Black Hole
For other uses,
see Blackhole
(disambiguation).
"Frozen star"
redirects here. For the hypothetical object, see Frozen
star (hypothetical star).
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). 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.
A black hole is a region
of spacetime where gravity is 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. 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, it has no locally detectable features. In many ways, a black
hole acts like an ideal black body, as it reflects no
light. 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. 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 not until 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. The matter that falls onto a black
hole can form an external accretion disk heated by
friction, forming quasars, some of the brightest objects in the
universe. Stars passing too close to a supermassive black hole can be shred
into streamers that shine very brightly before being "swallowed."[13] 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. 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). 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 center.
History
The idea of a body so massive that even the 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. 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.
If light were a wave rather than a "corpuscle", it is unclear what, if any,
influence gravity would have on escaping light waves. Modern physics 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.
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. 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. This solution
had a peculiar behavior 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.[27] 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 redshift 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).
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. His arguments
were opposed by many of his contemporaries like Eddington and Lev Landau, who argued that some yet unknown a mechanism would stop the collapse.[31] They were partly correct: a white dwarf slightly more massive than the
Chandrasekhar limit will collapse into a neutron star, which in 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. 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☉.Observations of the neutron star
merger GW170817, which is thought to have generated a
blackhole shortly afterward, have refined the TOV limit estimate to
~2.17 M☉.[35][36][37][38][39]
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 froze in time at the instant where its collapse takes it to the
Schwarzschild radius
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