The Perfect Theory (11 page)

Every summer Oppenheimer would head down to Southern California with his crowd of students and researchers and take up residence at Caltech, in sunny Pasadena. There he could talk to not only the other physicists but also the astronomers who had followed Hubble's success and had witnessed Lemaître's lectures on the primeval atom at first hand. There, they still held a flame for general relativity. It was in Pasadena that Oppenheimer first read a paper by the Russian physicist Lev Davidovich Landau on what would happen if the cores of stars were purely made of a compact mess of neutrons.

Landau was one of the leading lights in Soviet physics, growing up during the Russian Revolution, a truly brilliant physicist who benefited from the wave of modernization sweeping through the new Russia. Like Oppenheimer, he had spent time abroad, studying in the great laboratories of Europe and witnessing the birth of quantum physics. At nineteen he had already written a paper applying the new physics to the behavior of atoms and molecules, and when he returned to Leningrad, at twenty-three, he had earned the admiration of his older colleagues and was rapidly embraced by the Soviet system.

With his flair for solving difficult and complex physical systems with quantum physics, Landau had decided to look at a novel source for energy in stars: neutrons, the neutrally charged particles found in the nuclei of atoms. Over the previous decade, it had become clear that adding neutrons or protons to or removing them from nuclei could lead to a copious amount of
nuclear
energy. So Landau conjectured that if the cores of stars could be packed with neutrons, it might be possible to unleash enough nuclear energy to generate light. If the neutrons were packed together to a density that resembled that of the nucleus of an atom, they might just be the necessary fuel. This nuclear material would be impossibly heavy—a teaspoon of material would weigh tons. If an atom in the stars' bulk fell into the core, it would be smashed to smithereens, partly absorbed, and partly released as radiation. According to Landau, the neutron core fueled a star's brightness—it was what made the sun shine. Landau proceeded to work out how big the core had to be and that for such a core to be stable, it just had to be heavier than a thousandth of the weight of the sun. These cores could be tucked away at the center of stars, burning away and fueling starlight.

But as Landau was writing up his idea, he was also getting caught up in the wave of political repression that was sweeping the country. Two months after Landau published his short paper on neutron cores, “Origin of Stellar Energy,” in
Nature,
he was arrested by the NKVD. He had been caught editing an anti-Stalinist pamphlet to be distributed at the 1938 May Day parade in Moscow in which Stalin was accused of being a Fascist
“with his rabid hatred of genuine Socialism” who had “become like Hitler and Mussolini.” Landau was incarcerated for a year in the Lubyanka prison, just after his
Nature
paper was feted in
Izvestia,
one of the main Soviet newspapers, as a source of pride for Soviet physics.

Oppenheimer was intrigued by the brevity of Landau's paper and the simple idea being proposed, so he decided to redo Landau's calculations himself. It took three collaborations with three gifted students, but he eventually got where he wanted to go. His first collaborator was Robert Serber. Together, they gently pulled apart Landau's idea that the neutron core could be easily tucked away in the sun, shrouded by the hot gases that puff the stars up, and showed that it was wrong. Oppenheimer and Serber published their letter, almost as short as Landau's, in October 1938 in the
Physical Review,
while Landau languished in the Lubyanka. Oppenheimer then took the next step with another student, George Volkoff. The pair studied the stability of neutron cores. Their calculation, published in January 1939, is a beautiful mix of mathematics using clever simplifications of Einstein's theory, with insightful physical intuition and hard calculations. They showed that neutron cores were incredibly unstable configurations and hence couldn't even be invoked to fuel the energy of very large stars, yet another blow for Landau's idea.

At the end of their paper, Oppenheimer and Volkoff pointed out that
“a consideration of non-static solutions must be essential” to understand the long-term fate of the neutron cores. Then Oppenheimer set off to do the last piece with yet another student, Hartland Snyder, this time taking general relativity far beyond what anyone had ever attempted. Oppenheimer and Snyder calculated how space and time (and the neutron core) would evolve once the neutron star became unstable. To do so they used a clever idea to understand the results that they were getting: they placed a fictitious observer very far away from the implosion and another fictitious observer right on the surface of the neutron core and compared what those observers would see. They found that the two observers would see very different things.

A distant observer would see the neutron core implode. But as the surface of the neutron core got closer and closer to the strange shroud that Schwarzschild had found, the collapse would seem to proceed more and more slowly. At some point the implosion would be so slow that it would almost have ground to a halt. The wavelength of any light beam trying to escape from the neutron core would be stretched, redshifting more and more the closer the surface of the neutron core contracted to the critical surface. It would be as if space and time had stopped evolving, and the star would cease to communicate with the outside world. It was very similar to what Eddington himself had said more than a decade before in his book
The Internal Constitution of the Stars:
“The mass would produce so much curvature . . . that space would close up round the star, leaving us outside (i.e. nowhere).”

An observer riding the surface of the star as it imploded would see something completely different. He or she would witness the inexorable collapse of the neutron core, see the surface of the neutron core actually
cross
the critical radius and fall into the inner region of Schwarzschild's magic surface. And furthermore, this poor, doomed observer would see the formation of the dreaded surface that Schwarzschild had found, the point of no return from which nothing could exit. In other words, if you could sit at the right (or wrong) place, you could see the actual formation of Schwarzschild's solution.

Oppenheimer and Snyder had completed Eddington's life story of stars by showing that, indeed, if they were massive enough, they would collapse to form Schwarzschild's strange solution. It meant that Schwarzschild's solution might not be just an interesting, exotic solution to the general theory of relativity. These strange objects might actually exist in nature and had to be included in astrophysics, just like the study of stars, planets, and comets. Once again, general relativity had potentially revealed something unexpected and wonderful about the universe.

 

Oppenheimer and Snyder's paper was published on September 1, 1939, in the
Physical Review,
on the day Nazi troops marched across the Polish border. In the exact same issue was another paper, this one by a Danish physicist named Niels Bohr and his young
American collaborator, John Archibald Wheeler. While they were also interested in neutrons and how they interact in extreme situations, the topic of “The Mechanism of Nuclear Fission” was completely different. Bohr and Wheeler were interested in modeling the structure of very heavy nuclei, such as those of uranium and its isotopes. If they could get this right, it might be possible to figure out how to extract the enormous amounts of energy locked up inside.

Throughout the 1930s, the zoo of atomic nuclei had begun to be understood in ever-increasing detail. Eddington had proposed that hydrogen nuclei could fuse together to form helium in the cores of stars, fueling starlight. This is known as nuclear fusion. On the other end of the range, it was believed that very heavy nuclei could be split into smaller nuclei, also releasing energy—in this case the process is known as nuclear fission. A question that was on everyone's mind was how to make nuclear fission efficient. Would it be possible to trigger nuclear fission in a clump of heavy atoms with a small amount of energy so that as each individual atom split, it would trigger yet another split? In other words, was it possible to trigger a chain reaction?

Bohr and Wheeler's paper pointed the way to nuclear fission and helped other physicists understand why uranium-235 and plutonium-239 might be the elements of choice to work, the sweet spot in the periodic table where fission might actually be easier to accomplish. Nuclear fission would dominate physics during the years that followed, eclipsing almost all other fields. An army of brilliant scientists turned their intellects to trying to understand how to harness fission, and Robert Oppenheimer was among them.

Oppenheimer, during his stay at Berkeley, had built a stunning group of young researchers and students who were willing to tackle any problem. He had developed a formidable reputation as an organizer and group leader and would deploy his leadership skill to marshal his team to solve problems that were of interest to him. His colleagues at Berkeley were beginning to synthesize the heavier, unstable elements in the cyclotron up on the Berkeley Hills. In 1941, one of his colleagues, Glenn Seaborg, discovered plutonium, opening one of the routes to fission. Oppenheimer was being caught up in the whirlwind of events and discoveries that characterized the development of nuclear physics during the Second World War.

Oppenheimer was also outraged. The reported treatment of Jews in Germany and the diaspora of brilliant scientists fleeing Nazi oppression who were washing up on American shores shocked him. As he developed his group at Berkeley, he also started to look around him, tentatively engaging with the teeming intellectual activity of the influx of European refugees. Although he refrained from being too active politically, he began paying attention. And with the onset of the war, nuclear fission became one of Oppenheimer's main concerns.

In 1942, Oppenheimer was asked to lead a task force of physicists based in Los Alamos, New Mexico, whose sole purpose was to produce and control a chain reaction of nuclear fission. The task force included a host of young and not-so-young brilliant minds, from John von Neumann, Hans Bethe, and Edward Teller to the young Richard Feynman. The Manhattan Project focused its resources on producing the first atomic bomb, and in just under three years they had achieved their goal. When the two atomic bombs, “Little Boy” and “Fat Man,” were dropped on Hiroshima and Nagasaki in August of 1945, around two hundred thousand people were killed. These devastating consequences were a harrowing testament to Oppenheimer's ability to harness the nuclear force in such a short period of time. With the success of the atomic bomb, the quantum firmly took center stage in the world of physics.

With so much attention focused on the war and the nuclear project, Oppenheimer and Snyder's seminal paper on black holes was kicked into the long grass, to be ignored and forgotten for years to come. What could have been the auspicious birth of one of general relativity's greatest concepts was indefinitely put off. The two grand old men of general relativity, Albert Einstein and Arthur Eddington, did nothing to save Oppenheimer and Snyder's finding from obscurity.

Eddington continued to insist that Chandra's calculation was wrong and misguided and that white dwarfs were the quiet endpoint of stellar evolution for stars of any mass. The continued unfettered collapse of a star until
“gravity becomes strong enough to hold in the radiation” was simply absurd. Chandra recalled, almost half a century later, “For my part I shall only say that I find it hard to understand why Eddington, who was one of the earliest and staunchest supporters of the general theory of relativity, should have found the conclusion that black holes may form during the natural course of evolution of the stars, so unacceptable.”

Einstein himself continued resisting the idea that the extreme form of Schwarzschild's solution—black holes—had any place in the natural world. He reacted in much the same way as he had to Friedmann and Lemaître's proposal of an expanding universe: it was beautiful mathematics but abominable physics all over again. After more than twenty years dismissing the more outlandish features of Schwarzschild's solution, he finally sat down and tried to come up with a reasoned argument for why they were of no physical significance in nature. In 1939, the same year Oppenheimer and Snyder devoted to determining the consequences of gravitational collapse, Einstein published a paper in which he worked out how a swarm of particles would behave as they collapsed through gravity. He argued that particles would never fall too close to the critical radius. He was too stubborn, setting up the problem in such a way that he got the answer he wanted: no black holes. He was wrong, once again, and just like Eddington he missed an opportunity to explore the full glory of his general theory of relativity.

Almost everyone's attention was elsewhere now, enthralled by the triumph of quantum physics. Most of the talented young physicists were focusing their efforts on pushing the quantum theory further, looking for more spectacular discoveries and applications. Einstein's general theory of relativity, with all its odd predictions and exotic results, had been elbowed out of the way and sentenced to a trek in the wilderness.

Chapter 5

D
URING HIS FINAL YEARS
, Albert Einstein lived a simple life. He would wake up late in his white clapboard house on Mercer Street near the heart of Princeton, New Jersey, where he lived with his sister, Maja. (His wife, Elsa, died in 1936, shortly after his arrival.) During the week, he would walk to Fuld Hall at the Institute for Advanced Study, where he had been based since 1933. Over the years he had become a familiar presence on the Princeton campus. Yet while he was more famous than ever before, he cut a lonely figure.