Black hole

A black hole is a special kind of astronomical body with gravity so strong that even light cannot escape. These amazing objects were predicted by Albert Einstein's theory of general relativity. This theory shows how mass and energy can bend the shape of spacetime. When a lot of mass gets squeezed into a tiny space, it creates a black hole. The edge of a black hole is called the event horizon, and it marks the point where nothing can come back out.

Black holes usually form when very big stars collapse at the end of their life cycle. After they form, they can grow by pulling in nearby matter. There are also huge black holes, called supermassive black holes, found at the centers of most galaxies, including our own Milky Way.

Scientists can find black holes by watching how they affect matter and light around them. For example, material falling toward a black hole often forms a hot, glowing disk called an accretion disk. When black holes merge, they create ripples in spacetime called gravitational waves, which we can now detect. By studying the movements of stars, astronomers have found many black holes in binary systems and confirmed the presence of a giant black hole at the center of our galaxy.

History

Main article: History of black hole physics

The idea of a body so massive that even light could not escape was first proposed in the late 18th century by English astronomer and clergyman John Michell and independently by French scientist Pierre-Simon Laplace. Both scholars proposed very large stars instead of the modern idea of an extremely dense object.

Michell's idea, in a short part of a letter published in 1784, calculated that a star with the same density but 500 times the radius of the Sun would not let any emitted light escape; the surface escape velocity would exceed the speed of light. Michell correctly thought that such invisible bodies might be found through their gravitational effects on nearby visible bodies. In 1796, while thinking about the origin of the Solar System in his book Exposition du Système du Monde, Laplace made a suggestion that a star could be invisible if it were very large.

General relativity

Singular solutions in general relativity

Only a few months after Einstein published the field equations describing general relativity, astrophysicist Karl Schwarzschild set out to apply the idea to stars. He assumed spherical symmetry with no spin and found a solution to Einstein's equations. A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution. At a certain radius from the center of the mass, the Schwarzschild solution became singular, meaning that some of the terms in the Einstein equations became infinite. The nature of this radius, which later became known as the Schwarzschild radius, was not understood at the time.

Many physicists of the early 20th century were sceptical of the existence of black holes. In a 1926 popular science book, Arthur Eddington critiqued the idea of a star with mass compressed to its Schwarzschild radius as a flaw in the theory of general relativity. In 1939, Einstein used his theory of general relativity in an attempt to prove that black holes were impossible. His work relied on increasing pressure or increasing centrifugal force balancing the force of gravity so that the object would not collapse beyond its Schwarzschild radius. He missed the possibility that implosion would drive the system below this critical value.

Gravity vs degeneracy pressure

By the 1920s, astronomers had found a number of white dwarf stars that were too cool and dense to be explained by the gradual cooling of ordinary stars. In 1926, Ralph Fowler showed that these stars are not like main-sequence stars, where thermal pressure balances gravity. Instead, a type of quantum-mechanical pressure balances gravity at these temperatures and densities. In 1931, Subrahmanyan Chandrasekhar studied the new state of matter that results from this balance, called electron-degenerate matter, discovering that it is stable below a certain limiting mass. By 1934 he showed that this explained the catalogue of white dwarf stars. When Chandrasekhar announced his results, Eddington pointed out that stars above this limit would radiate until they were sufficiently dense to prevent light from exiting, a conclusion he considered absurd. Eddington and, later, Lev Landau argued that some yet unknown mechanism would stop the collapse.

In the 1930s, Fritz Zwicky and Walter Baade studied stellar novae, focusing on exceptionally bright ones they called supernovae. Zwicky promoted the idea that supernovae produced stars with the density of atomic nuclei—neutron stars—but this idea was largely ignored at the time. In 1939, based on Chandrasekhar's reasoning, but working within general relativity rather than Newtonian gravity, J. Robert Oppenheimer and George Volkoff predicted that neutron stars below a certain mass limit, later called the Tolman–Oppenheimer–Volkoff limit, would be stable due to neutron degeneracy pressure. Above that limit, they reasoned that either their model would not apply or that gravitational contraction would not stop.

John Archibald Wheeler and two of his students resolved questions about the model behind the Tolman–Oppenheimer–Volkoff (TOV) limit. In 1965, Harrison and Wheeler developed the equations of state relating density to pressure for cold matter all the way through electron degeneracy and neutron degeneracy. Masami Wakano and Wheeler then used the equations to compute the equilibrium curve for stars, relating mass to circumference. They found no additional features that would invalidate the TOV limit. This meant that the only thing that could prevent black holes from forming was a dynamic process ejecting sufficient mass from a star as it cooled.

Birth of modern model

The modern concept of black holes was formulated by Robert Oppenheimer and his student Hartland Snyder in 1939. In the paper, Oppenheimer and Snyder solved Einstein's equations of general relativity for an idealised imploding star, in a model later called the Oppenheimer–Snyder model, then described the results from far outside the star. The implosion starts as one might expect: the star material rapidly collapses inward. However, as the density of the star increases, gravitational time dilation increases and the collapse, viewed from afar, seems to slow down further and further until the star reaches its Schwarzschild radius, where it appears frozen in time.

In 1958, David Finkelstein identified the Schwarzschild surface as an event horizon, calling it "a perfect unidirectional membrane: causal influences can cross it in only one direction". This means that events that occur inside the black hole cannot affect events that occur outside the black hole. Finkelstein created a new reference frame to include the point of view of infalling observers. Finkelstein's new frame of reference allowed events at the surface of an imploding star to be related to events far away. By 1962 the two points of view were reconciled, convincing many sceptics that implosion into a black hole made physical sense.

Golden age

The era from the mid-1960s to the mid-1970s was the "golden age of black hole research", when general relativity and black holes became mainstream subjects of research.

In this period, solutions to the equations of general relativity under various different physical constraints were discovered. In 1963, Roy Kerr found the exact solution for a rotating black hole. Two years later, Ezra Newman found the axisymmetric solution for a black hole that is both rotating and electrically charged.

In the late 1960s and early 1970s scientists from research groups formed by Yakov Zeldovich, John Archibald Wheeler and Dennis W. Sciama discovered a series of important mathematical properties of black hole models dubbed "a black hole has no hair" by Wheeler. The first hints came from work by Vitaly Ginzburg who studied a series of increasing compact stars threaded with intense magnetic fields. He discovered that the fields get trapped on the black hole surface. In 1967, Werner Israel showed that any non-spinning, uncharged collapsing star gives a spherically symmetric black hole: any asymmetry must somehow vanish. In 1972, Richard H. Price found that the asymmetry was converted into gravitational waves. It took another 15 years and many physicists to produce a body of work that became known as the no-hair theorem, which states that a stationary black hole is completely described by the three parameters of the Kerr–Newman metric: mass, angular momentum, and electric charge.

At first, it was suspected that the strange mathematical singularities found in each of the black hole solutions only appeared due to the assumption that a black hole would be perfectly spherically symmetric, and therefore the singularities would not appear in generic situations where black holes would not necessarily be symmetric. This view was held in particular by Vladimir Belinski, Isaak Khalatnikov and Evgeny Lifshitz, who tried to prove that no singularities appear in generic solutions, although they would later reverse their positions. However, in 1965, Roger Penrose proved that general relativity predicts that singularities appear in all black holes, although this may not still hold when quantum mechanics is taken into account.

Astronomical observations also made great strides during this era. In 1967, Antony Hewish and Jocelyn Bell Burnell discovered pulsars and by 1969, these were shown to be rapidly rotating neutron stars. Until that time, neutron stars, like black holes, were regarded as just theoretical curiosities, but the discovery of pulsars showed their physical relevance and spurred a further interest in all types of compact objects that might be formed by gravitational collapse. However, experimental evidence confirming a black hole was very difficult to obtain and ultimately required efforts from many astronomers. X-ray telescope observations by Riccardo Giacconi's team in 1971 showed that Cygnus X-1 emitted x-rays in rapid, sporadic fashion consistent with a compact source. This became the first candidate black hole.

Image by the Event Horizon Telescope of the supermassive black hole in the center of Messier 87

Work by James Bardeen, Carter, and Hawking in the early 1970s led to the formulation of black hole thermodynamics. These laws describe the behaviour of a black hole in a manner analogous to the laws of thermodynamics. Jacob Bekenstein strengthened this analogy with the properties of mass, surface area, and surface gravity for a black hole related to the thermodynamical concepts of energy, entropy, and temperature respectively. The analogy was completed when Hawking, in 1974, showed that quantum field theory implies that black holes should radiate like a black body with a temperature proportional to the surface gravity of the black hole, predicting the effect now known as Hawking radiation.

Modern research and observation

While Cygnus X-1, a stellar-mass black hole, was generally accepted by the scientific community as a black hole by the end of 1973, it would be decades before a supermassive black hole would gain the same broad recognition. The idea that such objects might exist began with models suggesting that powerful quasars or active galactic nuclei in the center of galaxies were powered by accreting supermassive black holes. When the Hubble Space Telescope launched in the 1990s, optical studies of the rotation of center of galaxy M87 showed it must have a large concentration of mass. The two candidates for this mass were a black hole and a dense cluster of stars. In 1995, interferometric microwave spectra from the Very Long Baseline Array observed H
₂O masers as they orbited the center of NGC 4258. The orbital parameters eliminated dense stellar clusters as possible gravitational source.

In 1999, David Merritt proposed the M–sigma relation, which related the dispersion of the velocity of matter in the center bulge of a galaxy to the mass of the supermassive black hole at its core. Subsequent studies confirmed this correlation. Around the same time, based on telescope observations of the velocities of stars at the center of the Milky Way galaxy, independent work groups led by Andrea Ghez and Reinhard Genzel concluded that the compact radio source in the center of the galaxy, Sagittarius A*, was likely a supermassive black hole.

In late 2015, the LIGO Scientific Collaboration and Virgo Collaboration made the first direct detection of gravitational waves, named GW150914, representing the first observation of a black hole merger. At the time of the merger, the black holes were approximately 1.4 billion light-years away from Earth and had masses roughly 30 and 35 times that of the Sun. In 2017, Rainer Weiss, Kip Thorne, and Barry Barish, who had spearheaded the project, were awarded the Nobel Prize in Physics for their work. Since the initial discovery in 2015, hundreds more gravitational waves have been observed.

On 10 April 2019, the first direct image of a black hole and its vicinity was published, following observations made by the Event Horizon Telescope (EHT) of the supermassive black hole in Messier 87's galactic centre. In 2022, the Event Horizon Telescope collaboration released an image of the black hole in the center of the Milky Way galaxy, Sagittarius A*; the data had been collected in 2017.

In 2020, the Nobel Prize in Physics was awarded for work on black holes. Andrea Ghez and Reinhard Genzel shared one-half for their discovery that Sagittarius A* is a supermassive black hole. Penrose received the other half for his work showing that the mathematics of general relativity requires the formation of black holes. Cosmologists lamented that Hawking's extensive theoretical work on black holes would not be honoured since he had died in 2018.

Etymology

In December 1967, someone in the audience reportedly suggested the phrase black hole at a lecture by John Wheeler; Wheeler adopted the term for its brevity and "advertising value", and Wheeler's stature in the field ensured it quickly caught on, leading some to credit Wheeler with coining the phrase. However, the term was used by others around that time. Science writer Marcia Bartusiak traces the term black hole to physicist Robert H. Dicke, who in the early 1960s reportedly compared the phenomenon to the Black Hole of Calcutta, notorious as a prison where people entered but never left alive. The term was used in print by Life and Science News magazines in 1963, and by science journalist Ann Ewing in her article "'Black Holes' in Space", dated 18 January 1964, which was a report on a meeting of the American Association for the Advancement of Science held in Cleveland, Ohio.

Definition

A black hole is a special place in space where gravity is very strong. Nothing, not even light, can escape from it. Scientists find black holes by looking for objects that are very small but very heavy — heavier than four Suns! These spaces pull everything inward very quickly.

Properties

The no-hair theorem says that a black hole has only three main features: its mass, its electric charge, and how fast it spins. Two black holes with the same mass, charge, and spin would look the same.

The simplest black hole is called a Schwarzschild black hole. It has only mass and no charge or spin. Other types include charged black holes, rotating black holes, and the most complex ones that have both charge and spin, described by the Kerr–Newman metric.

All black holes have mass. From far away, their gravity feels like any object of the same mass. Black holes can spin very fast — some spin thousands of times per second! Scientists study the light from stars and gas around a black hole to learn how fast it spins. Most black holes have very little electric charge. This helps keep them balanced.

Classification

Black holes are grouped by how they form and how heavy they are. One type is called a stellar black hole. These come from stars that have collapsed. They can be about 10 to 100 times the mass of our Sun.

There are also much bigger black holes called supermassive black holes. These can be millions or even billions of times the mass of the Sun. These huge black holes are found at the centers of most big galaxies, like our own Milky Way.

Black hole classifications
Class	Approx. mass	Approx. radius
Ultramassive black hole	10⁹–10¹¹ M_☉	>1,000 AU
Supermassive black hole	10⁶–10⁹ M_☉	0.001–400 AU
Intermediate-mass black hole	10²–10⁵ M_☉	10³ km ≈ R_Earth
Stellar black hole	2–150 M_☉	30 km
Micro black hole	up to M_Moon	up to 0.1 mm

Structure

Relativistic jets from the supermassive black hole in Centaurus A extend perpendicularly from the galaxy.

Black holes are special objects in space with gravity so strong that even light cannot escape. This strong gravity changes how nearby objects move and can pull in gas, making those areas very bright.

Some black holes send out thin streams of charged particles called relativistic jets. These jets move away from the black hole at speeds close to the speed of light. Black holes can also have accretion disks, which are flat, swirling disks of gas and dust. These disks get very hot and glow brightly as they move toward the black hole.

Formation

Black holes are formed when very large stars collapse under their own gravity. This happens when a star runs out of fuel and can no longer support itself, leading to a powerful explosion called a supernova.

Sometimes, two neutron stars can merge and form a black hole, too.

Scientists also think black holes might have formed in the very early universe from special conditions right after the Big Bang. These are called primordial black holes. In these early times, parts of space that were denser than others could have collapsed into black holes.

Evolution

After a black hole forms, it can change in several ways. One way is through mergers. This is when black holes combine with other objects like stars or even other black holes. This helped supermassive black holes grow in the early universe.

Another way a black hole changes is through accretion of matter. When matter gets close to a black hole, it heats up and glows brightly. This glowing matter can be seen from far away and powers some of the brightest objects in the universe, like active galactic nuclei and quasars.

Finally, black holes may evaporate over time through a process called Hawking radiation. This means very small black holes could eventually disappear, but large ones would take much longer than the current age of the universe to evaporate.

Observational evidence

A Chandra X-Ray Observatory image of Cygnus X-1, which was the first strong black hole candidate discovered

Many black holes may exist in our Milky Way galaxy. They form when stars collapse. Even small galaxies might have many of them. Because black holes don’t shine, scientists look at objects nearby for clues.

One way to study black holes is by watching stars that move around an invisible object. For example, stars near the center of the Milky Way seem to orbit a supermassive black hole called Sagittarius A*. Another way uses special telescopes like the Event Horizon Telescope to picture black holes. Scientists also find black holes by watching for ripples in space, called gravitational waves, which happen when black holes move around each other and crash together.

Areas of investigation

Two galaxies from the first billion years after the Big Bang. The galaxy on the left hosts a luminous quasar at its center.

Scientists wonder what happens to information inside a black hole. A black hole is usually described by three things: its mass, its charge, and how it spins. This makes it seem like other details about what formed the black hole are lost. We used to think black holes lasted forever, but now we know they slowly lose energy and shrink by sending out special kinds of radiation, called Hawking radiation. This radiation doesn’t carry extra details about what made the black hole, so scientists wonder if that information is truly gone. Solving this mystery could help us understand how tiny particles and the forces that control space and time work together.

Observations of very distant galaxies show that extremely bright objects, powered by giant black holes, existed when the universe was very young. Scientists are still learning how these giant black holes formed so early. Some ideas include smaller black holes merging together, or special conditions allowing huge clouds of gas to collapse directly into black holes. These early giant black holes are still a fascinating topic for scientists.

In fiction

The idea of black holes has captured the imaginations of both artists and scientists. In her book Conjuring the Void: the Art of Black Holes, Lynn Gamwell shows how art and science work together. She uses black holes to help us picture scientific ideas.

The black hole and accretion disk used in the movie Interstellar, without lens flare. Interstellar's visual effects team used relativity to visualize gravitational lensing around the black hole.

Black holes are popular in science fiction stories and movies. Even before scientists used the term "black hole," stories like the 1928 novel The Skylark of Space and the 1935 short story Starship Invincible had mysterious dark objects in space. Modern films like Interstellar and High Life show black holes using real science. These stories often explore fun ideas like time moving differently near black holes.