Black hole
A black hole is an object whose mass and size are such that nothing, not even light, can escape its gravity. The term was coined by theoretical physicist John Wheeler in 1967 [1], but the concept was developed, on the basis of Newtonian gravity, by the French mathematician Pierre Laplace in 1796.
Overview
Black holes are believed to form from the gravitational collapse of astronomical objects containing about two solar masses or more. Astronomical observations suggest that the centers of most galaxies, including our own Milky Way, contain supermassive black holes containing millions to billions of solar masses.
Black holes are a prediction of Einstein's theory of general relativity, but were not completely recognized as such for almost half a century. They occur in the Schwarzschild metric, one of the earliest and simplest solutions to Einstein's equations, found by Karl Schwarzschild in 1915. This solution describes the curvature of spacetime in the vicinity of a static and spherically symmetric object.
According to Schwarzschild's solution, a gravitating object will collapse into a black hole if its radius is smaller than a characteristic distance, known as the Schwarzschild radius. Below this radius, spacetime is so strongly curved that any light ray emitted in this region, regardless of the direction in which it is emitted, will travel towards the center of the system. Because relativity forbids anything from travelling faster than light, anything below the Schwarzschild radius - including the constituent particles of the gravitating object - will collapse into the center.
According to classical general relativity, a gravitational singularity, a region of theoretically infinite density will eventually form. Because not even light can escape from within the Schwarzschild radius, a black hole would truly appear black according to classical general relativity. Many astrophysicists believe that the prediction of a gravitational singularity reveals that general relativity is flawed and that a full theory of quantum gravity will give different answers. Indeed, Stephen Hawking gained fame in the 1970's for showing that when considering with quantum mechanics, black holes are not black, but release radiation and evaporate over time.
The Schwarzschild radius is given by
where G is the gravitational constant, M is the mass of the object, and c is the speed of light. For an object with the mass of the Earth, the Schwarzschild radius is a mere 9 millimeters. By a mathematical coincidence, one can derive the Schwarzschild radius from Newtonian physics.
The mean density inside the Schwarzschild radius decreases as the mass of the black hole increases, so while an earth-mass black hole would have a density of 6.2 x 1024 kg/m3, a supermassive black hole of (109 Msun) has a density of around 20 kg/m3, less than water! The mean density is given by
Since the mean radius of the Earth is around 6371 kilometers, the Earth would have to be compressed to a ludicrous 4 × 1026 times its current density for it to collapse into a black hole. For an object with the mass of the Sun, the Schwarzschild radius is approximately three kilometers, much smaller than the Sun's current radius of about 700,000 kilometers. It is also significantly smaller than the radius to which the Sun will ultimately shrink after exhausting its nuclear fuel, which is several thousand kilometers. More massive stars can collapse into black holes at the end of their lifetimes (see the section on "Black Hole Formation" below.)
More general black holes are also predicted by other solutions to Einstein's equations, such as the Kerr metric for a rotating black hole, which possesses a ring singularity, and the Reissner-Nordstrøm metric for charged black holes. The generalization of the Schwarzschild radius is known as the event horizon.
Although black holes follow from general relativity which was first proposed in the early 20th century, they were not taken seriously even as hypothetical objects by the astronomical community until the late 1960s. The general belief was that black holes were so odd that some physical process would stop the collapse of an object before it became a black hole. What motivated interest in black holes, was the discovery of pulsars in 1967 showing that small compact collapsed objects could exist. This naturally led to the question of what would happen if enough mass existed to overcome neutron denegerate pressure, and this led naturally to the idea of black holes.
Theoretical consequences
Black holes demonstrate some counter-intuitive properties of general relativity. Consider a hapless astronaut falling radially towards the center of a Schwarzschild black hole. The closer she comes to the event horizon, the longer the photons she emits take to escape to infinity. A distant observer will see her descent slowing as she approaches the event horizon, which she never appears to reach. However, this is something of an optical illusion in that in her own frame of reference, the astronaut crosses the event horizon and reaches the singularity in a finite amount of time.
Black holes produce other interesting results when applied in unison with other physical theories. A commonly stated proposition is that "black holes have no hair", meaning they have no observable external characteristics that can be used to determine what they are like inside. Black holes have only three measurable characteristics: mass, angular momentum, and electric charge, and can be completely specified by these three parameters.
The entropy of black holes is a fascinating subject, and an area of active research. In 1971, Hawking showed that the total event horizon area of any collection of classical black holes can never decrease.
This sounds remarkably similar to the Second Law of Thermodynamics, with area playing the role of entropy.
Therefore, Bekenstein proposed that the entropy of a black hole really is proportionate to its horizon area. In 1975, Hawking applied quantum field theory to a semi-classical curved spacetime and discovered that black holes can emit thermal radiation, known as Hawking radiation. This allowed him to calculate the entropy, which indeed was proportionate to the area, validating Bekenstein's hypothesis. It was later discovered that black holes are maximum-entropy objects, meaning that the maximum entropy of a region of space is the entropy of the largest black hole that can fit into it. This led to the proposal of the holographic principle.
One consequence of Hawking's work is the finding that black holes do not exist forever, but they shrink as they emit radiation. The amount of radiation emitted is small for solar mass sized black holes, but increases as the size for the black hole decreases. This eventually leads to a positive feedback loop in which the black hole evaporates. The time over which a solar mass sized black hole will evaporate is very large, much larger than the estimated age of the universe.
One other theoretical question is involves the cosmic censorship principle which states that no gravitational singularity can form without a event horizon hiding the event. This would be very convenient if true, because it would mean that a naked singularity is impossible, and that all singularities are conveniently hidden inside a black hole. However, computer simulations of collapsing material in the 1990's suggested that the cosmic censorship principle appears to be false.
Black holes and accretion disks
While black holes which are stellar sized or larger are predicted to emit neglible amounts radiation, they can be observed a number of ways.
If there is substantial amounts of gas and dust, frictional heating will cause the infalling material to be ionized into a plasma, the charged moving material then generates an intense magnetic field, which causes produces amounts of synchrotron radiation and jets. In addition, conservation of angular momentum causes the material to flatten to form an accretion disk. The interaction between all of these processes is highly complex, and a very active area of current research.
Where there is little infalling gas and dust, conclusions are drawn from objects, such as stars, that appear to be in orbit around space where there is no visible matter. In addition, black holes act as gravitational lenses and will bend light passing nearby. Although the probability of detecting a single black hole in this way is low, if the galaxy contains large numbers of black holes, then there is a large probability that a black hole will randomly pass in front of a background star and generate a distinctive optical signature. Efforts to detect these microlensing events are currently underway.
Accretion disks, jets, and orbital motions are found not only around black holes, but are common astrophysically around other objects such as neutron stars, and the physics of materials near these non-black hole objects is largely but not completely identical to the physics of materials around black holes,and is current a very complex and active field of research involving magnetic fields and plasma physics. Accretion disks, jets, and orbital motions are found everywhere we look in the universe. For the most part, observations of accretion disks and orbital motions merely indicate that there is some attractor nearby and do not provide any information about the nature of the attractor, merely its mass and size.
The belief that the central object is a black hole is due to the fact that the mass and size are such that general relativity predicts that there is a black hole. There have been some occasional papers which argue that some process will stop the collapse, and that the object being observed is not an actual black hole. However, this requires general relativity to be incorrect. General relativity predicts that above a certain mass, any force including electromagnetism that would stop the collapse would be so large that it would be equivalent to a certain mass and thereby exert its own gravity.
Note here that external observations tell us nothing about conditions within the black hole. It is possible (and some astrophysicist feel likely) that some effect of quantum gravity would stop the collapse and prevent the formation of a gravitational singularity. However, what happens inside the black hole is irrelevant to conditions outside of the black hole.
Despite the similarity in physics of matter around both black holes and non-black hole objects, there is some direct observational evidence that the objects at the center of observed accretion disks are indeed black holes. Accretion disks around low-mass objects will often experience flare-ups in which the energy released by the disk increase suddenly but briefly. These have been interpreted as being caused when material block by magnetic fields near the surface of the compact object changes allowing trapped material to suddenly fall onto the surface of the compact object. These flare-ups are not observed around high mass compact objects, and the standard explanation is that high mass compact objects are black holes with no surface.
One difficulty in generating predictions of the behavior of
gas and dust near a black hole comes from the mathematical
complexity of general relativity. One major advance in astrophysics was the development in 1986 of the membrane paradigm by Kip Thorne. Thorne and his colleagues were able to show that the behavior of a black hole in a magnetic plasma could be modelled as that of a spherical
conductive membrane with a resistance of 377 ohms. This paradigm allows astrophysicists to model the behavior of plasmas and magnetic fields near a black hole, without explicitly invoking general relativity.
Observational evidence
There is now a great deal of observational evidence for the existence of two types of high mass compact objects:
There is also some evidence for intermediate-mass black holes (IMBHs), those with masses of a few thousand times of the Sun. These black holes may be responsible for the formation of supermassive black holes.
A proposed additional type of black hole, primordial black holes, has not been observed.
In the case of both a stellar-size and supermassive black hole, matter can be drawn in, producing an accretion disk and large amounts of X-rays. As gas falls into a black hole, frictional heating causes large amounts of energy to be released. This heating is extremely efficient and can convert about 50% of the mass energy of an object into radiation as opposed to nuclear fusion which can only convert a few percent of the energy.
This efficient radiation mechanism is essential in order to produce that amount of energy that is observed in active galactic nuclei such as quasars, and its introduction in the 1970's removed a major objection to the belief that quasars were distant galaxies, namely that no physical mechanism could generate that much energy.
From observations in the 1980s of motions of stars around the galactic center, it is now believed that such supermassive black holes exist in the center of most galaxies, including our own Milky Way. Sagittarius A* is now agreed to be the most plausible candidate for the location of a supermassive black hole at the center of the Milky Way galaxy.
The current picture is that all galaxies may have a supermassive black hole in their center, and that this black hole swallows gas and dust in the middle of the galaxies generating huge amounts of radiation until there is no more, and the process shuts off. This picture also nicely explains why there are no nearby quasars.
They may be involved in gamma ray bursters, although observations of GRB's in association with supernova have reduced the possibility of a link.
Observations with Chandra and XMM-Newton, combined with earlier images from the German Roentgen satellite, detected a powerful X-ray outburst from the center of the galaxy RXJ1242-11. This outburst, one of the most extreme ever detected in a galaxy, was caused by gas, heated to millions of degrees Celsius, from the destroyed star being swallowed by the black hole. The energy liberated in the process was equivalent to a supernova. February 18, 2004 NASA press release
"Now, with all the data in hand, we have the smoking gun proof that this spectacular event has occurred," said coauthor Guenther Hasinger, also of MPE.
Black hole formation
Close to solar-mass black holes are created by the gravitational collapse of massive stars. When a star exhausts its nuclear fuel, the equilibrium between gravitation and radiation pressure is disturbed, and it collapses. If the mass of the star is greater than about 3 times the mass of the sun, the collapse cannot be stopped, and a black hole is created. (See stellar evolution.)
Instead of collapsing on themselves, black holes might also be created by compression of matter by extreme external pressure. Such black holes are called primordial black holes. The enormous pressures necessary for creating primordial black holes are thought to have existed in the very early stages of the universe. These black holes can have masses smaller than that of the sun. Unlike larger black holes, primordial black holes are predicted to emit non-negligible amounts of Hawking radiation, which has not been detected.
The formation of supermassive black holes is currently matter of very active research. Though the mechanism of formation is still not clear, there is
increasing evidence that the growth of the black hole is intimately related
to the growth of the spheroidal component (elliptical galaxy, or bulge of a spiral galaxy) in which it lives.
Related topics
External links
Black hole is also used to descripe a pipe which ends in /dev/null.
Referenced By
Active Galactic Nuclei | Active Galactic Nucleus | Active galaxy | Astronomical body | Astronomical object | Eddington coordinates | Electromagnetic spectrum | Entropy | List of astronomical topics | List of astronomical topics (N-Z) | List of physics topics A-E | Radio galaxy | Reciprocal System of Theory | Starbust galaxy | T-symmetry | Thermodynamic entropy | What is entropy?
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