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Black Hole

Black hole

A black hole, among the most mysterious elements in the universe, is all that remains of a massive star that has used up its nuclear fuel. Lacking energy to combat the force of its own gravity, the star compresses or shrinks in size to a single point, called a singularity. At this point, pressure and density are infinite. Any object or even light that gets too close to a black hole is pulled in, stretched to infinity, and trapped forever. Black holes, so named by American physicist John Wheeler in 1969, are impossible to see, but may account for 90 percent of the content of the universe.

English geologist John Michell and French astronomer Pierre-Simon Laplace first developed the idea of black holes in the eighteenth century. They theorized that if a celestial body were large enough and dense enough, it would exhibit so much gravity that nothing could escape its pull.

This idea can be explained by looking at the effects of gravity on known objects. To break free of Earth's gravity, a spaceship has to travel at a speed of at least 7 miles (11 kilometers) per second. To escape a larger planet like Jupiter, it would have to travel at 37 miles (60 kilometers) per second. And to escape the Sun, it would have to travel at 380 miles (611 kilometers) per second. A large and dense enough object could require the spaceship to go faster than the speed of light, 186,000 miles (299,000 kilometers) per second. However, since nothing can travel faster than the speed of light, nothing would be able to escape the gravity of such an object. Black holes, indeed, are such objects.

Black hole formation

Once a star's nuclear fuel is spent, it will collapse. Without the force of nuclear fusion pushing outward from its core to balance its immense gravity, a star will fall into itself. Average-sized stars, like the Sun, shrink to become white dwarfs (small, extremely dense stars having low brightness) about the size of Earth. Stars up to three times the mass of the Sun explode to produce a supernova. Any remaining matter of such stars ends up as densely packed neutron stars or pulsars (rapidly rotating stars that emit varying radio waves at precise intervals). Stars more than three times the mass of the Sun explode in a supernova and then, in theory, collapse to form a black hole.

When a giant star collapses, its remaining mass becomes so concentrated that it shrinks to an indefinitely small size and its gravity becomes completely overpowering. According to German-born American physicist Albert Einstein's (18791955) general theory of relativity, space becomes curved near objects or matter; the more concentrated or dense that matter is, the more space is curved around it. When a black hole forms, space curves so completely around it that only a small opening to the rest of normal space remains. The surface of this opening is called the event horizon, a theorized point of no-return. Any matter that crosses the event horizon is drawn in by the black hole's gravity and cannot escape, vanishing across the boundary like water down a drain.

Black hole evidence

Black holes cannot be seen because matter, light, and other forms of energy do not escape from them. They can possibly be detected, however, by their effect on visible objects around them. Scientists believe that as gaseous matter swirls in a whirlpool before plunging into a black hole, that heated matter emits fluctuating X rays. Discovery of such a condition in space, therefore, may indicate the existence of a black hole near the source of those X rays.

In 1971, an X-ray telescope aboard the satellite Uhuru detected the first serious black hole candidate in our galaxy, the Milky Way. A black hole is believed to be the companion star in a binary star called Cygnus X-1 (a binary star is a pair of stars in a single system that orbit each other, bound together by their mutual gravities). Cygnus X-1 is emitting intense amounts of X rays, possibly as a result of the unseen companion pulling in stellar material from the other star.

Words to Know

Binary star: Pair of stars in a single system that orbit each other, bound together by their mutual gravities.

General theory of relativity: A theory of gravity put forth by Albert Einstein in 1916 that describes gravity as a distortion or curvature of space-time caused by the presence of matter.

Light-year: Distance light travels in one solar year, roughly 5.9 trillion miles (9.5 trillion kilometers).

Pulsars: Rapidly rotating stars that emit varying radio waves at precise intervals; also known as neutron stars because much of the matter within has been compressed into neutrons.

White dwarf: Average-sized star that has collapsed to about the size of Earth and has extreme density and low brightness.

In the 1990s, the Hubble Space Telescope provided scientists with evidence that black holes probably exist in nearly all galaxies and in interstellar space between galaxies. The biggest black holes are those at the center of galaxies. In the giant galaxy M87, located in the constellation Virgo, swirling gases around a suspected massive black hole stretch a distance of 500 light-years, or 2,950 trillion miles (4,750 trillion kilometers).

In early 2001, scientists announced that data from the Chandra X-Ray Observatory and the Hubble Space Telescope provided the best direct evidence for the existence of the theorized event horizons. The two Earth-orbiting telescopes both surveyed matter surrounding suspected black holes that eventually disappeared from view. The Chandra telescope observed X-ray emissions that disappeared around six candidate black holes, while the Hubble telescope observed pulses of ultraviolet light from clumps of hot gas as they faded and then disappeared around Cygnus X-1.

Types of black holes

Until very recently, scientists believed there were only two types of black holes. The first kind, stellar black holes, form from the remains

of collapsed stars that have, at most, 10 times the mass of the Sun. The second type, supermassive black holes, are believed to have been formed when the universe was very young. It is also believed they are the most common, existing at the core of every galaxy in the universe. These gigantic black holes have masses up to that of a billion Suns. In 2000, astrophysicists from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, announced they had convincing evidence for a new class of black hole: midsize black holes. Using observations from the Chandra X-Ray Observatory of a galaxy about 12 million light-years from Earth, the scientists theorized the existence of a black hole in that galaxy with a mass of at least 500 Suns. How a midsize black hole like this forms, however, remains a puzzle for scientists.

[See also Relativity, theory of; Star; Supernova ]

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black hole

black hole, in astronomy, celestial object of such extremely intense gravity that it attracts everything near it and in some instances prevents everything, including light, from escaping. The term was first used in reference to a star in the last phases of gravitational collapse (the final stage in the life history of certain stars; see stellar evolution) by the American physicist John A. Wheeler.

Gravitational collapse begins when a star has depleted its steady sources of nuclear energy and can no longer produce the expansive force, a result of normal gas pressure, that supports the star against the compressive force of its own gravitation. As the star shrinks in size (and increases in density), it may assume one of several forms depending upon its mass. A less massive star may become a white dwarf, while a more massive one would become a supernova. If the mass is less than three times that of the sun, it will then form a neutron star. However, if the final mass of the remaining stellar core is more than three solar masses, as shown by the American physicists J. Robert Oppenheimer and Hartland S. Snyder in 1939, nothing remains to prevent the star from collapsing without limit to an indefinitely small size and infinitely large density, a point called the "singularity."

At the point of singularity the effects of Einstein's general theory of relativity become paramount. According to this theory, space becomes curved in the vicinity of matter; the greater the concentration of matter, the greater the curvature. When the star (or supernova remnant) shrinks below a certain size determined by its mass, the extreme curvature of space seals off contact with the outside world. The place beyond which no radiation can escape is called the event horizon, and its radius is called the Schwarzschild radius after the German astronomer Karl Schwarzschild, who in 1916 postulated the existence of collapsed celestial objects that emit no radiation. For a star with a mass equal to that of the sun, this limit is a radius of only 1.86 mi (3.0 km). Even light cannot escape a black hole, but is turned back by the enormous pull of gravitation.

It is now believed that the origin of some black holes is nonstellar. Some astrophysicists suggest that immense volumes of interstellar matter can collect and collapse into supermassive black holes, such as are found at the center of large galaxies. The British physicist Stephen Hawking has postulated still another kind of nonstellar black hole. Called a primordial, or mini, black hole, it would have been created during the "big bang," in which the universe was created (see cosmology). Unlike stellar black holes, primordial black holes create and emit elementary particles, called Hawking radiation, until they exhaust their energy and expire. It has also been suggested that the formation of black holes may be associated with intense gamma ray bursts. Beginning with a giant star collapsing on itself or the collision of two neutron stars, waves of radiation and subatomic particles are propelled outward from the nascent black hole and collide with one another, releasing the gamma radiation. Also released is longer-lasting electromagnetic radiation in the form of X rays, radio waves, and visible wavelengths that can be used to pinpoint the location of the disturbance.

Because light and other forms of energy and matter are permanently trapped inside a black hole, it can never be observed directly. However, a black hole can be detected by the effect of its gravitational field on nearby objects (e.g., if it is orbited by a visible star), during the collapse while it was forming, or by the X rays and radio frequency signals emitted by rapidly swirling matter being pulled into the black hole. The first discovery (1971) of a possible black hole was Cygnus X-1, an X-ray source in the constellation Cygnus. In 1994 astronomers employing the Hubble Space Telescope announced that they had found conclusive evidence of a supermassive black hole in the M87 galaxy in the constellation Virgo. Since then others have been found, and in 2011 astronomers announnced the discovery of one, in NGC 4889 in the constellation Coma, whose mass may be as great as 21 billion times that of the sun. The first evidence (2002) of a binary black hole, two supermassive black holes circling one another, was detected in images from the orbiting Chandra X-ray Observatory. Located in the galaxy NGC6240, the pair are 3,000 light years apart, travel around each other at a speed of about 22,000 mph (35,415 km/hr), and have the mass of 100 million suns each. As the distance between them shrinks over 100 million years, the circling speed will increase until it approaches the speed of light, about 671 million mph (1080 million km/hr). The black holes will then collide spectacularly, spewing radiation and gravitational waves across the universe. The Chandra observatory has also discovered that massive black holes were associated with galaxies that existed 13 billion years ago.

See S. W. Hawking, Black Holes and Baby Universes and Other Essays (1994); P. Strathern, The Big Idea: Hawking and Black Holes (1998); J. A. Wheeler, Geons, Black Holes, and Quantum Foam: A Life in Physics (1998); H. Falcke and F. W. Hehl, The Galactic Black Hole: Studies in High Energy Physics, Cosmology and Gravitation (2002).

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Black Hole

BLACK HOLE

Frances Tustin introduced the idea of black holes in her Autistic Barriers in Neurotic Patients (1986). The term was chosen by analogy with ideas in modern astrophysics, which has discovered zones of extraordinary density in the universe that are probably related to the condensation and fusion of several stars. Once formed, such hyperdense zones are thought to exert a sort of attraction upon other stars, which are thus at risk of plunging into the core of these vast concentrations of matter, which swallow them up and strip them of all individuality. It is not hard to see how the metaphor of a "black hole of the psyche" can help explain, or at least help us picture what happens at the core of the psyche of autistic children.

Indeed Tustin had already elaborated on a notion first proposed by Sydney Klein (1980), that of "autistic islands." And, most significantly, in her first book, Autism and Childhood Psychosis (1972), she had painstakingly recounted the case of John, who had described to her, on emerging from autism, what he himself called "the black hole w/the mechant piquant." What John was striving to verbalize in this way was all the pain and suffering he had felt on the occasion of far too brutal and premature a separation between the breast and the nipple, this at a time when nipple and mouth are inextricably conjoined (as described, albeit in a different way, by Piera Aulagnier, with her "complementary zone-object"). Naturally it is less a physical separation that is involved here than a mental oneor even, to be quite precise, the inscription in the psyche of the process of separation.

If, for one reason or another, this process turns out to be impossible or impeded, the child is liable to feel as if a part of him- or herself has been cut off.

This traumatic organization of the psyche leaves its mark in the shape of "autistic islands" which fail to become integrated into the cycles of deferred effects and historical time: Their massiveness and their intensity, in autistic children, are an obstacle to their becoming part of mental functioning, and they end up serving as pathological poles of attraction for a whole variety of psychic elements which accrete within their sphere of influence and thus become incapable of dispersing in a manner at once orderly and differentiated.

In the wake of Frances Tustin, the post-Kleinian tendency in psychoanalysis has made wide use of the concept of the black hole, extending it to nonpsychotic subjects in whom autistic islands are possible even if in such cases they are less significant and less serious in their implications.

Bernard Golse

See also: Autism; Autistic capsule/nucleus; Breakdown.

Bibliography

Klein, Sydney. (1980). Autistic phenomena in neurotic patients. International Journal of Psycho-Analysis, 61 (2), 395-401.

Tustin, Frances. (1972). Autism and childhood psychosis. London: Hogarth; New York: Science House. Reprinted, London: Karnac, 1995.

. (1986). Autistic barriers in neurotic patients. London: Karnac.

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black hole

black hole Postulated end-product of the total gravitational collapse of a massive star into itself following exhaustion of its nuclear fuel; the matter inside is crushed to unimaginably high density. It is an empty region of distorted space-time that acts as a centre of gravitational attraction; matter is drawn towards it, and once inside nothing can escape. Its boundary (the event horizon) is a demarcation line, rather than a material surface. Black holes can have an immense range of sizes. Since no light or other radiation can escape from black holes, they are extremely difficult to detect. Not all black holes result from stellar collapse. During the Big Bang, some regions of space might have become so compressed that they formed so-called primordial black holes Such black holes would not be completely black, because radiation could still ‘tunnel out’ of the event horizon at a steady rate, leading to the evaporation of the hole. Primordial black holes could therefore be very hot.

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Black Hole

Black Hole


Modern astronomy has produced a theory about the life of stars in which the fate of a star crucially depends on how massive it is. Lighter stars might end as red dwarfs, and heavier stars as enormously dense but tiny neutron stars. The heaviest stars collapse in upon themselves, creating black holes. Black holes are called black because the gravitational force associated with them is so strong that no light can escape. The infinite gravitational attraction at the edge of an event horizon such as a black hole not only warps space but also warps time for the hypothetical observer near the black hole.


See also Astrophysics; Cosmology, Physical Aspects; Gravitation; Singularity


mark worthing

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black hole

black hole • n. Astron. a region of space having a gravitational field so intense that no matter or radiation can escape. ∎ inf. a figurative place of emptiness or aloneness: they think he's sitting in a black hole with no interaction with his people. ∎ inf., chiefly humorous a place where money, lost items, etc., are supposed to go, never to be seen again: the moribund economy has been a black hole for federal funds. ∎ inf. (of a system, practice, or institution) a state of inadequacy or excessive bureaucracy in which hopes, progress, etc., become futile: juveniles lost for good in the black hole of the criminal justice system.

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Black hole

Black hole

The event horizon

Detection of black holes

Centerpiece of the galaxy

Quantum physics and black holes

Resources

A sufficiently intense gravitational field can prevent the escape not only of matter, but even of light. Bodies known as black holes produce such gravitational fields. In 2004, astronomers, while searching quasars, found over 30 possible black holes. A black hole was discovered in June of 2004. Scientists believe that its discovery helps to confirm that gigantic black holes were created early in the formation of the universe. In 2005, a black hole was discovered to be traveling at twice the escape velocity of the galaxy as it exited the Milky Way. Scientists think that such a black hole may help to support the theory that a black hole exists in the center of the Milky Way galaxy.

The maximum intensity of a spherical objects gravitational field is a function both of the amount of matter it contains and of its volume. The more matter is contained in an object and the smaller its volumein other words, the higher its densitythe more intense the gravitational field at its surface will be. If the Earth were compacted so that it had the same mass, but half its present radius, the force of gravity at its surface would be four times as great as it is now; if it were compacted further, a density would eventually be reached at which its constituent subatomic particles would be unable to support their own weight and would collapse to a state of (theoretically) infinite density, producing a black hole. Black holes can (and some do) contain very large amounts of mattermillions or billions of times the mass of the sunbut may be formed by even a small amount of matter sufficiently compressed.

The idea of black holes is not new. French mathematician Pierre Simon Laplace (17491847) reasoned in 1795 that if the corpuscular theory of light proposed by English physicist Isaac Newton (16421727) were correct, there could exist massive objects from which light could not escape. The theory of general relativity, put forward by German physicist Albert Einstein (16791959) in 1915 and today basic to physicists understanding of the universe, also predicts the existence of black holes, though on from rather different reasoning. In recent decades, much observational evidence has been gathered to support the existence of black holes; there is no debate among astronomers today about whether black holes exist, only regarding their precise properties.

The event horizon

According to general relativity, the path taken by a beam of light is the shortest distance between two points; such a path is called a geodesic. Furthermore, gravity warps space, bending geodesics; the stronger a gravitational field is in a certain region, the more bent the geodesics are in that region. Within a certain radius of a black hole, all geodesics are so warped that a photon of light cannot escape to another part of the universe. Essentially, there are no straight lines connecting any point that is within a certain radius of a black hole (which, in theory, means there is no dimension) to any point that is farther away. The spherical surface defined by this radius is termed the event horizon of the black hole because events inside the event horizon can have no effect on events outside it. Whatever is inside the event horizon is sealed off forever from the space-time of outside observers.

The event horizon, thus, imposes a form of censorship on the makeup of a black hole; the only properties of a black hole that can be ascertained from the outside are its mass, net charge, and rate of spin. No internal time-dependent processes can be detected in the external environment, for that would involve sending signals from inside the black hole to the outside which is impossible, for not even light can escape. This censorship, or inability to produce information from within the black hole, is what is responsible for the fewness of a black holes measurable properties: mass, spin, and charge.

Although there are complications in defining the size of a black holedue to the fact that the everyday concept of size assumes Euclidean three-dimensional space and such space does not exist even approximately in the near vicinity of a black holeone can uniquely specify a black holes circumference and thus its radius as the circumference divided by 2π, where π is a mathematical constant equal approximately to 3.1459. This value is known as the Schwarzschild radius (Rs) after German astronomer Karl Schwarzschild (18731916), who first defined it as Rs = 2GM / c 2 where G is the gravitational constant, M is the mass of the black hole, and c is the speed of light.

Rs, however, cannot be interpreted as the radius of a Euclidean spherethat is, as the distance from a spherical surface (the event horizon) to its center (the black hole). As mentioned above, the geometry of space-time in the interior of the black hole is so warped that Euclidean notions of distance no longer apply. Nevertheless, Rs does provide a measure of the space around a black hole of mass M. Rs for an object having the mass of the sun is about 3 kilometers (km). Thus, in order to turn the sun into a black hole, one would have to compress it from a sphere with a radius of 696,000 km to a sphere with a radius of just 3 km. Squeezing any mass into a volume dictated by its Schwarzschild radius presents a serious assembly problem. In fact, the only processes that might lead to the formation of a sizable black hole are the explosive death of a moderately massive star or the formation of a supermassive star by sheer accumulation. Physicists also speculate that extremely small black holes might be created by the collision of subatomic particles at high energies. In fact, they estimate that as many as 100 subatomic-size black holes may be produced in the atmosphere of the Earth every year by cosmic rays.

The European Laboratory for Particle Physics (CERN, for Conseil Europe´en pour la Recherche Nucleáire) hopes to produce such microscopic black holes on demand in its new Large Hadron Collider, due to begin operation in November 2007. As of 2006, the artificial formation of mini-black holes in particle accelerators has been reported, but their creation has not yet been confirmed.

Very small black holes are predicted by theory to be short-lived, however, due to a quantum phenomenon termed evaporation. Only large black holes are long-lived enough to have cosmic effectsto swallow millions of suns worth of mass, to squeeze sufficient energy from the matter approaching their event horizons to outshine entire galaxies, to organize the orbits of billions of stars into well-defined galaxies, and so forth. Large black holes are thought to form primarily from exploding stars or by direct gravitational accumulation of large quantities of matter. A black hole may be produced by an exploding star (nova) as follows: An older star eventually exhausts the nuclear fuel that enables it to produce energy at its core, thus supporting its own weight (and shining steadily for many millions of years). It then begins a rapid collapse. The crushing pressure of the collapsing matter may be sufficient to form a black hole with the mass of several times that of the sun. Such black holes would have Schwarzschild radii of several to a few tens of kilometers. Considering the amount of mass filling that space, such objects are truly tiny.

Detection of black holes

In their near vicinity, black holes produce bizarre effects; from a distance, however, they are well-behaved. If one were to replace the sun with a black hole of the same mass as the sun, there would be a region of space a few kilometers in size, located where the center of the sun currently resides, in which space would be extremely warped. The gravitational field of this object, measured at the distance of Earth, would be exactly that of the present-day sun. Earth and planets would continue in their orbits and the solar system would continue much as it does todayonly in the dark.

Normally, an observer must get within a few Schwarzschild radii in order to feel the distinctive effects of the black hole. Indeed, one of the observational tests for the presence of a black hole in binary (two-body) systems is to look for the characteristic radiations of matter being heated as it is squeezed during its final plunge toward the black holes event horizon. Such matter will emit fluctuating x rays because of being squeezed. The rate of fluctuation is tied to the size of the emitting region. Astronomers find that in such systems the x rays come from a volume of space only a few kilometers in diameter. In several instances, analysis of the orbital motions in a binary star system with only one visible member (a conventionally shining star) indicates that the dark, unseen member of the binary system is much more massive than the sun. A dark stellar component more massive than the sun confined to a volume smaller than a few kilometers is a prime candidate for a black hole.

There is at least one other situation in which astronomers suspect the existence of a black hole. Because a black hole that is not actively swallowing large amounts of matter does not radiate significantly, astronomers must detect it indirectly, through the effect of its gravitational field on neighboring objects. In the centers of many galaxies, the stars, gas, and dust of the galaxy are moving at very high speeds, suggesting they are orbiting some very massive, comparatively small object. If the object was a tightly packed collection of massive stars, it would shine so brightly as to dominate the light from the galactic center. The absence of light from the massive, central object suggests it is a black hole. Recent astronomical observations have confirmed many galaxies, including the Milky Way, have supermassive black holes at their centers. Some scientists believe that all galaxies may be organized around such black holes; after the Big Bang, supermassive black holes may have formed first, then gathered the galaxies around them.

In one galaxy, the Hubble Space Telescope (HST) has photographed a spiraling disk of matter that appears to be accreting onto a central massive dark object that is likely to be a black hole. Recently a large team of astronomers reported the results of a worldwide study involving the HST, the International Ultraviolet Explorersatellite, and many ground based telescopes. Instruments were able to detect light that was emitted by the accreting matter as it spiraled into the black hole that was subsequently absorbed and re-emitted by the orbiting clouds just a few light-days away from the central source. Mass estimates of the central source determined from the motion of these clouds suggests that the object has a mass of at least several million times the mass of the sun. So much material contained in a volume of space no larger than a few light-days in diameter provides some of the clearest evidence yet for the existence of a black hole at the center of any galaxy.

In another study with the HST and a ground-based telescope in Hawaii, scientists were able to observe a black hole in a two-star system in the constellation Cygnus. This black hole is sucking material from its companion star in a swirling disk of material and hot gases, swallowing nearly 100 times as much energy as it radiates. The material being pulled in toward the black hole stores its energy as heat until the critical moment. The observations show gas at temperatures of over a million degrees falling toward the event horizon of the black hole.

Centerpiece of the galaxy

The concept of massive black holes at the centers of some galaxies is supported by theoretical investigations of the formation of very massive stars. Stars of

KEY TERMS

Binary system Any system of two stellar-like objects that orbit one another under the influence of their combined gravity.

Galaxy A large collection of stars and clusters of stars, containing anywhere from a few million to a few trillion stars.

General relativity A theory of gravity put forth by Albert Einstein in 1915 that basically describes gravity as a distortion of space-time by the presence of matter.

Interstellar material Any material that resides between the stars. It makes up the material from which new stars form.

Perfect radiator Also known as a black body (not to be confused with a black hole). Any object that absorbs all radiant energy that falls upon it and subsequently re-radiates that energy. The radiated energy can be characterized by a single dependant variable, the temperature.

Quantum gravity A theory resulting from the application of quantum principles to interaction between two objects normally attributed to gravity.

Quantum mechanics The theory that has been developed from Max Plancks quantum principle to describe the physics of the very small. The quantum principle, basically, states that energy only comes in certain indivisible amounts designated as quanta. Any physical interaction in which energy is exchanged can only exchange integral numbers of quanta.

more than about a hundred times (and less than about a million times) the mass of the sun cannot form because they will explode from nuclear energy released during their contraction before the star can shrink enough for its self-gravity to hold it together. However, if a collapsing cloud of interstellar material contains more than about a million times the mass of the sun, the collapse will occur so fast the nuclear processes initiated by the collapse will not be able to stop the collapse. The collapse will continue unrestrained until the object forms a black hole.

Such objects appear to be required to understand the observed behavior of the material in the center of some galaxies. Indeed, it is now certain that black holes reside at the centers of many normal galaxies, including the Milky Way. Evidence comes from the motion of gas clouds near the galactic center and from the detection of x-rays bursts from the galactic center, such as would typically be produced by a supermassive black hole swallowing matter.

Quantum physics and black holes

All that has been said so far involves black holes as described by the general theory of relativity (as written by Einstein). However, in the realm of the very small, quantum mechanics has proved to be the proper theory to describe the physical world. To date, no one has successfully combined general relativity with quantum mechanics to produce a fully consistent theory of quantum gravity; however, in 1974, British physicist Stephen Hawking (1942) suggested that quantum principles showed that a black hole should radiate energy like a perfect radiator having a temperature inversely proportional to its mass. This radiationtermed Hawking radiationdoes not come about by the conventional departure of photons from the black holes surfacewhich is impossible but as a result of certain effects predicted by quantum physics. While the amount of radiation for any astrophysical black hole is very small (e.g., the radiation temperature for a black hole with the mass of the Sun would be 107K [Kelvin]), the suggestion that loss of energy from a black hole was possible at all was revolutionary. It suggested a link between quantum theory and general relativity. The suggestion has spawned a host of new ideas expanding the relationship between the two theories. It is the ability of a black hole to lose mass via Hawking radiation (i.e., to evaporate) that prevents microscopic black holes, such as those that physicists hope to produce at CERN, from swallowing up the Earth. These black holes evaporate faster than they can grow.

See also Relativity, general; Stellar evolution;Supernova.

Resources

BOOKS

Eckart, Andreas. The Black Hole at the Center of the Milky Way. London, UK: Imperial College Press, 2005.

Hawking, Stephen. W. The Illustrated A Brief History of Time. 2nd ed. New York: Bantam Books, 2001.

Kundt, Wolfgang. Astrophysics: A New Approach. Berlin and New York: Springer, 2005.

Raine, Derek J. Black Holes: An Introduction. London, UK: Imperial College Press, 2005.

Zelik, Michael. Astronomy: The Evolving Universe. Cambridge and New York: Cambridge University Press, 2002.

PERIODICALS

Glanz, James. Evidence Points to Black Hole At Center of the Milky Way. New York Times. October 17, 2002.

Johnson, George. Physicists Strive to Build a Black Hole. New York Times. September 11, 2001.

George W. Collins, II

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Black Hole

Black hole

A sufficiently intense gravitational field can prevent the escape not only of matter , but even of light . Such gravitational fields are produced by the bodies known as black holes.

The maximum intensity of a spherical object's gravitational field is a function both of the amount of matter it contains and of its volume . The more matter is contained in an object and the smaller its volume—in other words, the higher its density—the more intense the gravitational field at its surface will be. If the earth were compacted so that it had the same mass , but half its present radius, the force of gravity at its surface would be four times as great as it is now; if it were compacted further, a density would eventually be reached at which its constituent subatomic particles would be unable to support their own weight and would collapse to a state of (theoretically) infinite density, producing a black hole. Black holes can (and some do) contain very large amounts of matter—millions or billions of times the mass of the Sun—but may be formed by even a small amount of matter sufficiently compressed.

The idea of black holes is not new; the French mathematician Pierre Simon Laplace (1749–1847) reasoned in 1795 that if the corpuscular theory of light proposed by English physicist Isaac Newton (1642–1727) were correct, there could exist massive objects from which light could not escape. The theory of general relativity, put forward by German physicist Albert Einstein (1679–1959) in 1915 and today basic to physicists' understanding of the Universe, also predicts the existence of black holes, though on from rather different reasoning. In recent decades, much observational evidence has been gathered to support the existence of black holes; there is no debate among astronomers today about whether black holes exist, only regarding their precise properties.


The event horizon

According to general relativity, the path taken by a beam of light is the shortest distance between two points; such a path is called a geodesic . Furthermore, gravity warps space , bending geodesics; the stronger a gravitational field is in a certain region, the more bent the geodesics are in that region. Within a certain radius of a black hole, all geodesics are so warped that a photon of light cannot escape to another part of the Universe; essentially, there are no straight lines connecting any point that is within a certain radius of a black hole (which, in theory, has no dimension) to any point that is farther away. The spherical surface defined by this radius is termed the event horizon of the black hole because events inside the event horizon can have no effect on events outside it. Whatever is inside the event horizon is sealed off forever from the space-time of outside observers.

The event horizon thus imposes a form of censorship on the makeup of a black hole; the only properties of a black hole that can be ascertained from the outside are its mass, net charge, and rate of spin. No internal time-dependent processes can be detected in the external environment, for that would involve sending signals from inside the black hole to the outside—which is impossible, for not even light can escape. This "censorship" is what is responsible for the fewness of a black hole's measurable properties: mass, spin, and charge.

Although there are complications in defining the "size" of a black hole, due to the fact that our everyday concept of "size" assumes Euclidean three-dimensional space and such space does not exist even approximately in the near vicinity of a black hole, one can uniquely specify a black hole's circumference and thus. its radius as the circumference divided by 2PI. This value is known as the Schwarzschild radius (Rs) after German astronomer Karl Schwarzschild (1873–1916), who first defined it as Rs = 2GM/c 2 where G is the gravitational constant, M is the mass of the black hole, and c is the speed of light.

Rs, however, cannot be interpreted as the radius of a Euclidean sphere—that is, as the distance from a spherical surface (the event horizon) to its center (the black hole). As mentioned above, the geometry of space-time in the interior of the black hole is so warped that Euclidean notions of distance no longer apply. Nevertheless, Rs does provide a measure of the space around a black hole of mass M. Rs for an object having the mass of the Sun is about 3 km. Thus, in order to turn the Sun into a black hole, one would have to compress it from a sphere with a radius of 696,000 km to a sphere with a radius of just 3 km. Squeezing any mass into a volume dictated by its Schwarzschild radius presents a serious assembly problem; in fact, the only processes that might lead to the formation of a sizable black hole are the explosive death of a moderately massive star or the formation of a supermassive star by sheer accumulation. Physicists also speculate that extremely small black holes might be created by the collision of subatomic particles at high energies. In fact, they estimate that as many as 100 subatomic-size black holes may be produced in the atmosphere of the Earth every year by cosmic rays. The European Laboratory for Particle Physics (CERN, for Conseil Européen pour la Recherche Nucléaire) hopes to produce such microscopic black holes on demand in its new Large Hadron Collider, due to begin operation in 2006.

Very small black holes are predicted by theory to be short-lived, however, due to a quantum phenomenon termed "evaporation." Only large black holes are long-lived enough to have cosmic effects—to swallow millions of suns' worth of mass, to squeeze sufficient energy from the matter approaching their event horizons to outshine entire galaxies, to organize the orbits of billions of stars into well-defined galaxies, and so forth. Large black holes are thought to form primarily from exploding stars or by direct gravitational accumulation of large quantities of matter. A black hole may be produced by an exploding star (nova ) as follows: An older star eventually exhausts the nuclear fuel that enables it to produce energy at its core, thus supporting its own weight (and shining steadily for many millions of years). It then begins a rapid collapse. The crushing pressure of the collapsing matter may be sufficient to form a black hole with the mass of several times that of the Sun. Such black holes would have Schwarzschild radii of several to a few tens of kilometers. Considering the amount of mass filling that space, such objects are truly tiny.

Detection of black holes

In their near vicinity, black holes produce bizarre effects; from a distance, however, they are well-behaved. If one were to replace the Sun with a black hole of the same mass as the Sun, there would be a region of space a few kilometers in size, located where the center of the Sun currently resides, in which space would be extremely warped. The gravitational field of this object, measured at the distance of the earth, would be exactly that of the present-day Sun. The earth and planets would continue in their orbits and the solar system would continue much as it does today—only in the dark.

Normally, an observer must get within a few Schwarzschild radii in order to feel the distinctive effects of the black hole. Indeed, one of the observational tests for the presence of a black hole in binary systems is to look for the characteristic radiations of matter being heated as it is squeezed during its final plunge toward the black hole's event horizon. Such matter will emit fluctuating x rays as a result of being squeezed. The rate of fluctuation is tied to the size of the emitting region. Astronomers find that in such systems the x rays come from a volume of space only a few kilometers in diameter. In several instances, analysis of the orbital motions in a binary star system with only one visible member (a conventionally shining star) indicates that the dark, unseen member of the binary system is much more massive than the Sun. A dark stellar component more massive than the Sun confined to a volume smaller than a few kilometers is a prime candidate for a black hole.

There is at least one other situation in which astronomers suspect the existence of a black hole. Because a black hole that is not actively swallowing large amounts of matter does not radiate significantly, we must detect it indirectly, through the effect of its gravitational field on neighboring objects. In the centers of many galaxies, the stars, gas, and dust of the galaxy are moving at very high speeds, suggesting they are orbiting some very massive, comparatively small object. If the object was a tightly packed collection of massive stars, it would shine so brightly as to dominate the light from the galactic center. The absence of light from the massive, central object suggests it is a black hole. Recent astronomical observations have confirmed many galaxies, including our own, have supermassive black holes at their centers. Scientists believe that all galaxies may be organized around such black holes; after the Big Bang, supermassive black holes may have formed first, then gathered the galaxies around them.

In one galaxy, the Hubble Space Telescope (HST) has photographed a spiraling disk of matter that appear be accreting onto a central massive dark object that is likely to be a black hole. Recently a large team of astronomers reported the results of a worldwide study involving the HST, the International Ultraviolet Explorer satellite , and many ground based telescopes. Instruments were able to detect light that was emitted by the accreting matter as it spiraled into the black hole that was subsequently absorbed and re-emitted by the orbiting clouds just a few light-days away from the central source. Mass estimates of the central source determined from the motion of these clouds suggests that the object has a mass of at least several million times the mass of the Sun. So much material contained in a volume of space no larger than a few light-days in diameter provides some of the clearest evidence yet for the existence of a black hole at the center of any galaxy.

In another study with the HST and a ground-based telescope in Hawaii, scientists were able to observe a black hole in a two-star system in the constellation Cygnus. This black hole is sucking material from its companion star in a swirling disk of material and hot gases, swallowing nearly 100 times as much energy as it radiates. The material being pulled in toward the black hole stores its energy as heat until the critical moment. The observations show gas at temperatures of over a million degrees falling toward the event horizon of the black hole.


Centerpiece of the galaxy

The concept of massive black holes at the centers of some galaxies is supported by theoretical investigations of the formation of very massive stars. Stars of more than about a hundred times (and less than about a million times) the mass of the Sun cannot form because they will explode from nuclear energy released during their contraction before the star can shrink enough for its self-gravity to hold it together. However, if a collapsing cloud of interstellar material contains more than about a million times the mass of the Sun, the collapse will occur so fast the nuclear processes initiated by the collapse will not be able to stop the collapse. The collapse will continue unrestrained until the object forms a black hole.

Such objects appear to be required to understand the observed behavior of the material in the center of some galaxies. Indeed, it is now certain that black holes reside at the centers of many normal galaxies, including our own Milky Way . Evidence comes from the motion of gas clouds near the galactic center and from the detection of x-rays bursts from the galactic center such as would typically be produced by a supermassive black hole swallowing matter.


Quantum physics and black holes

All that has been said so far involves black holes as described by the general theory of relativity. However, in the realm of the very small, quantum mechanics has proved to be the proper theory to describe the physical world. To date, no one has successfully combined general relativity with quantum mechanics to produce a fully consistent theory of quantum gravity; however, in 1974, British physicist Stephen Hawking (1942–) suggested that quantum principles showed that a black hole should radiate energy like a perfect radiator having a temperature inversely proportional to its mass. This radiation—termed Hawking radiation—does not come about by the conventional departure of photons from the black hole's surface—which is impossible—but as a result of certain effects predicted by quantum physics. While the amount of radiation for any astrophysical black hole is very small (e.g., the radiation temperature for a black hole with the mass of the Sun would be 10-7K), the suggestion that loss of energy from a black hole was possible at all was revolutionary. It suggested a link between quantum theory and general relativity, and has spawned a host of new ideas expanding the relationship between the two theories. It is the ability of a black hole to lose mass via Hawking radiation (i.e., to evaporate) that prevents microscopic black holes, such as those that physicists hope to produce at CERN, from swallowing up the earth. These black holes evaporate faster than they can grow.

See also Relativity, general; Stellar evolution; Supernova.

George W. Collins, II

Resources

books

Hawking, Stephen. W. The Illustrated A Brief History of Time. 2nd ed. New York: Bantam Books, 2001.

periodicals

Cowan, John. "Supernova Birth for a Black Hole." Nature. (September 9, 1999): 124–125.

Glanz, James. "Evidence Points to Black Hole At Center of the Milky Way." New York Times. October 17, 2002.

Irion, Robert. "Galaxies, Black Holes Shared Their Youths." Science. (June 16, 2000): 1946–1947.

Johnson, George. "Physicists Strive to Build a Black Hole." New York Times. September 11, 2001.

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Binary system

—Any system of two stellar-like objects that orbit one another under the influence of their combined gravity.

Galaxy

—A large collection of stars and clusters of stars, containing anywhere from a few million to a few trillion stars.

General relativity

—A theory of gravity put forth by Albert Einstein in 1915 that basically describes gravity as a distortion of space-time by the presence of matter.

Interstellar material

—Any material that resides between the stars. It makes up the material from which new stars form.

Perfect radiator

—Also known as a black body (not to be confused with a black hole). Any object that absorbs all radiant energy that falls upon it and subsequently re-radiates that energy. The radiated energy can be characterized by a single dependant variable, the temperature.

Quantum gravity

—A theory resulting from the application of quantum principles to interaction between two objects normally attributed to gravity.

Quantum mechanics

—The theory that has been developed from Max Planck's quantum principle to describe the physics of the very small. The quantum principle basically states that energy only comes in certain indivisible amounts designated as quanta. Any physical interaction in which energy is exchanged can only exchange integral numbers of quanta.

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"Black Hole." The Gale Encyclopedia of Science. . Encyclopedia.com. 17 Aug. 2018 <http://www.encyclopedia.com>.

"Black Hole." The Gale Encyclopedia of Science. . Encyclopedia.com. (August 17, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/black-hole-0

"Black Hole." The Gale Encyclopedia of Science. . Retrieved August 17, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/black-hole-0

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Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

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The Chicago Manual of Style

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Notes:
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