Black Holes
Simulation
Simulation
A black hole is a star that has collapsed into a tiny point known as a singularity. It is so dense that it sucks in everything near it, including light. Black holes can be seen via the death throes of the matter being sucked in. Although it becomes invisible past a certain point, an accretion disk, which is visible, develops as the matter swirls toward the black hole. The collapsed star is so dense that nothing can escape its gravitational pull, not even light.
A black hole is a concentration of mass great enough that the force of gravity prevents anything from escaping from it except through quantum tunneling behavior. The gravitational field is so strong that the escape velocity near it exceeds the speed of light. This implies that nothing, not even light, can escape its gravity, hence the word "black." The term "black hole" is widespread, even though it does not refer to a hole in the usual sense, but rather a region of space from which nothing can return. Theoretically, black holes can have any size, from microscopic to near the size of the observable universe.
Black holes are predicted by general relativity. According to classical general relativity, neither matter nor information can flow from the interior of a black hole to an outside observer. For example, one cannot bring out any of its mass, or receive a reflection back by shining a light source such as a flashlight, or retrieve any information about the material that has entered the black hole. Quantum mechanical effects may allow matter and energy to radiate from black holes; however, it is thought that the nature of the radiation does not depend on what has fallen into the black hole in the past.
The existence of black holes in the universe is well supported by astronomical observation, particularly from studying supernovae and X-ray emissions from active galactic nuclei.
History
The concept of a body so massive that not even light could escape from it was put forward by the English geologist John Michell in a 1783 paper sent to the Royal Society. At that time, the Newtonian theory of gravity and the concept of escape velocity were well known. Michell computed that a body 500 times the radius of the Sun and of the same density would have at its surface an escape velocity equal to the speed of light, and therefore would be invisible. Although he thought it unlikely, Michell considered the possibility that many such objects that cannot be seen might be present in the cosmos.
In 1796, the French mathematician Pierre-Simon Laplace promoted the same idea in the first and second edition of his book Exposition du Systeme du Monde. It disappeared in later editions. The whole idea gained little attention in the 19th century, since light was thought to be a massless wave, not influenced by gravity.
In 1915, Einstein developed the theory of gravity called General Relativity. Earlier he had shown that gravity does influence light. A few months later, Karl Schwarzschild gave the solution for the gravitational field of a point mass, showing that something we now call a black hole could theoretically exist. The Schwarzschild radius is now known to be the radius of a non-rotating black hole, but was not well understood at that time. Schwarzschild himself thought it not to be physical.
In the 1920s, Subrahmanyan Chandrasekhar argued that special relativity demonstrated that a non-radiating body above a certain mass, now known as the Chandrasekhar limit, would collapse since there would be nothing that could stop the collapse. His arguments were opposed by Arthur Eddington, who believed that something would inevitably stop the collapse.
In 1939, Robert Oppenheimer and H. Snyder predicted that massive stars could undergo a dramatic gravitational collapse. Black holes could in principle be formed in nature. Such objects for a while were called frozen stars since the collapse would be observed to rapidly slow down and become heavily reddened near the Schwarzschild radius. However, these hypothetical objects were not the topic of much interest until the late 1960s. Most physicists believed that they were a peculiar feature of the highly symmetric solution found by Schwarzschild, and that objects collapsing in nature would not form black holes.
Interest in black holes was rekindled in 1967, due to theoretical and experimental progress. Stephen Hawking and Roger Penrose proved that black holes are a generic feature in Einstein's theory of gravity, and cannot be avoided in some collapsing objects. Interest was renewed in the astronomical community with the discovery of pulsars. Shortly thereafter, the use of the expression "black hole" was coined by theoretical physicist John Wheeler. Prior to that time, the term black star was used occasionally. The latter term appears in an early episode of Star Trek, and was still used occasionally after 1967. This is because some people found the term "black hole" obscene when translated into French or Russian, for example.
Qualitative Physics
Black holes require the general relativistic concept of a curved spacetime: their most striking properties rely on a distortion of the geometry of the space surrounding them.
The "surface" of a black hole is the so-called event horizon, an imaginary surface surrounding the mass of the black hole. Using the Gauss-Bonnet theorem, Stephen Hawking proved that the topology of the event horizon of a (four dimensional) black hole is a 2-sphere. At the event horizon, the escape velocity is equal to the speed of light. Thus, anything inside the event horizon, including a photon, is prevented from escaping across the event horizon by the extremely strong gravitational field. Particles from outside this region can fall in, cross the event horizon, and will never be able to leave.
According to classical general relativity, black holes can be entirely characterized according to three parameters: mass, angular momentum, and electric charge. This principle is summarized by the saying, coined by John Wheeler, "black holes have no hair."
Objects in a gravitational field experience a slowing down of time, called time dilation. This phenomenon has been verified experimentally in the Scout rocket experiment of 1976 [2], and is, for example, taken into account in the GPS system. Near the event horizon, the time dilation increases rapidly. From the point of view of an external observer, it takes an infinite amount of time for an object to approach the event horizon, at which point the light coming from it is infinitely red-shifted. To the distant observer, the object, falling slower and slower, approaches but never reaches the event horizon. The object itself might not even notice the point at which it crosses the event horizon, and will do so in a finite amount of proper time.
Singularity
At the center of the black hole, well inside the event horizon, general relativity predicts a singularity, a place where the curvature of spacetime becomes infinite and gravitational forces become infinitely strong. Spacetime inside the event horizon is peculiar in that the singularity is in every observer's future, so all particles within the event horizon move inexorably towards it. This means that there is a conceptual inaccuracy in the nonrelativistic concept of a black hole as originally proposed by John Michell in 1783.
It is expected that future refinements or generalizations of general relativity (in particular quantum gravity) will change what is thought about the nature of black hole interiors. Most theorists interpret the mathematical singularity of the equations as indicating that the current theory is not complete, and that new phenomena must come into play as one approaches the singularity. The question may be largely academic, as the cosmic censorship hypothesis asserts that there are no naked singularities in general relativity: Every singularity is hidden behind an event horizon and cannot be probed.
Another school of thought holds that no singularity occurs, because of a bubble-like local inflation in the interior of the collapsing star. Radii stop converging as they approach the event horizon, are parallel at the horizon, and begin diverging in the interior. The solution resembles a wormhole (from the exterior to the interior) in a neighborhood of the horizon, with the horizon as the neck.In Michell's theory, the escape velocity equals the speed of light, but it would still, for example, be theoretically possible to hoist an object out of a black hole using a rope. General relativity eliminates such loopholes, because once an object is inside the event horizon, its time-line contains an end-point to time itself, and no possible world-lines come back out through the event horizon.
Entering a black hole
The effects of a black hole's gravity as decribed by the Theory of Relativity cause a number of peculiar effects. An object approaching simple Schwarzschild-type (non-rotating) black hole's center will appear to distant observers as having an increasingly slow descent as the object approaches the event horizon. This is because a photon takes an increasingly long time to escape from the pull of the black hole to allow the distant observer to gain information on the object's fate.
From the object's frame of reference, it will cross the event horizon and reach the singularity, or center of the black hole, all within a finite amount of time. Once the object crosses over the event horizon, light will no longer escape the black hole, and the object can no longer be observed outside of the black hole. As the object continues to approach the singularity, it will elongate, and the parts closest to the singularity will begin to red shift, until they finally become invisible. Nearing the singularity, the gradient of the gravitational field from head to foot will become considerable, will stretch and tear because of tidal forces: the parts closest to the singularity feel disproportionatly stronger gravitational force than those parts farther away. This process is known as spaghettification.
According to theory, the event horizon of a black hole that is not spinning is spherical, and its singularity is (informally speaking) a single point. If the black hole carries angular momentum (inherited from a star that is spinning at the time of its collapse), it begins to drag space-time surrounding the event horizon in an effect known as frame-dragging. This spinning area surrounding the event horizon is called the ergosphere and has an ellipsoidal shape. Since the ergosphere is located outside the event horizon, objects can exist within the ergosphere without falling into the hole. However, because space-time itself is moving in the ergosphere, it is impossible for objects to remain in a fixed position. Objects grazing the ergosphere could in some circumstances be catapulted outwards at great speed, extracting energy (and angular momentum) from the hole, hence the name ergosphere ("sphere of work") because it is capable of doing work.
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. This sounded remarkably similar to the Second Law of Thermodynamics, with area playing the role of entropy. Classically, one could violate the second law of thermodynamics by material entering a black hole disappearing from our universe and 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. Since black holes do not classically emit radiation, the thermodynamic viewpoint was simply an analogy. However, in 1974, Hawking applied quantum field theory to the curved spacetime around the event horizon and discovered that black holes can emit thermal radiation, known as Hawking radiation.
Using the first law of black hole mechanics, it follows that the entropy of a black hole is one quarter of the area of the horizon. This is a universal result and can be extended to apply to cosmological horizons such as in de Sitter spacetime. It was later suggested that black holes are maximum-entropy objects, meaning that the maximum entropy of a region of space is the entropy of the largest black hole that can fit into it. This led to the holographic principle.
Hawking radiation originates just outside the event horizon and, so far as it is understood, does not carry information from its interior since it is thermal. However, this means that black holes are not completely black: the effect implies that the mass of a black hole slowly evaporates with time. Although these effects are negligible for astronomical black holes, they are significant for hypothetical very small black holes where quantum-mechanical effects dominate. Indeed, small black holes are predicted to undergo runaway evaporation and eventually vanish in a burst of radiation. Hence, every black hole that cannot consume new mass has a finite life that is directly related to its mass.
On 21 July 2004 Stephen Hawking presented a new argument that black holes do eventually emit information about what they swallow, reversing his previous position on information loss. He proposed that quantum perturbations of the event horizon could allow information to escape from a black hole, where it can influence subsequent Hawking radiation [5]. The theory has not yet been reviewed by the scientific community and if it is accepted it is likely to resolve the black hole information paradox. In the meantime, the announcement has attracted a lot of attention in the media.
Formation
General relativity (as well as most other metric theories of gravity) not only says that black holes can exist, but in fact predicts that they will be formed in nature whenever a sufficient amount of mass gets packed in a given region of space, through a process called gravitational collapse. As the mass inside that region increases, its gravity becomes stronger - or, in the language of relativity, the space around it becomes increasingly deformed. When the escape velocity at a certain distance from the center reaches the speed of light, an event horizon is formed within which matter must inevitably collapse onto a single point, forming a singularity.
A quantitative analysis of this idea led to the prediction that a star remaining about three times the mass of the Sun at the end of its evolution (usually as a neutron star), will almost inevitably shrink to the critical size needed to undergo a gravitational collapse. Once it starts, the collapse cannot be stopped by any physical force, and a black hole is created.
Stellar collapse will generate black holes containing at least three solar masses. Black holes smaller than this limit can only be created if their matter is subjected to sufficient pressure from some source other than self-gravitation. The enormous pressures needed for this are thought to have existed in the very early stages of the universe, possibly creating primordial black holes which could have masses smaller than that of the Sun.
Supermassive black holes containing millions to billions of solar masses could also form wherever a large number of stars are packed in a relatively small region of space, or by large amounts of mass falling into a "seed" black hole, or by repeated fusion of smaller black holes. The necessary conditions are believed to exist in the centers of some (if not most) galaxies, including our own Milky Way .
Observation
Theory says that we cannot detect black holes by light that is emitted or reflected by the matter inside them. However, those objects can be inductively detected from observation of phenomena near them, such as gravitational lensing and stars that appear to be in orbit around space where there is no visible matter.
The most conspicuous effects are believed to come from matter falling into a black hole, which (like water flowing into a drain) is predicted to collect into an extremely hot and fast-spinning accretion disk around the object before being swallowed by it. Friction between adjacent zones of the disk causes it to become extremely hot and emit large amounts of X-rays. This heating is extremely efficient and can convert about 50% of the mass energy of an object into radiation, as opposed to nuclear fusion which can only convert a few percent of the mass to energy. Other predicted effects are narrow jets of particles at relativistic speeds squirting off along the disk's axis.
However, accretion disks, jets, and orbiting objects are found not only around black holes, but also around other objects such as neutron stars; and the dynamics of bodies near these non-black hole attractors is largely similar to the dynamics of bodies around black holes, and is currently a very complex and active field of research involving magnetic fields and plasma physics.
Hence, for the most part, observations of accretion disks and orbital motions merely indicate that there is a compact object of a certain mass, and says very little about the nature of that object. The identification of an object as a black hole requires the further assumption that no other object (or bound system of objects) could be so massive and compact. Most astrophysicists accept that this is the case, since according to general relativity, any concentration of matter of sufficient density must necessarily collapse into a black hole.
One important observable difference between black holes and other compact massive objects is that any infalling matter will eventually collide with the latter, at relativistic speeds, leading to irregular intense flares of X-rays and other hard radiation. Thus the lack of such flare-ups around a compact concentration of mass is taken as evidence that the object is a black hole, with no surface onto which matter can be suddenly dumped.
Have we found them?
There is now a great deal of indirect astronomical observational evidence for black holes in two mass ranges:
Additionally, there is some evidence for intermediate-mass black holes (IMBHs), those with masses of a few thousand times that of the Sun. These black holes may be responsible for the formation of supermassive black holes.
Candidates for stellar-mass black holes were identified mainly by the presence of accretion disks of the right size and speed, without the irregular flare-ups that are expected from disks around other compact objects. Stellar-mass black holes may be involved in gamma ray bursts (GRBs), although observations of GRBs in association with supernovae or other objects that are not black holes have reduced the possibility of a link.
Candidates for more massive black holes were first provided by the active galactic nuclei and quasars, discovered by radioastronomers in the 1960s. The efficient conversion of mass into energy by friction in the accretion disk of a black hole seems to be the only explanation for the copious amounts of energy generated by such objects. Indeed the introduction of this theory in the 1970s removed a major objection to the belief that quasars were distant galaxies - namely, that no physical mechanism could generate that much energy.
From observations in the 1980s of motions of stars around the galactic center, it is now believed that such supermassive black holes exist in the center of most galaxies, including our own Milky Way. Sagittarius A is now agreed to be the most plausible candidate for the location of a supermassive black hole at the center of the Milky Way galaxy.
The current picture is that all galaxies may have a supermassive black hole in their center, and that this black hole swallows gas and dust in the middle of the galaxies generating huge amounts of radiation - until all the nearby mass has been swallowed and the process shuts off.
This explains why there are no nearby quasars. Though the details are still not clear, it seems that the growth of the black hole is intimately related to the growth of the spheroidal component - an elliptical galaxy, or the bulge of a spiral galaxy - in which it lives. Interestingly, there is no evidence for massive black holes in the center of globular clusters, suggesting that these are fundamentally different from galaxies.
Micro Black Holes
The formation of micro black holes on Earth in particle accelerators have been tentatively reported, but not yet confirmed. So far there are no observed candidates for primordial black holes.
References - Additional Theories - Links - Recent Discoveries
Burrowing black holes devoured first stars from within New Scientist - December 19, 2008
There is a giant black hole at the center of our galaxy, a study has confirmed. BBC - December 10, 2008
Supermassive black hole at the Center of the Milky Way NASA - December 11, 2008
Black Hole "Hearts" Warm Galaxies, Control Growth National Geographic - November 21, 2008
Bursts Spotted at Milky Way's Black Hole Live Science - November 20, 2008
Scientists find black hole 'missing link' PhysOrg - September 17, 2008
How big can a black hole grow? New Scientist - September 3, 2008
Milky Way's black hole gets extreme close-up New Scientist - September 3, 2008
Closest Look Ever at the Edge of a Black Hole PhysOrg - September 3, 2008
Closest Look Yet at Milky Way's Black Hole Live Science - September 3, 2008
Most Black Holes Might Come in Only Small and Large BBC - August 20, 2008
Photo: Star Portrait Reveals "Family Tree" National Geographic - August 22, 2008
How stars form amid black hole chaos MSNBC - August 21, 2008
Black hole star mystery 'solved' BBC - August 23, 2008
Astronomers have shed light on how stars can form around a massive black hole, defying conventional wisdom.
Physicists Demonstrate How Information Can Escape From Black Holes Science Daily - May 15, 2008
Smallest Known Black Hole Discovered National Geographic - April 2, 2008
World's First Movie Of Black Hole Birth Science Daily - March 28, 2008
Milky Way's antimatter linked to exotic black holes New Scientist - January 22, 2008
The "supermassive" black holes at the centres of most galaxies could be spinning at a dizzying rate BBC - January 12, 2008
Huge black hole tips the scales BBC - January 10, 2008
Black holes may harbour their own universes New Scientist - October 31, 2007
Massive Stellar Black Hole Smashes Record Space.com - October 30, 2007
Can Black Holes Transport You to Other Worlds? Live Science - October 8, 2007
Black Hole Eclipse NASA - April 13, 2007
Black hole found in ancient lair BBC - January 5, 2007
Cosmos packed with black holes News in Science - October 9, 2006
Black hole findings yield new mysteries Space.com - July 11, 2006
Magnetism nudges matter into black holes National Geographic - June 21, 2006
Black Hole Pair Sets Proximity Record and come very close Space.com - May 2, 2006
Older Black Holes Still Full of Energy Scientific American - April 25, 2006
Chandra finds black holes are 'green' National Geographic - April 24, 2006
Black hole mergers modelled in 3D BBC - April 19, 2006
Black Holes Bound to Merge Space.com - April 7, 2006
Most distant cosmic explosion was a star collapsing into a black hole PhysOrg - March 8, 2006
Animation: Slow Birth of a Black Hole Space.com - March 8, 2006
Hitching a Ride Out of a Gluttonous Black Hole PhysOrg - February 26, 2006
Black hole puts dent in space-time MSNBC - January 25, 2006
Vanishing Gas Confirms Black Hole Event Horizons Space.com - January 10, 2006
Supermassive Black Hole at Center of Milky Way "Sagittarius A Star" National Geographic - November 2, 2005
Images Reveal the Surroundings of a Super-massive Black Hole PhysOrg - October 17, 2005
The black hole at the center of the Milky Way is actually helping stars form Space.com - October 14, 2005
Black Hole Forges Invisible Bubble Space.com - August 17, 2005
Eye-to-Eye with a Black Hole Space.com - July 11, 2005
X-Ray "Vision" Unlocking Black Hole Mysteries National Geographic - May 25, 2005
Scientists Watch Black Hole Born In Split-Second Light Flash Science Daily - May 17, 2005
Creation of Black Hole Detected Space.com May 9, 2005
Rhode Island: Did they create a black hole in the lab? BBC - March 2005
Astronomers Measure Mass Of Smallest Black Hole In A Galactic Nucleus Science Daily - March 2005
Very largest black holes reach a certain point and then grow no more Science Daily - February 2005
Runt of the litter? Odd Black Hole Revealed Space.com - February 2005
Black Holes Spark Star Formation Space.com - February 2005
Milky Way's Center Packed with Black Holes Space.com - January 2005
Matter Rides Black Hole's Space-Time Wave Space.com - January 2005
Black Holes: Biggest Space Explosion Creates Giant Bubbles Space.com - January 2005
Twisted Physics: How Black Holes Spout Off Space.com - August 2004
Scientists Spot Doughnut-Shaped Cloud With a Black Hole Filling Goddard - July 2004
Dublin - Hawking: Black Holes Mangle Matter, Energy Space.com - July 2004
Massive Black Hole Stumps Researchers Space.com - July 2004
Youngest Possible Black Hole Spotted Near Birth Space.com - June 2004
Odd Black Hole Defies Explanation Space.com - June 2004
Swirling Dust Near Black Hole Too Thick for Theory Space.com - May 2004
Runaway Star Collisions Create Black Holes Space.com - April 2004
Dark Matter and Black Holes at the Galactic Center Space.com - March 2004
Black Holes: Fuzzy Tangles of Strings? Space.com - March 2004
Black hole tears star apart BBC - February 2004
The True Shape of Black Holes Space.com
How the release of energy from massive black holes are shaping two distant galaxies Science Daily - May 2003
The give and take of black holes BBC - March 2003
Most distant black hole weighed BBC March 2003
The New History of Black Holes: 'Co-evolution' Dramatically Alters Dark Reputation Space.com January 2003
Close-up on a quasar BBC - January 2003
Black hole 'on a diet' BBC - January 2003
Black hole hunter's first image BBC - December 2002
Black holes 'on collision course' BBC - Nov. 2002
Black hole's on-the-run snack BBC Nov. 2002
Old galaxies have youthful shine BBC - September 2002
Hubble rings a black hole BBC - November 30, 2000
Biggest black hole in cosmos discovered New Scientist - January 10, 2008
Video: "Death Star" Galaxy a Bully? National Geographic - December 19, 2007
NASA Announces Discovery of Assault by a Black Hole NASA - December 18, 2007
"Death Star" Galaxy Found Blasting Smaller Neighbor National Geographic - December 17, 2007
Black hole 'bully' blasts galaxy BBC - December 17, 2007
Monster Black Hole Busts Theory Space.com - October 18, 2007
Most Massive Stellar Black Hole Found in Binary System National Geographic - October 17, 2007
First black hole found in globular star cluster New Scientist - January 4, 2007
Indirect evidence from X-ray telescopes has revealed telltale
signs of thousands of black holes lurking in our own galaxy
and beyond. Many are the remnants of exploded stars.
This gives insight into how black holes generate energy and affect their environment.
Scientists simulate relativity's recipe for massive mergers
In a process similar to teleportation, quantum information inside the
black hole entangles itself with Hawking radiation. As the black hole
evaporates, the information is mostly preserved in the radiation ...
A spinning black hole in the constellation Scorpius has created a stable dent in the fabric of space-time, scientists say. The dent is the sort of thing predicted by Albert Einsteinšs theory of general relativity. It affects the movement of matter falling into the black hole.The space-time dent is invisible, but scientists deduced its existence after detecting two X-ray frequencies from the black hole that were identical to emissions noted nine years ago. The finding will allow scientists to calculate the black holešs spin, a crucial measurement necessary for describing the object's behavior.
Scientists Find Black Hole's 'Point of No Return'
The jets are the result of some really twisted physics, according to a new
computer model. And to unravel the mystery, a researcher invokes some
imaginary string, a corkscrew and a certain child's plaything - the Slinky.
Stephen Hawking - black holes, the mysterious massive vortexes formed
from collapsed stars, do not destroy everything they consume but
instead eventually fire out matter and energy in a mangled form.
Stephen Hawking's Website
A team of astronomers have found a colossal black hole so ancient,
they're not sure how it had enough time to grow to its current size,
about 10 billion times the mass of the Sun.
A close-up view of a donut-shaped disk of dust around a black hole
confirms several expectations about the massive structure but has
astronomers wondering how the disk could be so thick.
Black holes may not be the smooth.
Scientists think the doomed star drifted too close to a giant black
hole and gradually fell under the influence of its enormous gravity.
The tidal forces of the black hole pulled on the star, stretching it
until it tore apart. The black hole then swallowed some of the matter
left behind causing a flare of X-rays that was detected on Earth.
Scientists have found evidence of high-speed winds blowing away
vast amounts of gas from the cores of two quasar galaxies.
The colliding galaxies known as The Mice and their black holes
will eventually merge into a single giant galaxy. Such mergers
can generate a quasar phase of galactic evolution.
The most detailed view yet of a feeding black hole in
the centre of a remote quasar has been obtained.
It seems the supermassive black hole that sits at the
centre of our galaxy, the Milky Way, is famished.
For the first time two supermassive black holes have been seen at
the heart of one galaxy. One day there will be a devastating collision.
At the galaxy's core lies a powerful black hole
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