Black hole

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They are wild waves of the sea, foaming up their shame; wandering stars, for whom blackest darkness has been reserved forever.
Jude 1:13

A black hole is an astronomical body so dense that the escape velocity[1] is greater than the speed of light.

Contents

[edit] Existence and formation

There are almost certainly black holes of many (thousands of?) solar masses at the centre of our and most other galaxies[2][3] . These may have been created by the compression caused by the enormous central mass of stars mutually attracting each other until the gravity exceeded the radiation pressure. These supermassive black holes are so gravitationally powerful that their effect on the real motion of stars in close orbit around them can be (easily - with the right equipment) detected.

They have been conjectured to form through the collapse of large stellar objects when the gravity of the object becomes sufficiently greater than the radiation pressure [4] or by the huge compressive forces inside (exploding) supernovas.

It has been hypothesised that black holes might have been created at or about the time of the big bang. These black holes could be tiny in comparison with stellar black holes and many would probably have decayed to ordinary matter by now[5].

[edit] Observable properties

Black holes have, theoretically, many outré properties of which possibly the most notable is relativistic time and space distortion. They have almost certainly been detected, by observing the radiation emitted from streams of matter from binary stars flowing into a black hole at velocities approaching light speed and by the lensing effect on light from more distant bodies as it passes close by on its way to Earth. Another method of finding black holes is examining their effect on the orbits of nearby objects, particularly stars.

[edit] Non-observable properties

Black holes are characterised by an event horizon. This is an imaginary sphere around the object, of radius proportional to the mass of the object, at which communication from inside to outside becomes impossible (but see note[5] below). Many relativistic effects become apparent at this distance.

The distance of the event horizon from the center of the black hole is called the Schwarzschild Radius (rs), and is directly proportional to the black hole's mass. rs = 2.95 kilometers per solar mass.

[edit] Hypothetical properties

It is hypothetically possible that black holes have no "physical dimensions" - that is, the object inside the Event Horizon is a true quantum singularity: an infinitely dense, infinitely small point of mass. To date, though, we know of no way to test this.

One of the latest hypotheses ("Brane theory") regarding the basic stucture of the universe implies that black holes might form "junction points" between universes at which matter/energy is passed from one universe to another. This is pure conjecture and will probably remain so.

Another supposition is that the dark matter of which the universe is largely composed, could consist of a very large number of microscopic black holes.

[edit] Hawking radiation, and black hole evaporation

See the main article on this topic: Hawking radiation

Physicist Stephen Hawking theorized that black holes slowly give up mass through a process dubbed "Hawking Radiation". In his theory Hawking stated (using a very, very simple explanation here) that when anti-particle/particle pairs pop into existence near the event horizon of a black hole from the vacuum energy, that one of the pair will be drawn into the black hole while the other escapes into space. As the pair have been created from the vacuum energy, they need to annihilate each other immediately to repay the energy back into the vacuum in order to satisfy the first law of thermodynamics. When this does not happen the black hole repays this energy by slowly giving up some of its own. It is thought that through this process even the largest of black holes would eventually disappear (where 'eventually' refers to mindtwistingly long timescales, even by cosmological standards).

The time that it would take a black hole to completely evaporate via Hawking radiation depends entirely on its mass:

t_{\operatorname{ev}} = \frac{5120 \pi G^2 M_0^{3}}{\hbar c^4} \;

Thus, a black hole with one Solar mass (2 x 1030 kg) would last for 2 x 1067 years, while a black hole with a mass of 1 gram would last for 8 x 10-26 seconds.

[edit] Micro black holes

Hypothetically, any process -- at any scale -- that squeezes matter into a space smaller than its own Schwarzchild Radius will create a black hole. This could happen if subatomic particles were smashed into each other with sufficient energy, thereby creating a microscopic black hole.

Some opponents of the Large Hadron Collider predicted that it would create such a micro black hole, which would swallow the Earth and doom us all. They have nothing to worry about. For one, some of the cosmic rays bombarding the Earth's atmosphere impact with more energy than the collisions in the LHC, and none of those have swallowed the Earth yet. For another, even if a microscopic black hole did pop into existence, it would evaporate via Hawking radiation so rapidly that it wouldn't have time to swallow anything. (A micro black hole weighing 1000 proton masses would evaporate in ~10-88 seconds.)

At 10^32 K each particle of matter becomes its own black hole, and the usual understanding of space and time is at an beginning end." [6]

[edit] History

Surprisingly:

A paper written by the Rev John Michell in 1783 was discovered in the 1970s. This is the first known discussion of the concept of a black hole. John Michell (1724-1793) was born three years before the death of Isaac Newton. He became a well-known British geologist and astronomer and was later regarded as the 'Father of Seismology' in his study of Earthquakes. He is also credited with the idea of Binary Stars, the demonstration of an inverse square law in magnetism, and was the inventor of the torsion balance before instigating the experiment, later completed by Cavendish, to weigh the Earth.[7]

However, there are distinctions between Michell's "black hole" and modern black holes. In Michell's model, the star is still shining, but the particles of light are pulled back to the surface of the star. This is like a ball thrown on the surface of the earth with velocity less than the escape velocity. In general relativity, nothing can get past the event horizon. Moreover, in GR, all the mass is in a single point. In Michell's theory, it isn't.

[edit] External links

[edit] Footnotes

  1. Escape velocity = the speed (strictly of course velocity) which anything needs to have in order to escape the gravitational effect of another object.
  2. Galactic black holes a likely source of cosmic rays
  3. Black hole 18 billion solar masses.
  4. Radiation Pressure = general photon and particle pressure caused by nuclear and subnuclear reactions (heat) it fluctuates over the lifetime of a star.
  5. 5.0 5.1 Black holes were thought to be absolutely inviolable, but Stephen Hawking has shown that they are hairy and will, as long as there is space time, slowly decay through a phenomenon known as Hawking Radiation
  6. http://www.straightdope.com/columns/read/807/what-is-the-opposite-of-absolute-zero
  7. Description of Michell's theory of black holes

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