Scientific ideas can have a life of their own. They can be forgotten, lauded or reworked into something very different from their creators’ original expectations. Personalities and peccadilloes and the unexpected, buffeting currents of history can take scientific discoveries in very unpredictable directions. One very telling example of this is provided by a paper that appeared in the September 1, 1939 issue of the “Physical Review”, the leading American journal of physics.
The paper had been published by J. Robert Oppenheimer and his student, Hartland Snyder, at the University of California at Berkeley. Oppenheimer was then a 35-year-old professor and had been teaching at Berkeley for ten years. He was widely respected in the world of physics for his brilliant mind and remarkable breadth of interests ranging from left-wing politics to Sanskrit. He had already made important contributions to nuclear and particle physics. Over the years Oppenheimer had collected around him a coterie of talented students. Hartland Snyder was regarded as the best mathematician of the group.
Hartland Snyder (Image credit: Niels Bohr Library and Archives)
Oppenheimer and Snyder’s paper was titled “On Continued Gravitational Contraction”. It tackled the question of what happens when a star runs out of the material whose nuclear reactions make it shine. It postulated a bizarre, wondrous, wholly new object in the universe that must be created when massive stars die. Today we know that object as a black hole. Oppenheimer and Snyder’s paper was the first to postulate it (although an Indian physicist named Bishveshwar Datt had tackled a similar case before without explicitly considering a black hole). The paper is now regarded as one of the seminal papers of 20th century physics.
But when it was published, it sank like a stone.
To see why, let’s look at some background. A few months before the paper appeared, the German-American theoretical physicist Hans Bethe had published a paper in the same journal showing that stars like the sun generate energy from the fusion of hydrogen nuclei into helium. Four hydrogen atoms fuse together to form one helium atom, releasing an enormous amount of energy, enough to keep the sun shining for billions of years. But what happens when the hydrogen runs out? The inexorable pull of gravity will then take over, and the sun will get crushed into a dense ball of matter called a white dwarf. But in 1930, the Indian-American astrophysicist Subrahmanyan Chandrasekhar found that there was a limit to how big a star could be before it could settle into a more or less happy state as a white dwarf. That limit was about 1.4 solar masses.
What happens to a star that exceeded the Chandrasekhar limit was worked out by a number of other physicists, including the maverick Swiss-American physicist Fritz Zwicky and his German-American colleague, Walter Baade. Zwicky and Baade postulated that if a star is too massive, instead of settling down as a white dwarf, it would be unstable and undergo a catastrophic gravitational collapse which they called a supernova. This collapse would generate a shockwave that sloughs off the outer layers of the star in an incandescent flash that, in a few seconds, can emit more energy than that emitted by our sun during its entire lifetime. Zwicky and Baade speculated that the remnant of a supernova would be a neutron star, a body with a core made out of just neutrons. Such a neutron star would be incredibly dense – a teaspoon full of its material would weigh a billion tonnes.
But the question remained: what would happen if a star was so massive that even a dense neutron core could not withstand the force of gravity. In early 1939, assisted by his colleagues Richard Tolman and George Volkoff, Oppenheimer worked out a limit for such neutron cores that was about 0.7 solar masses. This was less than the Chandrasekhar limit because Oppenheimer and his colleagues had not modeled the nuclear force holding protons and neutrons together, a force insufficiently understood in 1939 ((modern work puts the Tolman-Oppenheimer-Volkoff limit at 1.5 to 3 solar masses).
But in science, answers only raise more questions. The persistent question still remained: what happens when a star’s mass exceeds the limit necessary to sustain even a neutron star? It was this question that Oppenheimer and Snyder tackled in their paper. The paper is remarkable in many ways, not just because of its scientific fate. It is a superb example of what we call a “model” in science, a construct where we ignore many complications and tackle a simple case. A good model keeps enough of reality inside it to give illuminating results. A good modeler knows exactly what aspects of reality can be eliminated and what aspects included to get these illuminating results. In this case Oppenheimer and Snyder ignored many aspects of a real star: effects due to spin, shock waves and radiation for instance. Perhaps most egregiously, they ignored pressure inside a massive star. One would have thought that ignoring all these real qualities of a star would make the situation hopeless, but one of the hallmarks of Oppenheimer’s style was an unerring ability to recognize just what must go inside a model to get to the core of the problem. So he and Snyder plowed on.
Even with their simplifications, the physicists discovered something striking. Stars greater than 0.7 solar masses will always implode under the force of their gravity. And the implosion will have some very strange and striking properties: For a far away observer the implosion past a certain point will take forever, getting slower and slower over time; measured by a clock, it would appear as if successive ticks of the clock get longer and longer. But for an observer who is located at that point, the implosion will be quick. They also found that once an observer goes past this point, they wouldn’t be able to send any light signals past the horizon; the horizon would thus appear “frozen” to an outside observer. This was bizarre behavior that nobody had modeled before. And what happens once this catastrophic implosion ends? As the authors say, “the star tends to close itself off from any communication with a distant observer; only its gravitational field persists.”
Without calling it that, Oppenheimer and Snyder were describing what we now call black holes. The point of no return is what we now call an event horizon. Since their paper was published, black holes have become some of the most important objects of study in the universe, no longer just exotic manifestations of the laws of physics but laboratories for understanding the very laws of physics themselves, possibly including the hitherto implacably impossible marriage between the two most important theories of physics – general relativity and quantum mechanics. Oppenheimer may well have received a Nobel Prize for his prediction had he lived long enough to see it experimentally validated. And yet after the paper was published, it virtually disappeared. Why?
The date makes it clear. The same day that the paper came out, Hitler invaded Poland and started World War II. With such a world-shifting political development, nobody was going to pay attention to what seemed like an obscure scientific prediction without experimental confirmation, no matter how exotic. But by sheer coincidence, there was another piece of scientific work that also appeared on the same day in the same issue of the Physical Review and that also upstaged the Oppenheimer-Snyder paper in a big way. The title of that paper was “The Mechanism of Nuclear Fission” and it was written by the Danish physicist Niels Bohr and his protege, the Princeton theorist John Wheeler. The Bohr-Wheeler paper sought to make sense of the sensational phenomenon of nuclear fission that had been discovered by the German physicists Otto Hahn and Fritz Strassman in December, 1938. Since then almost every physicist worth anything seemed to be engaged in fission work. Among many questions, one overriding question was the difference in fission behavior between the two isotopes of uranium – the minor isotope uranium-235 (occurring to the extent of 0.7% in natural uranium) and the major isotope uranium-238.
In their seminal paper Bohr and Wheeler answered this question and many others. Just like the Oppenheimer-Snyder paper, their analysis too is a high exemplar of a model. To model the subatomic nucleus of an element like uranium, one would think that the full machinery of quantum mechanics would be needed. But Bohr, based in part on his previous work, realized that one could largely ignore quantum effects and model the nucleus semi-classically, like a liquid drop. He and Wheeler found that whether an atom of uranium fissions or not depends on the balance between electrostatic and nuclear forces endemic in its nucleus and on how it exactly deforms. They found that instead of deforming like an orange cut into two equal pieces, the nucleus goes through an “orange to a cucumber to a peanut” transition, in Wheeler’s words, before it breaks up. They came up with many quantitative results, including a simple, crucial ratio of the square of the number of protons to the total number of nucleons (protons + neutrons) that indicated how easily uranium-235 relative to uranium-236 would split when bombarded by a neutron. Perhaps most importantly, the paper found an explanation for why uranium-235, unlike its more abundant cousin, would fission with neutrons of any energy. This conclusion, through the jagged arc of history, led directly to both nuclear reactors and nuclear bombs.
Oppenheimer and Snyder’s paper was speculative and abstract; with the discovery of fission ten months earlier, Wheeler and Bohr’s was practical and urgent. For the next few years, as the United States raced to build an atomic bomb and both Oppenheimer and Wheeler got involved in the Manhattan Project, fission took center stage; black holes were too speculative.
Ironically after the war, it was Wheeler who got interested in gravitationally collapsed stars while Oppenheimer became totally uninterested in them. By the 1950s Oppenheimer was deeply involved in national security, and on the scientific front he thought that black holes were too derivative compared to what he considered as fundamental, deep research in particle physics. But Wheeler, after having conquered nuclear physics and fission with Bohr, now decided to conquer the realms of the very large. He started a school of gravitational physics that became the foremost school of its kind in the world – among his students was Kip Thorne who won a Nobel Prize for his work on detecting gravitational waves a few years ago. It was Wheeler who in 1967 popularized the name “black hole”. Meanwhile, expect for a single confrontation between himself and Wheeler, Oppenheimer never again showed the slightest interest in his creations. But the Oppenheimer-Snyder paper, one of only three on astrophysics that Oppenheimer ever wrote, has become immortal in the history of science, even if it was sidelined by the quirks of historical and scientific fate.
I find the story of the Oppenheimer-Snyder paper fascinating because of many reasons: how great scientists know the right models to build, how historical events can overrun scientific accomplishments, how founders can abandon their own fields. But there’s an equally fascinating coda to the story. After the war ended, scientists like Wheeler started to work on the hydrogen bomb, a weapon that was hundreds of times more powerful than the bombs that destroyed Hiroshima and Nagasaki. As macabre as these weapons are, they rely on the same processes of fusion that power the stars and involve the same kinds of effects related to radiation and hydrodynamics. The complexities of these calculations led to a great demand for computers. By the 1950s and 1960s when work on these weapons was progressing in full swing, computer simulations had become available to model both the inner workings of hydrogen bombs and the inner workings of supernovas, neutron stars and black holes. Led by the physicist Sterling Colgate, this work allowed models of gravitational collapse to be built that would take into account all the effects that Oppenheimer and Snyder had deliberately ignored.
Gratifyingly, the models pointed to the exact same conclusions that the 1939 paper did – stars that were too massive beyond a point to support themselves against the crushing force of their gravity had to turn into black holes. In 2020, the British mathematical physicist Roger Penrose shared the Nobel Prize for physics “for the discovery that black hole formation is a robust prediction of the general theory of relativity”. It was the ultimate validation of Oppenheimer and Snyder’s work. As the historian of science George Dyson once wrote, “Bombs built computers, and computers build bombs.” Which might be slightly modified to say, “Bombs built computers, and computers built stars.”
Further reading:
1. Kip Thorne, “Black Holes and Time Warps: Einstein’s Outrageous Legacy” – Perhaps the best popular treatment of the history and scientific details of general relativity, with sharp character portraits of the principal characters. All from one of the world’s leading experts in general relativity.
2. John Wheeler and Kenneth Ford, “Geons, Black Holes and Quantum Foam” – A revealing autobiography of one of the most creative and important physicists of the 20th century, detailing his work in both nuclear and gravitational physics.
3. Stuart Shapiro and Saul Teukolsky, “Black Holes, White Dwarfs and Neutron Stars: The Physics of Compact Objects” – A comprehensive introduction to the basic physics of gravitationally collapsed objects. Includes details of nuclear and relativistic physics.
4. Arthur Miller, “Empire of the Stars: Obsession, Friendship and Betrayal in the Quest for Black Holes” – A lively and informative account of the rivalry between Subrahmanyan Chandrasekhar and Arthur Eddington in the former’s elucidation of gravitational collapse and the implications of that debate.
5. Steven Detweiler, “Black Holes: Selected Reprints” – A set of classic reprints of papers by Einstein, Minkowski, Oppenheimer and Snyder and others.
