Oppenheimer Calculates the Nature of Black Holes

Date February 15, 1939

J. Robert Oppenheimer calculated that stellar matter could collapse under intense gravitational pressure to form what would later become known as a black hole.

Locale Berkeley, California

Key Figures

• J. Robert Oppenheimer (1904-1967), American physicist
• George Michael Volkoff (1914-2000), Canadian physicist and student of Oppenheimer
• Karl Schwarzschild (1873-1916), German physicist
• Lev Davidovich Landau (1908-1968), Soviet physicist
• Hartland S. Snyder (1913-1962), American physicist and student of Oppenheimer

Summary of Event

Sir Isaac Newton first formulated the mathematical nature of gravity and its relationship to mass in 1692, when he theorized that the more massive an object, the more gravity it possesses. Such a relationship is true for all objects with mass, including a child’s marble, the earth, and massive stars. Shortly after Newton’s brilliant philosophical and mathematical treatment of gravity, scientists began to consider the limits of mass and gravity. In 1796, Pierre-Simon Laplace used the eighteenth century notion that light was made of microscopic particles, or corpuscles, and reasoned that if there were a sufficiently massive body somewhere in the universe, these light corpuscles could not escape from its surface. Laplace’s reasoning, however, was nothing more than armchair musings on the limits of Newtonian gravity.

In 1915, German physicistAlbert Einstein reconsidered Newton’s description of gravity with profound effect in a treatment he called the general theory of relativity. Einstein’s theory united such seemingly disparate ideas as light, energy, time, space, matter, and gravity into a single formulation, enabling all these concepts to be treated as unified elements of single conditions for the first time.

Later that year, German physicist Karl Schwarzschild considered the new philosophy of gravity proposed by Einstein. Schwarzschild began to ponder the relativistic mathematical implications of a point in space emanating an intense gravitational field and what an observer would see as that point in space was approached. He was contemplating Einstein’s notion that light is affected, or bent, when traveling through a gravitational field, a concept substantiated in 1913 when the light of a star was apparently bent while traveling through the gravitational field of the Sun.

Schwarzschild’s mathematics were designed to establish the limits of relativity and the degree of the bending effect on light, not to define what would later become known as a “black hole.” The significance of Schwarzschild’s work was not only that he had uncovered some extremely interesting concepts based on relativity but also that some remarkable effects were mathematically allowed by relativity that seemed to violate even common sense.

Schwarzschild discovered that as one approaches his theoretical focal point of intense gravity (such as Earth’s mass concentrated in a single point), space literally curves in on itself, and relativity dictates that not even light can escape such a point. More significantly, Schwarzschild discovered that the intense gravitational field need not be confined to a single point in space. His calculations demonstrated that such effects could be observed if one compressed the planet Earth to a sphere with a diameter of 1 centimeter. This relationship of mass to diameter has become known as the Schwarzschild radius.

In 1939, American physicist J. Robert Oppenheimer and his student George Michael Volkoff were doing calculations on the nature of extremely massive star cores at the University of California, Berkeley. They were contemplating the theory of Soviet physicist Lev Davidovich Landau, who had used Newton’s theory of gravity in the previous decade. Landau created the first theoretical treatment of the center of very dense stars known as neutron stars. Landau believed that if a star were massive enough, the core would contract and be composed of densely packed neutrons.

The discussion of stellar densities continued, and it was suggested to Landau that if the density were great enough, the core of the star would continue to collapse even beyond the neutron state to a single point. Landau dismissed this suggestion as ridiculous, insisting that his calculations demonstrated that this could never happen. However, Oppenheimer and Volkoff reasoned that there was nothing in the relativistic calculations that would prevent collapse beyond the neutron star state and added that Landau had used Newtonian concepts that had been superseded by relativistic concepts.

On February 15, 1939, Oppenheimer and Volkoff published a paper making these points in the Physical Review. The two scientists continued to speculate even after their paper was published: They teamed up with another one of Oppenheimer’s graduate students, mathematics prodigy Hartland S. Snyder, and formulated a more refined mathematical picture of such a hypothetical stellar collapse. In this treatment, published less than a year after the Oppenheimer-Volkoff paper, Oppenheimer and Snyder described in detail the effects of such a stellar collapse. They discussed the effects that would prevent such a collapse, including a rapid spin rate, stellar explosions, and internal pressure that would act to resist the collapse. Still, they speculated that a truly massive star could not help but collapse in on itself. Eventually, light would bend back into the star, as would any other form of radiation, until it could no longer escape. As they described it, “The star thus tends to close itself off from any communication with a distant observer; only its gravitational field persists.”

In these two papers, Oppenheimer and his students were the first to address the idea of a black hole as more than an academic exercise. They both introduced the idea of such an object and also related it to stellar concepts. Furthermore, they went on to mathematically define the limits of such an object. At no time during any of these discussions did the term “black hole” ever arise. Indeed, even Oppenheimer had no idea that such an object really existed, and if it did, he did not know it might ever be detected; by his own definition, the object would tend to cut itself off from any outside communication. The person credited with first using the term “black hole” (in 1967) was Princeton physicist John Archibald Wheeler.

In late 1963, a group of scientists convened in Dallas, Texas, for a meeting titled “An International Symposium on Gravitational Collapse and Other Topics in Relativistic Astrophysics.” Scientists discussed the relationship of very strong, high-energy point sources emanating from space, and most strongly suspected that these point sources of extraordinary energy could well be caused by the collapse of very massive stars up to or beyond the Schwarzschild radius. The meeting was chaired by Oppenheimer and was also attended by Martin Schwarzschild (son of Karl) and by Wheeler, who contributed his long-held convictions that black holes did in fact exist. Although no black hole has ever been directly observed, strong indirect evidence points to their existence.

Significance

The Oppenheimer-Volkoff paper was an important early use of Einstein’s relativity because it deliberately challenged Landau’s use of classical Newtonian physics within the same predictive environment. It clearly demonstrated the superiority of relativistic physics compared with classical physics, which became a practically useless measure of predicting stellar conditions. Oppenheimer employed relativistic physics—and made its superiority obvious—in his now-famous discussion of the core of neutron stars.

Oppenheimer’s paper made an important comparison between Newtonian and classical physics and justified his view of neutron stars, but it also gave rise to a wildly conjectural concept. It proposed that there could be a class of star so dense that it devoured itself and became, in essence, a hole in space and time, where space literally curved in on itself. In his follow-up paper, Oppenheimer used Snyder’s mathematical genius to further refine the concept of a black hole to the point that it became a well-defined physical entity. It was Oppenheimer’s refinement of the idea that took the concept of black holes from a physical abstraction to a physical entity.

The Oppenheimer-Snyder work defined the black hole in terms that are still used. Twenty years after their paper was published, scholars determined that a class of bizarre stellar objects emitted prodigious quantities of energy, and Oppenheimer’s studies were applied to further understand these objects’ strange environment and makeup. Oppenheimer’s work discussed the relativistic concepts of observing a black hole from nearby space and even within the direct physical influence of a black hole. Today, his work remains an important tool for explaining the extremes of relativistic physics.

Black holes are a vital piece of the universe’s vast puzzle, and ideas about black holes are widely used in a variety of cosmological and astrophysical theories. They are blamed for everything from hot jets of matter seen ejected from the center of some galaxies to energetic X-ray pulses emitted from star groups. There may be a black hole lurking at the heart of nearly every galaxy, and black holes may hide a significant portion of the universe’s mass. The knowledge gained in studying these ideas is vital to predicting whether the universe will continue expanding indefinitely or will ultimately collapse on itself. The science and theory of these enigmatic objects remain some of the most startling and interesting ideas in science in the late twentieth century.

Bibliography

Asimov, Isaac. The Collapsing Universe. New York: Walker, 1977. Asimov provides an easy-to-grasp look at the story of black holes as seen from the layperson’s perspective. In his readable style, Asimov attacks the discussion from both the historical and scientific points of view.

Crease, Robert P., and Charles C. Mann. The Second Creation: Makers of the Revolution in Twentieth Century Physics. 1986. Reprint. New Brunswick, N.J.: Rutgers University Press, 1996. In this book, Crease and Mann follow the making of twentieth century physics from its nineteenth century roots to the most enigmatic mysteries of the late 1980’s. Examines characters and personalities as well as the issues of physics. Although this work makes little mention of Oppenheimer’s famous black hole paper of 1939, it offers a unique and fascinating glimpse into his character and personality.

Harwit, Martin. Cosmic Discovery. New York: Basic Books, 1981. This book offers a readable style and approach that details the development of the black hole concept and Oppenheimer’s contribution. The book is written somewhat stiffly from the lay perspective, and it contains valuable information, photographs, and illustrations.

Miller, Arthur I. Empire of the Stars: Obsession, Friendship, and Betrayal in the Quest for Black Holes. Boston: Houghton Mifflin, 2005. Provides background on the history of the idea of black holes and describes the debate concerning the nature of black holes as well as the implications of that debate for science.

Shipman, Harry L. Black Holes, Quasars, and the Universe. New York: Houghton Mifflin, 1976. In this excellent book, black holes are covered extensively in a readable style and related to their cosmic cousins, the pulsars and quasars. The book relates how Oppenheimer’s work with pulsars dovetailed into the black hole theory and how both are related to quasars. The book is thoroughly illustrated and is quite readable by those with a good background in the sciences.

Sullivan, Walter. Black Holes: The Edge of Space, the End of Time. Garden City, N.Y.: Doubleday, 1979. This excellent book is well illustrated and easy to read, offering a clear picture of the revolution in physics that led to the theoretical discovery of black holes by Oppenheimer and his colleagues. It details the pioneering efforts of Einstein and Schwarzschild and describes the historic 1963 Dallas conference, where the concept of black holes was first announced to the public.

Susskind, Leonard, and James Lindesay. An Introduction to Black Holes, Information, and the String Theory Revolution: The Holographic Universe. Hackensack, N.J.: World Scientific, 2004. Explains concepts that physicists of the early twenty-first century have developed in relation to black holes and thinking about space, time, matter, and information.

Wheeler, John A. A Journey into Gravity and Spacetime. New York: W. H. Freeman, 1990. Wheeler, the Princeton physicist who coined the term “black hole,” depicts gravity from its simplest forms to the black hole. The book is written for the armchair scientist but is full of interesting stories and is lavishly illustrated in color so that anyone can enjoy it piecemeal or in its entirety.