experimental apparatus and the unsuspecting Blondlot continued to observe the N-rays, N-rays were exposed as a fiction. Blondlot was made a laughingstock.
There was one type of radiation, though, that did not fit neatly into the pattern scientists had been seeing. High-energy light, such as x-rays or gamma rays, penetrates matter easily; its beams are hard to block. Fragments of atoms—charged particles like protons and electrons and alpha particles—tend not to penetrate matter much at all. Because of their charge, they get tangled in the electrons and protons in a given hunk of matter and quickly slow to a halt. But a new type of radiation, discovered in the 1930s, seemed like a weird cross between light and atom fragment. Scientists generated this bizarre radiation by shining a beam of alpha particles upon certain kinds of atoms (such as beryllium atoms). This new kind of radiation did not have an electric charge: it was unaffected by electric or magnetic fields. It penetrated matter as readily as gamma rays did, but it did not behave as a light beam should. It behaved like a heavy particle: it would hit a block of paraffin and knock protons out; mere light couldn’t do that so easily. In 1932, the British physicist James Chadwick concluded, correctly, that this new type of radiation consisted of particles almost identical to protons but for one major difference: they had no electric charge. Chadwick won the Nobel Prize for his discovery: the neutron.
The neutron is just a tiny fraction of a percent heavier than a proton, so it has quite a bit of oomph. But because it is electrically neutral, it doesn’t “feel” the electrical charges of the electrons and protons in a material. It is only affected by an atom when it slams directly into the nucleus. However, since atoms are mostly empty space and atomic nuclei are very small, a neutron can zoom straight through a chunk of matter without ever encountering something that deflects it. Neutrons penetrate matter extremely well, going through lead bricks almost as if they didn’t exist. But when a neutron does, by chance, hit an atomic nucleus, it packs a punch. A light atom (such as hydrogen) might be kicked out of the substance altogether. A heavy atom (such as uranium) might shiver and break apart when struck with the right amount of force. (As described in chapter 1, neutrons doing just this is what causes the chain reaction at the heart of the atom bomb.)
The discovery of the neutron also solved a puzzle that was beginning to vex physicists. When chemists and physicists used their newfound knowledge of protons and electrons to understand the nature of the elements, they were surprised by a strange inconsistency. They discovered that an atom of a given element did not have a fixed weight. For example, in 1932 scientists found that hydrogen came in several flavors. There was ordinary hydrogen—which was thought to be made up of one proton (and one very light electron, whose weight is negligible). Then there was a heavier form of hydrogen that weighed twice as much. They called it deuterium. Soon, they realized there was yet another version that weighed almost exactly three times as much as hydrogen: tritium. All three of these varieties had the same chemistry as hydrogen, but they all had different weights. (And tritium, as it turned out, was radioactive.) Until the neutron was discovered, nothing could explain why a single element could have multiple weights.
Once Chadwick discovered the neutron, though, the answer to the puzzle was obvious. Scientists already knew that the number of protons determined the chemical properties of an atom; hydrogen, deuterium, and tritium each had a single proton in the nucleus, so they were almost identical, chemically speaking. But neutrons can also sit in an atom’s nucleus. Because neutrons don’t have a charge (and don’t attract extra electrons), they don’t affect an atom’s chemical behavior; an extra neutron doesn’t turn hydrogen into a different element. But an extra neutron makes that hydrogen weigh more than before.
Ordinary hydrogen’s nucleus is simply one proton. It weighs as much as one proton, so it is known as hydrogen-1, or 1H. Deuterium’s nucleus, too, has one proton. But it also has a neutron that weighs roughly the same as the proton; the mass of the nucleus (hence, the mass of the atom) is doubled. Deuterium is thus known as hydrogen-2, 2H. Tritium has a single proton in its nucleus, but in addition it has two neutrons, making it three times as heavy as ordinary hydrogen. Tritium is therefore designated hydrogen-3, 3H. All these atoms are considered to be varieties, or isotopes, of hydrogen. In a chemical reaction, all three behave more or less the same way. But they have slightly different physical properties by virtue of their nuclei’s different weights.
Scientists were thrilled when they discovered the neutron because it gave them a complete model to explain an atom’s chemical behavior. Just figure out how many protons and neutrons are in a given atom and you can predict its properties extremely well.
Despite the spectacular success of atomic theory, scientists, in some sense, were astonished that atoms could exist at all. Nuclei are finicky things, and it is amazing that any of them are stable. By rights, they should fly apart instantly. They are filled with positively charged protons, and positively charged things repel one another. If the protons in a nucleus were to obey their electrical urges, they would flee each other’s presence, and the nucleus would explode in all different directions. But something forces the protons to stay put and in close proximity to one another. A very strong force—stronger than gravity, stronger than electromagnetism—glues nuclei together, trapping protons inside. In a great burst of creativity, scientists dubbed this strong force . . . the strong force. This force holds the secret to nuclear fusion.
The strong force is powerful enough to overcome the natural repulsion that protons have for other protons. However, it can do so only under a fairly narrow range of conditions. If there is the right balance of particles in the nucleus—the correct number of protons and neutrons—the strong force keeps the nucleus stable (or nearly so), preventing the nucleus from exploding. If there are too many neutrons or too few, the nucleus will be unstable. An unstable atom will destroy itself somehow, changing the balance of particles in its nucleus until the nucleus reaches a more stable state. A nucleus can break apart, spit a particle out, or swallow one to get closer to an ideal, stable balance of protons and neutrons.
For example, hydrogen (one proton) and deuterium (one proton and one neutron) are stable. Left to their own devices, they would not change at all. But add a second neutron to the mix, making tritium, and the atom has too many neutrons for comfort. It is no longer stable. Eventually, a tritium atom will, spontaneously, transmute one of its neutrons into a proton (and spit out an electron in the process). The substance left behind is no longer tritium; it has become helium-3, a stable if rare isotope of helium that has two protons and one neutron. (Most helium is helium-4, which has two protons and two neutrons.) It takes an average of twelve years or so for any given tritium atom to undergo this decay process, but over time, if you have a jar full of hydrogen-3, you will find that it slowly transforms itself into helium-3.
This transformation process releases energy, because the helium- 3 does not weigh exactly the same as the tritium did. The neutron that disintegrated weighed more than the proton and the electron that it turned into.17 There is mass missing: it was converted into energy, just as E = mc2 says. As the unstable tritium changed itself into the stable helium-3, it lost a little bit of mass and released a bunch of energy.
This is an example of a general rule. When a nucleus converts itself from a less-stable variety to a more-stable one, it releases a little bit of energy because some of its mass disappears. And nuclei always “want” to become more stable, just as a ball perched on a hill “wants” to roll down to the bottom. In the process of getting more stable, an atom releases energy, just as a ball rolling down a hill picks up more and more speed as it goes.
Marie Curie was seeing this process with radium. Radium-226 is a heavy atom with 88 protons and 138 neutrons. It is almost stable ... but not quite. On average, after 1,600 years, a radium-226 nucleus spits out an alpha particle (a helium-4 nucleus: two protons and two neutrons), leaving behind 86 protons and 136 neutrons—radon-222—and releasing a bunch of energy. This energy heats the hunk of radium, and it is why Curie observed that chilled radium would warm itself. It is also why a hunk of radium emits radon and helium. It so happens that radon, itself, is unstable; it decays into thorium, releasing energy, which, in turn, decays into another species and another and another, emitting energy at each step. Like a ball rolling down a bumpy hill, it keeps rolling and rolling until it reaches a stable place to rest: in this case, lead-206, which is much more stable than radium-226. The ball has rolled a long way down the hill. But it didn’t roll all the way down. There are, in fact, nuclei more stable than lead-206. At the very bottom of the valley are the most stable atoms of them all: the iron group.
Iron-56 (26 protons, 30 neutrons), nickel-62 (28 protons, 34 neutrons), and a few other nearby iron and nickel isotopes are the ne plus ultra of the nucleus world. They are the most stable elements of them all. They are at the very bottom of the valley. All other atoms “want” to be iron, just as a ball anywhere on the slope of a hill “wants” to be at the very bottom.
The landscape of nuclei is very much like a valley with a steep hill on one side and a shallow hill on the other (see the graph on page 47). At the very bottom of the valley is iron. Heavy nuclei, like radium and uranium, have more protons and neutrons than iron—they are high up on the shallow hill. To roll down to iron, they have to get lighter, shedding those extra protons and neutrons. Sometimes they do it in small steps, like radium does. Sometimes they do it violently, by breaking into two or more parts: fission. (Indeed, fission is little more than a way for heavy atoms to roll quickly down the shallow hill, losing mass and releasing energy in the process.) Light elements, on the other hand, are on the steep hill. They have to get bigger if they want to get into the valley of iron. This is what fusion is all about. Two light nuclei, if they slam together, can stick to one another to create a larger nucleus.
CURVE OF BINDING ENERGY: Iron is at the bottom of the valley. Light elements on the left and heavy elements on the right release energy when they move down the valley of iron by fusing or fissioning.
Fusion is a way for light atoms to roll down the steep hill toward iron. Since the fusion hill is much steeper than the fission one, a fusion reaction yields much more energy than an equivalent fission reaction. Fusion and fission are two sides of the same coin, but as Teller well knew, fusion is more powerful than fission.
In the 1930s, fusion was soon to solve the puzzle that had so vexed Darwin and Kelvin, and it would answer a question that had bothered humans for millennia: Why does the sun shine? Hans Bethe, then a physicist at Cornell University, would uncover the answer.
In retrospect, William Thomson was fundamentally correct. If the sun were, in fact, an incandescent ball of liquid, the energy released by infalling matter would only power it for a few tens of millions of years, far short of the time that Darwin’s theory needed to explain the diversity of life on Earth (and far short of the time that other scientists needed to explain geological processes). But Thomson’s work preceded E = mc2 by decades; nobody had yet puzzled over the nature of radioactivity or understood how fission and fusion turn matter into energy. Fusion held the solution to Thomson’s puzzle and vindicated Darwin. Fusion is the source of the energy that has powered the sun for billions of years.
The clues were already old by the time Bethe set to work on the problem. By carefully analyzing the colors of the light that streams from the sun, scientists already had a pretty good idea of what the sun was made of. Roughly 90 percent of its atoms are hydrogen. About 9 percent are helium atoms; in fact, it was by looking at the sun that scientists discovered helium in the first place. The remaining 1 percent is mostly carbon, nitrogen, oxygen, neon, and a tiny smattering of heavier elements, but almost all of these are lighter than iron. The sun bears all the hallmarks of being powered by fusion. Bethe figured out precisely how that power is generated.
A star begins its life as a cloud of gas: mostly hydrogen and a little bit of helium. Because atoms have mass, they attract each other gravitationally, and because of this mutual attraction, the cloud begins to collapse under its own gravity. As gravity compresses the cloud, the cloud heats up.
If gravity were the only force at play, the cloud would simply get smaller and smaller and eventually collapse into a tiny, massive point. But that is not what happens. As the gas cloud gets denser, atoms of hydrogen bump into each other more and more frequently. The collision rate increases dramatically. And as the cloud heats up, its atoms have more energy and collide more violently. The hydrogen atoms jostle each other harder and harder.
Ordinarily, nuclei try to escape from one another. They are positively charged, so they find other nuclei repulsive. When two atoms “collide,” they don’t usually come into physical contact. Once they get within close range, the repulsive forces send them zooming in opposite directions before they actually touch—something like what happens when you try to make two powerful magnets touch each other despite their mutual repulsion. But if the nuclei are moving fast enough—if both atoms are hot enough—then even the mutual repulsion is not enough to keep the nuclei from hitting each other. The two nuclei slam together with great force. This is where fusion begins, and how a sun sparks to life.
Hans Bethe realized that with all these hydrogen nuclei constantly slamming into one another, two hydrogen nuclei—two protons—might smash together at the same time that one spits out a set of particles, turning itself from a proton into a neutron. They fuse, creating deuterium and releasing energy in the process. The deuterium slams into another proton, making helium-3, and again releasing energy. And when two helium-3 atoms collide with each other, they fuse, making helium-4 and releasing two protons and yet more energy. This process, known as the proton-proton chain, turns four hydrogen atoms into helium-4 and lots of energy. Bethe figured out that this was one way our sun generates power: by turning hydrogen into helium. He also realized that other processes are going on as well; for example, the trace amounts of carbon, nitrogen, and oxygen are involved in a cycle that has the same outcome as the proton-proton chain: this process takes four hydrogens and turns them into helium-4. Once a cloud of hydrogen gets hot enough and dense enough, it turns into a machine that converts hydrogen to helium, releasing energy. That is how the nuclear furnace at the heart of a star works.
FUSION REACTIONS IN THE SUN: Colliding protons release an electron and make deuterium, then deuterium and protons make helium-3, and finally helium-3s make helium-4, producing a lot of energy in the process.
The fusion energy released in the guts of the sun makes it shine. But it also threatens to blow the sun apart. The energy heats the hydrogen gas, making the nuclei slam together harder and harder, and the reaction speeds up, pouring more energy into the cloud. This would appear to lead to a runaway reaction; the furnace should run hotter and hotter and eventually get so energetic that the cloud explodes violently in all directions. However, it turns out that the hotter a cloud of gas is, the more it expands. So when the fusion engine runs hot, the star expands slightly. It becomes slightly less dense and the atoms slam into each other less and less often. The fusion engine slows, and the star cools. Gravity takes over once more, compressing the star, heating it up, and making the fusion energy run hot again. This means that a star is in a delicate equilibrium, caught between the force of gravity and the energy of fusion. The force of gravity tries to collapse the star while the energy of fusion tries to blow it apart.
EQUILIBRIUM IN A STAR: Two forces compete for dominance in every star; gravity tries to crush the star to a point, while the fusion explosion tries to blow the star apart. In a normal star, these two forces are in balance.
When that delicate equilibrium fails, the star dies. A fusion engine, no matter how well-balanced, can only run for as long as it has fuel. As a star gets older, its hydrogen supply begins to run out; the hydrogen fusion cycles sputter to a halt. A large star then turns to other light elements to keep itself from collapsing. It begins to fuse helium, turning it into yet heavier elements, such as carbon and oxygen. As the helium runs out, the star fuses heavier and heavier fuels: carbon, oxygen, silicon, sulfur. The fusion engine is rolling further and further down the fusion hill. Soon it hits bottom. The valley of iron.
Fusion gets its energy by making light elements roll down the hill toward iron. Fission gets its energy by making heavy elements roll down the hill toward iron. Iron, already at the bottom of the hill, can’t yield energy through fusion or fission. It is the dead ashes of a fusion furnace, utterly unable to yield more energy. When a star runs out of other fuels, its iron cannot burn in its fusion furnace. The fusion engine has nothing that it can turn into energy, so it shuts off and the star abruptly collapses. Depending on the star’s nature, it can die a fiery death: the final collapse ignites one last, violent burn of its remaining fuel, blowing up the star with unimaginable violence. A supernova, as such an explosion is called, is so energetic that a single one will typically outshine all the other stars in its galaxy combined. The star spews its guts into space, contaminating nearby hydrogen clouds in the process of collapsing into new stars. This is what happened before our sun was born; it got seeded with the nuclear ash of a supernova explosion. All the iron on Earth, all the oxygen, all the carbon—almost all the elements heavier than hydrogen and helium—are the remnants of a dead fusion furnace. We are all truly made of star stuff.18
When Hans Bethe solved the riddle of the sun’s energy, the idea of a fusion bomb seemed absurd. To ignite a fusion reaction, you need to have a bunch of light atoms that are extremely hot (so they have enough energy to overcome their mutual repulsion and slam into each other) and extremely dense (so they are close enough to one another that they collide frequently). The laws of nature seem to conspire against having those two conditions at the same time: hot things expand, reducing their density. The only reason a star can keep a stable fusion engine going is because it is so massive. It is held together by the intense force of its own gravity, resisting the explosive force of the fusion engine in its belly. A cloud of hydrogen smaller than a star doesn’t have the benefit of that gravitational girdle keeping the reaction from puffing out. Even if you are somehow able to start a fusion reaction, it will blow itself up and snuff itself out in moments.
It is extraordinarily hard to get fusion going outside a star. Even the biggest explosion of all—the big bang—couldn’t get fusion going for more than a few minutes. In the first seconds after the big bang, all the matter in the universe—lots of fundamental particles, including a whole bunch of protons—was contained in a relatively small, intensely hot space. Protons—hydrogen nuclei—fused to create helium. But the universe was expanding rapidly because of its own explosive energy. After about three minutes, the universe had expanded so much that the matter wasn’t dense enough to fuse anymore. As hot and dense as the early universe was, it could only sustain fusion for a measly three minutes. After that, there wasn’t any fusion going on in the universe, not until it occurred again in the heart of a solar furnace. To get fusion going on Earth, you must create conditions that are as hot as the first few minutes after the big bang. And without the massive gravity of a star, it is nearly impossible to keep those conditions going for very long.
This was the major obstacle to designing the hydrogen bomb. Even with deuterium and tritium (or lithium) as fuel—deuterium and tritium are relatively easy to fuse—it is hard to make the fuel hot enough and dense enough to get the nuclei fusing. And if you can initiate fusion, you have to maintain the high temperatures and high density long enough to generate an appreciable amount of energy from it.
Teller’s first design would not have worked. Even with the power of an atomic weapon behind him, he would have found it difficult to get the hydrogen fuel hot enough and dense enough to ignite. As a substance heats up, it radiates energy more rapidly. In fact, the radiation goes up as the fourth power of the temperature: double the temperature of an object and it radiates its energy sixteen times as fast. To ignite fusion, the fuel has to get to tens or hundreds of millions of degrees (depending on the density of the fuel). Yet even then, it will radiate its energy away at a tremendous rate; it is almost as if everything in the universe is trying to cool it down. And if he had been lucky enough to ignite fusion, he would have been unable to keep the reaction from blowing itself apart with its own energy just as it got going.19
The Alarm Clock design was the simplest way to get around these problems. This bomb was to be like a spherical layer cake with alternating layers of heavy, fissioning material and light, fusionable hydrogen isotopes. By imploding the whole thing symmetrically, making sure that nothing squirted out, the hydrogen would get hot and dense enough to ignite. It would fuse for a tiny fraction of a second before the whole package blew itself apart. Teller dreamed up the concept in 1946 and discarded it as impractical. In Russia, the physicist Andrei Sakharov came up with a very similar design and called it the “sloika” after a Russian layer cake. The sloika was the basis of the Joe-4 test, but the design was eventually abandoned because megaton-size bombs became too large to use as weapons. Within a few years, Sakharov, like Teller and Ulam in the United States, figured out a much cleverer way to ignite a fusion reaction for a short time.
On the surface, it might seem that America’s first fusion bomb, Ivy Mike, was little different from Teller’s original “bomb at the end of a tank of hydrogen” design. But in fact, it was impossible to ignite a cylinder full of fuel in the way that Teller had hoped; more energy was radiated away by the expanding fireball than the fusion was producing, so the reaction would snuff itself out very quickly. The Teller-Ulam design had some subtle techniques to avoid this problem.
In a bomb like Ivy Mike, the fission bomb that starts the reaction (the primary) is a distance away from the cylindrical vessel containing deuterium and tritium (the secondary). As the fission bomb explodes, it radiates a huge number of x-rays in all directions. These x-rays, being light waves, travel at light speed and move much faster even than the blast wave coming from the fission bomb. As the atom bomb explodes, the x-rays course through a channel left in the casing that houses the primary and secondary. The x-rays then vaporize a plastic shell, turning it into a plasma, a hot soup of nuclei and electrons. This superhot plasma radiates more x-rays, which strike a heavy pusher surrounding the fuel, compressing the fuel from the outside. As the fuel cylinder compresses, it heats up, getting denser and denser. The compressing plasma soon ignites a small “spark plug” of fissionable material at the center of the cylinder, generating a second fission explosion that squeezes the fuel from the inside. The deuterium and tritium are caught between a pusher that is pushing inward and the spark plug explosion pushing outward. The fuel compresses even further and, whoomp! The fusion reaction ignites. It’s as if there’s a tiny hunk of the sun on Earth.
DETONATION OF A HYDROGEN BOMB: (a) A fission bomb explodes at one end of the device, sending x-rays in all directions. (b) The x-rays strike the walls of the container, causing them to evaporate and radiate more x-rays. These x-rays hit the container containing deuterium and tritium, causing it to implode. (c) The compressing deuterium and tritium fuel heats up and ignites a fission “spark plug” at the center of the device, causing it to explode outward. Trapped between the imploding container and the exploding spark plug, the fuel ignites in a fusion reaction.
The reaction only lasts for a fraction of a second before it blows itself apart, but in the process it releases an enormous amount of energy. Ivy Mike was equivalent to ten megatons of TNT, but there was no reason why the whole device could not have been scaled up by adding a third stage . . . or a fourth. In 1961, the Russians detonated a (roughly) fifty-megaton whopper nicknamed the Tsar Bomba, the most powerful weapon ever built by man.
In theory, there was no end to the power of fusion. But the race to build ever-bigger hydrogen bombs crept to a halt, because it had diminishing returns. As early as 1949, scientists realized that after about 150 megatons, hydrogen bombs simply take a huge column of air and lift it into outer space, punching a hole in the atmosphere about fourteen miles across. Bigger bombs would not do much more than that. They would radiate most of their energy uselessly into space. So after 150 megatons, there was no point in getting bigger, unless you wanted to build a fusion device large enough to destroy the Earth. Not even the most rabid hawks were in favor of that.
Nevertheless, with Ivy Mike and its successors, the fusion bomb scientists had succeeded at creating a tiny star on Earth. For a fraction of a second, scientists were able to get a fusion reaction going. They had figured out how to use that energy for war. It would be much, much harder to harness that energy for peace.