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4,800 | On 11 February 2016, the LIGO Scientific Collaboration and the Virgo collaboration announced the first direct detection of gravitational waves, representing the first observation of a black hole merger. On 10 April 2019, the first direct image of a black hole and its vicinity was published, following observations made ... | https://en.wikipedia.org/wiki?curid=4650 |
4,801 | The idea of a body so big that even light could not escape was briefly proposed by English astronomical pioneer and clergyman John Michell in a letter published in November 1784. Michell's simplistic calculations assumed such a body might have the same density as the Sun, and concluded that one would form when a star's... | https://en.wikipedia.org/wiki?curid=4650 |
4,802 | Modern physics discredits Michell's notion of a light ray shooting directly from the surface of a supermassive star, being slowed down by the star's gravity, stopping, and then free-falling back to the star's surface. | https://en.wikipedia.org/wiki?curid=4650 |
4,803 | In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion. Only a few months later, Karl Schwarzschild found a solution to the Einstein field equations that describes the gravitational field of a point mass and a spherical mass. A few months aft... | https://en.wikipedia.org/wiki?curid=4650 |
4,804 | In 1931, Subrahmanyan Chandrasekhar calculated, using special relativity, that a non-rotating body of electron-degenerate matter above a certain limiting mass (now called the Chandrasekhar limit at ) has no stable solutions. His arguments were opposed by many of his contemporaries like Eddington and Lev Landau, who arg... | https://en.wikipedia.org/wiki?curid=4650 |
4,805 | Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were cal... | https://en.wikipedia.org/wiki?curid=4650 |
4,806 | In 1958, David Finkelstein identified the Schwarzschild surface as an event horizon, "a perfect unidirectional membrane: causal influences can cross it in only one direction". This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. Finkelstein's sol... | https://en.wikipedia.org/wiki?curid=4650 |
4,807 | These results came at the beginning of the golden age of general relativity, which was marked by general relativity and black holes becoming mainstream subjects of research. This process was helped by the discovery of pulsars by Jocelyn Bell Burnell in 1967, which, by 1969, were shown to be rapidly rotating neutron sta... | https://en.wikipedia.org/wiki?curid=4650 |
4,808 | In this period more general black hole solutions were found. In 1963, Roy Kerr found the exact solution for a rotating black hole. Two years later, Ezra Newman found the axisymmetric solution for a black hole that is both rotating and electrically charged. Through the work of Werner Israel, Brandon Carter, and David Ro... | https://en.wikipedia.org/wiki?curid=4650 |
4,809 | At first, it was suspected that the strange features of the black hole solutions were pathological artifacts from the symmetry conditions imposed, and that the singularities would not appear in generic situations. This view was held in particular by Vladimir Belinsky, Isaak Khalatnikov, and Evgeny Lifshitz, who tried t... | https://en.wikipedia.org/wiki?curid=4650 |
4,810 | Work by James Bardeen, Jacob Bekenstein, Carter, and Hawking in the early 1970s led to the formulation of black hole thermodynamics. These laws describe the behaviour of a black hole in close analogy to the laws of thermodynamics by relating mass to energy, area to entropy, and surface gravity to temperature. The analo... | https://en.wikipedia.org/wiki?curid=4650 |
4,811 | John Michell used the term "dark star" in a November 1783 letter to Henry Cavendish, and in the early 20th century, physicists used the term "gravitationally collapsed object". Science writer Marcia Bartusiak traces the term "black hole" to physicist Robert H. Dicke, who in the early 1960s reportedly compared the pheno... | https://en.wikipedia.org/wiki?curid=4650 |
4,812 | The term "black hole" was used in print by "Life" and "Science News" magazines in 1963, and by science journalist Ann Ewing in her article Black Holes' in Space", dated 18 January 1964, which was a report on a meeting of the American Association for the Advancement of Science held in Cleveland, Ohio. | https://en.wikipedia.org/wiki?curid=4650 |
4,813 | In December 1967, a student reportedly suggested the phrase "black hole" at a lecture by John Wheeler; Wheeler adopted the term for its brevity and "advertising value", and it quickly caught on, leading some to credit Wheeler with coining the phrase. | https://en.wikipedia.org/wiki?curid=4650 |
4,814 | The no-hair theorem postulates that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, electric charge, and angular momentum; the black hole is otherwise featureless. If the conjecture is true, any two black holes that share the same values for these... | https://en.wikipedia.org/wiki?curid=4650 |
4,815 | These properties are special because they are visible from outside a black hole. For example, a charged black hole repels other like charges just like any other charged object. Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law (through the AD... | https://en.wikipedia.org/wiki?curid=4650 |
4,816 | When an object falls into a black hole, any information about the shape of the object or distribution of charge on it is evenly distributed along the horizon of the black hole, and is lost to outside observers. The behavior of the horizon in this situation is a dissipative system that is closely analogous to that of a ... | https://en.wikipedia.org/wiki?curid=4650 |
4,817 | The simplest static black holes have mass but neither electric charge nor angular momentum. These black holes are often referred to as Schwarzschild black holes after Karl Schwarzschild who discovered this solution in 1916. According to Birkhoff's theorem, it is the only vacuum solution that is spherically symmetric. T... | https://en.wikipedia.org/wiki?curid=4650 |
4,818 | Solutions describing more general black holes also exist. Non-rotating charged black holes are described by the Reissner–Nordström metric, while the Kerr metric describes a non-charged rotating black hole. The most general stationary black hole solution known is the Kerr–Newman metric, which describes a black hole with... | https://en.wikipedia.org/wiki?curid=4650 |
4,819 | While the mass of a black hole can take any positive value, the charge and angular momentum are constrained by the mass. The total electric charge "Q" and the total angular momentum "J" are expected to satisfy the inequality | https://en.wikipedia.org/wiki?curid=4650 |
4,820 | for a black hole of mass "M". Black holes with the minimum possible mass satisfying this inequality are called extremal. Solutions of Einstein's equations that violate this inequality exist, but they do not possess an event horizon. These solutions have so-called naked singularities that can be observed from the outsid... | https://en.wikipedia.org/wiki?curid=4650 |
4,821 | Due to the relatively large strength of the electromagnetic force, black holes forming from the collapse of stars are expected to retain the nearly neutral charge of the star. Rotation, however, is expected to be a universal feature of compact astrophysical objects. The black-hole candidate binary X-ray source GRS 1915... | https://en.wikipedia.org/wiki?curid=4650 |
4,822 | Black holes are commonly classified according to their mass, independent of angular momentum, "J". The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is proportional to the mass, "M", through | https://en.wikipedia.org/wiki?curid=4650 |
4,823 | where "r" is the Schwarzschild radius and is the mass of the Sun. For a black hole with nonzero spin and/or electric charge, the radius is smaller, until an extremal black hole could have an event horizon close to | https://en.wikipedia.org/wiki?curid=4650 |
4,824 | The defining feature of a black hole is the appearance of an event horizon—a boundary in spacetime through which matter and light can pass only inward towards the mass of the black hole. Nothing, not even light, can escape from inside the event horizon. The event horizon is referred to as such because if an event occur... | https://en.wikipedia.org/wiki?curid=4650 |
4,825 | As predicted by general relativity, the presence of a mass deforms spacetime in such a way that the paths taken by particles bend towards the mass. At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole. | https://en.wikipedia.org/wiki?curid=4650 |
4,826 | To a distant observer, clocks near a black hole would appear to tick more slowly than those farther away from the black hole. Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow as it approaches the event horizon, taking an infinite time to reach it. At the same... | https://en.wikipedia.org/wiki?curid=4650 |
4,827 | On the other hand, indestructible observers falling into a black hole do not notice any of these effects as they cross the event horizon. According to their own clocks, which appear to them to tick normally, they cross the event horizon after a finite time without noting any singular behaviour; in classical general rel... | https://en.wikipedia.org/wiki?curid=4650 |
4,828 | The topology of the event horizon of a black hole at equilibrium is always spherical. For non-rotating (static) black holes the geometry of the event horizon is precisely spherical, while for rotating black holes the event horizon is oblate. | https://en.wikipedia.org/wiki?curid=4650 |
4,829 | At the centre of a black hole, as described by general relativity, may lie a gravitational singularity, a region where the spacetime curvature becomes infinite. For a non-rotating black hole, this region takes the shape of a single point; for a rotating black hole it is smeared out to form a ring singularity that lies ... | https://en.wikipedia.org/wiki?curid=4650 |
4,830 | Observers falling into a Schwarzschild black hole (i.e., non-rotating and not charged) cannot avoid being carried into the singularity once they cross the event horizon. They can prolong the experience by accelerating away to slow their descent, but only up to a limit. When they reach the singularity, they are crushed ... | https://en.wikipedia.org/wiki?curid=4650 |
4,831 | In the case of a charged (Reissner–Nordström) or rotating (Kerr) black hole, it is possible to avoid the singularity. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a wormhole. The possibility of travel... | https://en.wikipedia.org/wiki?curid=4650 |
4,832 | The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory. This breakdown, however, is expected; it occurs in a situation where quantum effects should describe these actions, due to the extremely high density and therefore particle interactions. To date, it has... | https://en.wikipedia.org/wiki?curid=4650 |
4,833 | The photon sphere is a spherical boundary of zero thickness in which photons that move on tangents to that sphere would be trapped in a circular orbit about the black hole. For non-rotating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. Their orbits would be dynamically unstable, hence ... | https://en.wikipedia.org/wiki?curid=4650 |
4,834 | While light can still escape from the photon sphere, any light that crosses the photon sphere on an inbound trajectory will be captured by the black hole. Hence any light that reaches an outside observer from the photon sphere must have been emitted by objects between the photon sphere and the event horizon. For a Kerr... | https://en.wikipedia.org/wiki?curid=4650 |
4,835 | Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere. This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it. Any objec... | https://en.wikipedia.org/wiki?curid=4650 |
4,836 | The ergosphere of a black hole is a volume bounded by the black hole's event horizon and the "ergosurface", which coincides with the event horizon at the poles but is at a much greater distance around the equator. | https://en.wikipedia.org/wiki?curid=4650 |
4,837 | Objects and radiation can escape normally from the ergosphere. Through the Penrose process, objects can emerge from the ergosphere with more energy than they entered with. The extra energy is taken from the rotational energy of the black hole. Thereby the rotation of the black hole slows down. A variation of the Penros... | https://en.wikipedia.org/wiki?curid=4650 |
4,838 | In Newtonian gravity, test particles can stably orbit at arbitrary distances from a central object. In general relativity, however, there exists an innermost stable circular orbit (often called the ISCO), inside of which, any infinitesimal perturbations to a circular orbit will lead to inspiral into the black hole. The... | https://en.wikipedia.org/wiki?curid=4650 |
4,839 | and decreases with increasing black hole spin for particles orbiting in the same direction as the spin. | https://en.wikipedia.org/wiki?curid=4650 |
4,840 | Given the bizarre character of black holes, it was long questioned whether such objects could actually exist in nature or whether they were merely pathological solutions to Einstein's equations. Einstein himself wrongly thought black holes would not form, because he held that the angular momentum of collapsing particle... | https://en.wikipedia.org/wiki?curid=4650 |
4,841 | Penrose demonstrated that once an event horizon forms, general relativity without quantum mechanics requires that a singularity will form within. Shortly afterwards, Hawking showed that many cosmological solutions that describe the Big Bang have singularities without scalar fields or other exotic matter (see "Penrose–H... | https://en.wikipedia.org/wiki?curid=4650 |
4,842 | Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature through stellar nucleosynthesis, or because a star that would have been stable receives extra ma... | https://en.wikipedia.org/wiki?curid=4650 |
4,843 | The collapse may be stopped by the degeneracy pressure of the star's constituents, allowing the condensation of matter into an exotic denser state. The result is one of the various types of compact star. Which type forms depends on the mass of the remnant of the original star left if the outer layers have been blown aw... | https://en.wikipedia.org/wiki?curid=4650 |
4,844 | If the mass of the remnant exceeds about (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. No known mechanism (except possibl... | https://en.wikipedia.org/wiki?curid=4650 |
4,845 | The gravitational collapse of heavy stars is assumed to be responsible for the formation of stellar mass black holes. Star formation in the early universe may have resulted in very massive stars, which upon their collapse would have produced black holes of up to . These black holes could be the seeds of the supermassiv... | https://en.wikipedia.org/wiki?curid=4650 |
4,846 | While most of the energy released during gravitational collapse is emitted very quickly, an outside observer does not actually see the end of this process. Even though the collapse takes a finite amount of time from the reference frame of infalling matter, a distant observer would see the infalling material slow and ha... | https://en.wikipedia.org/wiki?curid=4650 |
4,847 | Gravitational collapse requires great density. In the current epoch of the universe these high densities are found only in stars, but in the early universe shortly after the Big Bang densities were much greater, possibly allowing for the creation of black holes. High density alone is not enough to allow black hole form... | https://en.wikipedia.org/wiki?curid=4650 |
4,848 | Despite the early universe being extremely dense—far denser than is usually required to form a black hole—it did not re-collapse into a black hole during the Big Bang. Models for the gravitational collapse of objects of relatively constant size, such as stars, do not necessarily apply in the same way to rapidly expandi... | https://en.wikipedia.org/wiki?curid=4650 |
4,849 | Gravitational collapse is not the only process that could create black holes. In principle, black holes could be formed in high-energy collisions that achieve sufficient density. As of 2002, no such events have been detected, either directly or indirectly as a deficiency of the mass balance in particle accelerator expe... | https://en.wikipedia.org/wiki?curid=4650 |
4,850 | Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb gas and interstellar dust from its surroundings. This growth process is one possible way through which some supermassive black holes may have been formed, although the formation of supermassive b... | https://en.wikipedia.org/wiki?curid=4650 |
4,851 | In 1974, Hawking predicted that black holes are not entirely black but emit small amounts of thermal radiation at a temperature ℏ"c"/(8π"GM""k"); this effect has become known as Hawking radiation. By applying quantum field theory to a static black hole background, he determined that a black hole should emit particles t... | https://en.wikipedia.org/wiki?curid=4650 |
4,852 | A stellar black hole of has a Hawking temperature of 62 nanokelvins. This is far less than the 2.7 K temperature of the cosmic microwave background radiation. Stellar-mass or larger black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation and thus will grow instead of ... | https://en.wikipedia.org/wiki?curid=4650 |
4,853 | If a black hole is very small, the radiation effects are expected to become very strong. A black hole with the mass of a car would have a diameter of about 10 m and take a nanosecond to evaporate, during which time it would briefly have a luminosity of more than 200 times that of the Sun. Lower-mass black holes are exp... | https://en.wikipedia.org/wiki?curid=4650 |
4,854 | The Hawking radiation for an astrophysical black hole is predicted to be very weak and would thus be exceedingly difficult to detect from Earth. A possible exception, however, is the burst of gamma rays emitted in the last stage of the evaporation of primordial black holes. Searches for such flashes have proven unsucce... | https://en.wikipedia.org/wiki?curid=4650 |
4,855 | If black holes evaporate via Hawking radiation, a solar mass black hole will evaporate (beginning once the temperature of the cosmic microwave background drops below that of the black hole) over a period of 10 years. A supermassive black hole with a mass of will evaporate in around 2×10 years. Some monster black holes ... | https://en.wikipedia.org/wiki?curid=4650 |
4,856 | By nature, black holes do not themselves emit any electromagnetic radiation other than the hypothetical Hawking radiation, so astrophysicists searching for black holes must generally rely on indirect observations. For example, a black hole's existence can sometimes be inferred by observing its gravitational influence o... | https://en.wikipedia.org/wiki?curid=4650 |
4,857 | On 10 April 2019, an image was released of a black hole, which is seen magnified because the light paths near the event horizon are highly bent. The dark shadow in the middle results from light paths absorbed by the black hole. The image is in false color, as the detected light halo in this image is not in the visible ... | https://en.wikipedia.org/wiki?curid=4650 |
4,858 | The Event Horizon Telescope (EHT) is an active program that directly observes the immediate environment of black holes' event horizons, such as the black hole at the centre of the Milky Way. In April 2017, EHT began observing the black hole at the centre of Messier 87. "In all, eight radio observatories on six mountain... | https://en.wikipedia.org/wiki?curid=4650 |
4,859 | On 12 May 2022, the EHT released the first image of Sagittarius A*, the supermassive black hole at the centre of the Milky Way galaxy. The published image displayed the same ring-like structure and circular shadow as seen in the M87* black hole, and the image was created using the same techniques as for the M87 black h... | https://en.wikipedia.org/wiki?curid=4650 |
4,860 | The brightening of this material in the 'bottom' half of the processed EHT image is thought to be caused by Doppler beaming, whereby material approaching the viewer at relativistic speeds is perceived as brighter than material moving away. In the case of a black hole, this phenomenon implies that the visible material i... | https://en.wikipedia.org/wiki?curid=4650 |
4,861 | In 2015, the EHT detected magnetic fields just outside the event horizon of Sagittarius A* and even discerned some of their properties. The field lines that pass through the accretion disc were a complex mixture of ordered and tangled. Theoretical studies of black holes had predicted the existence of magnetic fields. | https://en.wikipedia.org/wiki?curid=4650 |
4,862 | On 14 September 2015, the LIGO gravitational wave observatory made the first-ever successful direct observation of gravitational waves. The signal was consistent with theoretical predictions for the gravitational waves produced by the merger of two black holes: one with about 36 solar masses, and the other around 29 so... | https://en.wikipedia.org/wiki?curid=4650 |
4,863 | More importantly, the signal observed by LIGO also included the start of the post-merger ringdown, the signal produced as the newly formed compact object settles down to a stationary state. Arguably, the ringdown is the most direct way of observing a black hole. From the LIGO signal, it is possible to extract the frequ... | https://en.wikipedia.org/wiki?curid=4650 |
4,864 | The observation also provides the first observational evidence for the existence of stellar-mass black hole binaries. Furthermore, it is the first observational evidence of stellar-mass black holes weighing 25 solar masses or more. | https://en.wikipedia.org/wiki?curid=4650 |
4,865 | The proper motions of stars near the centre of our own Milky Way provide strong observational evidence that these stars are orbiting a supermassive black hole. Since 1995, astronomers have tracked the motions of 90 stars orbiting an invisible object coincident with the radio source Sagittarius A*. By fitting their moti... | https://en.wikipedia.org/wiki?curid=4650 |
4,866 | Due to conservation of angular momentum, gas falling into the gravitational well created by a massive object will typically form a disk-like structure around the object. Artists' impressions such as the accompanying representation of a black hole with corona commonly depict the black hole as if it were a flat-space bod... | https://en.wikipedia.org/wiki?curid=4650 |
4,867 | Within such a disk, friction would cause angular momentum to be transported outward, allowing matter to fall farther inward, thus releasing potential energy and increasing the temperature of the gas. | https://en.wikipedia.org/wiki?curid=4650 |
4,868 | When the accreting object is a neutron star or a black hole, the gas in the inner accretion disk orbits at very high speeds because of its proximity to the compact object. The resulting friction is so significant that it heats the inner disk to temperatures at which it emits vast amounts of electromagnetic radiation (m... | https://en.wikipedia.org/wiki?curid=4650 |
4,869 | As such, many of the universe's more energetic phenomena have been attributed to the accretion of matter on black holes. In particular, active galactic nuclei and quasars are believed to be the accretion disks of supermassive black holes. Similarly, X-ray binaries are generally accepted to be binary star systems in whi... | https://en.wikipedia.org/wiki?curid=4650 |
4,870 | In November 2011 the first direct observation of a quasar accretion disk around a supermassive black hole was reported. | https://en.wikipedia.org/wiki?curid=4650 |
4,871 | X-ray binaries are binary star systems that emit a majority of their radiation in the X-ray part of the spectrum. These X-ray emissions are generally thought to result when one of the stars (compact object) accretes matter from another (regular) star. The presence of an ordinary star in such a system provides an opport... | https://en.wikipedia.org/wiki?curid=4650 |
4,872 | If such a system emits signals that can be directly traced back to the compact object, it cannot be a black hole. The absence of such a signal does, however, not exclude the possibility that the compact object is a neutron star. By studying the companion star it is often possible to obtain the orbital parameters of the... | https://en.wikipedia.org/wiki?curid=4650 |
4,873 | The first strong candidate for a black hole, Cygnus X-1, was discovered in this way by Charles Thomas Bolton, Louise Webster, and Paul Murdin in 1972. Some doubt, however, remained due to the uncertainties that result from the companion star being much heavier than the candidate black hole. Currently, better candidates... | https://en.wikipedia.org/wiki?curid=4650 |
4,874 | The X-ray emissions from accretion disks sometimes flicker at certain frequencies. These signals are called quasi-periodic oscillations and are thought to be caused by material moving along the inner edge of the accretion disk (the innermost stable circular orbit). As such their frequency is linked to the mass of the c... | https://en.wikipedia.org/wiki?curid=4650 |
4,875 | Astronomers use the term "active galaxy" to describe galaxies with unusual characteristics, such as unusual spectral line emission and very strong radio emission. Theoretical and observational studies have shown that the activity in these active galactic nuclei (AGN) may be explained by the presence of supermassive bla... | https://en.wikipedia.org/wiki?curid=4650 |
4,876 | 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 incl... | https://en.wikipedia.org/wiki?curid=4650 |
4,877 | It is now widely accepted that the centre of nearly every galaxy, not just active ones, contains a supermassive black hole. The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M–sigma relation, strongly suggests a connection between the ... | https://en.wikipedia.org/wiki?curid=4650 |
4,878 | Another way the black hole nature of an object may be tested is through observation of effects caused by a strong gravitational field in their vicinity. One such effect is gravitational lensing: The deformation of spacetime around a massive object causes light rays to be deflected, such as light passing through an opti... | https://en.wikipedia.org/wiki?curid=4650 |
4,879 | Another possibility for observing gravitational lensing by a black hole would be to observe stars orbiting the black hole. There are several candidates for such an observation in orbit around Sagittarius A*. | https://en.wikipedia.org/wiki?curid=4650 |
4,880 | The evidence for stellar black holes strongly relies on the existence of an upper limit for the mass of a neutron star. The size of this limit heavily depends on the assumptions made about the properties of dense matter. New exotic phases of matter could push up this bound. A phase of free quarks at high density might ... | https://en.wikipedia.org/wiki?curid=4650 |
4,881 | Since the average density of a black hole inside its Schwarzschild radius is inversely proportional to the square of its mass, supermassive black holes are much less dense than stellar black holes (the average density of a black hole is comparable to that of water). Consequently, the physics of matter forming a superma... | https://en.wikipedia.org/wiki?curid=4650 |
4,882 | The evidence for the existence of stellar and supermassive black holes implies that in order for black holes to not form, general relativity must fail as a theory of gravity, perhaps due to the onset of quantum mechanical corrections. A much anticipated feature of a theory of quantum gravity is that it will not feature... | https://en.wikipedia.org/wiki?curid=4650 |
4,883 | A few theoretical objects have been conjectured to match observations of astronomical black hole candidates identically or near-identically, but which function via a different mechanism. These include the gravastar, the black star, and the dark-energy star. | https://en.wikipedia.org/wiki?curid=4650 |
4,884 | In 1971, Hawking showed under general conditions that the total area of the event horizons of any collection of classical black holes can never decrease, even if they collide and merge. This result, now known as the second law of black hole mechanics, is remarkably similar to the second law of thermodynamics, which sta... | https://en.wikipedia.org/wiki?curid=4650 |
4,885 | The link with the laws of thermodynamics was further strengthened by Hawking's discovery in 1974 that quantum field theory predicts that a black hole radiates blackbody radiation at a constant temperature. This seemingly causes a violation of the second law of black hole mechanics, since the radiation will carry away e... | https://en.wikipedia.org/wiki?curid=4650 |
4,886 | One puzzling feature is that the entropy of a black hole scales with its area rather than with its volume, since entropy is normally an extensive quantity that scales linearly with the volume of the system. This odd property led Gerard 't Hooft and Leonard Susskind to propose the holographic principle, which suggests t... | https://en.wikipedia.org/wiki?curid=4650 |
4,887 | 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 that have the same macroscopic qualities (such as mass, ch... | https://en.wikipedia.org/wiki?curid=4650 |
4,888 | Another promising approach is constituted by treating gravity as an effective field theory. One first computes the quantum gravitational corrections to the radius of the event horizon of the black hole, then integrates over it to find the quantum gravitational corrections to the entropy as given by the Wald formula. Th... | https://en.wikipedia.org/wiki?curid=4650 |
4,889 | Because a black hole has only a few internal parameters, most of the information about the matter that went into forming the black hole is lost. Regardless of the type of matter which goes into a black hole, it appears that only information concerning the total mass, charge, and angular momentum are conserved. As long ... | https://en.wikipedia.org/wiki?curid=4650 |
4,890 | The question whether information is truly lost in black holes (the black hole information paradox) has divided the theoretical physics community (see Thorne–Hawking–Preskill bet). In quantum mechanics, loss of information corresponds to the violation of a property called unitarity, and it has been argued that loss of u... | https://en.wikipedia.org/wiki?curid=4650 |
4,891 | One attempt to resolve the black hole information paradox is known as black hole complementarity. In 2012, the "firewall paradox" was introduced with the goal of demonstrating that black hole complementarity fails to solve the information paradox. According to quantum field theory in curved spacetime, a single emission... | https://en.wikipedia.org/wiki?curid=4650 |
4,892 | The Grumman F-14 Tomcat is an American carrier-capable supersonic, twin-engine, two-seat, twin-tail, variable-sweep wing fighter aircraft. The Tomcat was developed for the United States Navy's Naval Fighter Experimental (VFX) program after the collapse of the General Dynamics-Grumman F-111B project. The F-14 was the fi... | https://en.wikipedia.org/wiki?curid=11719 |
4,893 | The F-14 first flew on 21 December 1970 and made its first deployment in 1974 with the U.S. Navy aboard , replacing the McDonnell Douglas F-4 Phantom II. The F-14 served as the U.S. Navy's primary maritime air superiority fighter, fleet defense interceptor, and tactical aerial reconnaissance platform into the 2000s. Th... | https://en.wikipedia.org/wiki?curid=11719 |
4,894 | The Tomcat was retired by U.S. Navy on 22 September 2006, supplanted by the Boeing F/A-18E/F Super Hornet. Several retired F-14s have been put on display across the US. | https://en.wikipedia.org/wiki?curid=11719 |
4,895 | Having been exported to Iran under the Pahlavi dynasty in 1976, F-14s were used as land-based interceptors by the Islamic Republic of Iran Air Force during the Iran–Iraq War. Iran claimed their F-14s shot down at least 160 Iraqi aircraft during the war (only 55 of these confirmed, according to historian Tom Cooper), wh... | https://en.wikipedia.org/wiki?curid=11719 |
4,896 | Beginning in the late 1950s, the U.S. Navy sought a long-range, high-endurance interceptor to defend its carrier battle groups against long-range anti-ship missiles launched from the jet bombers and submarines of the Soviet Union. They outlined the idea of a Fleet Air Defense (FAD) aircraft with a more powerful radar a... | https://en.wikipedia.org/wiki?curid=11719 |
4,897 | The Navy was still looking for a long-range defensive aircraft, but one with higher performance than the Missileer. The Navy was directed to participate in the Tactical Fighter Experimental (TFX) program with the U.S. Air Force by Secretary of Defense Robert McNamara. McNamara wanted "joint" solutions to service aircra... | https://en.wikipedia.org/wiki?curid=11719 |
4,898 | The TFX had adequate speed, range and payload for the FAD role, but was designed as a tactical bomber that lacked the maneuverability and overall performance that the Navy expected. The Navy strenuously opposed the TFX as it feared compromises necessary for the Air Force's need for a low-level attack aircraft would adv... | https://en.wikipedia.org/wiki?curid=11719 |
4,899 | The name "Tomcat" was partially chosen to pay tribute to Admiral Thomas F. Connolly, as the nickname "Tom's Cat" had already been widely used within the program during development to reflect Connolly's involvement, and now the moniker was adapted into an official name in line with the Grumman tradition of giving its fi... | https://en.wikipedia.org/wiki?curid=11719 |
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