The Discovery of Black Holes: From Theory to Actuality

From the lecture series: What Einstein Got Wrong

By Dan Hooper, Ph.D., University of Chicago

Although Einstein was skeptical about the existence of black holes, his own theory suggested otherwise. Bringing us closer to the discovery of black holes was an observation regarding the behavior of collapsing stars. A mysterious object called Cygnus X-1 at last delivered proof.

Cygnus X-1
The black hole named Cygnus X-1 pulls matter from the super giant blue star near it. (Image:NASA-CXC-M.Weiss/

Einstein Dismisses Black Holes

Although astronomers and physicists had learned much about stars in the fifteen years since the discovery of general relativity—including the likely existence of white dwarfs and neutron stars—Albert Einstein’s opinions on the subject had not changed substantially.

In 1939, he wrote his first and only paper about black holes, entitled “On a Stationary System with Spherical Symmetry Consisting of Many Gravitating Masses.” In this article, Einstein set out to calculate how a large group of particles would behave as they collapsed under the force of gravity.

This is a transcript from the video series What Einstein Got Wrong. Watch it now, on The Great Courses Plus.

Albert Einstein 1931
Albert Einstein’s opinion that black holes could never exist in nature led most physicists specializing in relativity to dismiss all talk of black holes for many years. (Image: Doris Ulmann/Public domain)

Einstein argued that the particles’ angular momentum would prevent them from collapsing indefinitely and that this would prevent a black hole from ever forming. In this conclusion, he was completely wrong.

Einstein’s prejudice that black holes could never exist in nature blinded him from all of the arguments to the contrary, leading him to reject one of the most incredible facets of his theory.

Worse still, Einstein’s opinion was so highly regarded that most physicists specializing in relativity tended to dismiss all talk of black holes for many years afterward. For decades, such objects were seldom mentioned in scientific literature.

Furthermore, interest in general relativity declined considerably during this period. It wasn’t so much that physicists doubted the validity of Einstein’s theory; they simply hadn’t found many practical uses for it.

The predictions made by general relativity are, in most cases, similar to the old Newtonian predictions. There wasn’t much that could be done in a laboratory either to test the theory further or explore its implications.

Learn more about what Einstein Got Right: Special Relativity

General Relativity’s Renaissance

A few years after Einstein’s death in 1955, interest in general relativity began to see a resurgence. All around the world, small groups of physicists started to actively explore the deeper—and stranger—implications of Einstein’s general theory.

One of the key figures in general relativity’s renaissance was the young British physicist and mathematician, Roger Penrose. Penrose first became interested in relativity while he was an undergraduate at University College London.

During this period, however, few physicists knew very much about general relativity. Penrose had little choice but to teach himself about the subject, managing to learn general relativity from books and papers instead of from his professors.

Penrose then went on to study mathematics at Cambridge, where he earned his Ph.D., and then researched for brief stints at Princeton, London, Syracuse and the University of Texas at Austin.

At the time, Austin was the location of one of the few concentrations of physicists who were actively studying general relativity. Among others, Penrose met the physicist Roy Kerr in Austin.

Kerr was able to find a solution to Einstein’s field equations that is more general and more powerful than those found by Karl Schwarzschild. In particular, while Schwarzschild’s result only describes stationary objects, Kerr’s solution also allows for the possibility that black holes could be rotating.

Learn more about what Einstein got right: general relativity

Proving the Existence of Black Holes

Robert Penrose
British physicist and mathematician Roger Penrose proved that under certain circumstances, a collapsing star would be guaranteed to form a black hole. (Image: Biswarup Ganguly/Public domain)

At the time, few physicists thought that black holes genuinely existed—if they gave any thought to the matter at all. But in 1965, Penrose made a discovery that would upend that viewpoint.

Using a type of mathematics that was very different from anything Einstein had ever used, Penrose was able to rigorously prove that, under certain circumstances, a collapsing star would be guaranteed to form a black hole. In particular, if the collapsing star is massive enough, then the formation of a black hole is entirely inevitable.

In January of 1965, Penrose published a short, three-page paper, entitled “Gravitational Collapse and Space-Time Singularities.” At the time, Penrose’s argument went strongly against the conventional wisdom of the physics community.

Many argued, as Einstein had long done, that the complexities of real collapsing stars would prevent them from forming black holes.

But Penrose’s mathematical argument was compelling. Over the next few years, the opinions of many physicists were swayed. By the end of the 1960s, it had become a mainstream view that black holes were, in fact, likely—if not guaranteed—to exist in nature.

Learn more about the detection of gravity waves from distant colliding black holes

How Black Holes Affect Surrounding Stars

As more and more physicists became convinced that black holes exist, interest began to grow about the ways that these objects might be detected or observed. One of the first scientists to actively work on this question was the incredibly prolific and versatile Russian physicist Yakov Zel’dovich.

Throughout his career, Zel’dovich made major contributions to almost every field of physics and astronomy, including material science, particle physics, relativity, astrophysics, cosmology, and nuclear physics—including work that he did on the Soviet weapons program.

Yakov Zel’dovich (1914-1987)
Russian physicist Yakov Zel’dovich proposed that a black hole can be detected by studying the motion of nearby stars. (Image: MARKA Publishing & Trading Centre/Public domain)

In the early 1960s, Zel’dovich proposed that the presence of black holes could be indirectly inferred by studying the motion of other nearby stars. The invisible black hole, he argued, would cause another star within its own solar system to wobble back and forth with a regular period.

If scientists could somehow observe such a wobbling star, they could identify the black hole, and even measure its mass.

Alternatively, Zel’dovich argued that under certain circumstances, a black hole could have a dramatic impact on the material surrounding it. All astrophysical bodies attract and accumulate matter through the force of their gravity.

But unlike ordinary stars or planets, the matter that falls toward a black hole will be accelerated to nearly the speed of light as it approaches. Furthermore, this infalling material will spiral around the black hole like a fluid running down a drain.

Since this material moves at nearly the speed of light, it reaches temperatures in the millions of degrees. Zel’dovich argued that such systems would release huge amounts of energy and could be observed by astronomers, even at very great distances.

Learn more about cosmology and the cosmological constant

The Mystery of Cygnus X-1

The mystery of the strange astronomical object known as Cygnus X-1 left astronomers perplexed. First detected by astronomers in 1964, observations of this object in 1970, however, revealed some of its more bizarre characteristics.

Cygnus X-1 was observed to release very bright flashes of X-rays multiple times each second. The short duration of this X-ray light indicated that whatever was emitting them was not very big by astronomical standards—no more than a fraction of a light second across.

In other words, the object would be no more than 100,000 kilometers or so. X-rays are produced only in very hot environments at millions of degrees.

In the following year, radio observations in the direction of Cygnus X-1 discovered a blue supergiant star. This star, however, is far too big to generate the rapid X-ray flickering that had been observed.

To explain the production of the observed X-rays, astronomers deduced that a portion of this star’s gas was somehow being torn off, then heated to very high temperatures. Later in the same year, other observations began to detect the wobble of the blue supergiant—just as Zel’dovich had suggested a decade earlier.

From the observed wobble, it was clear to astronomers that the nearby object was massive—far too massive to even be a neutron star.

As the quality of the observations continued to improve over the years that followed, it became clearer that Cygnus X-1 was a black hole. By the late 1970s, most astrophysicists had come to accept this conclusion, as well as the conclusion that black holes indeed exist in our universe.

Cygnus X-1 binary system
Cygnus X-1, a black hole about 6000 light years away from us (right) sucks gas from the nearby super giant star named HDE 226868. (30 times the mass of our Sun) (Image : ESA-Hubble/

It is now known that Cygnus X-1 is a black hole about 6,000 light-years away from us, and about 15 times as massive as the Sun. At this mass, the Schwarzschild radius of this black hole is about 44 kilometers.

Anything within this radius is forever lost from our view. And, in a sense, is lost from our universe itself.

Learn more about how the cosmological constant led to the discovery of dark energy

The Black Holes Around Us

In the decades following the determination that Cygnus X-1 is a black hole, astronomers and astrophysicists discovered numerous other black holes in our universe. This includes dozens of black holes that were once thought to be massive stars, similar to Cygnus X-1.

Also, many larger and more massive black holes have been discovered. The center of the Milky Way galaxy, for example, is the host of an enormous black hole, with a mass equal to about four million times the mass of the Sun.

The center of our own Milky Way galaxy hosts an enormous black hole, with a mass equal to about four million times the mass of the Sun. (Image: ESA–C. Carreau/

It is now generally thought that most spiral and elliptical galaxies contain a supermassive black hole at their centers.

Although most of these supermassive black holes are similar in mass to the one at the center of the Milky Way, some galaxies harbor even larger black holes, with masses that are measured in the billions, rather than mere millions, of solar masses.

Learn more about the paradox of quantum entanglement

Black holes are a consequence of Einstein’s Theory of General Relativity. Yet Einstein never came to accept that black holes did—or even could—exist in our universe.

Common Questions About the Discovery of Black Holes

Q: Who exactly discovered black holes?

Even though Einstein’s general relativity predicted black holes, Karl Schwarzschild is often credited with discovering them. Even this fact is tricky to state with absolute certainty, though, as Kerr after him better defined what black holes were. It was Roger Penrose who proved their existence as collapsed stars.

Q: How many black holes are in the universe?

Scientists estimate that nearly all large galaxies have super massive black holes in their center, which would result in billions of billions.

Q: Does the Milky Way have a black hole in the center?

Yes. Scientists have confirmed a super massive black hole at the center of the Milky Way.

Q: When do black holes form?

Scientists believe that when a galaxy forms, its black hole is formed at the same time.

This article was updated on December 2, 2019

Keep Reading
The Great Courses Professors Remember Stephen Hawking
Searching For The Theory Of Everything: The Torch Podcast
Dark Matter: MACHOs or WIMPs?