Black Hole
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Soon after Baade transferred to the Mount Wilson staff in 1931 to use the world’s largest telescopes to pursue his observations of supernovae (among a bevy of other interests), physicist Fritz Zwicky from the nearby California Institute of Technology joined up with him as a collaborator. Born in Bulgaria of Swiss parents in 1898, Zwicky was educated in Zurich and remained a Swiss national all of his life. He had arrived at Caltech in 1925 as a research fellow in the physics department, where he studied the physical properties of liquids and crystals. But that was just for starters. Restless in his interests, he eventually rose to a professorship and published over his lifetime nearly six hundred scientific papers, with a sweeping range of topics: cosmic rays, the extragalactic distance scale, age of galaxy clusters, aerial propulsion, meteors, ionization of gases, quantum theory, elasticity in solids, crystal lattices, electrolytes, gravitational lensing, propellants, and quasars.
Fritz Zwicky (Photograph by Fred Stein, courtesy of the American Institute of Physics Emilio Segrè Visual Archives)
Despite Caltech’s relaxed campus atmosphere, a hallmark of the California lifestyle, Zwicky retained the authoritative air of a nineteenth-century European professor. He was an aggressive, original, and stubbornly opinionated man, the supreme scientific individualist. He regularly annoyed his physics and astronomy colleagues by studying anything he pleased (he called astronomy his “hobby”) and championing along the way some pretty wild ideas, some that waited decades to be proven true. In 1933 he was the first to propose, for example, the existence of cosmic “dark matter” (what he called in German “dunkle Materie”), today one of astronomy’s outstanding mysteries. “Zwicky was one of those people,” recalled Caltech astronomer Wallace Sargent, “who was determined to show the other guy was wrong. His favorite phrase was, ‘I’ll show those bastards,’” which he did to the fullest.
Baade and Zwicky were astronomy’s odd couple: where Zwicky was cranky, imperious, and for the most part a solitary researcher, Baade was soft-spoken, even-tempered, and a team player. Yet, sharing a common language and cultural heritage, the two became fast friends and could often be seen about town talking endlessly about novae (until they had a terrible falling out years later).
Some of their best work together took place in 1933. While Chandrasekhar in England was reluctant to speculate on what might happen to a star more massive than the typical white dwarf star, Zwicky was quick to offer a suggestion. Just the year before in Great Britain, James Chadwick had bombarded atomic nuclei with high-energy radiation and succeeded in wrenching out some particles, which had all the properties of a particle that theorists were suggesting might exist. Each had just about the same mass as a proton but with no electrical charge. This particle was neutral—hence its name, the neutron.
On hearing this news from the world of particle physics, Zwicky, in his usual madcap way, figured he could use this newfound particle to explain how a supernova ignited. Somehow (Zwicky didn’t yet know the exact route), a stellar core would get squeezed and squeezed over time, until it reached a tremendous density, just like that of an atomic nucleus. The negatively charged electrons and positively charged protons in the stellar core, in this event, would be pressed inward to form a naked sphere of neutrons. “Such a star,” he and Baade wrote in the Proceedings of the National Academy of Sciences, “may possess a very small radius and an extremely high density.” In fact, a width not much longer than a city. It seemed natural for Zwicky to call it a “neutron star.”
Since Zwicky’s day, astronomers have now worked out that route to a supernova. It all depends on how massive the original star is. An average star over its lifetime carries out an amazing balancing act. Gravity is continually pulling the matter inward, trying to squeeze it down tighter and tighter. But at the same time the tremendous pressure of the star’s hot gases pushes outward. What results is a stable star emitting light and energy into the universe. Our Sun has walked this tightrope for some five billion years and will do so again for another five billion. But there’s an end to this road. The hydrogen atoms being fused into helium are eventually exhausted, gravity takes over, and the core shrinks. With this gravitational energy liberated, the outer envelope of the star expands outward and cools, thus creating a giant star, no longer yellow but now a cooler red. At this point, helium takes over as the fuel.
For our Sun the helium, too, will ultimately be used up, yet the nuclei will fuse further—generating carbon and oxygen. But that is the Sun’s endpoint. The Sun is not massive enough to fuse those atoms into heavier elements. It will run out of fuel. When that happens, its red giant envelope will eventually whisk away, and what is left—the hot core—will remain behind as a white dwarf star, about as big as the Earth. With the nuclear engine turned off, this stellar nugget, around three-fifths the mass of the present-day Sun, will begin slowly to cool. A spoonful of Earth has an average density of five grams per cubic centimeter. In a white dwarf, it can range from ten thousand to a hundred million grams per cubic centimeter. Gravity is the ultimate cosmic vise, capable of packing the mass of a high-rise building into the space of a sugar cube. Ultimately, “electron pressure” keeps the white dwarf from compacting further. The electrons don’t budge; they’re still powerful enough to hold gravity at bay.
But what happens if the star is more massive than the Sun? First of all, the star can continue to burn beyond carbon and oxygen. The atoms fuse into neon and magnesium; these in turn serve as the raw materials for constructing even heavier elements, such as silicon, sulfur, argon, and calcium. If the star is massive enough, this can go on and on until ultimately iron is formed. But that’s the end of the line. The star’s terminus. Its final depot. Fusing iron takes more energy than it will release. So at this critical moment the star faces its waterloo. Unable to generate any more energy, gravity takes charge. In fact, it enters the scene like gangbusters. As soon as a stellar core turns to iron, it collapses catastrophically. In less than a second, a core that was once the size of the Moon is squeezed down to the size of a city. That’s right—in less than a second.
How could this be? It’s because the electrons can no longer hold up against gravity. In the course of the titanic stellar collapse, each electron ends up merging with a proton to form a neutron, a neutral particle, releasing a flood of tiny neutrinos in the process. What forms is a neutron star about a dozen miles (twenty kilometers) wide. This sphere is so dense that it’s essentially one humongous atomic nucleus, more than 100,000,000,000,000 times denser than the Earth. (It was once estimated that your bathroom sink could hold all the water of the Great Lakes if the water were compressed to the density of a neutron star.) If a mountain did form on the neutron star, it couldn’t get any higher than a few centimeters, given the strength of its gravitational field. In this situation it is now the strong nuclear force that begins to play a role in holding the star in place, resisting the pull of further gravitational squeezing. Nuclear forces are a major defense against the relentless tug of gravity to make the star even smaller.
The evidence for all this is in the grand announcement of this event: the shock wave sent out from the collapse, along with the torrent of neutrinos, speeds through the remaining stellar envelope. When they emerge at the surface, we see the result as a spectacular explosion—a supernova, the birth announcement for the neutron star, just as Zwicky had predicted in the 1930s. And in the process, elements beyond iron are forged within the chaotic turbulence of the explosion’s cloud.
Zwicky, of course, did not know all these details at the time. He just figured that the supernova was fueled by the tremendous energy loss as the stellar core got smaller and smaller, somehow releasing that power in a stupendous burst. But his embryonic vision of the process was still an astounding and prophetic prediction; the neutron star wouldn’t be confirmed for another three decades. In Zwicky’s day, neutron stars remained theoretical fabrications, which astronomers figured would never be seen even if they did exist, due to their extremely small size. (That al
l changed when the first bona fide neutron star, beeping away as a radio pulsar, was at last discovered by the British astronomer Jocelyn Bell in 1967.)
Baade and Zwicky first presented their prescient ideas at a meeting of the American Physical Society at Stanford University in December 1933. Astronomers liked the idea of the supernova—an extra powerful stellar explosion—but considered the concept of the neutron star far-fetched and wildly speculative. They figured that supernovae allowed extra-massive stars to eject enough mass to settle down as white dwarfs. Astronomers had barely gotten comfortable with the idea that matter could be crushed to huge densities within a white dwarf star. As a consequence, hardly anyone took the neutron star seriously—except for a few brave souls. Chandra remarked on the possibility at a Paris conference in 1939, agreeing that the formation of a neutron core “may be the origin of the Supernova.” But he didn’t immediately follow up. Those who did included Lev Landau in the Soviet Union and, at the University of California, Berkeley, J. Robert Oppenheimer, who went on to become the father of the atomic bomb. That these two physicists didn’t dismiss the neutron-star idea right off, but instead pursued it, was a crucial turning point for the black-hole story. It led researchers, at least a few at first, to suspect that the cosmos might well be generating those danged singularities.
6
Only Its Gravitational Field Persists
It was a bleak and despairing time for the Soviet Union in the late 1930s. The Great Purge initiated by Joseph Stalin was at its zenith, and Lev Landau figured he was in the crosshairs, despite being a fervent Marxist. When rules for Soviet scientists were looser in the 1920s, Landau had spent time in Western Europe, visiting the top university centers in physics. He was the topsy-turvy-haired wunderkind whose research articles on a wide range of physics problems were noted for their creative insight and mathematical dexterity. Landau’s genius was recognized by everyone who met him. But soon after he returned in 1931, it became a crime for Soviet scientists to maintain any contacts with the West, for fear of capitalist contamination. Just having visited the West—even in an earlier, more liberal era—made Landau suspect.
By 1937 the purge was reaching beyond the Communist Party into the intelligentsia. As a result Landau, twenty-nine years old at the time, decided to put his current work on atomic physics, magnetism, and superconductivity on hold and to take another look at the problem of stellar energy, hoping for a breakthrough in one of physics’ greatest challenges. An astute practitioner in the art of academic politics, Landau reckoned the scientific glory that was bound to come if he figured out how a star was powered might protect him from getting arrested. Many of his colleagues had already been caught up in the sweep. The worldwide attention he’d receive for a scientific triumph, he believed, would force officials to spare him.
Striving for a completely new approach, one that didn’t involve astrophysicists’ standard stellar models, Landau arrived at the conclusion that stars have a “neutronic core.” While Zwicky considered that the formation of a neutron star was the trigger for a supernova, Landau concluded that atomic nuclei and electrons combine to form neutrons in the dense heart of every normal star. According to Landau, this made the core more compact, which liberated enough energy to power a star over eons. Landau’s friend the physicist George Gamow, in a book he published that year on atomic physics, calculated that the density of such a stellar core would be around a hundred trillion grams per cubic centimeter, “analogous to the conditions inside an atomic nucleus.” The gravitational energy liberated as the core squeezed down to such an immense density, Gamow went on, would “be quite enough to secure the life of the star for a very long period of time.”
Lev Landau (American Institute of Physics Emilio Segrè Visual Archives, Margrethe Bohr Collection)
Landau mailed his manuscript to Niels Bohr in Copenhagen. As Bohr was an honorary member of the Soviet Academy of Sciences, this was still an allowable route for getting work noticed in the West. Bohr passed it along to the scientific journal Nature, which published the paper on 19 February 1938. In it, Landau claimed that “we can regard a star as a body which has a neutronic core the steady growth of which liberates the energy which maintains the star at its high temperature.”
Once in print, Landau shrewdly devised a public relations campaign to spread the word. Through contacts in high places, he got one of the USSR’s most influential newspapers to praise his scientific paper, which described it as a “bold idea [that] gives new life to one of the most important processes in astrophysics.”
It was a good try—but not good enough. Landau’s political strategy—Bohr’s backing, the glowing press coverage, publication in a prestigious journal—failed miserably. (That he had prepared an anti-Stalinist leaflet to hand out during 1938’s May Day parade likely contributed to the outcome.) Landau was eventually arrested and imprisoned for a year on the ludicrous charges that he, a Jew, had been spying for Nazi Germany. Not until the intervention of the noted Soviet physicist Pyotr Kapitsa, who stridently told Soviet authorities that only Landau could explain a newly discovered phenomenon called superfluidity, was he released. And Kapitsa was right. Landau did come to solve how some supercooled liquids can flow without friction, for which he received the Nobel Prize in 1962.
Although Landau proved brilliant on superfluids, he failed on stellar energy. His physics in that arena was seriously flawed. The very next year in 1939, the German-American physicist Hans Bethe cracked the mystery of how the Sun shines. He was the first to devise a plausible pathway for stars to generate their immense energies through the fusion of atoms rather than the release of gravitational energy.
Still, Landau’s Nature article was highly influential in advancing the story of the black hole. The paper arrived at the desk of Caltech theorist Richard Tolman, a world expert on general relativity, who enthusiastically embraced the idea of a neutron star. Tolman saw it as a problem crying out to be solved, and he urged his colleague J. Robert Oppenheimer to apply Einstein’s space-time equations to collapsing stars. Oppenheimer was already aware of Landau’s model of high concentrations of neutron matter and intrigued by it. Encouraged by Bethe, who was visiting Berkeley, Oppenheimer worked with Robert Serber in the summer of 1938 to check Landau’s paper out. They quickly figured that normal stars, such as the Sun, could not possibly harbor neutronic cores; otherwise, they would look very different. The Sun, for example, would be far smaller, owing to the huge gravitational pull of an ultradense center.
Even though Landau’s central idea failed when it came to explaining stellar power, Oppenheimer was inspired by its description of dense stellar cores. If they couldn’t resolve how stars shine (Bethe took care of that), Oppenheimer began to wonder whether Landau’s neutron cores played a role at the end of a star’s life. Could Zwicky be right after all?
Like any good physicist, Oppenheimer reduced the problem to its essentials. He ignored any talk of stars blowing up. To Oppenheimer, people like Zwicky, a physicist he did not greatly admire, did that sort of grandstanding. Oppenheimer focused solely on the neutron star itself. What was its physics? Chandrasekhar had discovered that a white dwarf star needs to stay under a certain mass before it transforms into something else: Did a neutron star have a similar limit? Though Oppenheimer dealt with these stellar questions only briefly in his professional life, they led to some of his greatest achievements in theoretical physics, a choice of fields one might not have expected based on his family background.
Oppenheimer had grown up on the Upper West Side of New York City amid privilege and comfort. His father had secured the family’s wealth from the textile trade. As a young boy, Oppenheimer was driven to private school by a uniformed chauffeur in a limousine. A solitary child (a younger brother didn’t arrive until he was eight), young Robert was particularly fascinated by rocks and minerals, and he filled the family’s Manhattan apartment with specimens.
At Harvard as an undergraduate, he was first drawn to chemistry. Finding himself inept
at experimental lab work, a failing he freely acknowledged, he was soon attracted to more theoretical pursuits, especially in physics. Quantum mechanics was revolutionizing physics in the mid-1920s, and Oppenheimer, eager to jump on the bandwagon, arranged to do his graduate work in Europe, first at Cambridge University in England and then at the University of Göttingen in Germany, becoming acquainted with the field’s all-stars, among them Paul Dirac, Niels Bohr, and Max Born.
Although Oppenheimer missed the first wave of quantum mechanical revelations, the sort that led to bevies of Nobel Prizes, he did become immersed in the second wave: the attempt to join special relativity theory to quantum mechanics. Dirac’s prediction that “antimatter” existed came out of these explorations.
After obtaining his doctorate, Oppenheimer returned to the United States just as business and government were pushing to enhance graduate science programs in the nation’s universities. With his golden European credentials, Oppenheimer was a hot prospect, and in 1929 he secured a joint position with the California Institute of Technology and the University of California at Berkeley. What he ultimately established on the West Coast through the 1930s and into the early 1940s was one of the best “schools” for theoretical physics in the world, with the most gifted students in the field flocking to both campuses to work with him. He and his students dealt with either the new particles and forces revealed by Ernest Lawrence’s cyclotron at Berkeley or the astronomical and astrophysical discoveries unveiled by Caltech professors. “What really made this school a success was Oppenheimer himself—not so much for his brilliant physics or his classroom skill or his administrative maneuvering, none of which was of the first rank, but for his unique, European-acquired ability to select the most promising problems for his group and to inspire and guide his charges to the leading edge of research in these areas,” explains science historian David Cassidy.