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. Soon 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.

In 1939, scientists J. Robert Oppenheimer and Hartland Snyder showed that stars of any size could collapse into black holes, making the idea more real. 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. New technology has 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 ago, some smart thinkers had 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 that if a star were very big and dense, its gravity might be so strong that even light couldn’t escape. He thought we might still see these stars because they would pull on other stars nearby.

In 1796, Laplace also talked about stars so big that we couldn’t see them. He put this idea in a book about our Solar System. But he decided not to keep that idea later, 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 rules of electromagnetism work the same for everyone, no matter how fast they are moving. This idea became known as special relativity. Later, Einstein worked to include gravity in his theory. By 1917, he finished his general theory of relativity. This theory explains how matter changes space and time, and how this affects how other matter moves. This theory later helped us understand black holes.

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

Gravity vs degeneracy pressure

By the 1920s, scientists found that some white dwarf stars were too cool and dense to explain with normal star cooling. They learned that a special pressure, called quantum-mechanical degeneracy pressure, was stronger than heat pressure at these densities.

Later, a scientist named Subrahmanyan Chandrasekhar worked out 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 called the Chandrasekhar limit. Some famous scientists thought this idea was wrong, but they were only partly right. 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 thought 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. Scientists Robert Oppenheimer and Hartland Snyder studied what happens when a star collapses. They found that as the star gets smaller, time seems to slow down for people far away. When the star gets very small, it seems to stop collapsing from far away.

In 1958, David Finkelstein helped us learn more about black holes. He talked about 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, with earlier ideas from Martin Kruskal, helped prove that black holes are real and can be understood.

Golden age

The years from the mid-1960s to the mid-1970s were a busy time for learning about black holes. Scientists used ideas about space and time to find new facts.

In 1963, Roy Kerr described how a spinning black hole might look. In 1965, Ezra Newman described a black hole that spins and also has an electric charge. We now know that black holes can be described by just a few simple things, such as mass, how fast they spin, and their charge. This idea is called the "no-hair theorem."

During this time, scientists also found objects called pulsars, showing that tiny, dense stars called neutron stars exist. In the early 1970s, Cygnus X-1 became the first object most scientists agreed was a black hole. Scientists also learned that black holes follow rules like those for heat and energy, and that they can slowly give off energy. This process is called Hawking radiation.

Modern research and observation

The first strong evidence for black holes came from looking at Cygnus X-1 in 1972. This object is in the Cygnus constellation. We found it using special rockets because Earth's atmosphere blocks certain types of light. Scientists saw that Cygnus X-1 was paired with a large star. Its mass was too big to be a white dwarf or a neutron star. This made them think it might be a black hole.

Later, the Hubble Space Telescope helped scientists learn that most galaxies have very big black holes in the middle. In 2016, the LIGO Scientific Collaboration and Virgo Collaboration detected waves from space caused by black holes crashing together. This proved that black holes exist and can join together. In 2019, the Event Horizon Telescope took the first picture of a black hole. The picture showed its shadow and a bright ring of material around it.

2020 Nobel

In 2020, the Nobel Prize in Physics was given for important work on black holes. Andrea Ghez and Reinhard Genzel shared part of the prize for proving that Sagittarius A* is a supermassive black hole. The rest of the prize went to Penrose for showing that math proves black holes must exist.

The word black hole was first used in the late 1960s. John Wheeler helped make the name well-known, although others had used it earlier. The name comes from an old story about a prison called the Black Hole of Calcutta.