Simulated view of a black hole in front of the Milky Way. The hole has 10 solar masses and is viewed from a distance of 600 km. An acceleration of about 400 million g is necessary to sustain this distance constantly. g-force (also G-force, g-load) is a measurement of an object's Acceleration expressed in g s ^[1]

General relativity

Einstein field equations

Introduction to... Mathematical formulation of... Phenomena Kepler problem · Lenses · Waves Frame-dragging · Geodetic effect Event horizon · Singularity Black hole

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A black hole is a region of space in which the gravitational field is so powerful that nothing, not even light, can escape its pull after having fallen past its event horizon. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 The Einstein field equations ( EFE) or Einstein's equations are a set of ten equations in Einstein 's theory of General relativity in which the General relativity (GR is a Theory of Gravitation that was developed by Albert Einstein between 1907 and 1915 The mathematics of general relativity refers to various mathematical structures and techniques that are used in studying Albert Einstein 's theory of General The Kepler problem in general relativity involves solving for the motion of two spherical bodies interacting with one another by Gravitation, as described by the theory of A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of In Physics, a gravitational wave is a Fluctuation in the Curvature of Spacetime which propagates as a wave, traveling outward from Albert Einstein 's theory of General relativity predicts that rotating bodies drag Spacetime around themselves in a phenomenon referred to as frame-dragging The geodetic effect represents the effect of the curvature of Spacetime, predicted by General relativity, on a spinning moving body In General relativity, an event horizon is a boundary in Spacetime, an area surrounding a Black hole or a Wormhole, inside which events cannot A gravitational singularity (sometimes spacetime singularity) is approximately a place where quantities which are used to measure the Gravitational field become A gravitational field is a model used within Physics to explain how gravity exists in the universe In General relativity, an event horizon is a boundary in Spacetime, an area surrounding a Black hole or a Wormhole, inside which events cannot The term "Black Hole" comes from the fact that, at a certain point, even electromagnetic radiation (e. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. g. visible light) is unable to break away from the attraction of these massive objects. This renders the hole's interior invisible or, rather, black like the appearance of space itself. Outer space, often simply called space, comprises the relatively empty regions of the Universe outside the escape velocities of Celestial bodies.

Despite its interior being invisible, a black hole may reveal its presence through an interaction with matter that lies in orbit outside its event horizon. For example, a black hole may be perceived by tracking the movement of a group of stars that orbit its center. Alternatively, one may observe gas (from a nearby star, for instance) that has been drawn into the black hole. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and earth-orbiting telescopes. Radiation, as in Physics, is Energy in the form of waves or moving Subatomic particles emitted by an atom or other body as it changes from a higher energy ^[2][3][4] Such observations have resulted in the general scientific consensus that — barring a breakdown of our understanding nature— black holes do exist in our universe. The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy ^[5]

The idea of an object with gravity strong enough to prevent light from escaping was proposed in 1783 by the Reverend John Mitchell, an amateur British astronomer. Gravitation is a natural Phenomenon by which objects with Mass attract one another In 1795, Pierre-Simon Laplace, a French physicist independently came to the same conclusion. ^[6][7] Black holes, as currently understood, are described by Einstein's general theory of relativity, which he developed in 1916. Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 Year 1916 ( MCMXVI) was a Leap year starting on Saturday (link will display the full calendar of the Gregorian calendar (or a Leap year This theory predicts that when a large enough amount of mass is present in a sufficiently small region of space, all paths through space are warped inwards towards the center of the volume, preventing all matter and radiation within it from escaping. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object The Hoop Conjecture, proposed by Kip Thorne in 1972, states that an imploding object forms a Black hole when and only when a circular hoop In physics the world line of an object is the unique path of that object as it travels through 4- Dimensional Spacetime. The volume of any solid plasma vacuum or theoretical object is how much three- Dimensional space it occupies often quantified numerically

While general relativity describes a black hole as a region of empty space with a pointlike singularity at the center and an event horizon at the outer edge, the description changes when the effects of quantum mechanics are taken into account. A gravitational singularity (sometimes spacetime singularity) is approximately a place where quantities which are used to measure the Gravitational field become Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons Research on this subject indicates that, rather than holding captured matter forever, black holes may slowly leak a form of thermal energy called Hawking radiation. Hawking radiation (also known as Bekenstein-Hawking radiation) is a Thermal radiation with a black body spectrum predicted to be emitted by Black holes ^[8][9][10] However, the final, correct description of black holes, requiring a theory of quantum gravity, is unknown. Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature

What makes it impossible to escape from black holes?

Far away from the black hole a particle can move in any direction. It is only restricted by the speed of light.

Closer to the black hole spacetime starts to deform. There are more paths going towards the black hole than paths moving away.

Inside of the event horizon all paths bring the particle closer to the center of the black hole. It is no longer possible for the particle to escape.

Popular accounts commonly try to explain the black hole phenomenon by using the concept of escape velocity, the speed needed for a vessel starting at the surface of a massive object to completely clear the object's gravitational field. In Physics, escape velocity is the speed where the Kinetic energy of an object is equal to the magnitude of its Gravitational potential energy Using Newton's law of gravity it is straight forward to show that if you take a sufficiently dense object its escape velocity will equal or even exceed the speed of light. Newton 's law of universal Gravitation is a physical law describing the gravitational attraction between bodies with mass Citing that nothing can exceed the speed of light they then infer that nothing would be able escape such a dense object. Of course, this argument has a flaw in that it doesn't explain why light would even be affected by a gravitating body, let alone why it wouldn't be able to escape. Some argue that in general relativity light is affected by gravity and that indeed the energy required to escape a black hole is infinite. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός This makes the argument for the attraction of light stronger but still leaves needed explanation.

Two concepts introduced by Albert Einstein help us understand this situation. Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical The first is that time and space are not two independent concepts, but are interrelated forming a single continuum, spacetime. SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS This continuum has some special properties. An object is not free to move around spacetime at will, instead it must always move forwards in time. In fact, not only must an object move forwards in time, it also cannot change its position faster than the speed of light. This is the main result of the theory of special relativity. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial

The second lesson is the main message of general relativity, mass deforms the structure of spacetime. Loosely speaking, the effect of a mass on spacetime is to slightly tilt the direction of time towards the mass. As a result, objects tend to move towards masses; we experience this as gravity. As you get closer to a mass this tilting effect becomes stronger. At some point close to the mass this effect becomes so strong that all the possible paths an object can take lead towards the mass. That is, no matter how hard you try—even if you used the most powerful rocket available—your path will only bring you closer to the mass; you are trapped. This is precisely what happens at the event horizon of a black hole.

So, to put it succinctly, the reason you cannot escape a black hole is because you cannot move backwards in time (or faster than the speed of light).

Properties: mass, charge and angular momentum

According to the "No Hair" theorem a black hole has only three independent physical properties: mass, charge and angular momentum. The no-hair theorem in Astrophysics postulates that all Black hole solutions of the Einstein-Maxwell equations of Gravitation and Electromagnetism Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. In Physics, the angular momentum of a particle about an origin is a vector quantity equal to the mass of the particle multiplied by the Cross product of the position ^[11] Any two black holes that share the same values for these properties are completely indistinguishable. This contrasts with other astrophysical objects such as stars, which have very many —possibly infinitely many— parameters. Consequently, a great deal of information is lost when a star collapses to form a black hole. Since in most physical theories information is (in some sense) preserved, this loss of information in black holes is puzzling. Physicists refer to this as the black hole information paradox. The black hole information paradox results from the combination of Quantum mechanics and General relativity.

The "No Hair" theorem does make some assumptions about the nature of our universe and the matter it contains. Other assumptions would lead to different conclusions. For example, if nature also allows magnetic monopoles to exist —which appears to be theoretically possible, but has never been observed— then it should also be possible for a black hole to have a magnetic charge. In Physics, a magnetic monopole is a hypothetical particle that is a Magnet with only one pole (see Maxwell's equations for more on magnetic In Physics, a magnetic monopole is a hypothetical particle that is a Magnet with only one pole (see Maxwell's equations for more on magnetic If the universe has more than four dimensions (as string theories, a controversial but apparently possible class of theories, would require), or has a global anti-de Sitter structure, the theorem could fail completely, allowing many sorts of "hair". String theory is a still-developing scientific approach to Theoretical physics, whose original building blocks are one-dimensional extended objects called strings In Mathematics and Physics, n -dimensional anti de Sitter space, sometimes written AdS_n is a maximally symmetric Lorentzian manifold But in our apparently four-dimensional, very nearly flat universe, the theorem should hold.

Black hole types

The simplest possible black hole is one that has mass but neither charge nor angular momentum. These black holes are often referred to as Schwarzschild black holes after the physicist Karl Schwarzschild who discovered this solution in 1915. In Einstein's theory of General relativity, the Schwarzschild solution (or the Schwarzschild vacuum) describes the Gravitational field outside Karl Schwarzschild ( October 9, 1873 - May 11, 1916) was a German Jewish Physicist and Astronomer. It was the first (non-trivial) exact solution to the Einstein equations to be discovered, and according to Birkhoff's theorem, the only vacuum solution that is spherically symmetric. The Einstein field equations ( EFE) or Einstein's equations are a set of ten equations in Einstein 's theory of General relativity in which the For real world physics 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 —for example a spherical star or planet— once you are in the empty space outside the object. The popular notion of a black hole "sucking in everything" in its surroundings is therefore incorrect; the external gravitational field, far from the event horizon, is essentially like that of ordinary massive bodies.

More general black hole solutions were discovered later in the 20th century. The Reissner-Nordström solution describes a black hole with electric charge, while the Kerr solution yields a rotating black hole. In Physics and Astronomy, the Reissner-Nordström metric is a solution to the Einstein field equations in empty space which corresponds to the gravitational In General relativity, the Kerr metric (or Kerr vacuum) describes the geometry of Spacetime around a rotating massive body The most general known stationary black hole solution is the Kerr-Newman metric having both charge and angular momentum. The Kerr-Newman metric is a solution of Einstein's General relativity field equation that describes the spacetime geometry in the region surrounding a charged All these general solutions share the property that they converge to the Schwarzschild solution at distances that are large compared to the ratio of charge and angular momentum to mass (in natural units). In Physics, natural units are Physical units of Measurement defined in terms of universal Physical constants, such that some chosen physical

While the mass of a black hole can take any (positive) value, the other two properties —charge and angular momentum— are constrained by the mass. In natural units , the total charge Q and the total angular momentum J are expected to satisfy Q²+(J/M)² ≤ M² for a black hole of mass M. Black holes saturating this inequality are called extremal. In Theoretical physics, an extremal black hole is a Black hole with the minimal possible mass that can be compatible with the given charges and Angular Solutions of Einstein's equation violating the inequality do exist, but do not have an horizon. These solutions have naked singularities and are thus deemed unphysical. The cosmic censorship hypothesis states that it is impossible for such singularities to form in due to gravitational collapse. The weak and the strong Cosmic Censorship Hypotheses are two mathematical conjectures about the structure of singularities arising in General relativity. This is supported by numerical simulations.

Black holes forming from the collapse of stars are expected —due to the relatively large strength of electromagnetic force— to retain the nearly neutral charge of the star. Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of Rotation, however, is expected to be a common feature of compact objects, and the black-hole candidate binary X-ray source GRS 1915+105^[12] appears to have an angular momentum near the maximum allowed value. GRS 1915+105 or V1487 Aquilae is an X-ray binary star system which features a regular star and a Black hole.

Sizes

Black holes occurring in nature are commonly classified according to their mass, independent of angular momentum J. The size of black hole as determined by the radius of the event horizon, or Schwarzschild radius, is proportional to the mass through where is the Schwarzschild radius and is the mass of the Sun. The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is a characteristic Radius associated with every Mass. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. Thus size and mass have a simple relationship, which is independent of rotation. Black hole#Major features of rotating black holes A rotating black hole is a Black hole that possesses Angular momentum. According to this mass/size criterion then, black holes are commonly classified as:

• Supermassive black holes that contain hundreds of thousands to billions of Solar masses are believed to exist in the center of most galaxies, including our own Milky Way. A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1 The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply They are thought to be responsible for active galactic nuclei, and presumably form either from the coalescence of smaller black holes, or by the accretion of stars and gas onto them. An active galactic nucleus ( AGN) is a compact region at the centre of a Galaxy which has a much higher than normal luminosity over some or all of the Electromagnetic

• Intermediate-mass black holes, whose sizes are measured in thousands of solar masses, probably exist. An Intermediate-mass black hole (IMBH is a Black hole whose mass is significantly more than Stellar black holes (a few tens of the mass of the Sun) yet They have been proposed as a possible power source for the ultra-luminous X ray sources. An ultra-luminous X-ray source ( ULX) is an astronomical source of X-rays that is not in the nucleus of a Galaxy, and is more luminous than 1039 There is no known mechanism for them to form directly, so they most probably form via collisions of lower mass black holes, either in the dense stellar cores of globular clusters or galaxies. A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. Such creation events should produce intense bursts of gravitational waves, which may be observed in the near- to mid-term. In Physics, a gravitational wave is a Fluctuation in the Curvature of Spacetime which propagates as a wave, traveling outward from For the Latvian holiday Ligo see Jāņi. LIGO stands for Laser Interferometer Gravitational-Wave Observatory. The boundary limit between super- and intermediate-mass black holes is a matter of convention. Their lower mass limit, the maximum mass for direct formation of a single black hole from collapse of a massive star, is poorly known at present.

• Stellar-mass black holes have masses ranging from a lower limit of about 1. A stellar black hole is a Black hole formed by the Gravitational collapse of a massive Star (20 or more Solar masses, though the exact amount 5-3. 0 solar masses (the Tolman-Oppenheimer-Volkoff limit for the maximum mass of neutron stars) up to perhaps 15—20 solar masses, and are created by the collapse of individual stars, or by the coalescence (inevitable, due to gravitational radiation) of binary neutron stars. The Tolman-Oppenheimer-Volkoff ( TOV) limit is an upper bound to the mass of stars composed of neutron-degenerate matter ( Neutron stars. In Physics, a gravitational wave is a Fluctuation in the Curvature of Spacetime which propagates as a wave, traveling outward from A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type Stars may form with initial masses up to ~100 solar masses, or possibly even higher, but these shed most of their outer massive layers during earlier phases of their evolution, either blown away in stellar winds during the red giant, AGB, and Wolf-Rayet stages, or expelled in supernova explosions for stars that turn into neutron stars or black holes. The initial mass function (IMF is an Empirical function that describes the Mass distribution (the histogram of stellar masses of a population of stars in terms A red giant is a luminous Giant star of low or intermediate mass (roughly 0 A supernova (plural supernovae or supernovas) is a stellar Explosion. Being known mostly by theoretical models for late-stage stellar evolution, the upper limit for the mass of stellar-mass black holes is somewhat uncertain at present. The cores of still lighter stars form white dwarfs. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter.

• Micro black holes (also mini black holes) have masses much less than that of a star. Micro black holes, are the tiny hypothetical Black holes also called quantum mechanical black holes or mini black holes, for which quantum mechanical At these sizes the effects of quantum mechanics are expected to come into play. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons There is no known mechanism for them to form via normal processes of stellar evolution, but certain inflationary scenarios predicted their production during the early stages of the evolution of the universe. In Physical cosmology, cosmic inflation is the idea that the nascent Universe passed through a phase of exponential expansion that According to some theories of quantum gravity they may also be produced in the highly energetic reaction produced by cosmic rays hitting the atmosphere or even in particle accelerators such as the Large Hadron Collider. Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature For the 1962 Bruce Conner film see Cosmic Ray (film Cosmic rays are energetic particles originating from space that impinge on An atmosphere (from Greek ατμός - atmos, " Vapor " + σφαίρα - sphaira, " Sphere " The theory of Hawking radiation predicts that such black holes will evaporate in bright flashes of gamma radiation. Hawking radiation (also known as Bekenstein-Hawking radiation) is a Thermal radiation with a black body spectrum predicted to be emitted by Black holes Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions NASA's GLAST satellite, to be launched in 2008, will search for such flashes as one of its scientific objectives. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program

Features

Event horizon

Main article: Event horizon

The defining feature of a black hole, the event horizon is a surface in spacetime that marks a point of no return. In General relativity, an event horizon is a boundary in Spacetime, an area surrounding a Black hole or a Wormhole, inside which events cannot Once an object has crossed this surface there is no way that it can return to the other side. Consequently, anything inside this surface is completely hidden from observers outside. Other than this the event horizon is a completely normal part of space, with no special features that would allow someone falling into the a black hole to know when he would cross the horizon. The event horizon is not a solid surface, and does not obstruct or slow down matter or radiation that is traveling towards the region within the event horizon.

Outside of the event horizon, the gravitational field is identical to the field produced by any other spherically symmetric object of the same mass. The popular conception of black holes as "sucking" things in is false: objects can maintain an orbit around black holes indefinitely, provided they stay outside the photon sphere (described below), and also ignoring the effects of gravitational radiation, which causes orbiting objects to lose energy, similar to the effect of electromagnetic radiation. In Physics, a gravitational wave is a Fluctuation in the Curvature of Spacetime which propagates as a wave, traveling outward from Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter.

Singularity

According to general relativity, a black hole's mass is entirely compressed into a region with zero volume, which means its density and gravitational pull are infinite, and so is the curvature of space-time that it causes. Infinity (symbolically represented with ∞) comes from the Latin infinitas or "unboundedness These infinite values cause most physical equations, including those of general relativity, to stop working at the center of a black hole. So physicists call the zero-volume, infinitely dense region at the center of a black hole a singularity. A gravitational singularity (sometimes spacetime singularity) is approximately a place where quantities which are used to measure the Gravitational field become

The singularity in a non-rotating black hole is a point, in other words it has zero length, width, and height. The singularity of a rotating black hole is smeared out to form a ring shape lying in the plane of rotation. Black hole#Major features of rotating black holes A rotating black hole is a Black hole that possesses Angular momentum. In General relativity the Gravitational singularity at the center of a rotating Black hole (a " Kerr black hole " is supposed to form a circle The ring still has no thickness and hence no volume.

The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 This breakdown is not unexpected, as it occurs in a situation where quantum mechanical effects should become important, since densities are high and particle interactions should thus play a role. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons Unfortunately, till date it has not been possible to combine quantum and gravitation effects in a single theory. It is however quite generally expected that a theory of quantum gravity will feature black holes without singularities. Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature

Photon sphere

Main article: Photon sphere

The photon sphere is a spherical boundary of zero thickness such that photons moving along tangents to the sphere will be trapped in a circular orbit. A photon sphere is a Spherical region of space where Gravity is strong enough that Photons of light are forced to travel in orbits For the tangent function see Trigonometric functions. For other uses see Tangent (disambiguation. For non-rotating black holes, the photon sphere has a radius 1. 5 times the Schwarzschild radius. The orbits are dynamically unstable, hence any small perturbation (maybe caused by some in falling matter) will grow over time, allowing the photon to escape or sending it spiraling to its doom. Instability in systems is generally characterized by some of the Outputs or internal states growing without Bounds.

While light can still escape from inside the photon sphere, any light that crosses the photon sphere on an inbound trajectory will be captured by black hole. Hence any light reaching an outside observer from inside the photon sphere must have been emitted by objects inside the photon sphere but still outside of the event horizon.

Other compact objects, such as neutron stars, can also have photon spheres. In Astronomy, the term compact star (sometimes compact object) is used to refer collectively to White dwarfs Neutron stars other exotic A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type ^[13] This follows from the fact gravitation field of an object does not depend on its actual size, hence any object that is smaller than 1. 5 times the Schwarzschild radius corresponding to its mass will in fact have a photon sphere.

Ergosphere

Two important surfaces around a rotating black hole. The inner sphere is the static limit (the event horizon). It is the inner boundary of a region called the ergosphere. The ergosphere is a region located outside a Rotating black hole. The oval-shaped surface, touching the event horizon at the poles, is the outer boundary of the ergosphere. Within the ergosphere a particle is forced (dragging of space and time) to rotate and may gain energy at the cost of the rotational energy of the black hole (Penrose process). The Penrose process (also called Penrose mechanism) is a process theorised by Roger Penrose wherein energy can be extracted from a Rotating black hole.

Main article: Ergosphere

Rotating black holes are surround by a region, called the ergosphere, of spacetime in which it is impossible to stand still. The ergosphere is a region located outside a Rotating black hole. This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slight "drag" along the spacetime immediately surrounding spacetime. Albert Einstein 's theory of General relativity predicts that rotating bodies drag Spacetime around themselves in a phenomenon referred to as frame-dragging Any object near the rotating mass will tend to start moving in the direction of rotation. For a rotating black hole this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.

The ergosphere of black hole is bounded by

• on the outside, an oblate spheroid, which coincides with the event horizon at the poles and is noticeably wider around the "equator". An oblate Spheroid is a rotationally symmetric Ellipsoid having a polar axis shorter than the diameter of the equatorial circle whose plane This boundary is sometimes called the "ergosurface", but it is just a boundary and has no more solidity than the event horizon. At points exactly on the ergosurface, spacetime is "dragged around at the speed of light. "
• on the inside, the (outer) event horizon.

Within the ergosphere, space-time is dragged around faster than light—general relativity forbids material objects to travel faster than light (so does special relativity), but allows regions of space-time to move faster than light relative to other regions of space-time. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial

Objects and radiation (including light) can stay in orbit within the ergosphere without falling to the center. But they cannot hover (remain stationary, as seen by an external observer), because that would require them to move backwards faster than light relative to their own regions of space-time, which are moving faster than light relative to an external observer.

Objects and radiation can also escape from the ergosphere. In fact the Penrose process predicts that objects will sometimes fly out of the ergosphere, obtaining the energy for this by "stealing" some of the black hole's rotational energy. The Penrose process (also called Penrose mechanism) is a process theorised by Roger Penrose wherein energy can be extracted from a Rotating black hole. If a large total mass of objects escapes in this way, the black hole will spin more slowly and may even stop spinning eventually.

Hawking radiation

Main article: Hawking radiation

In 1974, Stephen Hawking showed that black holes are not entirely black but emit small amounts of thermal radiation. Hawking radiation (also known as Bekenstein-Hawking radiation) is a Thermal radiation with a black body spectrum predicted to be emitted by Black holes Stephen William Hawking CH, CBE, FRS, FRSA (born 8 January 1942 is a British theoretical physicist. ^[14] He got this result by applying quantum field theory in a static black hole background. In quantum field theory (QFT the forces between particles are mediated by other particles The result of his calculations is that a black hole should emit particles in a perfect black body spectrum. In Physics, a black body is an object that absorbs all light that falls on it This effect has become known as Hawking radiation. Hawking radiation (also known as Bekenstein-Hawking radiation) is a Thermal radiation with a black body spectrum predicted to be emitted by Black holes Since Hawking's result many others have verified the effect through various methods. ^[15]

The temperature of the emitted black body spectrum is proportional to the surface gravity of the black hole. The surface gravity, g, of an astronomical or other object is the Gravitational acceleration experienced at its surface For a Schwarzschild black hole this is inversely proportional to the mass. Consequently, large black holes are very cold and emit very little radiation. A stellar black hole of 10 solar masses, for example, would have a Hawking temperature of several nanokelvin, much less than the 2. Hawking radiation (also known as Bekenstein-Hawking radiation) is a Thermal radiation with a black body spectrum predicted to be emitted by Black holes 7K produced by the Cosmic Microwave Background. Micro black holes on the other hand could be quite bright producing high energy gamma rays.

Due to low Hawking temperature of stellar black holes, Hawking radiation has never been observed at any of the black hole candidates.

Effects of Falling into a Black Hole

This section describes what happens when something falls into a Schwarzschild (i. e. non-rotating and uncharged) black hole. Rotating and charged black holes have some additional complications when falling into them, which are not treated here.

Spaghettification

Main article: spaghettification

An object in any very strong gravitational field feels a tidal force stretching it in the direction of the object generating the gravitational field. In Astrophysics, spaghettification is the stretching of objects into long thin shapes (rather like Spaghetti) in a very strong gravitational field and is The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced This is because the inverse square law causes nearer parts of the stretched object to feel a stronger attraction than farther parts. In Physics, an inverse-square law is any Physical law stating that some physical Quantity or strength is inversely proportional Near black holes, the tidal force is expected to be strong enough to deform any object falling into it, even atoms or composite nucleons; this is called spaghettification. The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced In Physics a nucleon is a collective name for two Baryons the Neutron and the Proton. In Astrophysics, spaghettification is the stretching of objects into long thin shapes (rather like Spaghetti) in a very strong gravitational field and is The process of spaghettification is as follows. First, the object that is falling into the black hole splits in two. Then the two pieces each split themselves, rendering a total of four pieces. Then the four pieces split to form eight. This process of bifurcation continues up to and past the point in which the split-up pieces of the original object are at the order of magnitude of the constituents of atoms. An order of magnitude is the class of scale or magnitude of any amount where each class contains values of a fixed ratio to the class preceding it At the end of the spaghettification process, the object is a string of elementary particles. In Particle physics, an elementary particle or fundamental particle is a particle not known to have substructure that is it is not known to be made

The strength of the tidal force of a black hole depends on how gravitational attraction changes with distance, rather than on the absolute force being felt. The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced This means that small black holes cause spaghettification while infalling objects are still outside their event horizons, whereas objects falling into large, supermassive black holes may not be deformed or otherwise feel excessively large forces before passing the event horizon. In General relativity, an event horizon is a boundary in Spacetime, an area surrounding a Black hole or a Wormhole, inside which events cannot A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1

Before the falling object crosses the event horizon

An object in a gravitational field experiences a slowing down of time, called gravitational time dilation, relative to observers outside the field. For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of Gravitational time dilation is the effect of time passing at different rates in regions of different Gravitational potential; the higher the local distortion of Spacetime The outside observer will see that physical processes in the object, including clocks, appear to run slowly. As a test object approaches the event horizon, its gravitational time dilation (as measured by an observer far from the hole) would approach infinity.

From the viewpoint of a distant observer, an object falling into a black hole appears to slow down, approaching but never quite reaching the event horizon: and it appears to become redder and dimmer, because of the extreme gravitational red shift caused by the gravity of the black hole. In Physics, Light or other forms of Electromagnetic radiation of a certain wavelength originating from a source placed in a region of stronger gravitational Eventually, the falling object becomes so dim that it can no longer be seen, at a point just before it reaches the event horizon. All of this is a consequence of time dilation: the object's movement is one of the processes that appear to run slower and slower, and the time dilation effect is more significant than the acceleration due to gravity; the frequency of light from the object appears to decrease, making it look redder, because the light appears to complete fewer cycles per "tick" of the observer's clock; lower-frequency light has less energy and therefore appears dimmer, as well as redder. Frequency is a measure of the number of occurrences of a repeating event per unit Time.

From the viewpoint of the falling object, distant objects generally appear blue-shifted due the gravitational field of the black hole. Blue shift is the shortening of a transmitted signal's Wavelength, and/or an increase in its Frequency, due to the Doppler Effect, which indicates that This effect may be partly (or even entirely) negated by the red shift caused by the velocity of the infalling object with respect to the object in the distance. In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by

As the object passes through the event horizon

From the viewpoint of the falling object, nothing particularly special happens at the event horizon. In fact, the Earth could be passing through an event horizon at just this moment without our ever noticing. An infalling object takes a finite proper time (i. In relativity, proper time is Time measured by a single Clock between events that occur at the same place as the clock e. measured by its own clock) to fall past the event horizon. This in contrast with the infinite amount of time it takes for a distant observer to see the infalling object cross the horizon.

Inside the event horizon

The object reaches the singularity at the center within a finite amount of proper time, as measured by the falling object. In relativity, proper time is Time measured by a single Clock between events that occur at the same place as the clock An observer on the falling object would continue to see objects outside the event horizon, blue-shifted or red-shifted depending on the falling object's trajectory. Blue shift is the shortening of a transmitted signal's Wavelength, and/or an increase in its Frequency, due to the Doppler Effect, which indicates that In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by Objects closer to the singularity aren't seen, as all paths light could take from objects farther in point inwards towards the singularity.

The amount of proper time a faller experiences below the event horizon depends upon where they started from rest, with the maximum being for someone who starts from rest at the event horizon. A paper in 2007 examined the effect of firing a rocket pack with the black hole, showing that this can only reduce the proper time of a person who starts from rest at the event horizon. However, for anyone else, a judicious burst of the rocket can extend the lifetime of the faller, but overdoing it will again reduce the proper time experienced. However, this cannot prevent the inevitable collision with the central singularity. ^[16]

Hitting the singularity

As an infalling object approaches the singularity, tidal forces acting on it approach infinity. The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced All components of the object, including atoms and subatomic particles, are torn away from each other before striking the singularity. History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny A subatomic particle is an elementary or composite Particle smaller than an Atom. At the singularity itself, effects are unknown; it is believed that a theory of quantum gravity is needed to accurately describe events near it. Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature Regardless, as soon as an object passes within the hole's event horizon, it is lost to the outside universe. An observer far from the hole simply sees the hole's mass, charge, and angular momentum change slightly, to reflect the addition of the infalling object's matter. After the event horizon all is unknown. Anything that passes this point cannot be retrieved to study.

Formation and evolution

From the exotic nature of black holes, it is natural to question if such bizarre objects could actual exist in nature or that they are merely pathological solutions to Einstein's equations. Nature could very well conspire against the formation of such anomalies. However in 1970, Hawking and Penrose proved the opposite; under generic conditions black holes are expected to form in any universe. ^[17] The primary formation process for black holes is expected to be the gravitational collapse of heavy objects such as stars, but there are also more exotic processes that can lead to the production of black holes. Gravitational collapse in Astronomy is the inward fall of a massive body under the influence of the force of Gravity.

Gravitation collapse

Main article: gravitational collapse

Gravitational collapse occurs when an object's internal pressure is insufficient to rest the object's own gravity. Gravitational collapse in Astronomy is the inward fall of a massive body under the influence of the force of Gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature, or because a star which would have been stable receives a lot of extra matter in a way which does not raise its core temperature. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in Stars to build the nuclei of the heavier elements. In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight (the ideal gas law explains the connection between pressure, temperature, and volume). The ideal gas law is the Equation of state of a hypothetical Ideal gas, first stated by Benoît Paul Émile Clapeyron in 1834

The collapse may be stopped by the degeneracy pressure of the star's constituents, condensing the matter in an exotic denser state. Degenerate matter is matter which has sufficiently high Density that the dominant contribution to its Pressure rises from the Pauli Exclusion Degenerate matter is matter which has sufficiently high Density that the dominant contribution to its Pressure rises from the Pauli Exclusion The result is one of the various types of compact star. In Astronomy, the term compact star (sometimes compact object) is used to refer collectively to White dwarfs Neutron stars other exotic Which type of compact star is formed depends on the mass of the remnant - the matter left over after changes triggered by the collapse (such as supernova or pulsations leading to a planetary nebula) have blown away the outer layers. A supernova (plural supernovae or supernovas) is a stellar Explosion. A planetary nebula is an Emission nebula consisting of a glowing shell of Gas and plasma formed by certain types of Stars when they die Note that this can be substantially less than the original star - remnants exceeding 5 solar masses are produced by stars which were over 20 solar masses before the collapse.

If the mass of the remnant of exceeds ~3-4 solar masses (the Tolman-Oppenheimer-Volkoff limit)—either because the original star was very heavy or because the remnant collected additional mass through accretion of matter)—even the degeneracy pressure of neutrons is insufficient to stop the collapse. The Tolman-Oppenheimer-Volkoff ( TOV) limit is an upper bound to the mass of stars composed of neutron-degenerate matter ( Neutron stars. This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. After this no known mechanism (except maybe the quark degeneracy pressure, see quark star) is powerful enough to stop the collapse and the object will inevitably collapse to a black hole. A quark star or strange star is a hypothetical type of Exotic star composed of Quark matter, or Strange matter.

This gravitational collapse of heavy stars is assumed to be responsible for the formation of most (if not all) stellar mass black holes. A stellar black hole is a Black hole formed by the Gravitational collapse of a massive Star (20 or more Solar masses, though the exact amount

Creation of primordial black holes in the big bang

Gravitational collapse requires great densities. In the current epoch of the universe these high densities are only found in stars, but in the early universe shortly after the big bang densities were much greater, possibly allowing for the creation of black holes. The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. The high density alone is not enough to allow the formation of black holes since a uniform mass distribution will not allow the mass to bunch up. In order for primordial black holes to form in such a dense medium, there must be initial density perturbations which can then grow under their own gravity. A primordial black hole is a hypothetical type of Black hole that is formed not by the Gravitational collapse of a star but by the extreme density of matter present Different models for the early universe vary widely in their predictions of the size of these perturbations. Various models predict the creation of black holes, ranging from a Planck mass to hundreds of thousands of solar masses. The Planck mass is the unit of Mass, denoted by m P in the system of Natural units known as Planck units. ^[18] Primordial black holes could thus account for the creation of any type of black hole.

Production in high energy collisions

Gravitational collapse is not the only process that could create black holes. In principle, black holes could also be created in high energy collisions that create sufficient density. Since classically black holes can take any mass one would expect micro black holes to be created in any such process no matter how low the energy. Micro black holes, are the tiny hypothetical Black holes also called quantum mechanical black holes or mini black holes, for which quantum mechanical However, to date, no such events have ever been detected either directly or indirectly as a deficiency of the mass balance in particle accelerator experiments. This suggests that there must be a lower limit for the mass of black holes.

Theoretically this bound is expect to lie around the Planck mass (~10¹⁹ GeV/c²), where quantum effects are expected to make the theory of general relativity break down completely. The Planck mass is the unit of Mass, denoted by m P in the system of Natural units known as Planck units. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 This would put the creation of black holes firmly out of reach of any high energy process occurring on or near the Earth. Certain developments in quantum gravity however suggest that this bound could be much lower. Some braneworld scenarios for example put the Planck mass much lower, may be even as low as 1 TeV. Brane cosmology refers to several theories in Particle physics and cosmology motivated by but not exclusively derived from Superstring theory and This would make it possible for micro black holes to be created in the high energy collisions occuring when cosmic rays hit the earth's atmosphere, or even maybe in new Large Hadron Collider at CERN. The European Organization for Nuclear Research (Organisation Européenne pour la Recherche Nucléaire known as CERN These theories are however very speculative, and the creation of black holes in these processes is deemed unlikely by many specialists.

Growth

Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb interstellar dust from its direct surroundings and omnipresent cosmic background radiation, but neither of these processes should significantly affect the mass of a stellar black hole. More significant contributions can occur when the black hole formed in a binary star system. A binary star is a Star system consisting of two Stars orbiting around their Center of mass. After formation the black hole can then leech significant amounts of matter from its companion.

Much larger contributions can be obtained when a black hole merges with other stars or compact objects. The supermassive black holes suspected in the center of most galaxies are expected to have formed from the coagulation of many smaller objects. A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1 The process has also been proposed as the origin of some intermediate-mass black holes. An Intermediate-mass black hole (IMBH is a Black hole whose mass is significantly more than Stellar black holes (a few tens of the mass of the Sun) yet

Evaporation

If Hawking's theory of black hole radiation is correct then black holes are expected to emit a thermal spectrum of radiation, and thereby lose mass, because according to Einstein's Theory of Relativity mass is just highly condensed energy (e = mc²). ^[14] Black holes will thus shrink and evaporate over time. The temperature of this spectrum ([[Hawking temperature) is proportional to the surface gravity of the black hole, which in turn is inversely proportional to the mass. The surface gravity, g, of an astronomical or other object is the Gravitational acceleration experienced at its surface Large black holes thus emit less radiation than small black holes.

A stellar black hole of 5 solar masses has a Hawking temperature of about 12 nanoKelvin. nano- is a prefix (symbol n) in the SI system of units denoting a factor of 10−9 The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic This is far less than the 2. 7 K produced by the Cosmic microwave background. Stellar mass (and larger) black holes thus receive more mass from the CMB than they emit through Hawking radiation and will thus grow instead of shrink. In order to have a Hawking temperature larger than 2. 7 K (and thus be able to evaporate) a black hole needs to be lighter than the Moon (and thus have diameter of less than a tenth of a millimeter).

On the other hand if a black hole is very small the radiation effects are expected to become very strong. Even a black hole that is heavy compared to a human would evaporate in an instant. A black hole the weight of a car (~ 10^-24 m) would only take a nanosecond to evaporate, during which time it would briefly have a luminosity more than 200 times that of the sun. Lighter black holes are expected to evaporate even faster, for example a black hole of mass 1 TeV/c² would less than 10^-88 seconds to evaporate completely. Of course, for such a small black hole quantum gravitation effects are expected to play an important role and could even —although current developments in quantum gravity do not indicate so— hypothetically make such a small black holes stable. Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature

Techniques for finding black holes

Accretion disks and gas jets

Formation of extragalactic jets from a black hole's accretion disk

Most accretion disks and gas jets are not clear proof that a stellar-mass black hole is present, because other massive, ultra-dense objects such as neutron stars and white dwarfs cause accretion disks and gas jets to form and to behave in the same ways as those around black holes. An accretion disc (or accretion disk) is a structure (often a Circumstellar disk) formed by diffuse material in orbital motion around a central body An accretion disc (or accretion disk) is a structure (often a Circumstellar disk) formed by diffuse material in orbital motion around a central body The lower-energy non-relativistic version of this phenomenon is described at Polar jet. A stellar black hole is a Black hole formed by the Gravitational collapse of a massive Star (20 or more Solar masses, though the exact amount A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. But they can often help by telling astronomers where it might be worth looking for a black hole.

On the other hand, extremely large accretion disks and gas jets may be good evidence for the presence of supermassive black holes, because as far as we know any mass large enough to power these phenomena must be a black hole. A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1

Strong radiation emissions

A "Quasar" Black Hole.

Steady X-ray and gamma ray emissions also do not prove that a black hole is present, but can tell astronomers where it might be worth looking for one - and they have the advantage that they pass fairly easily through nebulae and gas clouds. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions A nebula (from Latin: "mist" pl nebulae or nebulæ, with ligature or nebulas) is an Interstellar cloud of

But strong, irregular emissions of X-rays, gamma rays and other electromagnetic radiation can help to prove that a massive, ultra-dense object is not a black hole, so that "black hole hunters" can move on to some other object. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Neutron stars and other very dense stars have surfaces, and matter colliding with the surface at a high percentage of the speed of light will produce intense flares of radiation at irregular intervals. Black holes have no material surface, so the absence of irregular flares round a massive, ultra-dense object suggests that there is a good chance of finding a black hole there.

Intense but one-time gamma ray bursts (GRBs) may signal the birth of "new" black holes, because astrophysicists think that GRBs are caused either by the gravitational collapse of giant stars^[19] or by collisions between neutron stars,^[20] and both types of event involve sufficient mass and pressure to produce black holes. Gamma-ray bursts ( GRB s are the most luminous electromagnetic events occurring in the Universe since the Big Bang. Gravitational collapse in Astronomy is the inward fall of a massive body under the influence of the force of Gravity. But it appears that a collision between a neutron star and a black hole can also cause a GRB,^[21] so a GRB is not proof that a "new" black hole has been formed. All known GRBs come from outside our own galaxy, and most come from billions of light years away^[22] so the black holes associated with them are actually billions of years old. A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by

Some astrophysicists believe that some ultraluminous X-ray sources may be the accretion disks of intermediate-mass black holes. An ultra-luminous X-ray source ( ULX) is an astronomical source of X-rays that is not in the nucleus of a Galaxy, and is more luminous than 1039 An accretion disc (or accretion disk) is a structure (often a Circumstellar disk) formed by diffuse material in orbital motion around a central body An Intermediate-mass black hole (IMBH is a Black hole whose mass is significantly more than Stellar black holes (a few tens of the mass of the Sun) yet ^[23]

Quasars are thought to be the accretion disks of supermassive black holes, since no other known object is powerful enough to produce such strong emissions. A quasar (contraction of QUASi-stellAR radio source) is an extremely powerful and distant Active galactic nucleus. A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1 Quasars produce strong emission across the electromagnetic spectrum, including UV, X-rays and gamma-rays and are visible at tremendous distances due to their high luminosity. The electromagnetic (EM spectrum is the range of all possible Electromagnetic radiation frequencies Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays X-radiation (composed of X-rays) is a form of Electromagnetic radiation. Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions Luminosity has different meanings in several different fields of science Between 5 and 25% of quasars are "radio loud," so called because of their powerful radio emission. Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light. ^[24]

Gravitational lensing

Gravitational lensing of a black hole caused from going by a galaxy in the background . A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter

A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is "bent" around a massive object (such as a black hole) between the source object and the observer. A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of A quasar (contraction of QUASi-stellAR radio source) is an extremely powerful and distant Active galactic nucleus. The process is known as gravitational lensing, and is one of the predictions of Albert Einstein's general theory of relativity. At its introduction in 1915 the general theory of relativity did not have a solid empirical foundation Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 According to this theory, mass "warps" space-time to create gravitational fields and therefore bend light as a result. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS A gravitational field is a model used within Physics to explain how gravity exists in the universe Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700

A source image behind the lens may appear as multiple images to the observer. In cases where the source, massive lensing object, and the observer lie in a straight line, the source will appear as a ring behind the massive object.

Gravitational lensing can be caused by objects other than black holes, because any very strong gravitational field will bend light rays. Some of these multiple-image effects are probably produced by distant galaxies.

Objects orbiting possible black holes

See also: Kepler problem in general relativity

Some large celestial objects are almost certainly orbiting around black holes, and the principles behind this conclusion are surprisingly simple if we consider a circular orbit first (although all known closed astronomical orbits are elliptical):

• The radius of the central object round which the observed object is orbiting must be less than the radius of the orbit, otherwise the two objects would collide. The Kepler problem in general relativity involves solving for the motion of two spherical bodies interacting with one another by Gravitation, as described by the theory of In Mathematics, an ellipse (from the Greek ἔλλειψις literally absence) is a Conic section, the locus of points in a Remote Authentication Dial In User Service ( RADIUS) is a networking protocol that provides centralized access authorization and accounting management for people or computers
• The orbital period and the radius of the orbit make it easy to calculate the centrifugal force created by the orbiting object. The orbital period is the time taken for a given object to make one complete Orbit about another object Strictly speaking, the centrifugal force also depends on the orbiting object's mass, but the next two steps show why we can get away with pretending this is a fixed number: e. g. , 1.
• The gravitational attraction between the central object and the orbiting object must be exactly equal to the centrifugal force, otherwise the orbiting body would either spiral into the central object or drift away.
• The required gravitational attraction depends on the mass of the central object, the mass of the orbiting object, and the radius of the orbit. But we can simplify the calculation of both the centrifugal force and the gravitational attraction by pretending that the mass of the orbiting object is the same fixed number: e. g. , 1. This makes it very easy to calculate the mass of the central object.
• If the Schwarzschild radius for a body with the mass of the central object is greater than the maximum radius of the central object, the central object must be a black hole whose event horizon's radius is equal to the Schwarzschild radius. The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is a characteristic Radius associated with every Mass. In General relativity, an event horizon is a boundary in Spacetime, an area surrounding a Black hole or a Wormhole, inside which events cannot

Unfortunately, since the time of Johannes Kepler, astronomers have had to deal with the complications of real astronomy:

• Astronomical orbits are elliptical. Johannes Kepler (ˈkɛplɚ ( December 27 1571 &ndash November 15 1630) was a German Mathematician, Astronomer In Mathematics, an ellipse (from the Greek ἔλλειψις literally absence) is a Conic section, the locus of points in a This complicates the calculation of the centrifugal force, the gravitational attraction, and the maximum radius of the central body. But Kepler could handle this without needing a computer.
• The orbital periods in this type of situation are several years, so several years' worth of observations are needed to determine the actual orbit accurately. The "possibly a black hole" indicators (accretion disks, gas jets, radiation emissions, etc. ) help "black hole hunters" to decide which orbits are worth observing for such long periods.
• If there are other large bodies within a few light years, their gravity fields will perturb the orbit. Adjusting the calculations to filter out the effects of perturbation can be difficult, but astronomers are used to doing it.

Determining the mass of black holes

Quasi-periodic oscillations can be used to determine the mass of black holes]]. In high-energy (X-ray astronomy, quasi-periodic oscillations (QPOs refer to the way the X-ray light from an astronomical object flickers about certain frequencies ^[25] The technique uses a relationship between black holes and the inner part of their surrounding disks, where gas spirals inward before reaching the event horizon. As the gas collapses inwards, it radiates X-rays with an intensity that varies in a pattern that repeats itself over a nearly regular interval. This signal is the Quasi-Periodic Oscillation, or QPO. A QPO’s frequency depends on the black hole’s mass; the event horizon lies close in for small black holes, so the QPO has a higher frequency. For black holes with a larger mass, the event horizon is farther out, so the QPO frequency is lower.

Black hole candidates

Supermassive black holes at the centers of galaxies

The jet originating from the center of M87 in this image comes from an active galactic nucleus that may contain a supermassive black hole. Messier 87 (also known as M87, Virgo A or NGC 4486) is a giant Elliptical galaxy. An active galactic nucleus ( AGN) is a compact region at the centre of a Galaxy which has a much higher than normal luminosity over some or all of the Electromagnetic A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1 Credit: Hubble Space Telescope/NASA/ESA. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member

According to the American Astronomical Society, every large galaxy has a supermassive black hole at its center. The black hole’s mass is proportional to the mass of the host galaxy, suggesting that the two are linked very closely. The Hubble and ground-based telescopes in Hawaii were used in a large survey of galaxies.

For decades, astronomers have used the term "active galaxy" to describe galaxies with unusual characteristics, such as unusual spectral line emission and very strong radio emission. An active galactic nucleus ( AGN) is a compact region at the centre of a Galaxy which has a much higher than normal luminosity over some or all of the Electromagnetic A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light. ^[26][27] However, theoretical and observational studies have shown that the active galactic nuclei (AGN) in these galaxies may contain supermassive black holes. An active galactic nucleus ( AGN) is a compact region at the centre of a Galaxy which has a much higher than normal luminosity over some or all of the Electromagnetic A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1 ^[26][27] The models of these AGN consist of a central black hole that may be millions or billions of times more massive than the Sun; a disk of gas and dust called an accretion disk; and two jets that are perpendicular to the accretion disk. The Sun (Sol is the Star at the center of the Solar System. An accretion disc (or accretion disk) is a structure (often a Circumstellar disk) formed by diffuse material in orbital motion around a central body The lower-energy non-relativistic version of this phenomenon is described at Polar jet. ^[27]

Although supermassive black holes are expected to be found in most AGN, only some galaxies' nuclei have been more carefully studied in attempts to both identify and measure the actual masses of the central supermassive black hole candidates. Some of the most notable galaxies with supermassive black hole candidates include the Andromeda Galaxy, M32, M87, NGC 3115, NGC 3377, NGC 4258, and the Sombrero Galaxy. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda Messier 32 (also known as NGC 221 and Le Gentil) is a Dwarf elliptical galaxy about 2 Messier 87 (also known as M87, Virgo A or NGC 4486) is a giant Elliptical galaxy. NGC 3115 (also called the Spindle Galaxy) is a lenticular (S0 galaxy in the Constellation Sextans. Messier 106 (also known as NGC 4258) is a Spiral galaxy about in the Constellation Canes Venatici. The Sombrero Galaxy (also known as M 104 or NGC 4594) is an unbarred Spiral galaxy in the Constellation ^[28]

Astronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center, in a region called Sagittarius A*:

• A star called S2 (star) follows an elliptical orbit with a period of 15. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply Sagittarius A* (pronounced "A-star" standard abbreviation Sgr A*) is a bright and very compact source of Radio emission at the center of S2 (also known as S0—2 is a star that is located close to the radio source Sagittarius A*, orbiting it with an orbital period of 15 In Mathematics, an ellipse (from the Greek ἔλλειψις literally absence) is a Conic section, the locus of points in a The orbital period is the time taken for a given object to make one complete Orbit about another object 2 years and a pericenter (closest) distance of 17 light hours from the central object. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from A light-hour (also written light hour) is a unit of Length. It is the distance travelled by Light in Vacuum in one Hour
• The first estimates indicated that the central object contains 2. 6M (2. 6 million) solar masses and has a radius of less than 17 light hours. Only a black hole can contain such a vast mass in such a small volume.
• Further observations^[29] strengthened the case for a black hole, by showing that the central object's mass is about 3. 7M solar masses and its radius no more than 6. 25 light-hours.

Intermediate-mass black holes in globular clusters

In 2002, the Hubble Space Telescope produced observations indicating that globular clusters named M15 and G1 may contain intermediate-mass black holes. A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. Globular Cluster M15 (also known as Messier Object 15 or NGC 7078) is a Globular cluster in the Constellation Mayall II ( M31 G1) also known as NGC-224-G1, SKHB 1, GSC 27882139, HBK 0-1, M31GC J003247+393440 An Intermediate-mass black hole (IMBH is a Black hole whose mass is significantly more than Stellar black holes (a few tens of the mass of the Sun) yet ^[30][31] This interpretation is based on the sizes and periods of the orbits of the stars in the globular clusters. But the Hubble evidence is not conclusive, since a group of neutron stars could cause similar observations. A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type Until recent discoveries, many astronomers thought that the complex gravitational interactions in globular clusters would eject newly-formed black holes.

In November 2004 a team of astronomers reported the discovery of the first well-confirmed intermediate-mass black hole in our Galaxy, orbiting three light-years from Sagittarius A*. An Intermediate-mass black hole (IMBH is a Black hole whose mass is significantly more than Stellar black holes (a few tens of the mass of the Sun) yet This black hole of 1,300 solar masses is within a cluster of seven stars, possibly the remnant of a massive star cluster that has been stripped down by the Galactic Centre. ^[32][33] This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.

In January 2007, researchers at the University of Southampton in the United Kingdom reported finding a black hole, possibly of about 400 solar masses, in a globular cluster associated with a galaxy named NGC 4472, some 55 million light-years away. ^[34]

Stellar-mass black holes in the Milky Way

Artist's impression of a binary system consisting of a black hole and a main sequence star. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness The black hole is drawing matter from the main sequence star via an accretion disk around it, and some of this matter forms a gas jet. An accretion disc (or accretion disk) is a structure (often a Circumstellar disk) formed by diffuse material in orbital motion around a central body The lower-energy non-relativistic version of this phenomenon is described at Polar jet.

Our Milky Way galaxy contains several probable stellar-mass black holes which are closer to us than the supermassive black hole in the Sagittarius A* region. A stellar black hole is a Black hole formed by the Gravitational collapse of a massive Star (20 or more Solar masses, though the exact amount Sagittarius A* (pronounced "A-star" standard abbreviation Sgr A*) is a bright and very compact source of Radio emission at the center of These candidates are all members of X-ray binary systems in which the denser object draws matter from its partner via an accretion disk. X-ray binaries are a class of Binary stars that are luminous in X-rays The X-rays are produced by matter falling from one component (usually a relatively normal The probable black holes in these pairs range from three to more than a dozen solar masses. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. ^[35][36] The most distant stellar-mass black hole ever observed is a member of a binary system located in the Messier 33 galaxy. The Triangulum Galaxy (also known as Messier 33 or NGC 598) is a Spiral galaxy approximately 3 million light-years away in the ^[37]

Micro black holes

In theory there is no smallest size for a black hole. Once created, it has the properties of a black hole. Stephen Hawking theorized that primordial black holes could evaporate and become even tinier, i. Stephen William Hawking CH, CBE, FRS, FRSA (born 8 January 1942 is a British theoretical physicist. A primordial black hole is a hypothetical type of Black hole that is formed not by the Gravitational collapse of a star but by the extreme density of matter present e. micro black holes. Micro black holes, are the tiny hypothetical Black holes also called quantum mechanical black holes or mini black holes, for which quantum mechanical Searches for evaporating primordial black holes are proposed for the GLAST satellite to be launched in 2008. However, if micro black holes can be created by other means, such as by cosmic ray impacts or in colliders, that does not imply that they must evaporate.

The formation of black hole analogs on Earth in particle accelerators has been reported. These black hole analogs are not the same as gravitational black holes, but they are vital testing grounds for quantum theories of gravity. ^[38]

They act like black holes because of the correspondence between the theory of the strong nuclear force, which has nothing to do with gravity, and the quantum theory of gravity. For the relation of the AdS/CFT correspondence to the general context of string theory see String theory. They are similar because both are described by string theory. So the formation and disintegration of a fireball in quark gluon plasma can be interpreted in black hole language. A quark-gluon plasma (QGP is a phase of Quantum chromodynamics (QCD which exists at extremely high Temperature and/or Density. The fireball at the Relativistic Heavy Ion Collider [RHIC] is a phenomenon which is closely analogous to a black hole, and many of its physical properties can be correctly predicted using this analogy. The Relativistic Heavy Ion Collider (RHIC pronounced like " Rick " ˈrɪk is a heavy- Ion Collider located at and operated by Brookhaven The fireball, however, is not a gravitational object. It is presently unknown whether the much more energetic Large Hadron Collider [LHC] would be capable of producing the speculative large extra dimension micro black hole, as many theorists have suggested.

History of the black hole concept

The Newtonian conceptions of Michell and Laplace are often referred to as "dark stars" to distinguish them from the "black holes" of general relativity. A dark star is a theoretical object compatible with Newtonian mechanics that due to its large mass has a surface Escape velocity that equals or exceeds the

Newtonian theories (before Einstein)

The concept of a body so massive that even light could not escape was put forward by the geologist John Michell in a letter written to Henry Cavendish in 1783 and published by the Royal Society. A geologist is a contributor to the Science of Geology, studying the physical structure and processes of the Earth and planets of the solar system John Michell ( December 25, 1724 – April 29, 1793) was an English natural philosopher and Geologist, whose work spanned Henry Cavendish, FRS (10 October 1731 - 24 February 1810 was a British Scientist noted for his discovery of Hydrogen or what he called "inflammable The Royal Society of London for the Improvement of Natural Knowledge, known simply as The Royal Society, is a Learned society for science that was founded in 1660 ^[39]

This assumes that light is influenced by gravity in the same way as massive objects.

In 1796, the mathematician Pierre-Simon Laplace promoted the same idea in the first and second editions of his book Exposition du système du Monde (it was removed from later editions).

The idea of black holes was largely ignored in the nineteenth century, since light was then thought to be a massless wave and therefore not influenced by gravity. Unlike a modern black hole, the object behind the horizon is assumed to be stable against collapse.

Theories based on Einstein's general relativity

In 1915, Albert Einstein developed the theory of gravity called general relativity, having earlier shown that gravity does influence light (although light has zero rest mass, it is not the rest mass that is the source of gravity but the energy). Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 A few months later, Karl Schwarzschild gave the solution for the gravitational field of a point mass and a spherical mass,^[40][41] showing that a black hole could theoretically exist. Karl Schwarzschild ( October 9, 1873 - May 11, 1916) was a German Jewish Physicist and Astronomer. In Einstein's theory of General relativity, the Schwarzschild solution (or the Schwarzschild vacuum) describes the Gravitational field outside The Schwarzschild radius is now known to be the radius of the event horizon of a non-rotating black hole, but this was not well understood at that time, for example Schwarzschild himself thought it was not physical. The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is a characteristic Radius associated with every Mass. In General relativity, an event horizon is a boundary in Spacetime, an area surrounding a Black hole or a Wormhole, inside which events cannot Johannes Droste, a student of Lorentz, independently gave the same solution for the point mass a few months after Schwarzschild and wrote more extensively about its properties. Hendrik Antoon Lorentz ( July 18, 1853 &ndash February 4, 1928) was a Dutch Physicist who shared the 1902 Nobel

In 1930, the astrophysicist Subrahmanyan Chandrasekhar argued that, according to special relativity, a non-rotating body above 1. Astrophysics is the branch of Astronomy that deals with the Physics of the Universe, including the physical properties ( Luminosity, Padma Vibhushan Subrahmanyan Chandrasekhar, FRS ( Tamil: சுப்பிரமணியன் சந்திரசேகர் English ˌtʃʌndrəˈʃeɪkɑr( Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial 44 solar masses (the Chandrasekhar limit), would collapse since there was nothing known at that time could stop it from doing so. The Chandrasekhar limit limits the mass of bodies made from Electron-degenerate matter, a dense form of matter which consists of nuclei immersed in a gas of Electrons His arguments were opposed by Arthur Eddington, who believed that something would inevitably stop the collapse. Sir Arthur Stanley Eddington, OM (28 December 1882 – 22 November 1944 was an English Astrophysicist of the early 20th century Eddington was partly right: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type But in 1939, Robert Oppenheimer published papers (with various co-authors) which predicted that stars above about three solar masses (the Tolman-Oppenheimer-Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar. The Tolman-Oppenheimer-Volkoff ( TOV) limit is an upper bound to the mass of stars composed of neutron-degenerate matter ( Neutron stars. ^[42]

Oppenheimer and his co-authors used Schwarzschild's system of coordinates (the only coordinates available in 1939), which produced mathematical singularities at the Schwarzschild radius, in other words the equations broke down at the Schwarzschild radius because some of the terms were infinite. In Einstein's theory of General relativity, the Schwarzschild solution (or the Schwarzschild vacuum) describes the Gravitational field outside In Mathematics, a singularity is in general a point at which a given mathematical object is not defined or a point of an exceptional set where it fails to be The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is a characteristic Radius associated with every Mass. Infinity (symbolically represented with ∞) comes from the Latin infinitas or "unboundedness This was interpreted as indicating that the Schwarzschild radius was the boundary of a "bubble" in which time "stopped". For a few years the collapsed stars were known as "frozen stars" because the calculations indicated that 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. But many physicists could not accept the idea of time standing still inside the Schwarzschild radius, and there was little interest in the subject for over 20 years.

In 1958 David Finkelstein broke the deadlock over "stopped time" and introduced the concept of the event horizon by presenting the Eddington-Finkelstein coordinates, which enabled him to show that "The Schwarzschild surface r = 2 m is not a singularity but acts as a perfect unidirectional membrane: causal influences can cross it but only in one direction". David Ritz Finkelstein (born July 19, 1929, New York City) is currently an emeritus professor of Physics at the Georgia Institute of Technology In General relativity, an event horizon is a boundary in Spacetime, an area surrounding a Black hole or a Wormhole, inside which events cannot In General relativity Eddington-Finkelstein coordinates, named for Arthur Stanley Eddington and David Finkelstein, are a pair of Coordinate systems ^[43] Note that at this stage all theories, including Finkelstein's, covered only non-rotating, uncharged black holes.

In 1963 Roy Kerr extended Finkelstein's analysis by presenting the Kerr metric (coordinates) and showing how this made it possible to predict the properties of rotating black holes. Roy Patrick Kerr (born 1934 is a New Zealander Mathematician who is best known for discovering the Kerr vacuum, an exact solution to the In General relativity, the Kerr metric (or Kerr vacuum) describes the geometry of Spacetime around a rotating massive body Black hole#Major features of rotating black holes A rotating black hole is a Black hole that possesses Angular momentum. ^[44] In addition to its theoretical interest, Kerr's work made black holes more believable for astronomers, since black holes are formed from stars and all known stars rotate.

In 1967 astronomers discovered pulsars, and within a few years could show that the known pulsars were rapidly rotating neutron stars. Pulsars are highly magnetized rotating Neutron stars that emit a beam of Electromagnetic radiation in the form of radio waves A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type Until that time, neutron stars were also regarded as just theoretical curiosities. So the discovery of pulsars awakened interest in all types of ultra-dense objects that might be formed by gravitational collapse.

In December 1967 the theoretical physicist John Wheeler coined the expression "black hole" in his public lecture Our Universe: the Known and Unknown, and this mysterious, slightly menacing phrase attracted more attention than the static-sounding "frozen star". John Archibald Wheeler ( July 9, 1911 &ndash April 13, 2008) was an eminent American Theoretical physicist. The phrase was probably coined with the awareness of the Black Hole of Calcutta incident of 1756 in which 146 Europeans were locked up overnight in punishment cell of barracks at Fort William by Siraj ud-Daulah, and all but 23 perished. The Black Hole of Calcutta was a small Dungeon in Fort William where troops of the Nawab of Bengal, Siraj ud-Daulah, held British Fort William is a Fort built in Calcutta on the Eastern banks of the river Hooghly, the major distributary of river Ganges during the Mîrzâ Mohammad Sirâjud Dawla, more popularly known as Siraj ud-Daulah (1733 &ndash July 2, 1757) was the last independent Nawab of ^[45]

In 1970, Stephen Hawking and Roger Penrose proved that black holes are a feature of all solutions to Einstein's equations of gravity, not just of Schwarzschild's, and therefore black holes cannot be avoided in some collapsing objects. Stephen William Hawking CH, CBE, FRS, FRSA (born 8 January 1942 is a British theoretical physicist. Sir Roger Penrose, PhD, OM, FRS (born 8 August 1931) is an English Mathematical physicist and Emeritus ^[46]

Black holes and Earth

Black holes are sometimes listed among the most serious potential threats to Earth and humanity,^[47][48] on the grounds that:

• A naturally-produced black hole could pass through our Solar System.
• Although it is purely hypothetical, a large particle accelerator might produce a micro black hole, and if this escaped it could gradually eat the whole of the Earth. Micro black holes, are the tiny hypothetical Black holes also called quantum mechanical black holes or mini black holes, for which quantum mechanical

Black hole wandering through our Solar System

Stellar-mass black holes travel through the Milky Way just like stars. Consequently, they may collide with the Solar System or another planetary system in the galaxy, although the probability of this happening is very small. Significant gravitational interactions between the Sun and any other star in the Milky Way (including a black hole) are expected to occur approximately once every 10¹⁹ years. The Sun (Sol is the Star at the center of the Solar System. A year (from Old English gēr) is the time between two recurrences of an event related to the Orbit of the Earth around the Sun ^[49] For comparison, the Sun has an age of only 5 × 10⁹ years, and is expected to become a red giant about 5 × 10⁹ years from now, incinerating the surface of the Earth. The Sun (Sol is the Star at the center of the Solar System. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 ^[27] Hence it is extremely unlikely that a black hole will pass through the Solar System before the Sun exterminates life on Earth.

Micro black hole escaping from a particle accelerator

There is a theoretical possibility that a micro black hole might be created inside a particle accelerator. Micro black holes, are the tiny hypothetical Black holes also called quantum mechanical black holes or mini black holes, for which quantum mechanical ^[50] Formation of black holes under these conditions (below the Planck energy) requires non-standard assumptions, such as large extra dimensions. In Physics, the unit of Energy in the system of Natural units known as Planck units is called the Planck energy, denoted by E P In Particle physics, the ADD model, also known as the model with large extra dimensions, is an alternative scenario to explain the weakness of Gravity relative In Physics, Kaluza–Klein theory (or KK theory, for short is a model that seeks to unify the two fundamental forces of Gravitation and

However, many particle collisions that naturally occur as the cosmic rays hit the edge of our atmosphere are often far more energetic than any collisions created by man. For the 1962 Bruce Conner film see Cosmic Ray (film Cosmic rays are energetic particles originating from space that impinge on If micro black holes can be created by current or next-generation particle accelerators, they have probably been created by cosmic rays every day throughout most of Earth's history, i. e. for billions of years, evidently without earth-destroying effects. However, such natural micro black holes would be relativistic relative to earth, and should zip safely through our planet in 1/4 second or less at 99. 99+% c. Collider produced micro black holes would be relatively "at rest" where they could become gravitationally bound, affording repeated opportunity to interact and grow larger, travelling at a tiny fraction of c, if Hawking Radiation is not real. This distinction between nature-made and man-made micro black holes has not yet been addressed in any of the safety studies on potential collider production of micro black holes.

If two protons at the Large Hadron Collider could merge to create a micro black hole, this black hole would be unstable, and would evaporate due to Hawking radiation before it had a chance to propagate. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive Hawking radiation (also known as Bekenstein-Hawking radiation) is a Thermal radiation with a black body spectrum predicted to be emitted by Black holes For a 14 TeV black hole (the center-of-mass energy at the Large Hadron Collider), the Hawking radiation formula indicates that it would evaporate in 10^-100 seconds.

The European Organization for Nuclear Research (CERN) conducted a study assessing the risk of producing dangerous objects such as black holes at the Large Hadron Collider, and concluded that there is "no basis for any conceivable threat. The European Organization for Nuclear Research (Organisation Européenne pour la Recherche Nucléaire known as CERN "^[51] However, due to renewed concerns about both potential negative strangelet production, and LHC micro black holes that are "at rest" compared to natural micro black holes that are relativistic, CERN commissioned another study in 2007, with the results to be published in early 2008. Essentially, the concern is that due to their tiny size, a relativistic micro black hole would barely interact while traversing earth, being very similar to a neutrino in having a low cross-section for interaction, and therefore harmless. Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost Conversely, the relatively slow speed of collider-produced micro black holes and their gravitational binding to earth would allow for repeated opportunity to interact with matter, eventually allowing such micro black hole to grow larger. Those speculative scenarios also require that theoretical Hawking Radiation is not real.

Alternative models

Main article: Nonsingular black hole models

Several alternative models, which behave like a black hole but avoid the singularity, have been proposed. A nonsingular black hole model is a mathematical theory of Black holes that avoids certain theoretical problems with the standard black hole model including information However, most researchers judge these concepts artificial, as they are more complicated but do not give near term observable differences from black holes (see Occam's razor). Occam's razor (sometimes spelled Ockham's razor) is a principle attributed to the 14th-century English Logician and Franciscan Friar, The most prominent alternative theory is the Gravastar. A gravastar is an object hypothesized in Astrophysics as an alternative to the Black hole theory by Pawel Mazur and Emil Mottola

In March 2005, physicist George Chapline at the Lawrence Livermore National Laboratory in California proposed that black holes do not exist, and that objects currently thought to be black holes are actually dark-energy stars. George Frederick Chapline Jr is a Condensed matter Physicist, at Lawrence Livermore National Laboratory. The Lawrence Livermore National Laboratory ( LLNL) in Livermore California is a scientific research laboratory founded by the University of California in 1952 California ( is a US state on the West Coast of the United States, along the Pacific Ocean. A dark-energy star is a hypothetical compact astrophysical object which a minority of physicists feel might constitute an alternative explanation for observations of astronomical Black He draws this conclusion from some quantum mechanical analyses. Although his proposal currently has little support in the physics community, it was widely reported by the media. ^[52][53] A similar theory about the non-existence of black holes was later developed by a group of physicists at Case Western Reserve University in June 2007. Case Western Reserve University is a private research university located in Cleveland Ohio, United States, with some residence halls on the south end of campus ^[54]

Among the alternate models are magnetospheric eternally collapsing objects, clusters of elementary particles^[55] (e. A Magnetospheric Eternally Collapsing Object or MECO is a proposed alternative to a Black hole. In Particle physics, an elementary particle or fundamental particle is a particle not known to have substructure that is it is not known to be made g. , boson stars^[56]), fermion balls,^[57] self-gravitating, degenerate heavy neutrinos^[58] and even clusters of very low mass (~0. Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost 04 solar mass) black holes. ^[55]

More advanced topics

Entropy and Hawking radiation

In 1971, Stephen Hawking showed that the total area of the event horizons of any collection of classical black holes can never decrease, even if they collide and swallow each other; that is merge. Stephen William Hawking CH, CBE, FRS, FRSA (born 8 January 1942 is a British theoretical physicist. ^[59] This is remarkably similar to the Second Law of Thermodynamics, with area playing the role of entropy. In Physics, thermodynamics (from the Greek θερμη therme meaning " Heat " and δυναμις dynamis meaning " In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy As a classical object with zero temperature it was assumed that black holes had zero entropy; if so the second law of thermodynamics would be violated by an entropy-laden material entering the black hole, resulting in a decrease of the total entropy of the universe. Therefore, Jacob Bekenstein proposed that a black hole should have an entropy, and that it should be proportional to its horizon area. Jacob David Bekenstein (born May 1, 1947) is a Physicist who has contributed to the foundation of Black hole thermodynamics and to other aspects Since black holes do not classically emit radiation, the thermodynamic viewpoint seemed simply an analogy, since zero temperature implies infinite changes in entropy with any addition of heat, which implies infinite entropy. However, in 1974, Hawking applied quantum field theory to the curved spacetime around the event horizon and discovered that black holes emit Hawking radiation, a form of thermal radiation, allied to the Unruh effect, which implied they had a positive temperature. In quantum field theory (QFT the forces between particles are mediated by other particles Hawking radiation (also known as Bekenstein-Hawking radiation) is a Thermal radiation with a black body spectrum predicted to be emitted by Black holes Thermal radiation is Electromagnetic radiation emitted from the surface of an object which is due to the object's Temperature. The Unruh effect, discovered in 1976 by Bill Unruh of the University of British Columbia, is the prediction that an accelerating observer will observe This strengthened the analogy being drawn between black hole dynamics and thermodynamics: using the first law of black hole mechanics, it follows that the entropy of a non-rotating black hole is one quarter of the area of the horizon. In Physics, black hole thermodynamics is the area of study that seeks to reconcile the Laws of thermodynamics with the existence of Black hole Event This is a universal result and can be extended to apply to cosmological horizons such as in de Sitter space. In Mathematics and Physics, n -dimensional De Sitter space, denoted dS_n is the Lorentzian analog of an ''n''-sphere (with its It was later suggested that black holes are maximum-entropy objects, meaning that the maximum possible entropy of a region of space is the entropy of the largest black hole that can fit into it. This led to the holographic principle. The holographic principle is a physical property of Quantum gravity theories proposed by Gerard 't Hooft and Leonard Susskind, which resolves the

The Hawking radiation reflects a characteristic temperature of the black hole, which can be calculated from its entropy. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature The more its temperature falls, the more massive a black hole becomes: the more energy a black hole absorbs, the colder it gets. A black hole with roughly the mass of the planet Mercury would have a temperature in equilibrium with the cosmic microwave background radiation (about 2. To help compare different orders of magnitude, the following list describes various Mass levels between 10&minus36&thinsp kg and 1053&thinspkg 73 K). More massive than this, a black hole will be colder than the background radiation, and it will gain energy from the background faster than it gives energy up through Hawking radiation, becoming even colder still. However, for a less massive black hole the effect implies that the mass of the black hole will slowly evaporate with time, with the black hole becoming hotter and hotter as it does so. Although these effects are negligible for black holes massive enough to have been formed astronomically, they would rapidly become significant for hypothetical smaller black holes, where quantum-mechanical effects dominate. Micro black holes, are the tiny hypothetical Black holes also called quantum mechanical black holes or mini black holes, for which quantum mechanical Indeed, small black holes are predicted to undergo runaway evaporation and eventually vanish in a burst of radiation.

If ultra-high-energy collisions of particles in a particle accelerator can create microscopic black holes, it is expected that all types of particles will be emitted by black hole evaporation, providing key evidence for any grand unified theory. Grand Unification, grand unified theory, or GUT refers to any of several very similar unified field theories or models in Physics that Above are the high energy particles produced in a gold ion collision on the RHIC. The Relativistic Heavy Ion Collider (RHIC pronounced like " Rick " ˈrɪk is a heavy- Ion Collider located at and operated by Brookhaven

Although general relativity can be used to perform a semi-classical calculation of black hole entropy, this situation is theoretically unsatisfying. In statistical mechanics, entropy is understood as counting the number of microscopic configurations of a system which have the same macroscopic qualities(such as mass, charge, pressure, etc. Statistical mechanics is the application of Probability theory, which includes mathematical tools for dealing with large populations to the field of Mechanics Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object In Physics, a charge may refer to one of many different quantities such as the Electric charge in Electromagnetism or the Color charge in Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface ). But without a satisfactory theory of quantum gravity, one cannot perform such a computation for black holes. Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature Some promise has been shown by string theory, however. String theory is a still-developing scientific approach to Theoretical physics, whose original building blocks are one-dimensional extended objects called strings There one posits that the microscopic degrees of freedom of the black hole are D-branes. In String theory, D-branes are a class of extended objects upon which open strings can end with Dirichlet boundary conditions after which they are named By counting the states of D-branes with given charges and energy, the entropy for certain supersymmetric black holes has been reproduced. In Particle physics, supersymmetry (often abbreviated SUSY) is a Symmetry that relates elementary particles of one spin to another particle that Extending the region of validity of these calculations is an ongoing area of research.

Black hole unitarity

An open question in fundamental physics is the so-called information loss paradox, or black hole unitarity paradox. The black hole information paradox results from the combination of Quantum mechanics and General relativity. Classically, the laws of physics are the same run forward or in reverse. That is, if the position and velocity of every particle in the universe were measured, we could (disregarding chaos) work backwards to discover the history of the universe arbitrarily far in the past. In Mathematics, chaos theory describes the behavior of certain dynamical systems – that is systems whose state evolves with time – that may exhibit dynamics that In quantum mechanics, this corresponds to a vital property called unitarity which has to do with the conservation of probability. In Functional analysis, a branch of Mathematics, a unitary operator is a Bounded linear operator U    H  →  ^[60]

Black holes, however, might violate this rule. The position under classical general relativity is subtle but straightforward: because of the classical no hair theorem, we can never determine what went into the black hole. The no-hair theorem in Astrophysics postulates that all Black hole solutions of the Einstein-Maxwell equations of Gravitation and Electromagnetism However, as seen from the outside, information is never actually destroyed, as matter falling into the black hole takes an infinite time to reach the event horizon.

Ideas about quantum gravity, on the other hand, suggest that there can only be a limited finite entropy (i. In Physics, the Bekenstein bound is a conjectured limit on the Entropy S or Information that can be contained within a region of space containing a known e. a maximum finite amount of information) associated with the space near the horizon; but the change in the entropy of the horizon plus the entropy of the Hawking radiation is always sufficient to take up all of the entropy of matter and energy falling into the black hole.

Many physicists are concerned however that this is still not sufficiently well understood. In particular, at a quantum level, is the quantum state of the Hawking radiation uniquely determined by the history of what has fallen into the black hole; and is the history of what has fallen into the black hole uniquely determined by the quantum state of the black hole and the radiation? This is what determinism, and unitarity, would require.

For a long time Stephen Hawking had opposed such ideas, holding to his original 1975 position that the Hawking radiation is entirely thermal and therefore entirely random, containing none of the information held in material the hole has swallowed in the past; this information he reasoned had been lost. Stephen William Hawking CH, CBE, FRS, FRSA (born 8 January 1942 is a British theoretical physicist. However, on 21 July 2004 he presented a new argument, reversing his previous position. Events 356 BC - Herostratus sets fire to the Temple of Artemis in Ephesus, one of the Seven Wonders of the World "MMIV" redirects here For the Modest Mouse album see " Baron von Bullshit Rides Again " ^[61] On this new calculation, the entropy (and hence information) associated with the black hole escapes in the Hawking radiation itself, although making sense of it, even in principle, is still difficult until the black hole completes its evaporation; until then it is impossible to relate in a 1:1 way the information in the Hawking radiation (embodied in its detailed internal correlations) to the initial state of the system. Once the black hole evaporates completely, then such an identification can be made, and unitarity is preserved.

By the time Hawking completed his calculation, it was already very clear from the AdS/CFT correspondence that black holes decay in a unitary way. This is because the fireballs in gauge theories, which are analogous to Hawking radiation are unquestionably unitary. Hawking's new calculation have not really been evaluated by the specialist scientific community, because the methods he uses are unfamiliar and of dubious consistency; but Hawking himself found it sufficiently convincing to pay out on a bet he had made in 1997 with Caltech physicist John Preskill, to considerable media interest. In 1997 the physics theorists Kip Thorne, Stephen Hawking and John Preskill made a public bet on the outcome of the Black hole John Phillip Preskill (born 19 January, 1953) is an American Theoretical physicist and a Professor at the California Institute

References

Further reading

Popular reading

• Ferguson, Kitty (1991). Black Holes in Space-Time. Watts Franklin. ISBN 0-531-12524-6.  
• Hawking, Stephen (1998). A Brief History of Time. A Brief History of Time is a Popular science Book written by Stephen Hawking and first published by the Bantam Dell Publishing Group Bantam Books, Inc. ISBN 0-553-38016-8.  
• Melia, Fulvio (2003). Fulvio Melia (born 2 August 1956) is an Italian - American Physicist / Astrophysicist and Author. The Black Hole at the Center of Our Galaxy. Princeton U Press. ISBN 978-0-691-09505-9.  
• Melia, Fulvio (2003). Fulvio Melia (born 2 August 1956) is an Italian - American Physicist / Astrophysicist and Author. The Edge of Infinity. Supermassive Black Holes in the Universe. Cambridge U Press. ISBN 978-0-521-81405-8.  
• Pickover, Clifford (1998). Black Holes: A Traveler's Guide. Wiley, John & Sons, Inc. ISBN 0-471-19704-1.  
• Thorne, Kip S. (1994). Black Holes and Time Warps. Black Holes and Time Warps Einstein's Outrageous Legacy (1994 is a Popular science book by Kip Thorne. Norton, W. W. & Company, Inc. ISBN 0-393-31276-3.  

University textbooks and monographs

• Carter, B. (1973). Black hole equilibrium states, in Black Holes, eds. DeWitt B. S. and DeWitt C.
• Chandrasekhar, Subrahmanyan (1999). Mathematical Theory of Black Holes. Oxford University Press. ISBN 0-19-850370-9.  
• Frolov, V. P. and Novikov, I. D. (1998), Black hole physics.
• Hawking, S. W. and Ellis, G. F. R. (1973), The large-scale structure of space-time, Cambridge University Press.
• Melia, Fulvio (2007). Fulvio Melia (born 2 August 1956) is an Italian - American Physicist / Astrophysicist and Author. The Galactic Supermassive Black Hole. Princeton U Press. ISBN 978-0-691-13129-0.  
• Taylor, Edwin F. ; Wheeler, John Archibald (2000). Exploring Black Holes. Addison Wesley Longman. ISBN 0-201-38423-X.  
• Thorne, Kip S. ; Misner, Charles; Wheeler, John (1973). Gravitation. W. H. Freeman and Company. ISBN 0-7167-0344-0.  
• Wald, Robert M. (1992). Space, Time, and Gravity: The Theory of the Big Bang and Black Holes. University of Chicago Press. ISBN 0-226-87029-4.  

Research papers

• Hawking, S. W. (July 2005), Information Loss in Black Holes, arxiv:hep-th/0507171. Stephen Hawking's purported solution to the black hole unitarity paradox, first reported at a conference in July 2004. In Functional analysis, a branch of Mathematics, a unitary operator is a Bounded linear operator U    H  → 
• Ghez, A.M. et al. Andrea Mia Ghez (born 1965 is an Astronomer and professor in the Department of Physics and Astronomy at UCLA. Stellar orbits around the Galactic Center black hole, Astrophysics J. 620 (2005). arXiv:astro-ph/0306130 More accurate mass and position for the black hole at the centre of the Milky Way.
• Hughes, S. A. Trust but verify: the case for astrophysical black holes, arXiv:hep-ph/0511217. Lecture notes from 2005 SLAC Summer Institute. The Stanford Linear Accelerator Center ( SLAC) is a United States Department of Energy National Laboratory operated by Stanford University under

External links

• Yale University Video Lecture: Introduction to Black Holes at Google Video
• Black Holes: Gravity's Relentless Pull - Award-winning interactive multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute
• FAQ on black holes
• Schwarzschild Geometry on Andrew Hamilton’s website
• Tufts University: Student Project (Great Kid's Section)
• Movie of Black Hole Candidate from Max Planck Institute
• UT Brownsville Group Simulates Spinning Black-Hole Binaries
• Black Hole Research News on ScienceDaily
• Scientific American Magazine (July 2003 Issue) The Galactic Odd Couple - giant black holes and stellar baby booms
• Scientific American Magazine (May 2005 Issue) Quantum Black Holes
• SPACE.com All About Black Holes - News, Features and Interesting Original Videos
• Black Holes Intro - Introduction to Black Holes
• Advanced Mathematics of Black Hole Evaporation
• HowStuffWorks: How Black Holes Work - Easy to consume guide to Black Holes
• Ted Bunn's Black Holes FAQ explains in simple language some other consequences of the way in which black holes bend space-time.