Learning Physics/ blackhole/15 may 2014.

https://en.wikipedia.org/wiki/Black_hole_information_paradox
black hole
Fundamentals of blackholes.updated 13 dec. 2017.

A black hole is defined as a region of spacetime from which gravity prevents anything, including light, from escaping.[1]
The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole.[2]
Around a black hole, there is a mathematically defined surface called an event horizon that marks the point of no return.
The hole is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics.[3][4]
Quantum field theory in curved spacetime predicts that event horizons emit radiation like a black body with a finite temperature.
This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater.

Objects whose gravity fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace.
The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958.
Long considered a mathematical curiosity, it was during the 1960s that theoretical work showed black holes were a generic prediction of general relativity.
The discovery of neutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.

Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle.
After a black hole has formed it can continue to grow by absorbing mass from its surroundings.
By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form
. There is general consensus that supermassive black holes exist in the centers of most galaxies.

Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as light.
Matter falling onto a black hole can form an accretion disk heated by friction, forming some of the brightest objects in the universe.
If there are other stars orbiting a black hole, their orbit can be used to determine its mass and location.
Such observations can be used to exclude possible alternatives (such as neutron stars).

In this way, astronomers have identified numerous stellar black hole candidates in binary systems, and established that the core of the Milky Way contains a supermassive black hole of about 4.3 million solar masses.
Far away from the black hole, a particle can move in any direction, as illustrated by the set of arrows.
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.[Note 1]
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.

15 may 2014

Properties and structure

The no-hair theorem states that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum.[28]

Any two black holes that share the same values for these properties, or parameters, are indistinguishable according to classical (i.e. non-quantum) mechanics.

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, the ADM mass, far away from the black hole.[34]

Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.

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 conductive stretchy membrane with friction and electrical resistance—the membrane paradigm.[35] This is different from other field theories like electromagnetism, which do not have any friction or resistivity at the microscopic level, because they are time-reversible.

Because a black hole eventually achieves a stable state with only three parameters, there is no way to avoid losing information about the initial conditions: the gravitational and electric fields of a black hole give very little information about what went in.
The information that is lost includes every quantity that cannot be measured far away from the black hole horizon, including approximately conserved quantum numbers such as the total baryon number and lepton number.
This behavior is so puzzling that it has been called the black hole information loss paradox.[36][37]
17 NOV. 2016.

Physical properties

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.[9] According to Birkhoff's theorem, it is the only vacuum solution that is spherically symmetric.[38]
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.
The popular notion of a black hole "sucking in everything" in its surroundings is therefore only correct near a black hole's horizon; far away, the external gravitational field is identical to that of any other body of the same mass.[39]

Solutions describing more general black holes also exist.
Charged black holes are described by the Reissner–Nordström metric, while the Kerr metric describes a rotating black hole.

The most general stationary black hole solution known is the Kerr–Newman metric, which describes a black hole with both charge and angular momentum.[40]

While the mass of a black hole can take any positive value, the charge and angular momentum are constrained by the mass.
In Planck units, the total electric charge Q and the total angular momentum J are expected to satisfy --------------

for a black hole of mass M. Black holes saturating 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 outside, and hence are deemed unphysical.

The cosmic censorship hypothesis rules out the formation of such singularities, when they are created through the gravitational collapse of realistic matter.[2]
This is supported by numerical simulations.[41]

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 common feature of compact objects.
The black-hole candidate binary X-ray source GRS 1915+105[42] appears to have an angular momentum near the maximum allowed value.

Black holes are commonly classified according to their mass, independent of angular momentum J or electric charge Q. The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is roughly proportional to the mass M through

------

where rsh is the Schwarzschild radius and MSun is the mass of the Sun.[43] This relation is exact only for black holes with zero charge and angular momentum; for more general black holes it can differ up to a factor of 2.

***

Event horizon

Main article: Event horizon

Far away from the black hole, a particle can move in any direction, as illustrated by the set of arrows.

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.[Note 1]

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.

The defining feature of a black hole is the appearance of an event horizon—a boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole.

Nothing, not even light, can escape from inside the event horizon.e

The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.[45]

As predicted by general relativity,
At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hthe presence of a mass deforms spacetime in such a way that the paths taken by particles bend towards the mass.[46]
ole.

To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.[47] Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.[48]
At the same time, all processes on this object slow down, for a fixed outside observer, causing emitted light to appear redder and dimmer, an effect known as gravitational redshift.[49]

Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.

On the other hand, an indestructible observer falling into a black hole does not notice any of these effects as he crosses the event horizon.

According to his own clock, which appears to him to tick normally, he crosses the event horizon after a finite time without noting any singular behaviour. In particular, he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations.[50]

The shape of the event horizon of a black hole is always approximately spherical.[Note 2][53]
For non-rotating (static) black holes the geometry is precisely spherical, while for rotating black holes the sphere is somewhat oblate.

Singularity

Main article: Gravitational singularity

At the center of a black hole as described by general relativity lies a gravitational singularity, a region where the spacetime curvature becomes infinite.[54]
For a non-rotating black hole, this region takes the shape of a single point and for a rotating black hole, it is smeared out to form a ring singularity lying in the plane of rotation.[55] In both cases, the singular region has zero volume.
It can also be shown that the singular region contains all the mass of the black hole solution.[56] The singular region can thus be thought of as having infinite density.

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 point; after attaining a certain ideal velocity, it is best to free fall the rest of the way.[57]
When they reach the singularity, they are crushed to infinite density and their mass is added to the total of the black hole.
Before that happens, they will have been torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the "noodle effect".[58]

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.[59]
The possibility of traveling to another universe is however only theoretical, since any perturbation will destroy this possibility.[60]
It also appears to be possible to follow closed timelike curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox.[61]

It is expected that none of these peculiar effects would survive in a proper quantum treatment of rotating and charged black holes.[62]

10 nov. 2014 uhm

The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.[63]
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 not been possible to combine quantum and gravitational effects into a single theory, although there exist attempts to formulate such a theory of quantum gravity.
It is generally expected that such a theory will not feature any singularities.[64][65]

15 may 2014.

10 nov. 2014/ uhm.
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Black hole information paradox

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Black_hole_information_paradox

Artist's representation of a black hole

The black hole information paradox[1] is a puzzle resulting from the combination of quantum mechanics and general relativity.

Calculations suggest that physical information could permanently disappear in a black hole, allowing many physical states to devolve into the same state.

This is controversial because it violates a commonly assumed tenet of science—that in principle complete information about a physical system at one point in time should determine its state at any other time.[2][3]

A fundamental postulate of quantum mechanics is that complete information about a system is encoded in its wave function up to when the wave function collapses.

The evolution of the wave function is determined by a unitary operator, and unitarity implies that information is conserved in the quantum sense.

Contents

1 Principles in action

2 Hawking radiation

3 Postulated solutions

4 Recent developments

5 See also

6 References

7 External links

Principles in action

There are two main principles in play:[citation needed]

Hawking radiation

Main article: Hawking radiation

The Penrose diagram of a black hole which forms, and then completely evaporates away. Information falling into it will hit the singularity.[clarification needed]
Time shown on vertical axis from bottom to top; space shown on horizontal axis from left (radius zero) to right (growing radius).

In 1975, Stephen Hawking and Jacob Bekenstein showed that black holes should slowly radiate away energy, which poses a problem.
From the no-hair theorem, one would expect the Hawking radiation to be completely independent of the material entering the black hole.
Nevertheless, if the material entering the black hole were a pure quantum state, the transformation of that state into the mixed state of Hawking radiation would destroy information about the original quantum state.
This violates Liouville's theorem and presents a physical paradox.[citation needed]

More precisely, if there is an entangled pure state, and one part of the entangled system is thrown into the black hole while keeping the other part outside, the result is a mixed state after the partial trace is taken into the interior of the black hole.
But since everything within the interior of the black hole will hit the singularity within a finite time, the part which is traced over partially might disappear completely from the physical system.[citation needed]

Hawking remained convinced that the equations of black-hole thermodynamics together with the no-hair theorem led to the conclusion that quantum information may be destroyed.
This annoyed many physicists, notably John Preskill, who bet Hawking and Kip Thorne in 1997 that information was not lost in black holes.
The implications that Hawking had opened led to a "battle" where Leonard Susskind and Gerard 't Hooft publicly 'declared war' on Hawking's solution, with Susskind publishing a popular book, The Black Hole War, about the debate in 2008.
(The book carefully notes that the 'war' was purely a scientific one, and that at a personal level, the participants remained friends.[6])
The solution to the problem that concluded the battle is the holographic principle, which was first proposed by 't Hooft but was given a precise string theory interpretation by Susskind. With this, "Susskind quashes Hawking in [the] quarrel over quantum quandary".[7]

There are various ideas about how the paradox is solved.
Since the 1997 proposal of the AdS/CFT correspondence, the predominant belief among physicists is that information is preserved and that Hawking radiation is not precisely thermal but receives quantum corrections.[clarification needed] Other possibilities include the information being contained in a Planckian remnant left over at the end of Hawking radiation or a modification of the laws of quantum mechanics to allow for non-unitary time evolution.[citation needed]

In July 2004, Stephen Hawking published a paper presenting a theory that quantum perturbations of the event horizon could allow information to escape from a black hole, which would resolve the information paradox.[8]

His argument assumes the unitarity of the AdS/CFT correspondence which implies that an AdS black hole that is dual to a thermal conformal field theory.
When announcing his result, Hawking also conceded the 1997 bet, paying Preskill with a baseball encyclopedia "from which information can be retrieved at will."[citation needed]

According to Roger Penrose, loss of unitarity in quantum systems is not a problem: quantum measurements are by themselves already non-unitary.
Penrose claims that quantum systems will in fact no longer evolve unitarily as soon as gravitation comes into play, precisely as in black holes.
The Conformal Cyclic Cosmology advocated by Penrose critically depends on the condition that information is in fact lost in black holes.
This new cosmological model might in future be tested experimentally by detailed analysis of the cosmic microwave background radiation (CMB): if true the CMB should exhibit circular patterns with slightly lower or slightly higher temperatures. In November 2010, Penrose and V. G. Gurzadyan announced they had found evidence of such circular patterns, in data from the Wilkinson Microwave Anisotropy Probe (WMAP) corroborated by data from the BOOMERanG experiment.[9] The significance of the findings was subsequently debated by others.[clarification needed]

Postulated solutions

Information is irretrievably lost[10][11]

Advantage: Seems to be a direct consequence of relatively non-controversial calculation based on semiclassical gravity.

Disadvantage: Violates unitarity. (Banks, Susskind and Peskin argued that it also violates energy-momentum conservation or locality, but the argument does not seem to be correct for systems with a large number of degrees of freedom.[12])

Information gradually leaks out during the black-hole evaporation[10][11]

Advantage: Intuitively appealing because it qualitatively resembles information recovery in a classical process of burning.

Disadvantage: Requires a large deviation from classical and semiclassical gravity (which do not allow information to leak out from the black hole) even for macroscopic black holes for which classical and semiclassical approximations are expected to be good approximations.

Information suddenly escapes out during the final stage of black-hole evaporation[10][11]

Advantage: A significant deviation from classical and semiclassical gravity is needed only in the regime in which the effects of quantum gravity are expected to dominate.

Disadvantage: Just before the sudden escape of information, a very small black hole must be able to store an arbitrary amount of information, which violates the Bekenstein bound.

Information is stored in a Planck-sized remnant[10][11]

Advantage: No mechanism for information escape is needed.

Disadvantage: To contain the information from any evaporated black hole, the remnants would need to have an infinite number of internal states. It has been argued that it would be possible to produce an infinite amount of pairs of these remnants since they are small and indistinguishable from the perspective of the low-energy effective theory.[13]

Information is stored in a large remnant[14][15]

Advantage: The size of remnant increases with the size of the initial black hole, so there is no need for an infinite number of internal states.

Disadvantage: Hawking radiation must stop before the black hole reaches the Planck size, which requires a violation of semi-classical gravity at a macroscopic scale.

Information is stored in a baby universe that separates from our own universe.[11][16]

Advantage: This scenario is predicted by the Einstein–Cartan theory of gravity which extends general relativity to matter with intrinsic angular momentum (spin). No violation of known general principles of physics is needed.

Disadvantage: It is difficult to test the Einstein–Cartan theory because its predictions are significantly different from general-relativistic ones only at extremely high densities.

Information is encoded in the correlations between future and past[17][18]

Advantage: Semiclassical gravity is sufficient, i.e., the solution does not depend on details of (still not well understood) quantum gravity.

Disadvantage: Contradicts the intuitive view of nature as an entity that evolves with time.

Recent developments

In 2014, Chris Adami, a physicist at the Michigan State University claimed to have solved the paradox.[19]
Since absolute loss of information is not allowed by quantum physics, Adami argues that the "lost" information is contained in stimulated emission that accompanies the Hawking radiation emitted by black holes.[20] His solution has not been confirmed.

In 2015, Modak, Ortíz, Peña and Sudarsky, have argued that the paradox can be dissolved by invoking foundational issues of quantum theory often referred as the Measurement problem of quantum mechanics.[21]
This work was built on an earlier proposal by Okon and Sudarsky on the benefits of Objective collapse theory in a much broader context.[22]
The original motivation of these studies was the long lasting proposal of Roger Penrose where collapse of the wave-function is said to be inevitable in presence of black holes (and even under the influence of gravitational field).[23][24] Experimental verification of collapse theories is an ongoing effort.[25]

Hawking et al. on 5 Jan 2016 proposed new theories of information moving in and out of a black hole.[26][27] The 2016 work posits that the information is saved in "soft particles", low-energy versions of photons and other particles that exist in zero-energy empty space.[28]

A Black Hole That Has Stopped Swallowing Stars

Trilobites

By NICHOLAS ST. FLEUR FEB. 22, 2016

Photo

A photo taken by the Hubble Space Telescope showing the galaxy NGC 4889 which houses a supermassive black hole. Credit European Space Agency and NASA

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The brightest orb in the center of this photograph captured by the Hubble Space Telescope is a galaxy some 300 million light-years away called NGC 4889.

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At its core rests a supermassive black hole that is about 21 billion times the size of the sun, according to NASA and the European Space Agency.

Though you cannot see it in this image because black holes swallow light, NASA said it was one of the largest star devourers ever observed. It dwarfs the black hole that twists at the center of our Milky Way, which researchers estimate is only four million times the size of the sun. The monster once feasted upon many of its host galaxy’s stars, gas and dust, but is no longer active.

The diameter of its event horizon, or the edge of where light is trapped and cannot escape, measured 80 billion miles, and its intense gravitational grasp was capable of heating material to millions of degrees, according to NASA.

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Glossary
ge·ner·ic

jəˈnerik/

adjective

adjective: generic

1.
characteristic of or relating to a class or group of things; not specific.
"chèvre is a generic term for all goat's milk cheese"

synonyms:

general, common, collective, nonspecific, inclusive, all-encompassing, broad, comprehensive, blanket, umbrella

"a generic classification for similar offenses"

antonyms:

specific

(of goods, especially medicinal drugs) having no brand name; not protected by a registered trademark.
"generic aspirin"

synonyms:

unbranded, nonproprietary, no-name

"generic drugs are cheaper than brand-name ones"

antonyms:

specific

noun

noun: generic; plural noun: generics

1.
a consumer product having no brand name or registered trademark.
"substituting generics for brand-name drugs"

2.

Biology

of or relating to a genus.

MAW

Tuesday, December 12, 2017

https://en.wikipedia.org/wiki/Black_hole_information_paradox black hole Fundamentals of blackholes. updated 13 dec. 2017.

A Black Hole That Has Stopped Swallowing Stars

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