For other uses, see Black hole (disambiguation) .
A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing—not even particles and electromagnetic radiation such as
light—can escape from inside it.[1] The theory of general relativity predicts that a sufficiently compact mass can deform
spacetime to form a black hole. [2][3] 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. In many ways a black hole acts like an ideal black body , as it reflects no light. [4][5] 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 .[6] 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 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 general consensus that supermassive black holes exist in the centers of most galaxies.
Despite its invisible interior, 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 our own 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 observation of gravitational waves ; because these waves were generated from a black hole merger it was the first ever direct detection of a binary black hole merger. [7] On 15 June 2016, a second detection of a gravitational wave event from colliding black holes was announced. [8]
Simulation of gravitational lensing by a black hole, which distorts the image of a
galaxy in the background
Gas cloud ripped apart by black hole at the centre of the Milky Way . [9]
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 John Michell in a letter published in 1783–84. Michell's simplistic calculations assumed that 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. [10][6][11] 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 around the early nineteenth century. [12] If light were a wave rather than a "corpuscle ", it became unclear what, if any, influence gravity would have on escaping light waves. [6][11] In any case, thanks to modern relativity, we now know that Michell's picture of a light ray shooting directly out 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, is fundamentally incorrect.
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. [13] 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. [14][15] 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 an unphysical
coordinate singularity . [16] 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)." [17][18]
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. [19] 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. [20] They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a
neutron star , [21] which is itself stable because of the Pauli exclusion principle . But in 1939, Robert Oppenheimer and others predicted that neutron stars above approximately 3 M ☉ (the
Tolman–Oppenheimer–Volkoff limit ) would collapse into black holes 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. [22]
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", [23] because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius.
Golden age
See also: History of general relativity
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". [24] 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. [25]
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 in 1967, [26][27] which, by 1969, were shown to be rapidly rotating neutron stars . [28] 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.
