For other uses, see Black hole (disambiguation).
An image of the core region of Messier 87, a supermassive black hole, processed from an array of eight radio telescopes known as the EHT with colours indicating brightness temperature[1][2]
Simulated view of a nonspinning, uncharged black hole in front of the Large Magellanic Cloud. The gravitational lensing effect produces two enlarged but distorted views of the Cloud. Across the top, the Milky Way disk appears distorted into an arc. Based on ray tracing.[3]
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, which describes gravitation as the curvature of spacetime, predicts that any sufficiently compact mass will form a black hole.[4] The boundary of no escape is called the event horizon. In general relativity, crossing a black hole’s event horizon traps an object inside but produces no locally detectable change. General relativity also predicts that every black hole should have a central singularity, where the curvature of spacetime is infinite.
Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century. In 1916, the first solution of general relativity that would characterise a black hole was found. By the late 1950s, this solution began to be interpreted physically as a region of space from which nothing can escape. Black holes were long considered a mathematical curiosity; it was not until the 1960s that theoretical work showed they were a generic prediction of general relativity. The first widely accepted black hole was Cygnus X-1, an x-ray source proposed as a black hole binary through several studies between 1971 and 1974.
Black holes typically form as part of a supernova event when massive stars collapse at the end of their life cycle. After a black hole has formed, it can grow by absorbing mass from its surroundings. Supermassive black holes of millions of solar masses may form by absorbing stars and merging with other black holes, or via direct collapse of gas clouds. There is consensus that supermassive black holes exist in the centres of most galaxies.
Quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the rate of emission being inversely proportional to the mass. This causes the black hole to lose mass very slowly, provided it is not accreting matter. However, even the smallest class of black holes observed, stellar black holes, are gaining mass from the cosmic microwave background faster than they are losing mass via Hawking radiation.
The presence of a black hole can be inferred through its interaction with matter and electromagnetic radiation such as visible light. Matter falling toward a black hole can form an accretion disk of infalling plasma, heated by friction and emitting light. In extreme cases, this creates a quasar, some of the brightest objects in the universe. Merging black holes can be detected by the gravitational waves they emit. If stars are orbiting a black hole, their motions can be used to determine the black hole’s mass and location. In this way, astronomers have identified numerous stellar black hole candidates in binary systems and established that the radio source known as Sagittarius A*, at the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses.
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 in contrast to the modern concept of an extremely dense object.[5]
Michell’s idea, in a short part of a letter published in 1784,[6] 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.[7]: 122 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 [de].[5]
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.[8]: 19
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.[8]: 19 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.[9][10] This formed the basis for black hole astrophysics.[11]
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.[7]: 124 [12] A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution.[13][14] At a certain radius from the centre 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.[15]
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.[16][7]: 134 In 1939, Einstein used his theory of general relativity in an attempt to prove that black holes were impossible.[17][18] 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.[7]: 135
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.[7]: 145 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.[7]: 151 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.[19]
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.[7]: 171 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.[7]: 193 Above that limit, they reasoned that either their model would not apply or that gravitational contraction would not stop.[20]: 380
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.[7]: 205
Birth of modern model #
The modern concept of black holes was formulated by Robert Oppenheimer and his student Hartland Snyder in 1939.[17][21]: 80 In the paper,[22] 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.[7]: 217
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.[23] Finkelstein created a new reference frame to include the point of view of infalling observers.[21]: 103 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.[7]: 226
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.[25][7]: 258
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.[26] Two years later, Ezra Newman found the axisymmetric solution for a black hole that is both rotating and electrically charged.[27]
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.[7]: 272 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.[7]: 285
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.[28] However, in 1965, Roger Penrose proved that general relativity predicts that singularities appear in all black holes,[29] although this may not still hold when quantum mechanics is taken into account.[30]
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.[31] 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.[21]: 153 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.)[32] By 1974 the object was widely considered to be a black hole, but 100% confidence for Cygnus X-1 may not be possible.[7]: 317
Work by James Bardeen, Brandon Carter, and Stephen 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[7]: 442 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.[33]: 415
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,[34] 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 centre of galaxies were powered by accreting supermassive black holes. When the Hubble Space Telescope launched in the 1990s, optical studies of the centre 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 centre 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.[35]
In 1999, David Merritt proposed the M–sigma relation, which related the dispersion of the velocity of matter in the centre bulge of a galaxy to the mass of the supermassive black hole at its core.[36] Subsequent studies confirmed this correlation.[37] Around the same time, based on telescope observations of the velocities of stars at the centre of the Milky Way galaxy, independent work groups led by Andrea Ghez and Reinhard Genzel concluded that the compact radio source in the centre of the galaxy, Sagittarius A*, was likely a supermassive black hole.[38][39]
