An acquaintance of Oppenheimer’s at Berkeley said that students considered the physicist “something like a god,” especially given his tall stature, arresting blue eyes, wealth, and extensive interests outside of science, including painting, Sanskrit, and reading Plato in the original Greek. He was a charismatic figure, generous in sharing credit with his students. Yet, he was also a man haunted by personal demons and insecurities.
J. Robert Oppenheimer (Los Alamos Scientific Laboratory, courtesy of American Institute of Physics Emilio Segrè Visual Archives)
Oppenheimer was not a mind-blowing, theoretical innovator—not in the way others, such as Werner Heisenberg, broke radically new ground in modern physics. There is no Oppenheimer Uncertainty Principle. He did work with Max Born to forge the Born-Oppenheimer approximation, still used to calculate the quantum behavior of molecules, but Oppenheimer mainly specialized on calculations that could explain ongoing atomic physics experiments—yeoman work that was necessary and vital but did not often gain the spotlight. Many of his theoretical papers are now out-of-date or largely forgotten. It was the fleeting detour that he took at the end of the 1930s into the world of astrophysics that generated the scientific papers that are most referred to today. What he and his students did was tug the Schwarzschild singularity into the real world.
It’s not surprising that his work in atomic and nuclear physics theory would draw him into the cosmic arena. As noted earlier, in the 1930s physicists were struggling to explain how the Sun and stars could be powered over billions of years, and it was already apparent that the source had to be a nuclear process that released energy as atoms combined. A number of physicists had made note of this possibility a decade earlier. The question was the precise pathway. Landau had proposed an entirely different scheme altogether, primarily because theorists were encountering so many obstacles in showing how atoms could fuse within the Sun. So much so that Landau was convinced that the physics done by leading astrophysicists, such as Eddington, was irrational.
For years, Oppenheimer kept his eye on the field and eventually helped organize a symposium on the astrophysical significance of nuclear transformations at the 1938 joint meeting of the American Physical Society and the American Association for the Advancement of Science. But at that very moment Hans Bethe at Cornell University was completing his historic paper that revealed the first bona fide route for a star to fuse its hydrogen into helium and generate nuclear energy, an achievement that led to Bethe’s receiving the Nobel Prize in 1967.
Seeing that he was scooped, Oppenheimer turned his attention to the other end of a star’s life—its death. By then Fritz Zwicky had suggested that a star blowing up would leave behind a dense ball of neutrons, as the protons and electrons in the collapsing core were crushed together. Landau at the same time was talking of stars having “neutronic cores.” Chandra had required only special relativity to arrive at his limit for the white dwarf star. But the neutron star was an arena where general relativity was necessary to get answers. With the neutron star’s high density and intense gravitational field, Newtonian laws of gravity were no longer adequate. To carry out this endeavor, Oppenheimer joined forces with his graduate student George Volkoff (with Tolman consulting from the sidelines). The pair worked out a full, general-relativistic treatment of how such a neutron core might form. In this precomputer era, Volkoff labored over a calculating machine to carry out the intricate number crunching. In the end, they proved that neutron stars, in all probability, were inhabiting the universe. Zwicky was right. But you wouldn’t know that from reading Oppenheimer’s writings on the topic. The very title of his 1939 Physical Review paper with Volkoff refers, not to neutron stars, but to “neutron cores,” favoring Landau’s description. And Oppenheimer made sure to not cite anything by Zwicky. It is Landau’s work that is generously credited throughout the eight-page paper.
Zwicky, hearing of their work, was furious that they were not conferring with him. The prickly physicist figured he was the world’s expert on this topic. By then he was carrying out systematic searches for supernovae, his suggested birthplace for neutron stars, using a special large-field telescope on California’s Palomar Mountain. He fired back at Oppenheimer with a pedestrian article that same year titled “On the Theory and Observation of Highly Collapsed Stars” in the same journal, with not one reference to Oppenheimer and Volkoff’s groundbreaking paper. That was Zwicky’s tit for Oppenheimer’s tat. Zwicky’s paper is largely forgotten today. He had the chutzpah and courage to speculate—properly getting the credit for imagining a neutron star emerging from the spectacular explosion of a star—but it was Oppenheimer and his brilliant student Volkoff, notes Caltech theorist Kip Thorne in his book Black Holes and Time Warps, who had the theoretical savvy to be the first to master the physics of this strange new stellar object. Thorne calls their paper “a tour de force, elegant, rich in insights, correct in all details.”
This pioneering paper was also titillating in its final conclusion. Volkoff and Oppenheimer found that there was an endpoint to the neutron star: past a certain mass, the neutron core would continue to contract—and contract indefinitely. Just as Chandrasekhar found a limit for a white dwarf’s size, Volkoff and Oppenheimer revealed a similar constraint for the neutron star. What becomes of the star if its mass steps past the boundary? “The question of what happens … remains unanswered,” they replied. But no one was panicking as yet. They knew they were just getting started on this problem. It was still possible that the physics of such condensed matter was not fully understood; perhaps new repulsive forces come into play to prevent the ultimate collapse.
To find out, Oppenheimer recruited another graduate student, Hartland Snyder, who had a reputation as a crackerjack mathematician and could handle general relativity with ease. It was a unique pairing. Snyder came from the working class. And “Oppie was extremely cultured; knew literature, art, music, Sanskrit. But Hartland—he was like the rest of us bums. He loved the … parties, where … we sang college songs and drinking songs. Of all of Oppie’s students, Hartland was the most independent,” Caltech physicist William Fowler once recalled. Oppenheimer asked Snyder to take the story further, to find out what happens to that collapsing neutron star, the one that goes past the limit. And the results of this endeavor, Oppenheimer later told a colleague, were “very odd.”
Oppenheimer and Snyder began with a star that has depleted its fuel. And to make the computation easier in that era of clunky, desktop calculating machines, they ignored certain pressures and the star’s rotation. Otherwise, the problem would have been impossible to solve.
With the heat from its nuclear fires gone, the star’s core cannot support itself against the pull of its own gravity, and the stellar corpse begins to shrink. Oppenheimer and Snyder determined that if this core is weightier than a certain mass (now believed to be around two to three solar masses, the kind of cores found in massive stars weighing twenty-five Suns or more), the stellar remnant would neither turn into a white-dwarf star (our own Sun’s fate) nor settle down as a ball of neutrons. That’s because once the material is squeezed to densities beyond four hundred billion pounds per cubic centimeter, the neutrons can no longer serve as an adequate brake against collapse. Degeneracy pressures, this time from neutrons, no longer do the job. Oppenheimer and Snyder calculated that the star would continue to contract indefinitely. There is no rest for the weary when gravity takes over. The matter within such a collapsing star is in a state of permanent free fall.
The last light waves to flee before the “door” is irrevocably shut get so extended by the enormous pull of gravity (from visible to infrared to radio and beyond) that the rays become invisible and the star vanishes from sight. Space-time is so warped around the collapsed star that it literally closes itself off from the rest of the universe. “Only its gravitational field persists,” the Berkeley physicists reported.
They figured that the star collapsed to a point, a singularity squeezed to infinite density and zero volume
(which seems impossible). Their equations indicated this, but they hesitated in saying it directly. That’s because singularities are a horror to physicists. It is a signal that something is wrong with the theory under these extreme conditions, that they had entered a realm where the mathematics being used ceases to be a valid description of the physics. It’s as bad as trying to divide a number by zero. How many zeroes are there in the numbers 8, 29, or 103? There is, of course, no definitive answer. There is a countless number of zeroes; in other words, 29 + 0 + 0 + 0 + 0 + (an endless series of zeroes) still equals 29. It’s a mathematical operation that leads nowhere. Zero divided into 29 equals infinity, which is not a satisfying answer. A singularity in a physics equation, where a parameter flies off to infinity, signals a similar breakdown. Given this predicament, Oppenheimer and Snyder were willing to go only so far. What they did report was bizarre enough. Werner Israel has called this “the most daring and uncannily prophetic paper ever published in the field. … There is nothing in this paper which needs revision today.”
In the title of their paper, Oppenheimer and Snyder called this phenomenon “continued gravitational contraction,” and it established the first modern description of a black hole. But few became aware of it, partly due to unfortunate circumstances. Oppenheimer and Snyder published their paper in the Physical Review on 1 September 1939, the day Hitler ordered his troops into Poland, triggering the start of World War II. No wonder it received little notice. More than that, the same journal issue held a seminal paper by Niels Bohr and John Wheeler on nuclear fission, then a far more urgent topic on physicists’ minds. Collapsing stars seemed of little import by comparison. It was the last paper that Oppenheimer wrote on the subject. With no physics yet developed to follow the collapsing matter into its abyss, what more could he do?
Professionally, it was a transitory detour in Oppenheimer’s scientific life, involving only three papers. Afterward, he went back to his work on nuclear particles and cosmic-ray physics and, by 1942, into the nation’s Manhattan Project aimed at manufacturing the world’s first atomic bomb. His students, after graduation, went off to university teaching positions, never returning to the topic. Most astronomers, if they thought of this problem at all, assumed that massive stars got rid of most of their mass over time—enough to safely keep them as white dwarf stars in old age. Only Fritz Zwicky kept banging the drum about neutron stars, publishing a few papers on the topic. No one took note.
Perhaps stellar winds, said astronomers, carried off a lot of an old star’s mass into space; maybe stellar explosions kept any stellar remnant at one solar mass or less. This was not an unreasonable assumption, for astronomers were just coming to recognize Wolf-Rayet stars, which do just that. These evolved and weighty stars eject tremendous amounts of mass each year, a billion times more than our own Sun, via strong stellar winds.
And even if a celestial object did gravitationally collapse somewhere in the heavens, it was still in essence invisible. No telescope at the time was capable of confirming the existence of a neutron star or black hole. They didn’t know it at the time, but astronomers had to wait for the development of new tools and new techniques for scanning the heavens—to capture electromagnetic waves beyond the visible spectrum.
And what about the general relativists? Weren’t they excited by this new and astounding finding coming out of general relativity? In truth, they weren’t even paying attention. Relativity experts at the time were more interested in the esoterics of curved space-time, not its practical use in astrophysics. The general theory of relativity was then virtually a playtoy for mathematical physicists, fun to delve into but unconnected in their minds to the celestial sky (except, maybe, for the bending of some starlight near the Sun). General relativity at the time was primarily taught as mathematics, not physics. They were interested not in experimental evidence or applications but rather in rigorous proofs.
Even though no one followed up on the gravitational collapse in the West, Landau in the Soviet Union was mightily impressed. He added the Oppenheimer-Snyder paper to his “golden list,” a running tally of classic papers he considered worth checking out.
Many years later physicist Freeman Dyson tried to talk to Oppenheimer about his work with black holes, but the father of the atomic bomb would have none of it. Because Oppenheimer believed that he was merely applying Einstein’s laws to collapsing stars and not unearthing a new law of physics, Dyson suspects that Oppenheimer thought of his accomplishment as worthy only of “graduate students or third-rate hacks.” He didn’t recognize his deed as a theoretical triumph. But Dyson strongly disagreed. He described Oppenheimer’s paper on continued gravitational contraction as his “most important contribution to science … a masterpiece of derived science, taking some of Einstein’s basic equations and showing that they give rise to startling and unexpected consequences in the real world of astronomy.”
Yet as the 1930s were coming to a close, most astronomers were not yet ready to believe that such bizarre objects could be generated in the real world. Even Einstein wrote a paper in 1939, published a month after Oppenheimer and Snyder’s, attempting to prove they were impossible to form. His calculations would have arrived at the same answer as the California theorists, except that Einstein stacked the deck and arranged his model in such an unrealistic way that his star could never collapse. “One could not be sure,” wrote Einstein, “that … assumptions have been made which contain physical impossibilities.” Okay, he was saying, you can create a singularity on paper mathematically, but matter likely acts in such a way to thwart the collapse. Einstein tried to prove this by imagining the mass as a “great number of small gravitating particles … resembling a spherical star cluster.” It was a sleight of hand in which the centrifugal force of the particles’ circular motion essentially keeps them from all collapsing to a singular point. And it was just that—an illusion. Infamous for not keeping up with the scientific literature, Einstein had not read Oppenheimer and Snyder’s paper before tackling the problem on his own.
Some historians have labeled Einstein’s 1939 disproof of the singularity as a “strong candidate for the dubious distinction of being his worst scientific paper.” That’s because Oppenheimer and company were spot on. Once a collapsing star gets small enough, nothing in the universe can stop gravity from creating a black hole—no amount of rotational motion or gas pressures. Gravity is the ultimate trump card, overwhelming any internal stellar forces. Why couldn’t Einstein recognize this simple physical fact? Because, says Caltech general relativist Kip Thorne, Einstein “was so firmly convinced they cannot exist (they ‘smelled wrong’; terribly wrong) that he had an impenetrable mental block against the truth—as did nearly all his colleagues.” It was the “mindset of nearly everybody in the 1920s and 1930s,” he says. In their heart of hearts, most physicists wanted to uncover a law of physics that forbids black holes from forming. Trained in Victorian times, these scientists had to step over formidable psychological hurdles before accepting something so unexpected in nature.
“There is a curious parallel between the histories of black holes and continental drift,” Werner Israel has noted. “Evidence for both was already non-ignorable by 1916, but both ideas were stopped in their tracks for half a century by a resistance bordering on the irrational.” Israel blames the threat each concept posed to our cherished faith in the permanence and stability of matter. Whole continents wandering about the Earth like chess pieces? Stars disappearing from space and time? Surely that must be poppycock!
Given today’s thriving interest in an Einsteinian universe, it’s now difficult to appreciate this line of thinking. But this dismissive attitude came about during a period when the theory of general relativity was pushed into the shadows, admired from afar for its mathematical beauty but largely ignored. Theorists revered Einstein’s equations (almost as sublime mathematical sculpture), but they were not actively engaged in working with them in any way. That was especially true in Germany once the Nazis rose to power. As p
art of their campaign against “Jewish physics,” they officially forbade relativity to be taught throughout the Third Reich. But, politics aside, universities around the world rarely offered general relativity as a course, and if they did, it was taught as mathematics, not physics. Most theoreticians at the time were more focused on quantum theory, with its new and revolutionary perspective on matter and energy. And outside the tight-knit world of theoretical physics, Einstein’s theory of gravitation was actually unpopular. “It was despised, occasionally even abhorred, by the specialists in other fields of physics,” says physicist and historian Jean Eisenstaedt. This is because it dealt “with some tricky concepts that the ordinary physicist finds difficult to understand.” It requires us to think of space and time in a way that fights against our everyday experiences of how the world works—“probably also our brain processes,” notes Eisenstaedt.
Moreover, the myth had arisen soon after general relativity’s introduction that less than a handful of people truly comprehended it. Arthur Eddington himself liked to tell the tale of being approached at a Royal Society meeting and told, “Professor Eddington, you must be one of three persons in the world who understands general relativity.” When Eddington hesitated, the questioner continued, “Don’t be modest.” To which Eddington replied, “On the contrary, I am trying to think who the third person is.”
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