An astronomical body consisting of a single, infinitely dense point. Although any object that has collapsed to within its own Schwarzschild radius will have the same external characteristics, once it has collapsed that far it is inevitable that it will collapse further to form a singularity. Oh, and something does come out, Hawking Radiation.

Black holes are caused by stars that have collapsed, with extreme density. Kinda like sticking Manhattan all into the head of a pin (only much bigger). The point at which nothing comes out is the Event Horizon, the area around the actual dense matter in the center, inside of which nothing can escape the pull of gravity, even light. The X-rays that seem to be emitted from a black hole are actually given off at the edge of the event horizon as a result of matter being violently pulled apart as it engulfs everything around it.

A black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby
object to escape its gravitational pull. Since our best theory of gravity at the moment is Einstein's general theory of relativity, we have to delve into some results of this theory to understand black holes in detail, but let's not.. :) perhaps I will find some way of posting this one REALLY long report I wrote once on why we think that black holes exist.. since they are dark matter and we must rely on indirect methods of detection.. yeah!

A black hole only has three characteristics one can measure from the outside:

Mass: The mass of the black hole determines the size of its event horizon. It also determines its gravitational pull at a given distance away from it. Thus, the mass can be measured.

Angular momentum: i.e. spin. A black hole can spin, yes. Due to the law of conservation of angular momentum, if a spinning mass turns into a black hole, the resultant hole must still spin. Just because it's a singularity doesn't mean it can escape the basic laws of nature! (well, at least outside the event horizon).

Charge: As above, the law of conservation of charge prevents charge from just disappearing. This means that if one found a small black hole, and shot enough charged particles into it, the hole would acquire a charge. You could then move it around with really, really big electrically charged plates, for example. Slowly, true, but so what? How many people could claim they had a black hole following them around?

Those are all the properties of the black hole. Thus the physicist's saying:

A black hole has no hair.
On The Scientific Origins Of Black Holes

Every gravitating body has associated with it an escape velocity. In 1783 John Michell calculated that if a star were to shrink, its escape velocity would rise, eventually reaching the speed of light. Using Newton's The Law of Gravity and the corpuscular theory of light, he reasoned that a particle of light emitted from such a star would rise a certain distance and then fall back to the surface. Obviously such a star would be dark.

As Newton's theory of gravity was replaced by Einstein's relativity theory, the effect a star had on the light it emitted had to be recalculated. Karl Schwarzschild performed the calculations, and first described the space-time geometry around such an object. This geometry showed that if a star shrinks below the Schwarzschild radius, then light would have an infinite redshift, effectively removing all the lights energy, again making the star dark.

However no physicists believed that such an object was physically possible and could never form naturally, in fact Einstein performed a calculation to show they were impossible. In 1935 Chandrasekhar showed that for a star greater than 1.4 solar masses, electron degeneracy pressure would not be enough to stop the star collapsing, at the time this implied that nothing would halt the collapse of the star. As this was considered impossible most physicists believed that at some point a real star would shed mass to bring it under this Chandrasekhar limit.

Zwicky hypothesised as early as early as 1933 that a star could be composed entirely of neutrons. A star so massive that its gravity overcomes the electron degeneracy pressure , forcing the electrons into the protons, converting them into neutrons. This neutron star could provide a explanation for the colossal energies released in a supernova explosion. If neutron stars existed, this would imply that there could well be another limit above 1.4 solar masses, where neutron degeneracy pressure is not enough to prevent further collapse. But nobody thought of this at first...

In 1938 J. Robert Oppenheimer and a student, Volkoff, used work by Tolman, to calculate if an upper limit for a neutron star's mass existed. They found there was a limit, and it was between half and several solar masses. No one knew if another limit existed, further preventing ultimate collapse.

The second world war intervened, but in 1956 John Archibald Wheeler considered the above work, and set out to calculate whether or not there would eventually be an ultimate collapse. One of his students calculated the final fate of all circumferences of masses, which showed finally that neutron star was the final stable state. They also showed that a star above three solar masses would be unstable as a neutron star, and would turn into.... what? Wheeler argued that the star would radiate mass away as it collapsed through some, as yet unknown, reason. His rationale was that infinite collapse was impossible, and somehow quantum mechanics could convert the mass into radiation which could escape. (This is not entirely incorrect, see Hawking Radiation) Oppenheimer took an opposing view; that only the theory of relativity was necessary to model the collapse, and a state if infinite density was inevitable. However this argument involved huge generalisations, and might not be true in the 'real world'. Wheeler didn't come around to the idea until computer simulations of a collapsing star,(adapted from simulations of fusion bombs), proved that the losses due to ejection of matter, radiation etc. could not prevent the gravitational collapse. Also a paper by Finklestein helped to resolve a paradox due to relativity; once the star has collapsed to the point at which light can no longer escape, shouldn't the time dilation effect become infinite, and 'freeze' the star forever at this point? Finklestein reformulated the model of the space-time in and around the star. He created a reference frame large enough to cover the area outside of the star, and the surface of the star. This showed that, yes from an external point of view, the star would appear to 'freeze', as the photons take an infinitely long time to crawl out of the gravity well, but someone sitting on the surface of the star would not see any freezing. They would ride the collapse with no pause at all, until the singularity...

Wheeler joined other physicists, such as Roger Penrose and Landau in believing such objects were possible, even inevitable. In 1967 Wheeler found the perfect name for such an object, he termed them Black Holes . The name stuck.

Of course science is often not content to leave things alone, and as black holes do not rely on the theories of quantum mechanics, their description is not yet complete. It is even possible Einstein was right, and a black hole in it's traditional formulation can not be created from a real object, if quantum machanics are taken into account. One such description of matter stressed to such a degree is that of a 'Gravitional Condensate Star' or a 'Gravastar'.

For more info, I'd recommend the book 'Black Holes and Timewarps : Einsteins Outrageous Legacy' By Kip S. Thorne ISBN 0-33-63969-3 which I used in the writing of this article.

Theoretically, since light bends when near a black hole, if you constructed a hollow ring around it near the event horizon you would be able to look down the tube and see your own back.

Hmmm, since someone here also said you can give a black hole a charge, perhaps you could "catch" a black hole and use it for a power source, similar to using a field to suspend anti-matter. Perhaps you could use it to propel a ship. I wouldn't want it orbiting the planet, though. I'd hate to have an "oops" that ate the Earth.

Black Holes

Members of a certain class of physical objects have a property known as the Schwarzschild radius. This class of objects includes (or is entirely occupied by - not sure) stars. The size of this radius is proportional to the mass within the object.

The occurence of black holes is a result of a star's state of collapse causing it to be smaller than its own Schwarzschild radius. When this happens, the Schwarzschild radius can be referred to as an event horizon and the star within it a "black hole".

For reasons unknown to me, any empty space on the inside of an event horizon causes space-time to bend in uncomfortable ways. Astrophysicists crapped themselves when they ran some of Einstein's equations over the event horizon scenario. It surely has something to do with the ridiculously high density of the object within. The specific outcome of this "bending" in space-time is that objects coming into contact with an event horizon are forced to move through time NOT through space. Objects can only move through time in the positive direction, and as it happens, the positive direction in this case is targetted at the centre of the event horizon sphere.

The understanding that black holes pull objects and light in because of their incredibly strong gravity is a myth and a simplification of the truth. Things don't fall into black holes because they are pulled by gravity. They fall into the black hole for the same reason that Thursday becomes Friday. Because time says you have to keep moving forward.

In a rather perplexing way, this rule also applies to the original star itself. So when the star collapses into the inside of its own event horizon, it is compelled by time to spontaneously collapse into a singularity. So, in a oddly theoretical fashion, the star officially obtains a radius of zero, and if you want to get mathematical about it, this means density is approaching infinity. (As if things weren't bad enough already).

There are a whole heap of suggestions as to what happens to objects captured by this bent sector of the continuum, but no real grasp of the situation. It is believed (well, by me anyway =) ) that objects being compelled toward a singularity are forced into the following conditions:

(a) Extreme acceleration
(b) velocity approaching the speed of lightTM, as a result of (a)
(c) mass approaching infinity and volume approaching zero as a result of (b)

A discussion of what happens to the object then can take a while. I've thought up some possible results, but I don't think they belong on this node.

That's my 0.02 cents.

With regard to the marvels of gravitational lensing around black holes, why would you even need a hollow ring to see your own back? It'd be there, just several million miles away.

Light can and will orbit a black hole: this happens at 1.5 times the Schwarzschild radius (the radius of the event horizon) and forms a sphere of light known as the photosphere.

So any light tangential to the photosphere will be trapped for eternity around the black hole, making it a fairly useful energy harvester, as this light is still outside the celestial censor that is the event horizon.

At this point, you whip your handy portable mass out of your multidimensional pockets (it could be anything, tame black hole, galaxy on a stick) and you wave it in a sorcerous fashion at the black hole.

Immediately (or close enough - I forget just how fast gravity waves, or gravitons or whatever, travel) radiation starts to peel off from the photosphere and sweep across the friendly vacuum towards your house, thanks to your precisely calculated actions.

As you smugly light your foot-long Havana cigar, the torrent of photons strikes the solar panels you installed on the roof of your house, and razes it to a mile-deep heat-glazed pit. You look around in case anyone saw that blunder.
Your oil-burning neighbour laughs like a drain.

black hat = B = black magic

black hole n.,vt.

[common] What data (a piece of email or netnews, or a stream of TCP/IP packets) has fallen into if it disappears mysteriously between its origin and destination sites (that is, without returning a bounce message). "I think there's a black hole at foovax!" conveys suspicion that site foovax has been dropping a lot of stuff on the floor lately (see drop on the floor). The implied metaphor of email as interstellar travel is interesting in itself. Readily verbed as `blackhole': "That router is blackholing IDP packets." Compare bit bucket and see RBL.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.

Black" hole` (?).

A dungeon or dark cell in a prison; a military lock-up or guardroom; -- now commonly with allusion to the cell (the Black Hole) in a fort at Calcutta, into which 146 English prisoners were thrust by the nabob Suraja Dowla on the night of June 20, 1756, and in which 123 of the prisoners died before morning from lack of air.

A discipline of unlimited autocracy, upheld by rods, and ferules, and the black hole.
H. Spencer.


© Webster 1913.

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