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.[4] In many ways a black hole acts like an ideal black body, as it reflects no light.[5][6] 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.[7] 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.[8] On 15 June 2016, a second detection of a gravitational wave event from colliding black holes was announced.[9]
Physical properties
The simplest static black holes have mass but neither electric charge nor angular momentum. These black holes are often referred to as Schwarzschild black holes after Karl Schwarzschild who discovered this solution in 1916.[15] According to Birkhoff's theorem, it is the only vacuum solution that is spherically symmetric.[49] This means that there is no observable difference between the gravitational field of such a black hole and that of any other spherical object of the same mass. The popular notion of a black hole "sucking in everything" in its surroundings is therefore only correct near a black hole's horizon; far away, the external gravitational field is identical to that of any other body of the same mass.[50]
Solutions describing more general black holes also exist. Non-rotating charged black holes are described by the Reissner–Nordström metric, while the Kerr metric describes a non-charged rotating black hole. The most general stationary black hole solution known is the Kerr–Newman metric, which describes a black hole with both charge and angular momentum.[51]
While the mass of a black hole can take any positive value, the charge and angular momentum are constrained by the mass. In Planck units, the total electric charge Q and the total angular momentum J are expected to satisfy
{\displaystyle Q^{2}+\left({\tfrac {J}{M}}\right)^{2}\leq M^{2},} Q^{2}+\left({\tfrac {J}{M}}\right)^{2}\leq M^{2},
for a black hole of mass M. Black holes with the minimum possible mass satisfying this inequality are called extremal. Solutions of Einstein's equations that violate this inequality exist, but they do not possess an event horizon. These solutions have so-called naked singularities that can be observed from the outside, and hence are deemed unphysical. The cosmic censorship hypothesis rules out the formation of such singularities, when they are created through the gravitational collapse of realistic matter.[2] This is supported by numerical simulations.[52]
Due to the relatively large strength of the electromagnetic force, black holes forming from the collapse of stars are expected to retain the nearly neutral charge of the star. Rotation, however, is expected to be a universal feature of compact astrophysical objects. The black-hole candidate binary X-ray source GRS 1915+105[53] appears to have an angular momentum near the maximum allowed value. That uncharged limit is[54]
{\displaystyle J\leq {\frac {GM^{2}}{c}},} {\displaystyle J\leq {\frac {GM^{2}}{c}},}
allowing definition of a dimensionless spin parameter such that[54]
{\displaystyle {\frac {cJ}{GM^{2}}}\leq 1.} {\displaystyle {\frac {cJ}{GM^{2}}}\leq 1.}[54][Note 1]
Black hole classifications
Class Mass Size
Supermassive black hole ~105–1010 MSun ~0.001–400 AU
Intermediate-mass black hole ~103 MSun ~103 km ≈ REarth
Stellar black hole ~10 MSun ~30 km
Micro black hole up to ~MMoon up to ~0.1 mm
Black holes are commonly classified according to their mass, independent of angular momentum, J. The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is roughly proportional to the mass, M, through
{\displaystyle r_{\mathrm {s} }={\frac {2GM}{c^{2}}}\approx 2.95,{\frac {M}{M_{\mathrm {Sun} }}}~\mathrm {km,} } {\displaystyle r_{\mathrm {s} }={\frac {2GM}{c^{2}}}\approx 2.95,{\frac {M}{M_{\mathrm {Sun} }}}~\mathrm {km,} }
where rs is the Schwarzschild radius and MSun is the mass of the Sun.[56] For a black hole with nonzero spin and/or electric charge, the radius is smaller,[Note 2] until an extremal black hole could have an event horizon close to[57]
{\displaystyle r_{\mathrm {+} }={\frac {GM}{c^{2}}}.} {\displaystyle r_{\mathrm {+} }={\frac {GM}{c^{2}}}.}