Where Does The Black Hole Get Its Name From?

Simulation of a non-rotating black hole — A model for Sagittarius A (Image from WikiCC)

Have you ever wondered why the black hole is called so? In my previous essay in the hitchhiker series, I laid out the intuition behind the notion of black holes.

In this essay, I will be answering the aforementioned question. But along the way, we will also be covering some of the rich history surrounding the discovery of black holes. If you are the kind of person who enjoys nostalgic views of scientific progress through the lens of time, you will certainly enjoy this one.

Without any further ado, let us begin!

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The Origins of the Black Hole

We did not discover black holes and then get interested in them after their discovery. In fact, it was the other way around.

On record, astronomer John Michell was the first person to mention “dark stars” which feature surface escape velocities that exceed the speed of light. He imagined a hypothetical star whose diameter exceeds that of the sun by a factor of 500. Like rocks on earth, the escaping light rays would drop back onto the surface of the star. Consequently, the star would appear “dark” to an outside viewer.

Although he was incorrect with a lot of the details, he was ahead of his time in predicting that such a supermassive object would be detectable via its gravitational effect on nearby objects.

This proposition initially stirred up a lot of excitement among the scholars of the time. However, in the early nineteenth century, it was shown that light had the properties of a wave. As the effect of gravity on light waves was unknown at that time, the excitement dwindled.

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The Dawn of General Relativity

Fast forward to the twentieth century, scientist Albert Einstein had come up with his theory of general relativity in 1915. A few months after this historic event, Karl Schwarzschild solved the Einstein field equations. This chain of events led to a never-before-seen explanation of gravity.

What Schwarzschild’s solutions also allowed for were theoretical entities that lead to singularities. Around a certain surface (now known as Schwarzschild’s radius), some of the terms in Einstein’s equations approached infinity. Scientists of the time argued that this was just the artificial limit of the theory, and some unknown physical phenomenon would stop these terms from approaching infinity in reality.

In 1933, Georges Lemaître figured out that the singularities at the Schwarzschild surface were non-physical (known as coordinate singularities). The equations suggested that there could exist an object of massive gravity. Such gravity would cause the entire spacetime around the object to close upon itself. This would leave us incapable of ever reaching the object.

The Mathematics of the Black Hole

Black holes existed as mathematical equations and numbers long before we discovered them. Nobel Prize winner Subrahmanyan Chandrasekhar showed in 1931 that non-rotating electron-degenerate matter can cause a singularity. In other words, according to Einstein’s equations, black holes can form under certain conditions. Chandrasekhar calculated the threshold for this “event” to be a mass of 1.4 M☉ (where M☉ is known as solar mass and depicts the mass of the sun) — known as the Chandrasekhar limit.

Scientific sceptics argued that there might be unknown phenomena that could stop this from happening. To reinforce this line of thought, it was later found out that white dwarf stars that are slightly over the Chandrasekhar limit would collapse into a neutron star — a stable configuration.

On the other hand, Robert Oppenheimer and his colleagues showed that neutron stars above a certain limit would follow Chandrasekhar’s calculations and no known physics would restrict a black hole from forming. One interesting thing to note here is that at this point, both neutron stars and black holes were purely mathematical features. No one had ever observed any of these objects until then!

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Event Horizon

In the decades that followed, a lot of scientific progress was made in the mathematics surrounding black holes. Even until this point, black holes were actually not known as black holes. In fact, the term “event horizon” came into existence before the term “black hole”. David Finkelstein named the Schwarzschild surface as “event horizon” in 1958.

He beautifully described it as the perfect unidirectional membrane where causal influences move only in one direction. This was the first time that mainstream interest was building up to understand what happens beyond the singularity.

Where Does the Black Hole Get its Name From?

Throughout the early 20th century, physicists predominantly used the term “gravitationally collapsed object” to describe the mysterious object. The term “black hole” was used for the first time by physicist Robert Dicke (as traced by science writer Marcia Bartusiak). Apparently, Dicke had compared this object to the “Black Hole of Calcutta” which was a prison in India. This prison was notoriously known as a place where people entered but never left alive.

Following this, the term “black hole” appeared in well-known science magazines in the 1960s. After a recommendation from a student, physicist John Wheeler saw the marketing potential in the term and adopted it in a scientific context in 1967. Consequently, he is widely attributed for coining the term.

Thus, the term “black hole” was established. This was a key turning point in the scientific history of this field as the newly coined term aided mainstream attention and research funding.

There we go! That’s where the black hole gets its name from. But hang on a minute! Even at this point, black holes only existed on paper and no one had observed them!

The First Observation of a Black Hole

In 1967, Jocelyn Bell Burnell discovered Pulsars. In the two years that followed, scientists and astronomers established that pulsars were none other than rapidly rotating neutron stars. This was the first time ever that a neutron star had been observed. Until this point, it existed only as numbers and symbols on paper.

This brought renewed interest in the notion of black holes. In 1971, several researchers independently identified the X-Ray source Cygnus X-1 (which was first discovered in 1964) as the first-ever black hole! This marked the dawn of the new age of black hole research.

Finally, it was not just a paper artifact anymore. The black hole not only established itself as a valid research topic but as an astrophysical reality as well!

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Further reading that might interest you: Why Is A Black Hole Really So Special? and How To Really Deal With The Friendship Paradox?

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