When was the existence of black holes predicted for the first time? And by whom?

For a long time, we already knew about these dense regions in space called black holes. These holes have intense gravity that even the speed of light could not escape. Black holes consume everything close to them. They grow in size and density as more matter is pulled through. Most black holes were formed from the remnants of a massive star after a supernova. These black holes are called stellar-mass. Some are believed to have formed after huge clouds of interstellar hydrogen collapsed. These black holes are called supermassive. There have been many scientists who speculated the possibility of black holes, but the first one who proved their existence was Karl Schwarzschild in 1915.

Pathways to Discovery

The most distinct characteristic of a black hole is its strong gravitational pull. In 1687, Isaac Newton proposed the law of universal gravitation wherein every mass attracts every other mass in the universe. John Michell, in 1783 assumed that there might be an object massive enough whose escape velocity would exceed that of the speed of light. A decade later, Simon Pierre LaPlace suggested a similar idea. He theorized a dark body where no light could escape and added that “the largest luminous bodies in the universe may be invisible.” Fast forward to 1915, Albert Einstein published the Theory of General Relativity that explains the spacetime curvature.

Karl Schwarzchild

In the same year, Schwarzchild used Einstein’s theory of relativity to prove and define black holes. He was an astronomy professor at Berlin University but volunteered for the German army during World War I. While he was stationed in Russia, he received a journal issue of Einstein’s equations. He then worked out a more straightforward answer for Einstein’s spacetime curvature. Spacetime curvature happens when the gravity of a massive body, like the Sun, creates a ‘valley’ in the spacetime around it. The Earth, alongside other planets in the solar system, travels around the slopes of this valley.

Schwarzschild further realized that if the mass of a star is squeezed into a much smaller volume, the valley of the spacetime around it will become steeper. Eventually, as more stars are squeezed into these regions, it would become so dense that the surrounding spacetime valley would steep until it becomes a bottomless pit from which nothing could escape.

More shreds of evidence

For the next few years, Schwarzschild’s concept of black holes changed the world of astrophysics. In 1935, Subrahmanyan Chandrasekhar pioneered the notion that there was a limit to the mass of a white dwarf. This limit, known as the Chandra limit, showed that if a dying star is more massive than our Sun, it would either become a neutron star or a black hole. Later in 1964, a visionary physicist named John Wheeler coined the term black holes, which we still use until today. Also, in 1964, Jocelyn Bell-Burnell verified Chandrasekhar’s claims when she provided the first direct evidence of a rapidly spinning neutron star. Stephen Hawking proposes the most modern theory for black holes in 1974. Hawking suggested that black holes will eventually evaporate entirely.

Cygnus X-1

Cygnus X-1 is the first object in space that was widely accepted as a black hole. This object is located at the constellation of Cygnus about 6000 lightyears away. It emitted copious amounts of X-rays believed to be a result of the material torn away from a star into a dark object, perhaps a black hole. Its companion star is smaller than the Earth but has a mass higher than a neutron star. Recent studies had calculated Cygnus X-1, which approximately weighs 14.8 suns, to gobble up an additional mass that is three times more than our Sun.

Black Hole Imaging

The Hubble Space Telescope provides the best evidence of the existence of supermassive black holes in the center of some galaxies. This NASA telescope studies black holes by observing the movements of the environment surrounding it. The Space Telescope Imaging Spectrograph (STIS) revealed velocities of large objects orbiting around the nucleus of these galaxies. This suggested that an object with a dense mass must be inside that small region, allowing it to pull many star clusters, dust, and other objects towards it. This telescope also captured our Milky Way galaxy’s central black hole called the Sagittarius A.