Black hole

A black hole is an astronomical body so compact that its gravity prevents anything, including light, from escaping. Albert Einstein’s theory of general relativity shows that when mass is squeezed into a very small space, it creates a region from which nothing can escape. The edge of this region is called the event horizon.

Black holes were first thought about in the 1700s, but it wasn’t until the 1960s that scientists realized they were real and common in the universe. The first black hole that most scientists agreed on was Cygnus X-1, found in 1971.

Most black holes are made when very massive stars collapse at the end of their life cycle. They can grow bigger by pulling in nearby matter. There are also huge black holes, called supermassive black holes, at the centers of most galaxies.

We can’t see black holes directly, but we can tell they are there by how they affect nearby matter and light. For example, matter falling toward a black hole can glow very brightly, and the movement of stars around an invisible object can show the presence of a black hole. This is how scientists discovered that Sagittarius A*, at the center of our Milky Way galaxy, is a supermassive black hole.

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 concept 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 hypothesized that such non-radiating bodies might be detectable through their gravitational effects on nearby visible bodies. In 1796, while speculating on the origin of the Solar System in his book Exposition du Système du Monde, Laplace made a qualitative suggestion that a star could be invisible if it were sufficiently large. Franz Xaver von Zach asked Laplace for a mathematical analysis, which Laplace provided and published in von Zach's journal Allgemeine Geographische Ephemeriden.

General relativity

See also: History of general relativity

In 1905, Albert Einstein showed that the laws of electromagnetism are identical for observers travelling at different velocities relative to each other. The laws of mechanics had already been shown to be invariant in this way. However, the theory of gravitation was yet to be included.

In 1907, Einstein published a paper proposing his equivalence principle, the hypothesis that inertial mass and gravitational mass have a common cause. Using the principle, Einstein predicted the redshift and the lensing effect of gravity on light; his prediction of gravitational lensing was one-half of the value that the full theory of general relativity would predict. By 1915, Einstein refined these ideas into his general theory of relativity, which explained how matter affects spacetime, which in turn affects the motion of other matter. This formed the basis for black hole physics.

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 then-poorly-understood 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 classified a number of white dwarf stars as 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. Optical spectroscopy and detailed astrophysical models for Cygnus X-1 were consistent with a binary system of a massive star and compact star generating x-rays as gas from the massive but ordinary star was sucked into its invisible compact companion. (In 2011, the masses of these stars was estimated to be 14.1±1.0 M_☉ for the black hole and 19.2±1.9 M_☉ for the optical stellar companion.) By 1974 the object was widely considered to be a black hole, but 100% confidence for Cygnus X-1 may not be possible.

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 center of galaxy Messier 87 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, a galaxy with a similar central mass. The orbital parameters ruled out dense stellar clusters as an explanation for galactic nuclei, making supermassive black holes the only plausible explanation.

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 place in space where gravity is so strong that nothing, not even light, can escape from it. Scientists identify black holes by measuring the mass of objects and using theories about how gravity can cause matter to collapse. If an object has more mass than three times that of the sun and is very compact, it is likely a black hole. Another way to think about a black hole is as a region where space is being pulled inward faster than light can travel.

Properties

Black holes are special objects in space where gravity is so strong that even light cannot escape. According to Einstein's theory of general relativity, a black hole is formed when a lot of mass is squeezed into a very small area. Once a black hole is formed and becomes stable, it has only three main properties: mass, electric charge, and how fast it spins. These three properties make all black holes that have the same values look exactly the same.

The simplest kind of black hole has only mass and does not spin or carry any electric charge. These are called Schwarzschild black holes. Other types include rotating black holes and black holes with electric charge. Scientists use different mathematical models to describe these different kinds of black holes. The way a black hole spins can be measured by looking at the light from stars and gas around it, or by studying waves released when black holes collide.

Classification

Black holes come in different sizes and form in different ways. One type, called a stellar black hole, forms when a big star collapses. These black holes can range in mass from about 10 to 100 times the mass of the Sun. Another type, called a supermassive black hole, is much larger and can be found at the center of most big galaxies, including our own Milky Way. These supermassive black holes have masses of more than a million times the mass of the Sun.

There are also ideas about smaller black holes called primordial black holes that might have formed very early in the universe, and even rarer ones called intermediate-mass black holes that are bigger than stellar black holes but smaller than supermassive ones. Scientists think these might form in special places in space or when smaller black holes combine together.

Structure

While black holes seem invisible because nothing can escape their strong gravity, they can make the area around them very bright. This happens because their gravity pulls nearby gas and stars very fast, creating heat and light that we can see.

External geometry

Some black holes shoot out narrow beams of fast-moving particles called relativistic jets. These jets can travel millions of light-years from the black hole and are often seen coming from spinning black holes with strong magnetic fields.

When gas falls toward a black hole, it usually forms a flat, spinning disk called an accretion disk. As the gas gets closer to the black hole, it heats up and glows, releasing lots of light. The colour of this light can change depending on how close the gas is to the black hole.

Very close to the black hole, there is a point called the innermost stable circular orbit (ISCO). Inside this point, anything that gets too close to the black hole will spiral in and be pulled in. For spinning black holes, this point moves closer if the object is orbiting in the same direction as the black hole spins.

Around a black hole, there is also a photon sphere where light can orbit the black hole. If light gets too close to this sphere, it will be pulled into the black hole. From far away, this creates a dark shadow that we can see, like the shadow of the black hole against the stars.

Near a spinning black hole, space and time get twisted, creating an area called the ergosphere. In this area, nothing can stay still; everything gets pulled along with the spinning black hole.

The part of space closest to the black hole’s edge is called the plunging region. Here, anything falling in moves very fast toward the black hole, getting very hot and glowing.

Radius

The size of a black hole’s edge, called the Schwarzschild radius, depends on its mass. For a black hole with the mass of our Sun, this edge would be about 3 kilometers across. But for a black hole with the mass of millions of Suns, this edge would be much larger.

Event horizon

The event horizon is the edge of the black hole past which nothing can escape, not even light. To someone far away, anything falling into a black hole would seem to slow down and fade away as it gets closer to this edge. But to someone falling in, they would not notice anything special as they cross the event horizon.

Internal geometry

Inside a rotating or charged black hole, there is another surface called the Cauchy horizon. If someone falls into a black hole, they might hit this inner surface and feel strong forces.

At the very center of a black hole, there is a point where space and time break down completely. This is called a singularity. For a non-spinning black hole, this is a single point, but for a spinning black hole, it looks more like a ring. Anything that falls in will eventually reach this point and be crushed.

Some theories suggest that black holes might not actually have singularities, but instead have a very dense, but not infinite, core.

Formation

Black holes are formed when very large stars collapse under their own gravity. This can happen when a star runs out of fuel and can no longer support itself, leading to a powerful explosion called a supernova. Sometimes, two smaller stars called neutron stars can merge and create a black hole, too.

Other ideas about how black holes might form include very dense areas in the early universe collapsing, or tiny black holes forming in high-energy collisions. These tiny black holes, if they ever formed, would disappear very quickly and not cause any danger.

Evolution

After a black hole forms, it can change in different ways. It might grow by taking in matter from nearby stars or other black holes, a process called accretion of matter. It can also shrink slowly over time through a process called Hawking radiation, where tiny particles escape from the black hole.

When matter gets very close to a black hole, it moves very fast and heats up. This creates bright light that we can sometimes see from Earth. Scientists study these bright lights to learn more about black holes. Some of the biggest and brightest lights in space are powered by supermassive black holes at the centers of galaxies.

Observational evidence

Millions of black holes are thought to exist in our galaxy, the Milky Way, formed when stars collapse. Even small galaxies likely have many of these invisible objects. Because black holes do not give off light, scientists look for clues from the stars and gas around them.

One way scientists study black holes is by watching how stars move around the center of our galaxy. These stars orbit an invisible object, suggesting the presence of a giant black hole. Another method uses special telescopes to look directly at the dark area around black holes. Scientists also detect black holes by watching how their gravity affects nearby matter and by measuring ripples in space caused by black holes moving together. These different approaches help confirm the existence of black holes.

Areas of investigation

Black holes are strange objects in space. According to some ideas, once a black hole forms, it can only be described by three things: its mass, its charge, and how it spins. This means that all the other details about what went into making the black hole might seem to disappear. But black holes can slowly lose energy and shrink by sending out special kinds of energy called Hawking radiation. This radiation doesn’t seem to carry any extra details about what made the black hole, so it looks like some information is gone forever. Scientists are still trying to understand this puzzle and how it connects to the rules that describe tiny particles and the rules that describe the shape of space and time.

We have found very bright objects called quasars that are powered by huge black holes, and they existed when the universe was very young—less than a billion years old. These giant black holes are a mystery because they grew so fast. Scientists think they might have started from smaller black holes that merged together, or from special clouds of gas that collapsed directly into black holes. There are also ideas that these black holes could have formed in very special ways right after the universe began. Scientists are still exploring all the different possibilities for how these giant black holes formed so early in the universe’s history.

See also: Exotic star

While most scientists agree that there are very large black holes, there are some ideas about other types of extremely dense objects that might look similar. For example, some think that objects made of special kinds of matter, like quark matter, could stop collapsing before becoming a black hole. There are also ideas about stars made of dark energy or very slowly collapsing stars that might look like black holes but work in different ways. These are still just theories, and scientists continue to study them to see if they could explain what we see in space.

In fiction

Black holes have inspired many artists and scientists. In her book Conjuring the Void: the Art of Black Holes, Lynn Gamwell explored how art and science work together, using black holes to show how art helps us understand science and how science influences art. Some science fiction films show black holes using ideas from real science, creating images that look like those from the Event Horizon Telescope. Stories about black holes are also used to help teach science.

In science fiction, black holes appear in many different ways. Even before we had the name "black hole," stories included objects that acted like them. Today, writers and filmmakers use the strange effects of black holes, such as time moving differently near them, and sometimes use black holes as gateways to travel faster than light.