History of black hole physics

Black holes have fascinated scientists and researchers since the early 1900s, especially after the development of general relativity by Albert Einstein. Just months after Einstein introduced his theory in 1917, a physicist named Karl Schwarzschild used it to describe how stars could collapse into extremely dense points, called black holes. At first, many thought these black holes were just imaginary or impossible, believing something would always stop a star from collapsing completely.

However, in 1939, scientists J. Robert Oppenheimer and Hartland Snyder showed that stars of any size could indeed collapse into black holes, making the idea more real and important. Over time, researchers discovered different kinds of black holes and learned that they could be described by just three basic properties: charge, mass, and spin.

The first black hole widely accepted by scientists was found in 1972, named Cygnus X-1. Since then, we’ve learned that huge black holes, called supermassive black holes, exist at the centers of most galaxies. Amazing new technology has even allowed scientists to take pictures of black holes and detect collisions between them using gravitational waves. Black holes remain one of the most exciting and mysterious objects in the universe.

Before general relativity

Long before we understood space and time the way we do today, some smart thinkers had interesting ideas about very big stars. Two of them were John Michell, an English scientist and priest, and Pierre-Simon Laplace, a French scientist. In 1784, Michell wrote a letter saying that if a star were very big and dense, its gravity might be so strong that even light couldn’t escape from it. He thought we might still see these invisible stars because they would pull on other stars nearby.

Later, in 1796, Laplace also talked about the idea of stars so big that we couldn’t see them. He put his idea in a book about how our Solar System might have started. But he decided not to keep that idea in later versions of his book, partly because another scientist, Thomas Young, had new ideas about how light behaves.

General relativity

See also: History of general relativity

In 1905, Albert Einstein discovered that the laws of electromagnetism work the same for everyone, no matter how fast they are moving. This idea became known as special relativity. Einstein then worked to include gravity in his theory. By 1917, he completed his general theory of relativity, which explained how matter shapes space and time, and how this, in turn, affects the movement of other matter. This theory later became the foundation for understanding black holes.

Soon after Einstein shared his ideas, an astronomer named Karl Schwarzschild used them to study stars. He found a special solution that described what would happen around a very dense object. This led to the idea of the Schwarzschild radius, a distance from the object where things get very strange. At first, many scientists did not believe such objects, called black holes, could really exist.

Gravity vs degeneracy pressure

By the 1920s, scientists noticed that some white dwarf stars were too cool and dense to be explained by normal star cooling. They discovered that a special pressure, called quantum-mechanical degeneracy pressure, was stronger than the usual pressure from heat at these densities.

Later, a scientist named Subrahmanyan Chandrasekhar calculated that stars made of a special kind of matter, called electron-degenerate matter, could only stay stable if they were not too heavy. This maximum weight is now known as the Chandrasekhar limit. Some famous scientists thought this idea was wrong, but they were only partly correct. If a white dwarf star was a little heavier than this limit, it would collapse into an even denser object called a neutron star.

Scientists also studied very bright explosions called supernovae and suggested these could create neutron stars. Later, they found that neutron stars could also have a maximum weight, called the Tolman–Oppenheimer–Volkoff limit, beyond which they might collapse further. Studies showed that nothing else could stop this collapse except if a lot of material was blown away from the star during its collapse.

Birth of modern model

The modern idea of black holes began in 1939 when scientists Robert Oppenheimer and Hartland Snyder studied what happens when a star collapses. They found that as the star shrinks, time seems to slow down for observers far away. Once the star reaches a certain size, called its Schwarzschild radius, it seems to stop collapsing completely from far away.

Later, in 1958, David Finkelstein helped us understand more about black holes by describing a boundary around them called the event horizon. He showed that once something passes this boundary, it cannot affect the world outside the black hole. His work, along with earlier ideas from Martin Kruskal, helped prove that black holes are real and understandable.

Golden age

The period from the mid-1960s to the mid-1970s is known as the "golden age of black hole research." During this time, scientists made many important discoveries about black holes using the theory of general relativity.

In 1963, Roy Kerr found a solution for a rotating black hole, and in 1965, Ezra Newman discovered a solution for a black hole that both rotates and has an electric charge. Later, scientists learned that black holes can be described by just a few simple properties, such as mass, spin, and charge. This idea is known as the "no-hair theorem." Observations also advanced, with the discovery of pulsars showing that neutron stars exist in space. In the early 1970s, Cygnus X-1 became the first object widely accepted to be a black hole. Scientists also discovered that black holes follow rules similar to the laws that govern heat and energy, and that they can slowly give off energy, a process called Hawking radiation.

Modern research and observation

The first strong evidence for black holes came from observations of Cygnus X-1 in 1972. This object, found in the Cygnus constellation, was discovered using suborbital rockets because Earth's atmosphere blocks X-rays. Scientists found that Cygnus X-1 was part of a binary system with a large star, and its mass was too great to be a white dwarf or neutron star, suggesting it was a black hole.

Later, the Hubble Space Telescope helped scientists discover that most galaxies have supermassive black holes at their centers. In 2016, the LIGO Scientific Collaboration and Virgo Collaboration made history by detecting gravitational waves from colliding black holes. This confirmed that black holes exist and can merge. In 2019, the Event Horizon Telescope (EHT) captured the first-ever image of a black hole, showing its shadow and the bright ring of material around it.

2020 Nobel

In 2020, the Nobel Prize in Physics was awarded for important discoveries about black holes. Andrea Ghez and Reinhard Genzel received half of the prize for proving that Sagittarius A* is a supermassive black hole. The other half went to Penrose for showing that the math behind general relativity means black holes must exist.

The term black hole was first used in the late 1960s. John Wheeler helped popularize the name, though others had used it before him. The name comes from an old story about a prison called the Black Hole of Calcutta.