# Black hole

The supermassive black hole at the core of supergiant elliptical galaxy Messier 87, with a mass ~7 billion times the Sun's,[1] as depicted in the first image released by the Event Horizon Telescope (10 April 2019).[2][3][4][5] Visible are the crescent-shaped emission ring and central shadow, which are gravitationally magnified views of the black hole's photon ring and the photon capture zone of its event horizon. The crescent shape arises from the black hole's rotation and relativistic beaming; the shadow is about 2.6 times the diameter of the event horizon.[3]

A black hole is a region of spacetime exhibiting gravitational acceleration so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it.[6] The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole.[7][8] The boundary of the region from which no escape is possible is called the event horizon. Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, no locally detectable features appear to be observed.[9] In many ways, a black hole acts like an ideal black body, as it reflects no light.[10][11] Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe.

Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace.[12] The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958. Black holes were long considered a mathematical curiosity; it was during the 1960s that theoretical work showed they were a generic prediction of general relativity. The discovery of neutron stars by Jocelyn Bell Burnell in 1967 sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.

Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses (M) may form. There is consensus that supermassive black holes exist in the centers of most galaxies.

The presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light. Matter that falls onto a black hole can form an external accretion disk heated by friction, forming some of the brightest objects in the universe. If there are other stars orbiting a black hole, their orbits can be used to determine the black hole's mass and location. Such observations can be used to exclude possible alternatives such as neutron stars. 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.

On 11 February 2016, the LIGO collaboration announced the first direct detection of gravitational waves, which also represented the first observation of a black hole merger.[13] As of December 2018, eleven gravitational wave events have been observed that originated from ten merging black holes (along with one binary neutron star merger).[14][15] On 10 April 2019, the first ever direct image of a black hole and its vicinity was published, following observations made by the Event Horizon Telescope in 2017 of the supermassive black hole in Messier 87's galactic centre.[3][16][17]

Simulation of gravitational lensing by a black hole, which distorts the image of a galaxy in the background
Gas cloud being ripped apart by black hole at the centre of the Milky Way (observations from 2006, 2010 and 2013 are shown in blue, green and red, respectively).[18]

## History

Simulated view of a black hole in front of the Large Magellanic Cloud. Note the gravitational lensing effect, which produces two enlarged but highly distorted views of the Cloud. Across the top, the Milky Way disk appears distorted into an arc.

The idea of a body so massive that even light could not escape was briefly proposed by astronomical pioneer and English clergyman John Michell in a letter published in November 1784. Michell's simplistic calculations assumed such a body might have the same density as the Sun, and concluded that such a body would form when a star's diameter exceeds the Sun's by a factor of 500, and the surface escape velocity exceeds the usual speed of light. Michell correctly noted that such supermassive but non-radiating bodies might be detectable through their gravitational effects on nearby visible bodies.[19][12][20] Scholars of the time were initially excited by the proposal that giant but invisible stars might be hiding in plain view, but enthusiasm dampened when the wavelike nature of light became apparent in the early nineteenth century.[21]

If light were a wave rather than a "corpuscle", it is unclear what, if any, influence gravity would have on escaping light waves.[12][20] Modern relativity discredits Michell's notion of a light ray shooting directly from the surface of a supermassive star, being slowed down by the star's gravity, stopping, and then free-falling back to the star's surface.[22]

### General relativity

In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion. Only a few months later, Karl Schwarzschild found a solution to the Einstein field equations, which describes the gravitational field of a point mass and a spherical mass.[23] A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote more extensively about its properties.[24][25] This solution had a peculiar behaviour at what is now called the Schwarzschild radius, where it became singular, meaning that some of the terms in the Einstein equations became infinite. The nature of this surface was not quite understood at the time. In 1924, Arthur Eddington showed that the singularity disappeared after a change of coordinates (see Eddington–Finkelstein coordinates), although it took until 1933 for Georges Lemaître to realize that this meant the singularity at the Schwarzschild radius was a non-physical coordinate singularity.[26] Arthur Eddington did however comment on the possibility of a star with mass compressed to the Schwarzschild radius in a 1926 book, noting that Einstein's theory allows us to rule out overly large densities for visible stars like Betelgeuse because "a star of 250 million km radius could not possibly have so high a density as the sun. Firstly, the force of gravitation would be so great that light would be unable to escape from it, the rays falling back to the star like a stone to the earth. Secondly, the red shift of the spectral lines would be so great that the spectrum would be shifted out of existence. Thirdly, the mass would produce so much curvature of the space-time metric that space would close up around the star, leaving us outside (i.e., nowhere)."[27][28]

In 1931, Subrahmanyan Chandrasekhar calculated, using special relativity, that a non-rotating body of electron-degenerate matter above a certain limiting mass (now called the Chandrasekhar limit at 1.4 M) has no stable solutions.[29] His arguments were opposed by many of his contemporaries like Eddington and Lev Landau, who argued that some yet unknown mechanism would stop the collapse.[30] They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star,[31] which is itself stable. But in 1939, Robert Oppenheimer and others predicted that neutron stars above another limit (the Tolman–Oppenheimer–Volkoff limit) would collapse further for the reasons presented by Chandrasekhar, and concluded that no law of physics was likely to intervene and stop at least some stars from collapsing to black holes.[32] Their original calculations, based on the Pauli exclusion principle, gave it as 0.7 M; subsequent consideration of strong force-mediated neutron-neutron repulsion raised the estimate to approximately 1.5 M to 3.0 M.[33] Observations of the neutron star merger GW170817, which is thought to have generated a black hole shortly afterward, have refined the TOV limit estimate to ~2.17 M.[34][35][36][37][38]

Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were called "frozen stars", because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it to the Schwarzschild radius.[39]

#### Golden age

In 1958, David Finkelstein identified the Schwarzschild surface as an event horizon, "a perfect unidirectional membrane: causal influences can cross it in only one direction".[40] This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. Finkelstein's solution extended the Schwarzschild solution for the future of observers falling into a black hole. A complete extension had already been found by Martin Kruskal, who was urged to publish it.[41]

These results came at the beginning of the golden age of general relativity, which was marked by general relativity and black holes becoming mainstream subjects of research. This process was helped by the discovery of pulsars by Jocelyn Bell Burnell in 1967,[42][43] which, by 1969, were shown to be rapidly rotating neutron stars.[44] 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.[citation needed]

In this period more general black hole solutions were found. 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.[45] Through the work of Werner Israel,[46] Brandon Carter,[47][48] and David Robinson[49] the no-hair theorem emerged, stating that a stationary black hole solution is completely described by the three parameters of the Kerr–Newman metric: mass, angular momentum, and electric charge.[50]

At first, it was suspected that the strange features of the black hole solutions were pathological artifacts from the symmetry conditions imposed, and that the singularities would not appear in generic situations. This view was held in particular by Vladimir Belinsky, Isaak Khalatnikov, and Evgeny Lifshitz, who tried to prove that no singularities appear in generic solutions. However, in the late 1960s Roger Penrose[51] and Stephen Hawking used global techniques to prove that singularities appear generically.[52]

Work by James Bardeen, Jacob Bekenstein, Carter, and Hawking in the early 1970s led to the formulation of black hole thermodynamics.[53] These laws describe the behaviour of a black hole in close analogy to the laws of thermodynamics by relating mass to energy, area to entropy, and surface gravity to temperature. 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.[54]

### Etymology

John Michell used the term "dark star",[55] and in the early 20th century, physicists used the term "gravitationally collapsed object". 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.[56]

The term "black hole" was used in print by Life and Science News magazines in 1963,[56] 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.[57][58]

In December 1967, a student reportedly suggested the phrase "black hole" at a lecture by John Wheeler;[57] Wheeler adopted the term for its brevity and "advertising value", and it quickly caught on,[59] leading some to credit Wheeler with coining the phrase.[60]

Other Languages
Afrikaans: Swartkolk
Alemannisch: Schwarzes Loch
العربية: ثقب أسود
aragonés: Forato negro
অসমীয়া: কৃষ্ণগহ্বৰ
asturianu: Furacu negru
Avañe'ẽ: Kuára hũ
azərbaycanca: Qara dəlik
تۆرکجه: قارادلیک
Bân-lâm-gú: O͘-khang
башҡортса: Ҡара упҡын
беларуская: Чорная дзірка
беларуская (тарашкевіца)‎: Чорная дзірка
भोजपुरी: ब्लैक होल
български: Черна дупка
Boarisch: Schwoaz Loch
བོད་ཡིག: ནག་ཁུང་།
bosanski: Crna rupa
brezhoneg: Toull du
буряад: Хара нүхэн
català: Forat negre
Чӑвашла: Хура хăвăл
čeština: Černá díra
Cymraeg: Twll du
dansk: Sort hul
eesti: Must auk
Ελληνικά: Μαύρη τρύπα
emiliàn e rumagnòl: Buś négar
español: Agujero negro
Esperanto: Nigra truo
euskara: Zulo beltz
Fiji Hindi: Karia kund
føroyskt: Svørt hol
français: Trou noir
Frysk: Swart gat
Gaeilge: Dúpholl
Gaelg: Towl doo
Gàidhlig: Toll Dubh
galego: Burato negro
ગુજરાતી: કૃષ્ણ વિવર
한국어: 블랙홀
հայերեն: Սև խոռոչ
Արեւմտահայերէն: Սեւ խոռոչ
हिन्दी: कृष्ण विवर
hornjoserbsce: Čorna dźěra
hrvatski: Crna rupa
Bahasa Indonesia: Lubang hitam
interlingua: Foramine nigre
íslenska: Svarthol
italiano: Buco nero
עברית: חור שחור
къарачай-малкъар: Къара тешик
ქართული: შავი ხვრელი
kaszëbsczi: Czôrnô dzura
қазақша: Қара құрдым
kernowek: Toll du
Kiswahili: Shimo jeusi
Kreyòl ayisyen: Twou nwa
kurdî: Çala Reş
Кыргызча: Кара көңдөй
latviešu: Melnais caurums
Lëtzebuergesch: Schwaarzt Lach
lietuvių: Juodoji bedugnė
Limburgs: Zwart laok
Lingua Franca Nova: Buco negra
Livvinkarjala: Mustu loukko
lumbaart: Bux neger
magyar: Fekete lyuk
македонски: Црна дупка
Malagasy: Lavaka mainty
മലയാളം: തമോദ്വാരം
მარგალური: უჩა რხვილი
مصرى: خرم اسود
Bahasa Melayu: Lohong hitam
монгол: Хар нүх
မြန်မာဘာသာ: တွင်းနက်
Nederlands: Zwart gat
नेपाल भाषा: ब्ल्याक होल
нохчийн: Ӏаьржа Ӏуьрг
Nordfriisk: Suart hool
norsk: Sort hull
norsk nynorsk: Svart hòl
Novial: Nigri true
occitan: Trauc negre
oʻzbekcha/ўзбекча: Qora tuynuk
ਪੰਜਾਬੀ: ਬਲੈਕ ਹੋਲ
پنجابی: بلیک ہول
Picard: Noért treu
Piemontèis: Përtus nèir
português: Buraco negro
română: Gaură neagră
русиньскый: Чорна дїра
русский: Чёрная дыра
саха тыла: Хара дьөлөҕөс
Scots: Black hole
sicilianu: Pirtusu nìguru
සිංහල: කළු කුහර
Simple English: Black hole
سنڌي: بليڪ هول
slovenčina: Čierna diera
slovenščina: Črna luknja
کوردی: کونەڕەش
српски / srpski: Црна рупа
srpskohrvatski / српскохрватски: Crna rupa
svenska: Svart hål
Tagalog: Black hole
татарча/tatarça: Кара тишек
తెలుగు: కృష్ణబిలం
тоҷикӣ: Вартаи сиёҳ
Türkçe: Kara delik
Türkmençe: Gara girdap
удмурт: Сьӧд пась
українська: Чорна діра
اردو: بلیک ہول
Vahcuengh: Hwzdung
vèneto: Buxo nero
vepsän kel’: Must reig
Tiếng Việt: Lỗ đen

Winaray: Itom nga luho

ייִדיש: שווארצע לאך

žemaitėška: Jouduojė skīlie