The educator of this lesson is also a theoretical astrophysicist working on black holes. You can find out more about his contributions to the field on his website.

The mystery of black holes grips our collective imagination. Innumerable books, movies, songs, videogames, comics, and paintings explore the enigma of black holes. Check out the fascinating film Interstellar and this interview with director Christopher Nolan and Kip Thorne, the Nobel Laureate astrophysicist who acted as a scientific consultant during production.

To learn about the ways in which our understanding of black holes has evolved, read Marcia Bartusiak’s excellent book.

At the end of the 18^th century, John Michell and Pierre-Simon de Laplace proposed that very massive stars could become “obscure,” hiding the light that they produce. Their theory was based on the Newtonian theory of gravity, published in 1687. However, it would take until 1915, with Albert Einstein’s theory of general relativity, for a theoretical framework that accurately explained black holes.

In 1916, in the middle of World War I, Karl Schwarzschild discovered the first full solution to the equations of general relativity. This solution describes what we now call a black hole: a mass concentrated in a point, or singularity. At a certain distance from this point, now named Schwarzschild radius (or event horizon), space and time appear to stop.

At this point, black holes were only a mathematical oddity. They existed as solutions to the equations of general relativity, but who would believe that an object with infinite density could really exist? It took about 50 years to understand and prove that these mathematical oddities were real. The work of several scientists, including Ralph Fowler, Subrahmanyan Chandrasekhar, and Walter Baade, was necessary to reframe black holes from the mathematical to the physical realm. This paper, dated 1939, was one of the first to give a modern description of a black hole as a real object.

But until the 1960s, these oddities where not even named black holes. The name was coined by Robert Dicke, who compared the properties of these objects to the “black hole of Calcutta,” an infamous Indian dungeon where 146 British prisoners were held for one night in 1756. The term “black hole” slowly became the official designation of these objects.

Until the early 1960s, black holes had never been observed. In 1963, a very strange object, named 3C 273, was detected. This source had an extremely luminous point about two billion lightyears away. These objects are now known as quasars, or “quasi-stellar radio sources.”

In 1964, another strange object, this time in our own galaxy, was discovered and named Cygnus X-1. It took the work of several outstanding scientists, including Yakov Zel’dovich, Edwin Salpeter, Roger Penrose, Jocelyn Bell Burnell, Stephen Hawking, Kip Thorne, Roger Blandford and Roman Znajek, to show that these objects were black holes: 3C 273 is a super-massive black hole, while Cygnus X-1 a stellar-mass one.

Nowadays, black holes are recognized as a fundamental part of the universe and new ones are detected every month. A major breakthrough happened on September 14, 2015, when the experiment LIGO detected the first gravitational wave ever observed. This gravitational wave was caused by the merging of two black holes. This observation opened up a new, exciting window into the universe. Check out the historical detection paper, as well as at this TED-Ed lesson on gravitational waves.

It is worth mentioning that now there is even an institute dedicated exclusively to the study of black holes, called the Black Hole Initiative (BHI) at Harvard University. The BHI is an interdisciplinary center involving a collaboration between scientists from the fields of astronomy, physics, mathematics and philosophy. It is the first center worldwide to focus on the study of black holes—not bad for a space oddity.

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