oppenheimer research paper on black hole

June 26, 2014

Oppenheimer’s folly: On black holes, fundamental laws and pure and applied science

On September 1, 1939, the same day that Germany attacked Poland and started World War 2, a remarkable paper appeared in the pages of the journal Physical Review.

By Ashutosh Jogalekar

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

On September 1, 1939, the same day that Germany attacked Poland and started World War 2, a remarkable paper appeared in the pages of the journal Physical Review . In it J. Robert Oppenheimer and his student Hartland Snyder laid out the essential characteristics of what we today call the black hole. Building on work done by Subrahmanyan Chandrasekhar, Fritz Zwicky and Lev Landau, Oppenheimer and Snyder described how an infalling observer on the surface of an object whose mass exceeded a critical mass would appear to be in a state of perpetual free fall to an outsider. The paper was the culmination of two years of work and followed two other articles in the same journal.

Then Oppenheimer forgot all about it and never said anything about black holes for the rest of his life.

He had not worked on black holes before 1938, and he would not do so ever again. Ironically, it is this brief contribution to physics that is now widely considered to be Oppenheimer’s greatest, enough to have possibly warranted him a Nobel Prize had he lived long enough to see experimental evidence for black holes show up with the advent of radio astronomy.

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What happened? Oppenheimer’s lack of interest wasn’t just because he became the director of the Manhattan Project a few years later and got busy with building the atomic bomb. It also wasn't because he despised the free-thinking and eccentric Zwicky who had laid the foundations for the field through the discovery of black holes' parents - neutron stars. It wasn’t even because he achieved celebrity status after the war, became the most powerful scientist in the country and spent an inordinate amount of time consulting in Washington until his carefully orchestrated downfall in 1954. All these factors contributed, but the real reason was something else entirely – Oppenheimer just wasn’t interested in black holes. Even after his downfall, when he had plenty of time to devote to physics, he never talked or wrote about them. The creator of black holes basically did not think they mattered.

Oppenheimer’s rejection of one of the most fascinating implications of modern physics and one of the most enigmatic objects in the universe - and one he sired - is documented well by Freeman Dyson who tried to initiate conversations about the topic with him. Every time Dyson brought it up Oppenheimer would change the subject, almost as if he had disowned his own scientific children.

The reason, as attested to by Dyson and others who knew him, was that in his last few decades Oppenheimer was stricken by a disease which I call “fundamentalitis”. Fundamentalitis is a serious condition that causes its victims to believe that the only thing worth thinking about is the deep nature of reality as manifested through the fundamental laws of physics.

As Dyson put it:

“Oppenheimer in his later years believed that the only problem worthy of the attention of a serious theoretical physicist was the discovery of the fundamental equations of physics. Einstein certainly felt the same way. To discover the right equations was all that mattered. Once you had discovered the right equations, then the study of particular solutions of the equations would be a routine exercise for second-rate physicists or graduate students.”

Thus for Oppenheimer, black holes, which were particular solutions of general relativity, were mundane; the general theory itself was the real deal. In addition they were anomalies, ugly exceptions which were best ignored rather than studied. As Dyson mentions, unfortunately Oppenheimer was not the only one affected by this condition. Einstein, who spent his last few years in a futile search for a grand unified theory, was another. Like Oppenheimer he was uninterested in black holes, but he also went a step further by not believing in quantum mechanics. Einstein’s fundamentalitis was quite pathological indeed.

History proved that both Oppenheimer and Einstein were deeply mistaken about black holes and fundamental laws. The greatest irony is not that black holes are very interesting, it is that in the last few decades the study of black holes has shed light on the very same fundamental laws that Einstein and Oppenheimer believed to be the only thing worth studying. The disowned children have come back to haunt the ghosts of their parents.

Black holes took off after the war largely due to the efforts of John Wheeler in the US and Dennis Sciama in the UK. The new science of radio astronomy showed us that, far from being anomalies, black holes litter the landscape of the cosmos, including the center of the Milky Way . A decade after Oppenheimer’s death, the Israeli theorist Jacob Bekenstein proved a very deep relationship between thermodynamics and black hole physics. Stephen Hawking and Roger Penrose found out that black holes contain singularities; far from being ugly anomalies, black holes thus demonstrated Einstein’s general theory of relativity in all its glory. They also realized that a true understanding of singularities would involve the marriage of quantum mechanics and general relativity, a paradigm that’s as fundamental as any other in physics.

In perhaps the most exciting development in the field, Leonard Susskind, Hawking and others have found intimate connections between information theory and black holes, leading to the fascinating black hole firewall paradox that forges very deep connections between thermodynamics, quantum mechanics and general relativity. Black holes are even providing insights into computer science and computational complexity. The study of black holes is today as fundamental as the study of elementary particles in the 1950s.

Einstein and Oppenheimer could scarcely have imagined that this cornucopia of discoveries would come from an entity that they despised. But their wariness toward black holes is not only an example of missed opportunities or the fact that great minds can sometimes suffer from tunnel vision. I think the biggest lesson from the story of Oppenheimer and black holes is that what is considered ‘applied’ science can actually turn out to harbor deep fundamental mysteries . Both Oppenheimer and Einstein considered the study of black holes to be too applied, an examination of anomalies and specific solutions unworthy of thinkers thinking deep thoughts about the cosmos. But the delicious irony was that black holes in fact contained some of the deepest mysteries of the cosmos, forging unexpected connections between disparate disciplines and challenging the finest minds in the field. If only Oppenheimer and Einstein had been more open-minded.

The discovery of fundamental science in what is considered applied science is not unknown in the history of physics. For instance Max Planck was studying blackbody radiation, a relatively mundane and applied topic, but it was in blackbody radiation that the seeds of quantum theory were found. Similarly it was spectroscopy or the study of light emanating from atoms that led to the modern framework of quantum mechanics in the 1920s. Scores of similar examples abound in the history of physics; in a more recent case, it was studies in condensed matter physics that led physicist Philip Anderson to make significant contributions to symmetry breaking and the postulation of the existence of the Higgs boson. And in what is perhaps the most extreme example of an applied scientist making fundamental contributions, it was the investigation of cannons and heat engines by French engineer Sadi Carnot that led to a foundational law of science – the second law of thermodynamics.

Today many physicists are again engaged in a search for ultimate laws, with at least some of them thinking that these ultimate laws would be found within the framework of string theory. These physicists probably regard other parts of physics, and especially the applied ones, as unworthy of their great theoretical talents. For these physicists the story of Oppenheimer and black holes should serve as a cautionary tale. Nature is too clever to be constrained into narrow bins, and sometimes it is only by poking around in the most applied parts of science that one can see the gleam of fundamental principles.

As Einstein might have said had he known better, the distinction between the pure and the applied is often only a "stubbornly persistent illusion". It's an illusion that we must try hard to dispel.

Was Oppenheimer, the father of the atomic bomb, also the father of black holes?

The theoretical physicist conducted research into black holes before their discovery.

Before becoming "the father of the atom bomb," J. Robert Oppenheimer made a significant contribution to the science of black holes.

Oppenheimer will forever, for better or for worse, be associated with the incredible destructive power of the atomic bomb and the image of the mushroom cloud, a near-Biblical symbol of destruction. That association will only strengthen in the public eye with today's (July 21) release of " Oppenheimer ," Christopher Nolan's highly anticipated biopic about the physicist.

But before journeying to Los Alamos, New Mexico, in 1942 to contribute to the development of the atomic bomb , Oppenheimer was a theoretical physicist focusing on quantum physics. In 1939, he and his University of California, Berkeley colleague Hartland S. Snyder published a pioneering paper entitled " On Continued Gravitational Contraction ," which used the equations of Albert Einstein's theory of gravity, general relativity , to show how black holes could be born. 

"Oppenheimer proposed the very first collapse model to describe how a star could collapse into a black hole ," Xavier Calmet, a professor of physics at the University of Sussex in England, told Space.com. "This model explains the formation of black holes as a dynamical astrophysical process, the final stage of the evolution of heavy-enough stars. This model is still being used today."

Related: 'Oppenheimer' trailer reveals Cillian Murphy as Manhattan Project's genius bomb maker

Calmet said that he recently used the model himself, in a paper describing the collapse of black holes when considering quantum gravity .

"This model is very significant because it is analytically solvable — solving the equations can be done with pen and paper and does not require numerical work. All the physics is thus easily trackable," he said. "Yet, despite its simplicity and maybe even crudeness, it is complex enough to describe many of the features of a collapsing star."

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Ironically, as Oppenheimer and Snyder worked on the paper, which so heavily depended on the 1915 theory of general relativity, the father of that theory, Einsten, was himself completing research aimed at showing that black holes could not exist. 

History would show Oppenheimer to be right about black holes, of course.

Related: Albert Einstein: His life, theories and impact on science

Oppenheimer pushes the limit

Eight years before Oppenheimer's theory of star collapse and black hole birth, another theoretical physicist was thinking about what happens when stars run out of fuel for nuclear fusion .

When this fuel is exhausted, a star can no longer support itself against gravitational collapse. While the star's outer layers are shed, its core rapidly contracts, leaving an exotic stellar remnant. The nature of the remnant depends on the mass of the stellar core.

Indian-American physicist Subrahmanyan Chandrasekhar realized that, for stellar cores with a mass less than 1.4 times that of the sun , gravitational collapse would halt due to quantum effects that prevent particles from "squashing" too close together.

This would come to be known as the Chandrasekhar limit , and any star below it — unless it has a stellar companion feeding it material — is doomed to end its existence as a smoldering stellar remnant called a white dwarf . That will be the fate of our star, the sun, after it exhausts the hydrogen at its core in around 5 billion years. 

Related: When will the sun die?

For stellar cores at least 1.4 times more massive than the sun, there is enough pressure, and thus heat, generated during gravitational collapse that further bouts of nuclear fusion can be triggered, with the helium created by the fusion of hydrogen itself forging heavier elements like nitrogen, oxygen and carbon. 

The most massive stars undergo a series of these collapses and bouts of nuclear fusion. But Oppenheimer and his students wanted to know where this gravitational collapse path leads and, thus, what is the final state of the universe's biggest stars. 

This answer had already been delivered by a German physicist in 1916. Oppenheimer just had to find out how to get there. 

The two births of black holes 

In 1915, while serving on the front with the German army during the First World War, astronomer Karl Schwarzschild got his hands on a copy of Einstein's theory of general relativity. Astoundingly, and to the shock of Einstein, under these incredibly harsh conditions, Schwarzschild managed to calculate an exact mathematical solution to the field equations of general relativity.

In these solutions lurked two disturbing things — places known as "singularities" where physics as we know it completely breaks down. These singularities  indicated the existence of objects with gravity so intense that they could "swallow" light. One of the singularities was deemed a coordinate singularity, which could be removed with a little clever mathematical manipulation. This coordinate singularity would come to be known as the Schwarzschild radius — the point at which the gravity of a body becomes so great that the velocity needed to escape its clutches is greater than the speed of light. 

This one-way light-trapping surface is called the " event horizon ," and it represents the outer boundary of the black hole.

The other singularity, the true or gravitational singularity, could not be dealt with mathematically. Nothing could remove it, so it was, and still is, the point at which physics completely breaks down — the heart of the black hole. That was the theoretical birth of the black hole concept, but it didn't say anything about the creation of these cosmic titans — just that they can exist. While Einstein toiled in 1939 to destroy this gravitational singularity, and thus the concept of the black hole, Oppenheimer was delving into how such objects could come to exist.

Working with simple assumptions that neglect quantum effects and don't consider rotation, Oppenheimer set Snyder to work. And this paid off when the latter researcher discovered that what appears to happen to a collapsing star is dependent on an observer's point of view.

Snyder theorized that, at some distance from the collapsing star, light from a source close to the event horizon would have its wavelength stretched by gravity, a process called redshift , with it becoming ever more red. At the same time, the frequency of this light is being reduced from the observer's perspective. This frequency reduction continues until, for the distant observer, the light is effectively "frozen." 

Oppenheimer and collaborators realized the story is quite different for an observer unfortunate enough to be falling with the surface of the collapsing star. An observer in this position would fall beyond the event horizon without noticing anything significant about it.

Of course, in reality, an observer would be " spaghettified " by intense tidal forces caused by the difference in the gravitational pull on their upper and lower body. This would kill them before they hit the event horizon, at least for smaller black holes, in which the Schwarzschild radius is close to the gravitational singularity.

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This concept was initially referred to as a "frozen star" due to the apparent freezing of light at the event horizon. It wouldn't receive its more familiar and snappier name until 1967, when Princeton University physicist John Wheeler coined the term "black hole" during a lecture.

Oppenheimer and colleagues may have taken a different path than Schwarzschild, but still, the two teams of physicists arrived at the same destination: the concept of a stellar body so massive that its gravity traps light and causes infinite redshift. Schwarzschild had the theory, but Oppenheimer and colleagues were the first scientists who truly understood the physical birth of a black hole.

Three years later, Oppenheimer would travel to Los Alamos, cementing his place in history and in the perception of the public. But many, scientists especially, remember him as the father of black holes. 

"Oppenheimer made very significant contributions to black hole physics and physics as a whole," Calmet concluded. "While the general public may associate his name with the bomb and the Manhattan Project, his contributions to physics and astrophysics are well appreciated by the scientific community. 

"He was one of the leading physicists during his lifetime and was extremely influential, and his seminal work is still relevant today."

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Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

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• Frank Sterle Jr While Ronald Reagan postulated that “Of the four wars in my lifetime none came about because the U.S. was too strong,” who can know what may have historically come to fruition had the U.S. remained the sole possessor of atomic weaponry. There’s a presumptive, and perhaps even arrogant, concept of American leadership as somehow, unless directly militarily provoked, being morally/ethically above using nuclear weapons internationally. ... Cannot absolute power corrupt absolutely? I read that, after President Harry S. Truman relieved General Douglas MacArthur as commander of the forces warring with North Korea — for the latter’s remarks about using many atomic bombs to promptly end the war — Americans’ approval-rating of the president dropped to 23 percent. It was a record-breaking low, even lower than the worst approval-rating points of the presidencies of Richard Nixon and Lyndon Johnson. Had it not been for the formidable international pressure on Truman (and perhaps his personal morality) to relieve MacArthur as commander, Truman may have eventually succumbed to domestic political pressure to allow MacArthur’s command to continue. Reply
• billslugg The allowability of a nuclear first strike, in international law, is laid out in the Nuclear Non-Proliferation Treaty. known as NPT, which has been in effect since 1970. The only countries that have not signed it are Israel, South Sudan, Pakistan, India and North Korea. When a country signs it they agree never to make a nuke and in return are put on the "no first strike list". We would only use a nuke in retaliation. That covers the legal aspect. Morally, one could only launch a first strike in self defense with the usual caveats applied: Imminent existential threat, no retreat possible. It is hard to imagine such a scenario though. Reply
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April 1, 2007

12 min read

The Reluctant Father of Black Holes

Albert Einstein's equations of gravity are the foundation of the modern view of black holes; ironically, he used the equations in trying to prove these objects cannot exist

By Jeremy Bernstein

G reat science sometimes produces a legacy that outstrips not only the imagination of its practitioners but also their intentions. A case in point is the early development of the theory of black holes and, above all, the role played in it by Albert Einstein. In 1939 Einstein published a paper in the journal Annals of Mathematics with the daunting title On a Stationary System with Spherical Symmetry Consisting of Many Gravitating Masses. With it, Einstein sought to prove that black holes--celestial objects so dense that their gravity prevents even light from escaping--were impossible.

The irony is that, to make his case, he used his own general theory of relativity and gravitation, published in 1916--the very theory that is now used to argue that black holes are not only possible but, for many astronomical objects, inevitable. Indeed, a few months after Einstein's rejection of black holes appeared--and with no reference to it--J. Robert Oppenheimer and his student Hartland S. Snyder published a paper entitled On Continued Gravitational Contraction. That work used Einstein's general theory of relativity to show, for the first time in the context of modern physics, how black holes could form.

Perhaps even more ironically, the modern study of black holes, and more generally that of collapsing stars, builds on a completely different aspect of Einstein's legacy--namely, his invention of quantum-statistical mechanics. Without the effects predicted by quantum statistics, every astronomical object would eventually collapse into a black hole, yielding a universe that would bear no resemblance to the one we actually live in.

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Bose, Einstein and Statistics

EINSTEIN'S CREATION of quantum statistics was inspired by a letter he received in June 1924 from a then unknown young Indian physicist named Satyendra Nath Bose. Along with Bose's letter came a manuscript that had already been rejected by one British scientific publication. After reading the manuscript, Einstein translated it himself into German and arranged to have it published in the prestigious journal Zeitschrift fr Physik .

Why did Einstein think that this manuscript was so important? For two decades he had been struggling with the nature of electromagnetic radiation--especially the radiation trapped inside a heated container that attains the same temperature as its walls. At the start of the 20th century German physicist Max Planck had discovered the mathematical function that describes how the various wavelengths, or colors, of this black body radiation vary in intensity. It turns out that the form of this spectrum does not depend on the material of the container walls. Only the temperature of the radiation matters. (A striking example of black-body radiation is the photons left over from the big bang, in which case the entire universe is the container. The temperature of these photons has been measured at 2.726 0.002 kelvins.)

Somewhat serendipitously, Bose had worked out the statistical mechanics of black-body radiation--that is, he derived the Planck law from a mathematical, quantum-mechanical perspective. That outcome caught Einstein's attention. But being Einstein, he took the matter a step further. He used the same methods to examine the statistical mechanics of a gas of massive molecules obeying the same kinds of rules that Bose had used for the photons. He derived the analogue of the Planck law for this case and noticed something absolutely remarkable. If one cools the gas of particles obeying so-called Bose-Einstein statistics, then at a certain critical temperature all the molecules suddenly collect themselves into a degenerate, or single, state. That state is now known as Bose-Einstein condensation (although Bose had nothing to do with it).

An interesting example is a gas made up of the common isotope helium 4, whose nucleus consists of two protons and two neutrons. At a temperature of 2.18 kelvins, this gas turns into a liquid that has the most uncanny properties one can imagine, including frictionless ow (that is, superuidity). More than a decade ago U.S. researchers accomplished the difficult task of cooling other kinds of atoms to several billionths of a kelvin to achieve a Bose-Einstein condensate.

Not all the particles in nature, however, show this condensation. In 1925, just after Einstein published his papers on the condensation, Austrian-born physicist Wolfgang Pauli identified a second class of particles, which includes the electron, proton and neutron, that obey different properties. He found that no two such identical particles--two electrons, for example--can ever be in exactly the same quantum-mechanical state, a property that has since become known as the Pauli exclusion principle. In 1926 Enrico Fermi and P.A.M. Dirac invented the quantum statistics of these particles, making them the analogue of the Bose-Einstein statistics.

Because of the Pauli principle, the last thing in the world these particles want to do at low temperatures is to condense. In fact, they exhibit just the opposite tendency. If you compress, say, a gas of electrons, cooling it to very low temperatures and shrinking its volume, the electrons are forced to begin invading one another's space. But Pauli's principle forbids this, so they dart away from one another at speeds that can approach that of light. For electrons and the other Pauli particles, the pressure created by these eeing particles--the degeneracy pressure--persists even if the gas is cooled to absolute zero. It has nothing to do with the fact that the electrons repel one another electrically. Neutrons, which have no charge, do the same thing. It is pure quantum physics.

Quantum Statistics and White Dwarfs

BUT WHAT HAS quantum statistics got to do with the stars? Before the turn of the century, astronomers had begun to identify a class of peculiar stars that are small and dim: white dwarfs. The one that accompanies Sirius, the brightest star in the heavens, has the mass of the sun but emits about 1/360 the light. Given their mass and size, white dwarfs must be humongously dense. Sirius's companion is some 61,000 times denser than water. What are these bizarre objects? Enter Sir Arthur Eddington.

When I began studying physics in the late 1940s, Eddington was a hero of mine but for the wrong reasons. I knew nothing about his great work in astronomy. I admired his popular books (which, since I have learned more about physics, now seem rather silly to me). Eddington, who died in 1944, was a neo-Kantian who believed that everything of significance about the universe could be learned by examining what went on inside one's head. But starting in the late 1910s, when Eddington led one of the two expeditions that confirmed Einstein's prediction that the sun bends starlight, until the late 1930s, when Eddington really started going off the deep end, he was truly one of the giants of 20th-century science. He practically created the discipline that led to the first understanding of the internal constitution of stars, the title of his classic 1926 book. To him, white dwarfs were an affront, at least from an aesthetic point of view. But he studied them nonetheless and came up with a liberating idea.

In 1924 Eddington proposed that the gravitational pressure that was squeezing a dwarf might strip some of the electrons off protons. The atoms would then lose their boundaries and might be squeezed together into a small, dense package. The dwarf would eventually stop collapsing because of the Fermi-Dirac degeneracy pressure--that is, when the Pauli exclusion principle forced the electrons to recoil from one another.

The understanding of white dwarfs took another step forward in July 1930, when Subrahmanyan Chandrasekhar, who was 19, was onboard a ship sailing from Madras to Southampton. He had been accepted by British physicist R. H. Fowler to study with him at the University of Cambridge (where Eddington was, too). Having read Eddington's book on the stars and Fowler's book on quantum-statistical mechanics, Chandrasekhar had become fascinated by white dwarfs. To pass the time during the voyage, Chandrasekhar asked himself: Is there any upper limit to how massive a white dwarf can be before it collapses under the force of its own gravitation? His answer set off a revolution.

A white dwarf as a whole is electrically neutral, so all the electrons must have a corresponding proton, which is some 2,000 times more massive. Consequently, protons must supply the bulk of the gravitational compression. If the dwarf is not collapsing, the degeneracy pressure of the electrons and the gravitational collapse of the protons must just balance. This balance, it turns out, limits the number of protons and hence the mass of the dwarf. This maximum is known as the Chandrasekhar limit and equals about 1.4 times the mass of the sun. Any dwarf more massive than this number cannot be stable.

Chandrasekhar's result deeply disturbed Eddington. What happens if the mass is more than 1.4 times that of the sun? He was not pleased with the answer. Unless some mechanism could be found for limiting the mass of any star that was eventually going to compress itself into a dwarf, or unless Chandrasekhar's result was wrong, massive stars were fated to collapse gravitationally into oblivion.

Eddington found this intolerable and proceeded to attack Chandrasekhar's use of quantum statistics--both publicly and privately. The criticism devastated Chandrasekhar. But he held his ground, bolstered by people such as Danish physicist Niels Bohr, who assured him that Eddington was simply wrong and should be ignored.

A Singular Sensation

AS RESEARCHERS explored quantum statistics and white dwarfs, others tackled Einstein's work on gravitation, his general theory of relativity. As far as I know, Einstein never spent a great deal of time looking for exact solutions to his gravitational equations. The part that described gravity around matter was extremely complicated, because gravity distorts the geometry of space and time, causing a particle to move from point to point along a curved path. More important to Einstein, the source of gravity--matter--could not be described by the gravitational equations alone. It had to be put in by hand, leaving Einstein to feel the equations were incomplete. Still, approximate solutions could describe with sufficient accuracy phenomena such as the bending of starlight. Nevertheless, he was impressed when, in 1916, German astronomer Karl Schwarzschild came up with an exact solution for a realistic situation--in particular, the case of a planet orbiting a star.

In the process, Schwarzschild found something disturbing. There is a distance from the center of the star at which the mathematics goes berserk. At this distance, now called the Schwarzschild radius, time vanishes, and space becomes infinite. The equation becomes what mathematicians call singular. The Schwarzschild radius is usually much smaller than the radius of the object. For the sun, for example, it is three kilometers, whereas for a one-gram marble it is 10 28 centimeter.

Schwarzschild was, of course, aware that his formula went crazy at this radius, but he decided that it did not matter. He constructed a simplified model of a star and showed that it would take an infinite gradient of pressure to compress it to his radius. The finding, he argued, served no practical interest.

But his analysis did not appease everybody. It bothered Einstein, because Schwarzschild's model star did not satisfy certain technical requirements of relativity theory. Various people, however, showed that one could rewrite Schwarzschild's solutions so that they avoided the singularity. But was the result really nonsingular? It would be incorrect to say that a debate raged, because most physicists had little regard for these matters--at least until 1939.

To make his point, Einstein focused on a collection of small particles moving in circular orbits under the inuence of one another's gravitation--in effect, a system resembling a spherical star cluster. He then asked whether such a configuration could collapse under its own gravity into a stable star with a radius equal to its Schwarzschild radius. He concluded that it could not, because at a somewhat larger radius the stars in the cluster would have to move faster than light in order to keep the configuration stable. Although Einstein's reasoning is correct, his point is irrelevant: it does not matter that a collapsing star at the Schwarzschild radius is unstable, because the star collapses past that radius anyway. I was much taken by the fact that the then 60-year-old Einstein presents in this paper tables of numerical results, which he must have gotten by using a slide rule. But the paper, like the slide rule, is now a historical artifact.

From Neutrons to Black Holes

WHILE EINSTEIN was doing this research, an entirely different enterprise was unfolding in California. Oppenheimer and his students were creating the modern theory of black holes. The curious thing about the black hole research is that it was inspired by an idea that turned out to be entirely wrong. In 1932 British experimental physicist James Chadwick found the neutron, the neutral component of the atomic nucleus. Soon thereafter speculation began--most notably by Fritz Zwicky of the California Institute of Technology and independently by the brilliant Soviet theoretical physicist Lev D. Landau--that neutrons could lead to an alternative to white dwarfs.

When the gravitational pressure got large enough, they argued, an electron in a star could react with a proton to produce a neutron. (Zwicky even conjectured that this process would happen in supernova explosions; he was right, and these neutron stars we now identify as pulsars.) At the time of this work, the actual mechanism for generating the energy in ordinary stars was not known. One solution placed a neutron star at the center of ordinary stars, in somewhat the same spirit that many astrophysicists now conjecture that black holes power quasars.

The question then arose: What was the equivalent of the Chandrasekhar mass limit for these stars? Determining this answer is much harder than finding the limit for white dwarfs. The reason is that the neutrons interact with one another with a strong force whose specifics we still do not fully understand. Gravity will eventually overcome this force, but the precise limiting mass is sensitive to the details. Oppenheimer published two papers on this subject with his students Robert Serber and George M. Volkoff and concluded that the mass limit here is comparable to the Chandrasekhar limit for white dwarfs. The first of these papers was published in 1938 and the second in 1939. (The real source of stellar energy--fusion--was discovered in 1938 by Hans Bethe and Carl Friedrich von Weizscker, but it took a few years to be accepted, and so astrophysicists continued to pursue alternative theories.)

To simplify matters, Oppenheimer told Snyder to make certain assumptions and to neglect technical considerations such as the degeneracy pressure or the possible rotation of the star. Oppenheimer's intuition told him that these factors would not change anything essential. (These assumptions were challenged many years later by a new generation of researchers using sophisticated high-speed computers--poor Snyder had an old-fashioned mechanical desk calculator--but Oppenheimer was right. Nothing essential changes.) With the simplified assumptions, Snyder found out that what happens to a collapsing star depends dramatically on the vantage point of the observer.

Two Views of a Collapse

LET US START with an observer at rest a safe distance from the star. Let us also suppose that there is another observer attached to the surface of the star--co-moving with its collapse--who can send light signals back to his stationary colleague. The stationary observer will see the signals from his moving counterpart gradually shift to the red end of the electromagnetic spectrum. If the frequency of the signals is thought of as a clock, the stationary observer will say that the moving observer's clock is gradually slowing down.

Indeed, at the Schwarzschild radius the clock will slow down to zero. The stationary observer will argue that it took an infinite amount of time for the star to collapse to its Schwarzschild radius. What happens after that we cannot say, because, according to the stationary observer, there is no after. As far as this observer is concerned, the star is frozen at its Schwarzschild radius.

Indeed, until December 1967, when physicist John A. Wheeler of Princeton University coined the name black hole in a lecture he presented, these objects were often referred to in the literature as frozen stars. This frozen state is the real significance of the singularity in the Schwarzschild geometry. As Oppenheimer and Snyder observed in their paper, the collapsing star tends to close itself off from any communication with a distant observer; only its gravitational field persists. In other words, a black hole has been formed.

But what about observers riding with collapsing stars? These observers, Oppenheimer and Snyder pointed out, have a completely different sense of things. To them, the Schwarzschild radius has no special significance. They pass right through it and on to the center in a matter of hours, as measured by their watches. They would, however, be subject to monstrous tidal gravitational forces that would tear them to pieces.

The year was 1939, and the world itself was about to be torn to pieces. Oppenheimer was soon to go off to war to build the most destructive weapon ever devised by humans. He never worked on the subject of black holes again. As far as I know, Einstein never did, either. In peacetime, in 1947, Oppenheimer became the director of the Institute for Advanced Study in Princeton, N.J., where Einstein was a professor. From time to time they talked. There is no record of their ever having discussed black holes. Further progress would have to wait until the 1960s, when discoveries of quasars, pulsars and compact x-ray sources reinvigorated thinking about the mysterious fate of stars.

JEREMY BERNSTEIN is professor emeritus of physics at the Stevens Institute of Technology in Hoboken, N.J. He was a staff writer for the New Yorker from 1961 to 1995 and is the recipient of many science writing awards. He is a former adjunct professor at the Rockefeller University and a vice president of the board of trustees of the Aspen Center for Physics, of which he is now an honorary trustee. Bernstein has written 12 books on popular science and mountain travel. This article is adapted from his collection of essays, A Theory for Everything , published by Copernicus Books in 1996.

The Early Scientific Contributions of J. Robert Oppenheimer: Why Did the Scientific Community Miss the Black Hole Opportunity?

• Published: 09 March 2017
• Volume 19 , pages 60–75, ( 2017 )

Cite this article

• M. Ortega-Rodríguez 1 ,
• H. Solís-Sánchez 1 ,
• E. Boza-Oviedo 2 ,
• K. Chaves-Cruz 1 ,
• M. Guevara-Bertsch 1 ,
• M. Quirós-Rojas 1 ,
• S. Vargas-Hernández 1 &
• A. Venegas-Li 1  

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We assess the scientific value of Oppenheimer’s research on black holes in order to explain its neglect by the scientific community, and even by Oppenheimer himself. Looking closely at the scientific culture and conceptual belief system of the 1930s, the present article seeks to supplement the existing literature by enriching the explanations and complicating the guiding questions. We suggest a rereading of Oppenheimer as a figure both more intriguing for the history of astrophysics and further ahead of his time than is commonly supposed.

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Karl Schwarzschild and Frankfurt

On the Epistemology of Observational Black Hole Astrophysics

C. V. Vishveshwara (Vishu) on the Black Hole Trek

Freeman Dyson, Oppenheimer: The Shape of Genius , New York Times , August 15, 2013, accessed July 30, 2016, http://www.nybooks.com/articles/archives/2013/aug/15/oppenheimer-shape-genius/ .

Abraham Pais, J . Robert Oppenheimer: A Life (New York: Oxford University Press, 2006), 33.

This version of the paper can be read without having technical knowledge of general relativity.

This idea appears in Dyson, Oppenheimer (ref. 1), 18. Many German Jews were liberal idealists who failed to achieve their dreams of social reform in Germany and came to the United States with an intense commitment to the American dream of a free society and a patriotic, poetic vision of the United States, and idea going back to Goethe.

According to Dyson, Oppenheimer (ref. 1), 19, and literally meaning “sit still,” this term refers to Oppenheimer’s inability to sit still and work quietly to finish a difficult calculation.

Karl Hufbauer, “J. Robert Oppenheimer’s Path to Black Holes,” in Reappraising Oppenheimer, Centennial Studies and Reflections , ed. Cathryn Carson and David A. Hollinger (Berkeley: University of California, Berkeley, 2005), 31–47.

Jeremy Bernstein, Oppenheimer: Portrait of an Enigma (Chicago: Ivan R. Dee, 2004); David Cassidy, J. Robert Oppenheimer and the American Century (New York: Pi Press, 2005).

Kip Thorne, Black Holes and Time Warps: Einstein’s Outrageous Legacy (New York: Norton, 1994), 209; details of the confrontation can be found in Werner Israel, “Dark Stars: the Evolution of an Idea,” in 300 Years of Gravitation , ed. Stephen Hawking and Werner Israel (Cambridge: Cambridge University Press, 1987), 229, and citations therein.

Dyson, Oppenheimer (ref. 1); Ray Monk, Robert Oppenheimer: His Life and Mind (New York: Doubleday, 2013).

J. Robert Oppenheimer and Robert Serber, “On the Stability of Stellar Neutron Cores,” Physical Review 54 (1938), 540; J. Robert Oppenheimer and George M. Volkoff, “On Massive Neutron Cores,” Physical Review 55 (1939), 374–81; J. Robert Oppenheimer and Hartland Snyder, “On Continued Gravitational Contraction,” Physical Review 56 (1939), 455–59.

Landau’s ideas appear on the following two papers: Lev Landau, “On the Theory of Stars,” Physikalische Zeitschrift der Sowjetunion 1 (1932), 285; Lev Landau, “Origin of Stellar Energy,” Nature 141 (1938), 333–34.

Oppenheimer and Serber, Stability (ref. 10).

Thorne, Black Holes (ref. 8), 187–97, 209–19.

Yoshitsugu Nakagawa, “ Chushiro Hayashi (1920–2010) ,” American Astronomical Society, accessed September 15, 2016, https://aas.org/obituaries/chushiro-hayashi-1920-2010 .

Chushiro Hayashi, Reun Hoshi and Daiichiro Sugimoto, “ Evolution of the Stars, ” Progress of Theoretical Physics 22, supplement (1962), 95. See also ref. 17.

Albert Einstein, “On a Stationary System with Spherical Symmetry Consisting of Many Gravitating Masses,” Annals of Mathematics 40 (1939), 922–36. This paper was received by the journal on May 10, 1939.

By 1962, Chushiro Hayashi was already forty-two years old and a prestigious scholar who had received a professorship at Kyoto University five years before. This strongly reduces the probability of him having paid lip service to Oppenheimer.

Explained in Hufbauer, Oppenheimer’s Path (ref. 6), 46, n. 77; Thorne, Black Holes (ref. 8), 219.

Lev Landau and Evgeny Lifshitz, Statisticheskaya Fizika (Moscow: Fizmatgiz, 1951).

Lev Landau and Evgeny Lifshitz, Statistical Physics (Oxford: Pergamon, 1958). A few earlier papers cite Oppenheimer’s work with Snyder, but they do so in passing and refer not to star collapse but to more conventional stellar dynamics. See Martin Johnson, “Atomic Possibilities Underlying Stellar Catastrophe,” The Observatory 66 (1946), 248–54; Lyle B. Borst, “Supernovae,” Physical Review 78 (1950), 807–08; Prahalad Vaidya, “Nonstatic Solutions of Einstein’s Field Equations for Spheres of Fluids Radiating Energy,” Physical Review 83 (1951), 10–17.

Landau and Lifshitz, Statistical Physics (ref. 20), 343. Emphasis added.

Tullio Regge and John A. Wheeler, “Stability of a Schwarzschild Singularity,” Physical Review 108 (1957), 1063–69.

Amalkumar Raychaudhuri, “Arbitrary Concentrations of Matter and the Schwarzschild Singularity,” Physical Review 89 (1953), 417–21.

David Finkelstein, “Past-Future Asymmetry of the Gravitational Field of a Point Particle,” Physical Review 110 (1958), 965–67.

Martin Kruskal, “Maximal Extension of Schwarzschild Metric,” Physical Review 119 (1960), 1743–45.

John A. Wheeler and Kenneth W. Ford, Geons, Black Holes & Quantum Foam: A Life in Physics (New York: Norton, 2000), 745.

Thorne, Black Holes (ref. 8), 197, 240.

Part of the conundrum’s answer is clearly extra-scientific. To give but one example, take Oppenheimer and (his Caltech colleague) Zwicky’s refusal even to acknowledge each other’s papers. Oppenheimer never used the word “neutron star.” See Thorne, Black Holes (ref. 8), 206.

A general reference for the statements made in this section of the paper is Helge Kragh, Quantum Generations: A History of Physics in the Twentieth Century (Princeton, NJ: Princeton University Press, 2002).

We are grateful to Prof. James Bjorken (Stanford) for his comments on this particular issue and for his interest in this paper’s discussion. He recalls how, as late as 1950, the multi-galaxy idea was still hard to accept in general.

Wheeler proposed the term “black hole” in 1967. See John A. Wheeler, “Our Universe: The Known and the Unknown,” American Scholar 37 (1968), 248–74. It is important to note also that the term had appeared in print as early as 1964. See Ann Ewing, “‘Black Holes’ in Space,” Science News Letter 85 (1964), 39.

Cassidy, Oppenheimer (ref. 7), 176.

Kragh, Quantum Generations (ref. 29), 218–29. More directly, see the section “Defense of Mysticism” in Arthur Eddington, The Nature of the Physical World (1928; Cambridge: Cambridge University Press, 1948), 162. Even though this edition is from 1948, his peculiar ideas were already present by the time Oppenheimer was working on black holes in the late 1930s.

Quoted in Kragh, Quantum Generations (ref. 29), 362.

Thorne, Black Holes (ref. 8), 209; Israel, Dark Stars (ref. 8), 229.

Cassidy, Oppenheimer (ref. 7), 179.

We are grateful to Prof. Robert Wagoner (Stanford) for his comment on this particular issue and for his interest in this paper’s discussion. He mentioned that a similar opinion of special-case irrelevance surfaced with Kerr’s solution for black holes. Prof. Robert Wald (University of Chicago), to whom we are also grateful, offered further the case of big bang cosmology as an example along these lines of special cases.

Israel, Dark Stars (ref. 8), 217.

Hans Bethe and Enrico Fermi, “Über die Wechselwirkung von Zwei Elektronen,” Zeitschrift für Physik 77 (1932), 296–306.

Max Born and J. Robert Oppenheimer, “Zur Quantentheorie der Molekeln,” Annalen der Physik 389 (1927), 457–84; J. Robert Oppenheimer and Melba Phillips, “Note on the Transmutation Function for Deuterons,” Physical Review 48 (1935), 500–502; the citation data is from Google Scholar Citations.

The contents of this section have been inspired by the work of philosopher Michel Foucault on epistemes (Michel Foucault, The Order of Things (New York: Random/Vintage, 1970)), especially as expounded by David Hess for a more modern, English-speaking readership (David J. Hess, Science and Technology in a Multicultural World (New York: Columbia University Press, 1995), 87). Foucault uses the term “episteme” to refer to the implicit assumptions about how we know the world. More precisely, it refers to “the assumptions about knowledge, method, and theory which at any given time period are shared across “discursive formations” (which as a first approximation can be translated as ‘disciplines’)” (ibid., 88). An episteme differs from a Kuhnian paradigm in part because it is transdisciplinary.

Einstein, Stationary System (ref. 16); In 1916, Karl Schwarzschild found a solution of Einstein’s equations which were not well behaved at certain points. See Karl Schwarzschild, “Über das Gravitationsfeld eines Massenpunktes nach der Einsteinschen Theorie,” Sitzungsberichte Königlich Preus, Akad. Wiss. Berlin, Phys.-Math. Klasse (1916), 189–96.

Israel, Dark Stars (ref. 8), 219. Emphasis added.

Quoted in Thorne, Black Holes (ref. 8), 208. Emphasis added.

We are in debt to Profs. Robert Wald (University of Chicago) and Randall Espinoza (University of Illinois at Chicago) for this particular point.

John A. Wheeler, “On the Nature of Quantum Geometrodynamics,” Annals of Physics 2 (1957), 604–14.

Michel Janssen, “What Did Einstein Know and When Did He Know It?,” in The Genesis of General Relativity , vol. 2, ed. Jürgen Renn (Dordrecht: Springer, 2007), 787–837, on 825.

Benjamin Abbott et al., “Observation of Gravitational Waves from a Binary Black Hole Merger,” Physical Review Letters 116 (2016), 061102-1–16.

Cassidy, Oppenheimer (ref. 7), 177. Emphasis added.

This discussion actually originated in lively fashion during the January 2014 Stanford meeting. Albert Einstein, Boris Podolsky, and Nathan Rosen, “Can Quantum-Mechanical Description of Physical Reality be Considered Complete?,” Physical Review 47 (1935), 777–80; Fritz Zwicky, “Die Rotverschiebung von Extragalaktischen Nebeln,” Helvetica Physica Acta 6 (1933), 110–27.

Israel, Dark Stars (ref. 8), 226.

Thorne, Black Holes (ref. 8), 178.

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Acknowledgments

We have greatly benefited from discussions with Barton Bernstein, which led to the organization of a multidisciplinary conversation with historians, philosophers, and physicists, among others, at Stanford University’s Hansen Experimental Physics Laboratory on January 31, 2014, with a follow-up on January 30, 2015, in the History Department. These Stanford sessions were themselves continuations of earlier conversations at Universidad de Costa Rica. This work was supported by grant 805-A4-125 of the Universidad de Costa Rica’s Vicerrectoría de Investigación and the CIGEFI, and by grant FI-0204-2012 of the MICITT and the CONICIT.

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M. Ortega-Rodríguez, H. Solís-Sánchez, K. Chaves-Cruz, M. Guevara-Bertsch, M. Quirós-Rojas, S. Vargas-Hernández & A. Venegas-Li

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Appendix: Timeline of Events

Main events relevant to the discussion. Note the gap between 1939 and 1957.

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Ortega-Rodríguez, M., Solís-Sánchez, H., Boza-Oviedo, E. et al. The Early Scientific Contributions of J. Robert Oppenheimer: Why Did the Scientific Community Miss the Black Hole Opportunity?. Phys. Perspect. 19 , 60–75 (2017). https://doi.org/10.1007/s00016-017-0195-6

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On Continued Gravitational Contraction

J. r. oppenheimer and h. snyder, phys. rev. 56 , 455 – published 1 september 1939.

• Citing Articles (1,256)

When all thermonuclear sources of energy are exhausted a sufficiently heavy star will collapse. Unless fission due to rotation, the radiation of mass, or the blowing off of mass by radiation, reduce the star's mass to the order of that of the sun, this contraction will continue indefinitely. In the present paper we study the solutions of the gravitational field equations which describe this process. In I, general and qualitative arguments are given on the behavior of the metrical tensor as the contraction progresses: the radius of the star approaches asymptotically its gravitational radius; light from the surface of the star is progressively reddened, and can escape over a progressively narrower range of angles. In II, an analytic solution of the field equations confirming these general arguments is obtained for the case that the pressure within the star can be neglected. The total time of collapse for an observer comoving with the stellar matter is finite, and for this idealized case and typical stellar masses, of the order of a day; an external observer sees the star asymptotically shrinking to its gravitational radius.

• Received 10 July 1939

DOI: https://doi.org/10.1103/PhysRev.56.455

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Long before he was pitted against Barbie, J Robert Oppenheimer worked on the densest objects in the cosmos.

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The theoretical building blocks were there long before the observations of neutron stars and black holes.

Image Credit: Public Domain / Vadim Sadovski/shutterstock.com

The existence or not of a theoretical object or phenomenon doesn’t stop physicists from studying it. First of all, it builds the foundation for explaining different known events and it is possible that because math allows it, the universe does too. Black holes are such objects. For decades, they were mere oddities causing problems in Einstein’s general relativity until they were discovered out in the universe, revealing that the famous theory of gravity has its limits. 

There are many physicists who worked on them long before the first was observed – Cygnus X-1, in 1971. Among them, was J Robert Oppenheimer , who played an important role in estimating how dense an object can be before it turns into a black hole – a calculation that has major implications in some of the most groundbreaking observations of today. 

General relativity was published in 1915 and by 1916, German physicist Karl Schwarzschild found a solution to the Einstein field equations where things broke apart. His solution became singular at a certain radius, meaning that the terms of the equation became infinite. Now, from those first descriptions, we get the term singularity to describe the black hole and also the Schwarzschild radius, where the event horizon of a black hole is located.

The following decades had scientists discussing how “physical” this solution was. The assumption was that things don’t just collapse on themselves, internal forces would push back. A planet doesn’t collapse on itself simply because the forces between atoms are enough to keep it stable. A star can be much heavier but the energy released by nuclear fusion at its core balances out the effect of gravity.

But what happens when a star like the Sun is no longer fusing? It collapses. Still, that was not thought at the time to be unstoppable. Quantum mechanical effects would turn the object into a dense sphere made of electron-degenerate matter. The internal material is no longer in a classical plasma but in a new state where electrons, protons, and neutrons (which are types of fermions) interact.

Fermions cannot all be in the same energy state at the same time (this is known as Pauli’s exclusion principle) and this property is what creates a pressure that counteracts the gravitational pull towards a collapse. We call objects like these white dwarfs , and the Sun is destined to become one. This quantum pressure was not a hard limit though.

Back in 1931, Subrahmanyan Chandrasekhar calculated that you can’t have an indiscriminately big white dwarf. A non-rotating object made of electron-degenerate matter with a mass over 1.4 times that of the Sun (now called the Chandrasekhar limit) doesn’t have a stable solution. This is only partially correct.

The limit is now seen as how much material-thieving white dwarfs can steal from a companion before going supernova. This is known as Type Ia (pronounced one-A) supernova and they have all the same luminosity, making them a great standard candle to measure how far galaxies are. So what’s the stable solution that is even more dense than a white dwarf? Well, that’s a neutron star!

While white dwarfs were becoming known to science at the same time as these theoretical discussions were taking place, neutron stars had not been discovered just yet. We’ll need Joycelyn Bell Burnell in 1967 with the discovery of the first pulsar (pulsating neutron stars) to bring them from theory into reality.

Neutron stars allow for greater masses and densities, and that limit is now known as the Tolman–Oppenheimer–Volkoff limit named after Oppenheimer and George Volkoff who worked it out in 1939, thanks to the research of Richard Tolman.

For masses lower than that limit, the short-range repulsion between neutrons is enough to balance out the gravity. But for higher masses, the neutron star will collapse into a black hole. The limit tells how massive stars going supernova can turn into neutron stars or into black holes, depending on their original mass.

But recently, we have also had a way to test the Tolman–Oppenheimer–Volkoff limit with some of the most advanced experiments we have: gravitational wave observatories. The first historic observations of a collision between neutron stars (with the two objects turning into a black hole) allowed us to estimate the limit in a real setting.

While Oppenheimer worked on this theoretical problem long before we knew of neutron stars and black holes as real objects, knowing they exist has not solved all the mysteries that surround them. The neutron star collision puts the limit to between 2.01 and 2.17 solar masses. And yet the most massive known pulsar is 2.35 times the mass of the Sun.   

The road to understanding the densest objects in the universe is probably still long, but some of the most famous physicists of the 20th century played a role in what we know and understand so far.

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How Oppenheimer Proved Einstein Wrong About Black Holes

In addition to the Manhattan Project, J. Robert Oppenheimer also worked on many other areas of physics.

J. Robert Oppenheimer is largely known to history for his work on the Manhattan Project, the US government’s secret wartime nuclear bomb development project. That project would result in the bombs dropped on Japan at the end of World War II, and they ushered in the Atomic Age, the Cold War, and endless iteration of post-apocalyptic fiction. There is no Mad Max without Oppenheimer.

But the Manhattan Project has largely eclipsed Oppenheimer’s previous work on a force even more powerful than nuclear bombs — black holes.

“Everyone knows his name in connection with the Manhattan Project, and that's certainly how I first learned of his name, because I grew up in Oak Ridge, one of the Manhattan Project sites.” Loyola University astrophysicist Robert McNees tells Inverse . But Oppenheimer wasn’t such a specialist in his time before his work on the bomb, working on nuclear and particle physics.

In a 1939 paper, Oppenheimer, then a particle physicist, declared that black holes were the inevitable result of the Albert Einstein ’s theory of general relativity, contrary to the hopes of that scientist. Oppenheimer declared, like a few others before him,that black holes weren’t just a quirk of the mathematics, but likely real astrophysical objects — that a star massive enough is destined to implode, creating a trap from which what goes in cannot come back out.

“Mathematically, the work of Oppenheimer and his colleagues was really important to put these on firm theoretical footing,” Sheperd Doeleman , an astrophysicist at the Harvard & Smithsonian Center for Astrophysics and director of the Event Horizon Telescope, tells Inverse .

A black hole feeding on a star.

In the shadow of the bomb

While Oppenheimer may have worked on black holes and atomic chain reactions, McNees says he was a bit of an intellectual nomad.

“He strikes me as one of those scientists who kind of like went from topic to topic and just had very good taste in what he worked on,” McNees says. “He would jump into an area and pinpoint an interesting problem and contribute something, and then maybe end up going on to something else.”

For example, Oppenheimer early work helped establish the concept of the positron, the antimatter equivalent of the electron. He also defined the nuclear process — the Oppenheimer-Phillipis process — involved in the transmutation of isotopes, like Carbon-12 transmuting to Carbon-13.

And then in the late 1930s, Oppenheimer stepped in to tell Einstein he was both right and wrong about the universe.

“In 1915, Einstein posits his equations of general relativity, his magnum opus. A masterpiece that elevates space-time from a stage to a participant in the drama,” University of Waterloo astrophysicist Avery Broderick tells Inverse . The Einstein field equations explained how the mass of matter curved the fabric of space-time, and how space-time in turn told matter how to move.

A year later in 1916, German Physicist Karl Schwarzschild came up with the first solution to the Einstein field equations, “which was pretty impressive, because they're difficult equations to solve,” Broderick says.

But the Schwartzchild solution implies something weird: That you could have a large mass in a single point, a singularity, which would so warp space-time that anything that came within a certain radius of that point would never be able to get out again.

“Everyone kind of chortled and said, ‘Oh, ho-ho, isn't that mathematically curious! But it will never happen,’” Doeleman says. Schwartzchild’s calculations showed that “theoretically, you could have a region of space time that was like a knot that you couldn't untie. And Einstein rebelled against that.”

Despite his crafting the theory of general relativity, Einstein, like many other physicists at the time, assumed singularities were either a mathematical phantom of Schwartchild’s solution, or, at the least, were a condition that nature could never actually enter into in practice.

“Einstein's approach or response to this was very natural, very experienced. He was like,Ah!, this can't happen. Nature will stop this, we just don't know how,” Broderick says. “You know, the universe intercedes.”

But in a 1939 paper entitled “On Continued Gravitational Contraction,” Oppenheimer and his co-author Hartland Snyder showed that a sufficiently massive star, when it exhausts its nuclear fuel, will necessarily contract forever, forming what we now know as a black hole.

“A lot of the things that kind of show up as descriptors and popular accounts of black holes, at least as far as I'm aware, originate in this paper,” McNees says.

For instance: If I were to watch you fly into a black hole from a safe distance, and you were holding up a clock I could see through a telescope, your clock would seem to slow down more and more as you approached the point of no return, known as the event horizon.

“It’s just kind of frozen,” McNees says, such that your falling into the black hole seems to take infinite time, from my perspective.

And even if you did fall in, it would be hard to see, because your light gets stretched out to longer and longer wavelengths — redshifted – to the point of obscurity.

But your experience is different. An observer falling past the event horizon doesn’t notice anything change, McNees says. Their clock runs as normal, and “for them, it happens in a finite amount of time.”

This is an effect of time dilation, which Christopher Nolan incidentally depicted in his early film, Interstellar , when a crew of astronauts spend time much closer to a black hole than their colleagues.

Einstein and Oppenheimer sit down together in 1947.

Rediscovering the black hole

These weird findings wound up getting buried for years, thanks to the advent of World War II and the subsequent Manhattan Project, which closed out Oppenheimer’s work in astrophysics.. It would take the work of John Wheeler in the 1950s, and then Roger Penrose and Stephen Hawking in the 1960s and 70s, to pick back up on the work Oppenheimer started and fill in the understanding of black holes we have today.

“It was the 60s and 70s, where people began to see cosmic objects that looked like they might be black holes, and that started a huge observational push to see these things,” Doeleman says.

X-ray and other observations would continue to find evidence gesturing at black holes, but it wasn’t until 2019, when the Event Horizon Telescope (EHT) collaboration released the first image of a black hole, the supermassive black hole at the heart of the Messier 87 galaxy, that we could actually image the event horizon.

The next step in the EHT project is to move from still images to movies, getting a dynamic picture of how matter swirls into the maw of supermassive black holes, and looking for clues that might give to what lies behind the event horizon, according to Doeleman. It could help solve the biggest mystery in physics, how Einstein’s theory of gravity and quantum mechanics, the rules that govern the other forces of the universe, fit together.

“At the center of the black hole at the singularity, that is where quantum physics and gravitational physics, which have never been shown to be consistent with each other, they have to combine,” Doeleman says. “We know that they have to merge. And we have no idea how that happens.”

All of that research and the discoveries yet to be uncovered comes to us in part because of the work Oppenheimer did, and for which he is barely known.

Doeleman questions how Oppenheimer himself would see his legacy today, knowing what became of his work on both nuclear weapons and black holes.

”Would he have thought the Manhattan Project was the most important thing? Or maybe he would think, ‘this black hole stuff is pretty amazing. And I'm glad to have been a part of something that went far beyond the Earth, was less terrestrial and more cosmic,” Doeleman says. “We won’t know that, but we can wonder.”

This article was originally published on July 29, 2023

• Space Science

The Early Scientific Contributions of J. Robert Oppenheimer

Why did the scientific community miss the black hole opportunity?

M. Ortega-Rodríguez, H. Solís-Sánchez, E. Boza-Oviedo, K. Chaves-Cruz,

M. Guevara-Bertsch, M. Quirós-Rojas, S. Vargas-Hernández, and

A. Venegas-Li

Escuela de Física, Universidad de Costa Rica, 11501-2060 San José, Costa Rica

We aim to carry out an assessment of the scientific value of Oppenheimer’s research on black holes in order to determine and weigh possible factors to explain its neglect by the scientific community, and even by Oppenheimer himself. Dealing primarily with the science and looking closely at the scientific culture and the scientific conceptual belief system of the 1930s, the present article seeks to supplement the existent literature on the subject by enriching the explanations and possibly complicating the guiding questions. We suggest a rereading of Oppenheimer as a more intriguing, ahead-of-his-time figure. 1 1 1 We have greatly benefited from discussions with Barton Bernstein, which led to the organization of a multidisciplinary conversation with historians, philosophers, and physicists, among others, at Stanford University’s Hansen Experimental Physics Laboratory on January 31, 2014, with a follow-up on January 30, 2015 in the History Dept. These Stanford sessions were themselves continuations of earlier conversations at Universidad de Costa Rica. The insightful and thorough accompanying article by Barton Bernstein (which has the merit that does not shy away from the science) and this paper complement each other and are best read together. To the date March 12, 2017, Barton Bernstein’s paper has not been published yet.

The 1930s witnessed a tremendous growth in our understanding of stars. Not only did Hans Bethe and others solve the long-standing problem of stellar energy production by means of nuclear fusion, but the recently discovered neutron (1932) allowed for speculation about the existence of more extreme physics. In this way, Fritz Zwicky and Lev Landau considered the possibility of stars composed entirely of neutrons. Along similar lines, J. Robert Oppenheimer became deeply interested in the problem of stellar stability, leading to an acute interest in total stellar collapse. Oppenheimer invented the concept of black holes.

Apparently, though, Oppenheimer’s move was too extreme. Despite the fact that these ideas are considered milestones today, in 1939 and for reasons that are not completely understood they fell into oblivion for two decades, failing to capture the attention of most physicists (Landau being a notable exception).

Freeman Dyson calls Oppenheimer’s black hole work his “only revolutionary contribution to science.” Furthermore, Dyson considers “the outstanding mystery in Oppenheimer’s life” the fact that even Oppenheimer failed to grasp the importance of his own discovery. 2 2 2 Dyson, F. (2013, August 15), Oppenheimer: The shape of genius , retrieved from http://www.nybooks.com/articles/archives/2013/aug/15/oppenheimer-shape-genius/

Indeed, Oppenheimer never regained interest in the topic, a potential Nobel winner. When biographer Abraham Pais asked him what his most important contribution to science had been, he referred to his electron/positron work, not a word on astrophysics. 3 3 3 Pais, A. (2006), J. Robert Oppenheimer, a life , New York: Oxford University Press, 33.

Main Question: Why Did the Scientific Community Miss the Black Hole Opportunity?

The present multidisciplinary collaboration aims to carry out an assessment of the scientific value of Oppenheimer’s research on black holes in order to determine and weigh possible factors to explain its neglect by the scientific community, and even by Oppenheimer himself. 4 4 4 This version of the paper can be read without having technical knowledge of general relativity.

Not that there is a lack of easy ways to dismiss, or to address, this question. For example, by arguing that Oppenheimer’s discovery was beyond experimental/observational corroboration and thus scientifically uninteresting. But that answer ignores the fact that physics seems many times not to care about this circumstance, and that theoretical corroborations and elaborations were doable in the 1930s even when the observational ones were not feasible.

We believe, then, that much insight can be gained from plunging into the question of the present paper’s subtitle. This subtitle question hints at another question, namely: What would it have taken for the black hole concept to become an active field of research in 1939?

This article differs from previous treatments of the subject in its emphasis. This essay a ) a) deals primarily with the science, b ) b) attempts to be situated in time (i.e., forgetting what came after 1939), and c ) c) looks closely at the scientific culture and conceptual belief system of the 1930s, in particular it considers what “good science” meant back then.

This paper therefore complements studies using other perspectives, such as career choices, network analyses (including very counterproductive enmities), German-Jew frustrated liberal idealism, the whole “bag” of personality traits (Oppenheimer’s peculiar intellectual impatient style, his “pathological” interest in everything, his constant desire to be at the center of things, his fierce independence and Sitzfleisch 5 5 5 According to Dyson (ref.  2 ), 19, and literally meaning “sit still,” this term refers to Oppenheimer’s inability to sit still and work quietly to finish a difficult calculation. problem), in addition to purely contingent factors: war, anti-Semitism, nationality issues, etc.

Review of the Literature

In addition to the well-known biographies of Oppenheimer, the subject of the contextualized stellar science of Oppenheimer has been touched upon by several authors with different backgrounds.

The most detailed account, to our knowledge, is the one given by historian-of-science Karl Hufbauer, 6 6 6 Hufbauer, K. (2005), J. Robert Oppenheimer’s path to black holes, in C. Carson & D. A. Hollinger (Eds.), Reappraising Oppenheimer, Centennial Studies and Reflections (pp. 31–47), Berkeley: University of California, Berkeley. which constitutes our starting point (see next section). In addition, the black hole science is briefly commented on books by physicist/journalist Jeremy Bernstein 7 7 7 Bernstein, J. (2004), Oppenheimer: Portrait of an enigma , Chicago: Ivan R. Dee. and historian-of-science David Cassidy. 8 8 8 Cassidy, D. (2005), J. Robert Oppenheimer and the American century , New York: Pi Press.

Kip Thorne, an astrophysicist, presents a comprehensive view of the circumstances surrounding the black hole conception, including Oppenheimer’s confrontation with theoretical physicist John Wheeler in 1958 in Brussels. 9 9 9 Thorne, K. (1994), Black holes and time warps: Einstein’s outrageous legacy , New York: Norton, 209; details of the confrontation can be found in Israel, W. (1987), Dark stars: the evolution of an idea, in S. Hawking & W. Israel (Eds.), 300 Years of Gravitation (pp. 199–276), Cambridge: Cambridge University Press, 229. Thorne’s account is the most complete from a scientific point of view with the caveat of being seen through modern eyes.

Finally, we must mention Freeman Dyson’s lucid review 10 10 10 Dyson (ref.  2 ). of biographer Ray Monk’s book on Oppenheimer. 11 11 11 Monk, R. (2013), Robert Oppenheimer: His life and mind , New York: Doubleday. Dyson actually reviews, albeit briefly, the whole Oppenheimer’s science debate.

Summary of Hufbauer’s Article

The article by Hufbauer represents, to our knowledge, the most comprehensive historical study of the black hole quest by Oppenheimer.

Hufbauer carefully explicates the path of events that led to the publication of the three relevant papers (Oppenheimer & Serber 1938, Oppenheimer & Volkoff 1939, Oppenheimer & Snyder 1939 12 12 12 Oppenheimer, J. R., & Serber, R. (Oct 1, 1938), “On the stability of stellar neutron cores,” Physical Review, 54 , 540; Oppenheimer, J. R., & Volkoff, G. M. (Feb 15, 1939), “On massive neutron cores,” Physical Review, 55 , 374–381; and Oppenheimer, J. R., & Snyder, H. (Sept 1, 1939), “On continued gravitational contraction,” Physical Review, 56 , 455–459. ), including how Oppenheimer became interested as early as 1933 in high-density stellar physics. This was facilitated by his simultaneous interest and competence in both particle physics and astronomy, a rather American trait. (For the benefit of readers not familiar with the papers, there is a brief description of each in Appendix A.)

In addition, Hufbauer describes Oppenheimer’s efficient use of available resources, including talking to prominent figures like his Caltech colleague Richard Tolman. Hufbauer also describes the way in which Bethe “scooped” Oppenheimer on the topic of stellar energy. Hufbauer then discusses the main results of the Oppenheimer & Snyder paper: not only the surprising collapse, but also how time freezes at the Schwarzschild radius.

Finally, Hufbauer offers five reasons for the early neglect of Oppenheimer’s papers, in the form of a contrast with Bethe’s more successful experience. Unlike Bethe’s research on stars, Oppenheimer a ) a) was not addressing a well-defined problem with a large following; b ) b) had no data and was invoking the little-used theory of general relativity; c ) c) offered a solution that was completely counterintuitive; d ) d) did not reach out to potential audiences; e ) e)  published his paper just as the war began.

The Value of Oppenheimer’s Work

There is no real doubt that Oppenheimer’s work on black holes is considered good science according to our modern point of view. The internal logic of the decade-long development of the ideas about denser and denser astrophysical entities is very clearly expounded in Thorne’s book using nontechnical language. 13 13 13 Thorne (ref.  9 ), 187–197 and 209–219. In addition, the citation record of the paper by Oppenheimer & Snyder shows a clear delayed recognition of their ideas, in the 1960s.

The real question is whether Oppenheimer’s work was considered good science according to the standards of the time. A second, related question is whether he was preeminent or not among scientists along this line of research.

The following quote, taken from a long, authoritative (“a bible in the field”) stellar evolution review from 1962 (and therefore written more than two decades after Oppenheimer’s work) is helpful in this respect: 14 14 14 Hayashi, C., Hoshi, R., & Sugimoto, D. (1962), “Evolution of the Stars,” Progr. Theoret. Phys. Supp., 22 , 95. See also footnote 16 .

A possibility of stellar evolution leading to these extremely dense configurations [more dense than neutron stars] may not be denied, but it will be highly more probable that, before the star reaches such a configuration, its mass will be reduced below its Chandrasekhar limit by mass ejection from its surface, due to an increase in the centrifugal force in the course of contraction.

In the sentence immediately preceding this quote in the review, the work of Oppenheimer with Snyder and Volkoff is referenced, but not Albert Einstein’s related article 15 15 15 Einstein, A. (1939), “On a stationary system with spherical symmetry consisting of many gravitating masses,” Annals of Mathematics, 40 , 922–936. (described in Appendix A), or any other author’s. This shows that, even when the scientific community as a whole (if we take this comment as representative) still did not believe in black holes , Oppenheimer’s work was considered authoritative. The authors could have easily been more dismissive of Oppenheimer, who had after all disappeared from the field of (what we would now call) astrophysics. 16 16 16 By 1962, Chushiro Hayashi was already 42 years old and a prestigious scholar who had received a Professor appointment at Kyoto University five years before. This strongly reduces the probability of him having paid lip service to Oppenheimer. According to the American Astronomical Society, Hayashi’s review with Hoshi and Sugimoto was considered “…a bible in the field of stellar evolution for a long time, and may be so still.” See https://aas.org/obituaries/chushiro-hayashi-1920-2010.

Further proof of Oppenheimer being considered as a preeminent scientist is the fact that Landau allegedly included the Oppenheimer & Snyder paper in his “Golden List” of classic papers in 1939. 17 17 17 Explained in Hufbauer (ref.  6 ), 46 and footnote 77; Thorne (ref.  9 ), 219. What we do know for certain is that Landau and Lifshitz cite the work of Oppenheimer with Snyder in their widely read 1951 (Russian) edition of Statistical Physics . 18 18 18 Landau, L., & Lifshitz, E. (1951), Statisticheskaya Fizika , Moscow: Fizmatgiz. (The corresponding English edition 19 19 19 Landau, L., & Lifshitz, E. (1958), Statistical Physics , Oxford: Pergamon. came out in 1958, and constitutes to the best of our knowledge the first critical citation of Oppenheimer’s black hole work in the Western World. 20 20 20 There are a few earlier citations of the work of Oppenheimer with Snyder, but these are made in passing and refer not to star collapse but to more normal stellar dynamics. See Johnson, M. (1946), “Atomic possibilities underlying stellar catastrophe,” The Observatory, 66 , 248–254; Borst, L. B. (1950), “Supernovae,” Physical Review, 78 , 807–808; and Vaidya, P. C. (1951), “Nonstatic solutions of Einstein’s field equations for spheres of fluids radiating energy,” Physical Review, 83 , 10–17. )

In this Russian book, Landau and Lifshitz fully support the relevant ideas:

Such a study [the one by Oppenheimer and Snyder] has been carried out only for the simplest case of the equation of state P = 0 𝑃 0 P=0 , i.e. for a sphere consisting of a very thin substance; it probably gives also a correct indication of the nature of the process for the general case of an exact equation of state [emphasis added].

One should also point out that there is not a single published attack to Oppenheimer’s ideas on black holes until the publication of a paper by Tullio Regge and Wheeler in 1957 21 21 21 Regge, T., & Wheeler, J. A. (1957), “Stability of a Schwarzschild singularity,” Physical Review, 108 , 1063–1069. (see immediately below), in which the attack is tacit as Oppenheimer is not referenced. The only piece resembling an attack on black holes before 1957 was Einstein’s 1939 paper, but this article faded away quickly. It was not cited until 1953, and then only to be torn apart by Amalkumar Raychaudhuri. 22 22 22 Raychaudhuri, A. (1953), “Arbitrary concentrations of matter and the Schwarzschild singularity,” Physical Review, 89 , 417–421.

The fact that Oppenheimer’s ideas survived Einstein’s assault is significant. Also significant is the fact that no other scientist published anything else on the subject of black holes until the late 1950s. A careful scrutiny of the articles written by Landau, Zwicky, Bethe, Richard Tolman, George Gamow, Robert Serber, George Volkoff, and Hartland Snyder shows nothing on this.

The first paper dealing with the subject is the one by Regge and Wheeler in 1957 mentioned above, where the authors proposed wormholes as a way to avoid total collapse of the star. It is important to stress that even though this 1957 paper does not reference Oppenheimer explicitly, it is clear that the paper is presented as a criticism of Oppenheimer’s ideas on indefinite contraction: there is a bold and unnecessary emphasis on the concept of “stability” all throughout the paper, including the first word in the title and the last sentence of the paper. A casual reader could be thus forgiven for thinking that the paper is not so much about discussing wormhole physics as being a defense of stellar stability under extreme conditions.

David Finkelstein wrote a paper in 1958 where, though not directly addressing total collapse, he established the Schwarzschild radius as a surface of “no return.” 23 23 23 Finkelstein, D. (1958), “Past-future asymmetry of the gravitational field of a point particle,” Physical Review, 110 , 965–967. Finkelstein did not reference Oppenheimer either.

In 1960, Wheeler wrote a paper on behalf of Martin Kruskal in which black holes are finally acknowledged. 24 24 24 Kruskal, M. (1960), “Maximal extension of Schwarzschild metric,” Physical Review, 119 , 1743–1745. It is significant that Wheeler is not listed as co-author of the paper even though he did the actual writing, 25 25 25 Wheeler, J. A., & Ford, K. (2000), Geons, Black Holes & Quantum Foam: A Life in Physics , New York: Norton, 745. and that Oppenheimer went unreferenced one more time.

In addition to these publications, there is unpublished work of Wheeler and (independently) Yakov Zel’dovich in the late 1950s, using computers, as reported by Thorne. 26 26 26 Thorne (ref.  9 ), 197 and 240. One must also not forget about the 1958 Brussels confrontation of Wheeler with Oppenheimer mentioned above.

Four Arguments

We now plunge into the question in the subtitle of this paper: Why did the scientific community miss the black hole opportunity? We note that, even though the five reasons listed by Hufbauer are sensible and generally agreed upon, we believe that they could benefit from being elaborated (as in the “too esoteric” and “not earned the right” arguments below) and extended (as in the “wrong episteme” and “wrong relativistic ontology” arguments).

The last two arguments are of a Kuhnian, “history of ideas” flavor and are offered here to complement more conventional approaches. Even if somewhat Foucauldian, they try to provide a fresh perspective on how the conceptual framework of knowing and discovery could have been very different back then.

Before starting, we discuss some general considerations (in the next section) and make the perhaps unnecessary proviso that the four arguments below are not independent among themselves nor with extra-scientific factors. 27 27 27 Part of the conundrum’s answer is clearly extra-scientific. To give but one example, take Oppenheimer and (Caltech colleague) Zwicky’s refusal even to acknowledge each other’s papers. Oppenheimer never used the word “neutron star.” See Thorne (ref.  9 ), 206.

General Considerations

The past is a foreign country: they do things differently there. L. P. Hartley

For a trained scientist, the main difficulty in a project like this one is effectively situating oneself in the conceptual framework of the time, forgetting what came after 1939.

For us, a black hole is an exciting opportunity. Back then, it was a nuisance in need of quick repair. Just to begin, think how a physicist living and working in the 1930s would have perceived an intellectual world very different from ours:

In the first place, the disciplinary landscape was very different. There was no “solid-state” discipline, and there were no “astrophysics” or “cosmology” disciplines in the institutional sense, in sharp contrast with the prestigious particle and nuclear physics disciplines. This lack of disciplinary affiliation made it difficult for a person like Gamow, even as late as the 1950s, to find a scientific audience for his cosmology ideas. On top of everything, the United States was not a scientific power like it is today. 28 28 28 A general reference for the statements made in this section of the paper is Kragh, H. (2002), Quantum generations: A history of physics in the twentieth century , Princeton, NJ: Princeton University Press.

In the second place, most of the related basic knowledge we take for granted today was absent: The mechanism for stellar energy was unknown, only being teased out in 1938 by Bethe and Carl von Weizsäcker. The neutron was a new thing, and the muon did not appear until 1937. Astronomers had not quite finished digesting the fact that galaxies were not nebulas in the Milky Way. 29 29 29 We are grateful to Prof. James Bjorken (Stanford) for his comments on this particular issue and for his interest in this paper’s discussion. He recalls how, as late as 1950, the multi-galaxy idea was still hard to take in general.

In the third place, the name “black hole” with all its psychological and metaphorical implications (e.g., a hole lets you move somewhere else–perhaps into new physics) did not exist. 30 30 30 Wheeler thrust the term “black hole” in 1967. See Wheeler, J. A. (1968), “Our universe: The known and the unknown,” American Scholar, 37 , 248–274. It is important to note also that the term had appeared in print as early as 1964. See Ewing, A. (1964), “‘Black holes’ in space,” Science News Letter, 85 , 39. Instead, the literature would talk about “frozen stars,” a quite anticlimactic term.

The “Too Esoteric” Argument

General relativity was the “string theory” of the 1930s

“Very odd” is how Oppenheimer described in writing his new results to George Uhlenbeck. 31 31 31 Cassidy (ref.  8 ), 176. This is probably an understatement.

Three reasons made this odd situation even odder. In the first place, astronomy in general was much more distant from physics than it is today. It had a natural history ring to it. Oppenheimer was working on the margins of physics.

Secondly, the influential Arthur Eddington had given an esoteric twist to astronomy and cosmology, invoking arguments that at times were perceived as too philosophical.

Thirdly, and most importantly, the abstruse character of general relativity did not help either. As late as 1960, Alfred Schild said that “Einstein’s theory of gravitation … is moving from the realm of mathematics to that of physics” 32 32 32 Quoted in Kragh (ref.  28 ), 362. and even as late as 1958, Wheeler (the eventual champion of black holes) did not feel comfortable at all with the concept of black holes (which led to the famous Brussels confrontation with Oppenheimer that year). 33 33 33 See footnote ( 9 ).

This situation became worse in the United States, as Cassidy says, where theoretical research was supposed to aid experimentalists, not become involved in radical, creative, German-style speculations. 34 34 34 Cassidy (ref.  8 ), 179.

On top of everything, one has to add the fact that Oppenheimer worked with idealized spherical symmetry conditions in his treatment of black holes. Even though this approach is not considered particularly grave today, back then spherical symmetry was considered a special, probably physically irrelevant case. 35 35 35 We are grateful to Prof. Robert Wagoner (Stanford) for his comment on this particular issue and for his interest in this paper’s discussion. He mentioned that a similar opinion of special-case irrelevance surfaced with Kerr’s solution for black holes. Prof. Robert Wald (University of Chicago), to whom we are also grateful, offered further the case of Big Bang cosmology as an example along these lines of special cases.

One also has to keep in mind the precedent of Eddington’s bashing of Chandrasekhar’s ideas (referred to as “stellar buffoonery”) on the collapse of white dwarfs in 1935. 36 36 36 Israel (ref.  9 ), 217. One may wonder just how influential this case might have been as Oppenheimer was trying to expound his position.

The “Not Earned the Right” Argument

Intellectual seniority matters

We could phrase this argument thus: if you had already succeeded at prestigious physics (which in that time meant something nuclear or particle), as in the case of Bethe, 37 37 37 Bethe, H., & Fermi, E. (1932), “Über die Wechselwirkung von Zwei Elektronen,” Zeitschrift für Physik, 77 , 296–306. then you earned the right to do something unorthodox in the border of physics and be taken seriously.

Since Oppenheimer liked to be at the center of things, and was (intellectually) moving all the time, he had never quite achieved fame in anything before publishing his paper with Snyder (Oppenheimer’s papers with Max Born in 1927 38 38 38 Born, M., & Oppenheimer, R. (1927), “Zur Quantentheorie der Molekeln,” Annalen der Physik, 389 , 457–484. and Melba Phillips in 1935 39 39 39 Oppenheimer, J. R., & Phillips, M. (1935), “Note on the transmutation function for deuterons,” Physical Review 48 , 500–502. had presumably been his most famous, but these papers, with only 15 citations each 40 40 40 Google Scholar Citations. in their respective first ten years, could not be called truly revolutionary). This was made worse by his distancing from physics into philosophy, literature and left-wing politics, so this trend of going to extreme stellar physics could be seen as part of a movement away from mainstream physics.

The “Wrong Episteme” Argument

The scientific world had already enough infinities to deal with

Why is it that Einstein never accepted the black hole consequences of his theory? One possibility is that black holes did not belong to the correct episteme of the time.

Michel Foucault uses the term “episteme” to refer to the implicit assumptions about how we know the world. 41 41 41 Foucault, M. (1970), The order of things , New York: Random/Vintage. More precisely, it refers to “…the assumptions about knowledge, method, and theory which at any given time period are shared across “discursive formations” (which as a first approximation can be translated as “disciplines”).” 42 42 42 Hess, D. J. (1995), Science and technology in a multicultural world , New York: Columbia University Press, 87. An episteme differs from a Kuhnian paradigm in part in that it is transdisciplinary.

These statements are best explained by examples. According to Foucault’s ideas, not just physics but the whole realm of academic knowledge in the beginning of the twentieth century was marked by the episteme of equilibrium and closedness. One sees it in biology (population equilibrium theory), economics (classical, pre-Keynesian theory), linguistics (syntax rather than evolution), the social sciences (structuralism), and physics, as in Bohr’s atom. 43 43 43 Ibid., 94.

To these examples discussed by anthropologist of science David Hess, one may add how Einstein develops his general theory of relativity immersed in this episteme. Einstein’s model of the universe needs (by Einstein’s own later account) an artificial “cosmological term” in order to preserve the equilibrium episteme.

Oppenheimer’s stellar “indefinite contraction” did not belong to this episteme. Was he ahead of his time, sensing the forthcoming episteme of open processes?

It is revealing that Einstein published a paper 44 44 44 Einstein (ref.  15 ). with the intention of killing the Schwarzschild singularity 45 45 45 In 1916, Karl Schwarzschild found a solution of Einstein’s equations which were not well behaved at certain points. See Schwarzschild, K. (1916), “Über das Gravitationsfeld eines Massenpunktes nach der Einsteinschen Theorie,” Sitzungsberichte Königlich Preus , Akad. Wiss. Berlin, Phys.-Math. Klasse, 189–196. once and for all. The paper was entitled “On a stationary system with spherical symmetry consisting of many gravitating masses.” It used a stationary argument to show that black holes were impossible. What he actually proved was only that there are no stable solutions to Schwarzschild radius stars (and therefore his original intention was frustrated), but for some reason Einstein thought this proof was sufficient. A case could be thus made for his tacit commitment to the equilibrium episteme.

Along similar equilibrium-episteme lines, Eddington 46 46 46 Israel (ref.  9 ), 219.

…like virtually every relativist of the time, considered the Schwarz- schild radius to be both a singularity and an impassible barrier. The image that he conjures up of the star ‘at last finding peace’ is of a body frozen at the Schwarzschild radius… [emphasis added]

We might speculate what would an out-of-the-equilibrium-episteme attitude look like for a person living within the equilibrium episteme. Perhaps an “equilibrium epistemist” would simply consider a person like Oppenheimer as somewhat lost, not confident, confused. The following 1967 quotation from particle physicist Isidor Rabi (born in 1898, and therefore only six years Oppenheimer’s senior) is useful: 47 47 47 Quoted in Thorne (ref.  9 ), 208.

[I]t seems to me that in some respects Oppenheimer was overeducated in those fields which lie outside the scientific tradition, such as his interest in religion, in the Hindu religion in particular, which resulted in a feeling for the mystery of the Universe that surrounded him almost like a fog. He saw physics clearly, looking toward what had already been done, but at the border he tended to feel that there was much more of the mysterious and novel than there actually was . He was insufficiently confident of the power of the intellectual tools he already possessed and did not drive his thought to the very end because he felt instinctively that new ideas and new methods were necessary to go further than he and his students had already gone [emphasis added].

Rabi thus felt the need to explain Oppenheimer’s unassertiveness as something having nothing to do with science, but rather with his other, extra-scientific inclinations.

It is appropriate to finish this section with an intriguing comment. 48 48 48 We are in debt to Prof. Robert Wald (University of Chicago) and Randall Espinoza (University of Illinois at Chicago) for this particular point. A different reading of Wheeler’s initial attitude towards Oppenheimer could be made (and it is one that does not necessarily contradict this article’s main argument) in which it was Wheeler’s strong commitment to a particle physics point view which would have intensified his lack of interest in Oppenheimer’s work. We are referring in particular to Wheeler’s work on geometrodynamics 49 49 49 Wheeler, J. A. (1957), “On the nature of quantum geometrodynamics,” Annals of Physics, 2 , 604–614. and his aversion to singularities, and on how gravity, considered as part of the particle physics puzzle, could have helped to solve fundamental difficulties in the theory. In such a reading, Wheeler’s disregard of Oppenheimer’s ideas would be less dramatic and more of a pragmatical nature. The details of such a study are to be carried out elsewhere.

The “Wrong Relativistic Ontology” Argument

The spell of geometry

The set of ten equations of general relativity,

can be interpreted in different ways. If one reads them from right to left, then the matter (through the momentum-energy tensor T a ​ b subscript 𝑇 𝑎 𝑏 T_{ab} ) determines the geometry (described by Einstein tensor G a ​ b subscript 𝐺 𝑎 𝑏 G_{ab} ). If, on the other hand, one chooses to read them from left to right, then geometry would be ontologically primal: geometry dictates how matter must behave.

Even though in either interpretation one must have of course exactly the same equations, from a cognitive point of view, and even from a mathematical point of view , it makes a huge difference what interpretation you adhere to.

The original interpretation was the geometrical one, even to the point that Einstein’s crafting of general relativity is imbued with quite a bit of implicit space-time reification, 50 50 50 Janssen, M. (2007), What did Einstein know and when did he know it?, in J. Renn (Ed.), The Genesis of General Relativity , vol. 2 (pp. 785–837), Dordrecht: Springer, 825. called “substantivalism” in the literature, a curious state of affairs indeed since Einstein was an enemy of absolute space.

In any event, nowadays general relativity applications follow a more “matter first” approach. In this sense, Oppenheimer appears to be again ahead of his time. The crucial point is that the geometrical approach biases your understanding and your problem searching towards more static/stationary situations.

To make this point clearer, consider a system of two masses rapidly rotating around each other. This system will produce oscillating space-time ripples moving away from them. If one starts from the two masses, then it is straightforward to calculate the surrounding oscillating geometry. However, the opposite problem of reconstructing the masses’ movements from the geometrical ripples is a fantastically complicated problem. This is an example of a problem that does not lend itself to be formulated if one starts from a geometrical viewpoint.

The consequence of all this is that your aesthetical judgment (“geometry first”) is going to have an effect on the type of problems you tackle. If you unite this effect with the equilibrium episteme one (described in the previous section) the result is devastating for Oppenheimer, as collapsing stars are thus doubly denaturalized: they are not in equilibrium, and they are not “geometry first.”

Oppenheimer was an outsider to this geometrical ontology. As Cassidy says, “the few active general relativity theorists were interested not in the astrophysics of a star collapsing into a mathematically awkward singularity but in the more elegant and well-behaved geometry of continuous, nonsingular curved space-time” (emphasis added). 51 51 51 Cassidy (ref.  8 ), 177.

Discussion: Oppenheimer’s Black Hole vis-à-vis Einstein’s EPR Paradox and Zwicky’s Dark Matter

It is helpful to perform a comparison between the black hole idea as developed by Oppenheimer’s group (in 1939) with the Einstein-Podolsky-Rosen Paradox (in 1935) and Zwicky’s concept of dark matter or Dunkle Materie (in 1933). 52 52 52 This discussion actually originated in lively fashion during the January 2014 Stanford meeting. Einstein, A., Podolsky, B., & Rosen, N. (1935), “Can quantum-mechanical description of physical reality be considered complete?,” Physical Review, 47 , 777; Zwicky, F. (1933), “Die Rotverschiebung von extragalaktischen Nebeln,” Helvetica Physica Acta, 6 , 110–127. (For the benefit of readers not familiar with these scientific concepts, there is a brief description of each in Appendix B.)

All three theoretical concepts appeared in the 1930s. They all have in common that the related ideas were put aside for several decades before they were taken seriously, when one could say experiments made them inevitable.

The similarities stop there, though. The EPR Paradox was really not a discovery of a new entity, but rather a gedankenexperiment designed with the sole purpose of pointing out an inconsistency in the looming (for Einstein) conceptual edifice of quantum mechanics. Einstein would have been happy if he had caused the dismissal of the Copenhagen interpretation of quantum mechanics; there was no actual intent, or interest, of carrying out the experiment or having somebody else carrying it out.

Zwicky’s dark matter was also more about pointing out an inconsistency than discovering a new substance. The fact that we are currently, 80 years later, looking for dark matter should not distract us from this point.

Both Zwicky’s dark matter and Einstein’s EPR Paradox are more what one would call “anomalies” in the Kuhnian sense (as is the case of Mercury’s perihelion precession) than true proposals/discoveries of new physical entities or phenomena. This makes a huge difference, since anomalies tend to be treated with respect, and kept along in their unresolvedness.

This is, we believe, what makes the history of Oppenheimer’s black holes much more intriguing than Einstein’s and Zwicky’s counterparts.

Final Words

Something rather interesting happened in physics in the late 1930s. In what was to prove (judged in retrospect) as his last shot at intellectual glory, Oppenheimer, with what might be termed the tacit complicity of the whole physics community, missed a chance to fully discover black holes—not observationally, but theoretically. Says Werner Israel about Oppenheimer’s work with Snyder: “[it] has strong claims to be considered the most daring and uncannily prophetic paper ever published in the field.” 53 53 53 Israel (ref.  9 ), 226. Thorne says: “This line of reasoning [what happens when a neutron star cannot hold its own weight] is so obvious in retrospect that it seems amazing that Zwicky did not pursue it, Chandrasekhar did not pursue it, Eddington did not pursue it.” 54 54 54 Thorne (ref.  9 ), 178. We have to add that once it was initially pursued, by Oppenheimer, it was then ignored by the community until the late 1950s, many years after the war was over.

In this paper we have tried to address the issue of why is it that this new idea did not receive the benefit of the doubt in the same sense that other oddities did (such as many in particle physics, e.g., the uncertainty principle), even though there is plenty of evidence that Oppenheimer’s work was considered authoritative by at least some of his contemporaries, such as Hayashi in 1962 (in addition to what was discussed about Landau above).

In this paper we have tried to go beyond previous discussions on this topic. The way we did so was by adding an additional layer of a more history-of-ideas, Foucaldian nature. We entertained the possibility that at least some of the explanations might have to do with idiosyncratic aspects (of the scientific culture, that is), in particular to a tacit commitment to the equilibrium episteme and a geometry-first ontology. In contrast with the EPR Paradox and the ingenuity of the dark matter concept, the black hole idea is not so much about pointing out an anomaly as adding a new object to our universe.

In this essay, we have sought to supplement the existent literature on the subject by enriching the explanations and possibly complicating the guiding questions. We suggest a rereading of Oppenheimer as a more intriguing, human, ahead-of-his-time figure.

Acknowledgements

This work was supported by grant 805-A4-125 of the Universidad de Costa Rica’s Vicerrectoría de Investigación and the CIGEFI, and by grant FI-0204-2012 of the MICITT and the CONICIT.

Brief Description of the Relevant Papers

Oppenheimer & Serber, October 1, 1938

On the stability of stellar neutron cores

This one-page, no-formula letter is a critique of Landau’s work on “condensed neutron cores”—as it was believed back then that a neutron star (a “neutron core”) could lie in the interior of stars like the Sun. 55 55 55 Landau’s ideas appear on the following two papers: Landau, L. (1932), “On the theory of stars,” Phys. Z. Sowjetunion, 1 , 285; Landau, L. (1938), “Origin of stellar energy,” Nature, 141 , 333–334.

The main point raised by the authors had to do with the necessary inclusion of strong nuclear forces considerations (which were absent in Landau’s papers). This inclusion was problematic because it came in a moment in history in which there was “no existing nuclear experiment or theory [giving] a complete answer to this question.” 56 56 56 Oppenheimer & Serber (ref.  12 ), 540.

Oppenheimer & Volkoff, February 15, 1939 (received January 3)

On massive neutron cores

Here the authors continue commenting on improvements on Landau’s ideas, this time emphasizing the importance of using a general relativistic approach rather than a Newtonian one. The reason for this is that neutron cores have an extremely high density and require thus a relativistic approach. Stars that would be stable in a Newtonian world are unstable once general relativity is considered.

For the first time, the indefinite contraction fate for heavy enough stars in mentioned.

Oppenheimer & Snyder, September 1, 1939 (received July 10)

On continued gravitational contraction

In this paper, the authors apply the equations of general relativity to prove that, at least under some simplifying conditions (non-rotating star, no pressure, no outward radiation), a large enough star will contract indefinitely.

This is the debut of black holes. The authors describe how time freezes at the Schwarzschild radius (of a few kilometers), while it does not freeze for an infalling observer.

Einstein, October 1939 (received May 10)

On a stationary system with spherical symmetry consisting of many gravitating masses

After criticizing simpler treatments on the subject, Einstein uses a stationary argument (cluster of particles in circular paths) to argue that black holes are impossible. However, what he actually proves is that very compact stars are unstable.

Brief Description of the EPR Paradox and Zwicky’s Dark Matter

Einstein-Podolsky-Rosen Paradox

The EPR Paradox is a thought experiment designed to show that there is a theoretical inconsistency within quantum mechanics if one holds that it is a complete theory. Imagine a pair of particles originating from a common source. According to the Copenhagen interpretation of quantum mechanics, under some conditions the state of particles 1 and 2 remain fundamentally undetermined until one decides to measure one of them. When one does measure one of them, say particle 1, then either a ) a) particle 2 has a definite state which, however, is not included in the theory, rendering thus the theory incomplete , or b ) b) particle 2 acquires, immediately after performing the measurement on particle 1, certain definite physical property, thus provoking an action-at-a-distance effect, which is contrary to the principles of special relativity.

Einstein et al. assumed that option b ) b) is untenable and thus quantum mechanics must be incomplete—ruining thus the Copenhagen interpretation of the theory. Experiments performed from the 1980s on have, however, corroborated option b ) b) .

Zwicky’s Concept of Dark Matter

The concept of “dark matter” was postulated in order to solve a breach between theory and observation in astrophysics. As stars move around the center of galaxies (including our own), their speeds are higher than expected, as if there were a substantial amount of matter not accounted for: invisible matter—hence the name “dark.” More precisely, theory requires that 85% (by modern calculations) of the mass be in the form of dark matter. Or else, there is something fundamentally wrong with our theories of gravity. The dark matter problem has not been solved yet.

Timeline of Events

Main events relevant to the discussion. Note the gap between 1939 and 1957.

• Collections

• APS Journals

Landmarks —Forgotten Black Hole Birth

APS has put the entire Physical Review archive online, back to 1893. Focus Landmarks feature important papers from the archive.

Had J. Robert Oppenheimer not led the US effort to build the atomic bomb, he might still have been remembered for figuring out how a black hole could form. His 1939 Physical Review paper, written with graduate student Hartland Snyder, described how a star might collapse into an object so dense that not even light could escape its gravitational clutches. The paper was hardly noticed until the 1960s, when astrophysicists began to seriously consider that such extreme objects might exist. John Wheeler of Princeton University then came up with the name “black holes” for these now standard elements of astrophysics.

In the 1930s, as astronomers began to ponder the future of a star whose nuclear fuel had run out, they ran into difficulty describing its fate. When energy generation within a star falters, its own gravity begins to take over, compressing the star’s mass. Our own sun will end up as a white dwarf, whose core is a dense mix of atomic nuclei in a sea of electrons. But the fates of larger stars were less clear.

Researchers understood that a white dwarf with too much mass isn’t stable, and its own immense gravity should compress it into something denser, possibly made entirely of neutrons. In a paper published early in 1939 [1] Oppenheimer and George Volkoff, both of the University of California at Berkeley, proved that neutron cores, like white dwarfs, could not be indefinitely heavy. In this paper they combined general relativity–Einstein’s theory of gravity–with a quantum mechanical description of a pure neutron fluid. But they could find no stable solution to the equations if the neutron core had more than about 70% of the sun’s mass. Astrophysicists now know such large cores result from the deaths of stars several times as massive as the sun. For a larger core, collapse must proceed further. But into what?

Oppenheimer and Snyder answered that question in a paper later in 1939. Since static solutions to their equations seemed to be impossible, they looked for solutions where the structure of the object never stopped changing in time, representing continued contraction. Theoretical physicists at the time had little experience applying Einstein’s equations under such extreme conditions–not to mention a total lack of mathematical computer power–so the team had to make many approximations and educated guesses along the way. Still, their brief paper arrives at essentially correct conclusions about the formation of what we now call a black hole.

The matter in an extreme collapsing object, Oppenheimer and Snyder say, would retreat inexorably inward. Meanwhile, any escaping radiation would suffer an increasing gravitational redshift, drifting to ever longer wavelengths as it fought an uphill battle against gravity. An outside observer would see the collapsing object become redder and fainter. “The star thus tends to close itself off from any communication with a distant observer; only its gravitational field persists,” the authors conclude.

“It fascinates me why that paper was neglected for so long,” says Saul Teukolsky of Cornell University in Ithaca, New York. There were some loose ends, he explains. It seemed possible that some extraordinary source of pressure might prevent contraction all the way to a black hole, but Wheeler and others, reviving the subject in the 1960s, showed that there was no way out. Teukolsky says that among many astronomers and physicists there was an instinctual revulsion against unlimited collapse, and that Lev Landau of Moscow University even suggested modifying quantum mechanics to make sure it couldn’t happen. The Oppenheimer-Snyder paper “goes against the prevailing thought of the time,” he says. “It really stands out.”

–David Lindley

David Lindley is a freelance science writer in Alexandria, Virginia.

• J. R. Oppenheimer and G. M. Volkoff, “On Massive Neutron Cores,” Phys. Rev. 55 , 374 (1939)

On Continued Gravitational Contraction

J. R. Oppenheimer and H. Snyder

Phys. Rev. 56 , 455 (1939)

Published September 1, 1939

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Oppenheimer- How the 'father of the atomic bomb' contributed to the discovery of black holes

Did you know that oppenheimer, the man who developed the atom bomb dropped on japan during the second world war, played a significant role in the discovery of black hole.

Today when people are excited about the Oppenheimer movie and can only imagine J. Robert Oppenheimer as the ‘father of the atom bomb', very few people know that the astrophysicist also played a stellar role in other discoveries too. He conducted research on black holes even before they were discovered. He played a significant role in the discovery of black holes and described how the collapse of a massive star can lead to the formation of black holes.

Prior to his work at Los Alamos, New Mexico, in 1942, Oppenheimer was a prominent theoretical physicist focusing on quantum physics.

You may be interested in

Research on Black hole

In collaboration with his colleague Hartland S. Snyder at the University of California, Berkeley, Oppenheimer published a research paper in 1939 titled "On Continued Gravitational Contraction." He showed how black holes could be born by using Einstein's general theory of relativity.

Oppenheimer's model described how a massive star could collapse under its own gravitational force, leading to the formation of a black hole. This work was the foundation to understand black hole as a dynamic astrophysical process and representing the final stage in the evolution of sufficiently massive stars. Notably, this model is still employed in modern astrophysical research.

Before starting the black hole research, Oppenheimer had already explored the topic of neutron stars in a 1938 paper. His interest in astrophysics continued with the incorporation of Einstein's general theory of relativity in a 1939. On September 1, 1939, Oppenheimer published another paper specifically dedicated to black holes. However, its significance was largely overshadowed as it coincided with the outbreak of World War II when Germany invaded Poland.

Despite his significant contributions to the field of black hole, Oppenheimer's legacy remains closely tied to his involvement in the development of the atomic bomb. His work on black holes and his pioneering collapse model, though often overlooked by the public, continues to influence and shape our understanding of the physical world.

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Black holes are mysterious, yet also deceptively simple − a new space mission may help physicists answer hairy questions about these astronomical objects

Professor of Physics, University of Rhode Island

Disclosure statement

The article presents work carried out in collaboration with Stefanos Aretakis, Kevin Gonzalez-Quesada, Lior Burko, Subir Sabharwal and Som Bishoyi. This research was supported by the US National Science Foundation. All computations were performed at the Massachusetts Green High Performance Computing Center leveraging the resources of the URI Center for Computational Research. The author also acknowledges support from the UMass-URI Gravity Research Consortium (U2GRC).

University of Rhode Island provides funding as a member of The Conversation US.

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Physicists consider black holes one of the most mysterious objects that exist. Ironically, they’re also considered one of the simplest. For years, physicists like me have been looking to prove that black holes are more complex than they seem. And a newly approved European space mission called LISA will help us with this hunt.

Research from the 1970s suggests that you can comprehensively describe a black hole using only three physical attributes – their mass, charge and spin. All the other properties of these massive dying stars, like their detailed composition, density and temperature profiles, disappear as they transform into a black hole. That is how simple they are.

The idea that black holes have only three attributes is called the “no-hair” theorem, implying that they don’t have any “hairy” details that make them complicated.

Hairy black holes?

For decades, researchers in the astrophysics community have exploited loopholes or work-arounds within the no-hair theorem’s assumptions to come up with potential hairy black hole scenarios. A hairy black hole has a physical property that scientists can measure – in principle – that’s beyond its mass, charge or spin. This property has to be a permanent part of its structure.

About a decade ago, Stefanos Aretakis , a physicist currently at the University of Toronto, showed mathematically that a black hole containing the maximum charge it could hold – called an extremal charged black hole – would develop “hair” at its horizon. A black hole’s horizon is the boundary where anything that crosses it, even light, can’t escape.

Aretakis’ analysis was more of a thought experiment using a highly simplified physical scenario, so it’s not something scientists expect to observe astrophysically. But supercharged black holes might not be the only kind that could have hair.

Since astrophysical objects such as stars and planets are known to spin, scientists expect that black holes would spin as well , based on how they form. Astronomical evidence has shown that black holes do have spin, though researchers don’t know what the typical spin value is for an astrophysical black hole.

Using computer simulations, my team has recently discovered similar types of hair in black holes that are spinning at the maximum rate. This hair has to do with the rate of change, or the gradient, of space-time’s curvature at the horizon. We also discovered that a black hole wouldn’t actually have to be maximally spinning to have hair, which is significant because these maximally spinning black holes probably don’t form in nature.

Detecting and measuring hair

My team wanted to develop a way to potentially measure this hair – a new fixed property that might characterize a black hole beyond its mass, spin and charge. We started looking into how such a new property might leave a signature on a gravitational wave emitted from a fast-spinning black hole.

A gravitational wave is a tiny disturbance in space-time typically caused by violent astrophysical events in the universe. The collisions of compact astrophysical objects such as black holes and neutron stars emit strong gravitational waves. An international network of gravitational observatories, including the Laser Interferometer Gravitational-wave Observatory in the United States, routinely detects these waves.

Our recent studies suggest that one can measure these hairy attributes from gravitational wave data for fast-spinning black holes . Looking at the gravitational wave data offers an opportunity for a signature of sorts that could indicate whether the black hole has this type of hair.

Our ongoing studies and recent progress made by Som Bishoyi, a student on the team, are based on a blend of theoretical and computational models of fast-spinning black holes. Our findings have not been tested in the field yet or observed in real black holes out in space. But we hope that will soon change.

LISA gets a go-ahead

In January 2024, the European Space Agency formally adopted the space-based Laser Interferometer Space Antenna , or LISA, mission. LISA will look for gravitational waves, and the data from the mission could help my team with our hairy black hole questions.

Formal adoption means that the project has the go-ahead to move to the construction phase, with a planned 2035 launch. LISA consists of three spacecrafts configured in a perfect equilateral triangle that will trail behind the Earth around the Sun. The spacecrafts will each be 1.6 million miles (2.5 million kilometers) apart , and they will exchange laser beams to measure the distance between each other down to about a billionth of an inch.

LISA will detect gravitational waves from supermassive black holes that are millions or even billions of times more massive than our Sun. It will build a map of the space-time around rotating black holes, which will help physicists understand how gravity works in the close vicinity of black holes to an unprecedented level of accuracy. Physicists hope that LISA will also be able to measure any hairy attributes that black holes might have.

With LIGO making new observations every day and LISA to offer a glimpse into the space-time around black holes, now is one of the most exciting times to be a black hole physicist.

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Scientists discover bizarre region around black holes that proves Einstein right yet again

Einstein's general theory of relativity predicted that so-called 'plunging regions' around black holes would accelerate matter into them at the speed of light. Now, X-ray observations of a remote black hole have proved him right.

Astronomers have observed matter plunging into the mouth of a black hole at the speed of light, proving a key prediction made by Einstein right, yet again.

In 1915, Einstein's general theory of relativity predicted that once matter gets sufficiently close to a black hole, the immense force of the space-time tear's gravity should force it to abandon a circular orbit and plunge straight in. 

Now, X-ray observations made with NASA's NuSTAR and NICER space telescopes have finally confirmed that this so-called "plunging region" exists. The team, led by researchers at the Department of Physics at Oxford, behind the discovery say studying it could reveal some fundamental mysteries about black holes and the nature of space-time. The researchers published their findings May 16 in the journal Monthly Notices of the Royal Astronomical Society .

"This is the first look at how plasma, peeled from the outer edge of a star, undergoes its final fall into the [center] of a black hole, a process happening in a system around 10,000 light years away," lead author Andrew Mummery , a physicist at Oxford University, said in a statement . "What is really exciting is that there are many black holes in the galaxy, and we now have a powerful new technique for using them to study the strongest known gravitational fields."

Black holes are born from the collapse of giant stars and grow by gorging on gas, dust, stars and other black holes. The cosmic monsters have such a powerful gravitational pull that nothing (not even light) can escape their maws.

Related: 1st detection of 'hiccupping' black hole leads to surprising discovery of 2nd black hole orbiting around it

But this doesn't mean that black holes can't be seen. Active black holes are surrounded by accretion disks — vast plumes of material that is stripped from gas clouds and stars and heated to red-hot temperatures by friction as it spirals into the black holes' mouths.

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By pointing the two space telescopes at a black hole called MAXI J1820+070, located inside a binary system roughly 10,000 light-years from Earth, the researchers detected X-rays emitted by the scorching material of its accretion disk. Placing their X-ray data into mathematical models, they discovered that the two only matched if the models included light coming from matter in the plunging region — confirming its existence.

" Einstein's theory predicted that this final plunge would exist, but this is the first time we have been able to demonstrate it happening," Mummery said. "Think of it like a river turning into a waterfall — hitherto, we have been looking at the river. This is our first sight of the waterfall."

By collecting and studying more light from this cosmic cascade, the researchers say that they will gain unprecedented insights into the extreme conditions around black holes. Plunging regions sit just outside of black holes' event horizons — points of no return where gravity becomes so strong that not even light can escape.

"We believe this represents an exciting new development in the study of black holes, allowing us to investigate this final area around them. Only then can we fully understand the gravitational force," Mummery said. "This final plunge of plasma happens at the very edge of a black hole and shows matter responding to gravity in its strongest possible form."

Ben Turner is a U.K. based staff writer at Live Science. He covers physics and astronomy, among other topics like tech and climate change. He graduated from University College London with a degree in particle physics before training as a journalist. When he's not writing, Ben enjoys reading literature, playing the guitar and embarrassing himself with chess.

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First proof that “plunging regions” exist around black holes in space

An international team led by researchers at Oxford University Physics  have proved Einstein was correct about a key prediction concerning black holes. Using X-ray data to test Einstein’s theory of gravity, their study gives the first observational proof that a “plunging-region” exists around black holes: an area where matter stops circling the hole and instead falls straight in.  Furthermore, the team found that this region exerts some of the strongest gravitational forces yet identified in the galaxy. The findings have been published in Monthly Notices of the Astronomical Society .

Einstein’s theory predicted that this final plunge would exist, but this is the first time we have been able to demonstrate it happening. Think of it like a river turning into a waterfall – hitherto, we have been looking at the river. This is our first sight of the waterfall. Lead author Dr Andrew Mummery , Oxford University Physics.

The new findings are part of wide-ranging investigations into outstanding mysteries around black holes by astrophysicists at Oxford University Physics. This study focused on smaller black holes relatively close to Earth, using X-ray data gathered from NASA’s space-based Nuclear Spectroscopic Telescope Array (NuSTAR)  and Neutron star Interior Composition Explorer (NICER) telescopes. Later this year, a second Oxford team hopes to move closer to recording the first videos of larger, more distant black holes as part of a European initiative.  

Unlike in Newton’s theory of gravity, Einstein’s theory states that sufficiently close to a black hole it is impossible for particles to safely follow circular orbits. Instead they rapidly “plunge” toward the black hole at close to the speed of light. The Oxford study assessed this region in depth for the first-time, using X-ray data to gain a better understanding of the force generated by black holes.

‘This is the first look at how plasma, peeled from the outer edge of a star, undergoes its final fall into the centre of a black hole, a process happening in a system around ten thousand light years away,’ said Dr Andrew Mummery , of Oxford University Physics, who led the study. ‘What is really exciting is that there are many black holes in the galaxy, and we now have a powerful new technique for using them to study the strongest known gravitational fields.’

‘Einstein’s theory predicted that this final plunge would exist, but this is the first time we have been able to demonstrate it happening,’ Dr Mummery continued. ‘Think of it like a river turning into a waterfall – hitherto, we have been looking at the river. This is our first sight of the waterfall.’

‘We believe this represents an exciting new development in the study of black holes, allowing us to investigate this final area around them. Only then can we fully understand the gravitational force,’ Mummery added. ‘This final plunge of plasma happens at the very edge of a black hole and shows matter responding to gravity in its strongest possible form.’

Debate between astrophysicists has been underway for many decades as to whether the so-called plunging region would be detectable. The Oxford team has spent the last couple of years developing models for it and, in the study just published, demonstrate its first confirmed detection found using X-ray telescopes and data from the International Space Station.  

Whilst this study focuses on small black holes closer to Earth, a second study team from Oxford University Physics is part of a European initiative to build a new telescope, The Africa Millimetre Telescope , which would greatly enhance our ability to make direct images of black holes. Over 10 million Euro funding has already been secured, part of which will support several first PhDs in astrophysics for The University of Namibia, working closely with the Oxford Physics University team.

The new telescope is expected to enable observation, and filming, for the first time of large black holes at the centre of our own galaxy, as well as far beyond. As with the small black holes, large black holes are expected to have a so-called “event horizon”, dragging material from space toward their centre in a spiral as the black hole rotates. These represent almost unimaginable sources of energy and the team hope to observe – and film - them rotating for the first time.

The study “Continuum emission from within the plunging region of black hole discs” has been published in Monthly Notices of the Astronomical Society .

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