Black holes are such extraordinarily compact objects they require a theory that involves general relativity (the theory of gravitation) and quantum mechanics (the theory of microscopic particles), in short a theory of quantum gravity.

But the idea of a material object with so much mass that it could trap light was conceived centuries before general relativity and quantum mechanics. It was proposed in the 18th century by English astronomer and clergyman John Michell and independently by French scientist Pierre-Simon Laplace. Both scholars proposed very large stars in contrast to the modern concept of an extremely dense object. Michell called it a "dark star" and Laplace an étoile occlu.

The popular definition of a black hole is a region of space having a gravitational field so intense that no matter or radiation can escape. When the gravitational attraction of the mass at the center singularity is greater than the centrifugal force of a particle grazing the event horizon, the particle will be drawn into the black hole, never to escape.

The centrifugal force on a particle with mass m traveling around a circle with radius r at velocity v is ½(mv²/r). The gravitational attraction of a mass M at the center is GMm/r².

Equating these, and assuming v = c, the velocity of light, the Schwarzscild radius R = 2GM/c².

Jacob Bekenstein showed that adding a low-energy photon with a wavelength the size of a black hole would add an area one Planck-length squared to its event horizon.¹

Leonard Susskind argued that the photon adds one bit of lost (or hidden) information to the internal entropy of the black hole (i.e., in the singularity at it's center) and one bit of information at the event horizon. Susskind's "holographic universe" interprets the event horizon as a hologram of the lost (or hidden) information inside a black hole.

But the event horizon is not a material structure capable of storing information. As first calculated by Karl Schwarzschild and Albert Einstein in the 1920's, the horizon is simply an abstract mathematical surface where the pull of gravity from the condensed matter of the singularity at the center prevents anything, including light, from crossing the horizon and escaping the black hole.

Stephen Hawking and Roger Penrose showed that all the matter in a black hole would be crushed into its center at a singularity they said would be infinitesimal in size but infinite in density. There is nothing but empty space in a black hole between the singularity at the center and the event horizon. Any particle that enters the horizon will very rapidly fall to the center.

John Wheeler showed that the event horizon would be featureless, it would have "no hair."
This is strictly correct. No visible radiation is coming through the event horizon, but we will see that immense amounts of radiation are being produced by the extreme activity in the immediate vicinity of the black hole event horizon.

Hawking denied the idea that black holes have "no hair," that the event horizon is featureless.

He said that black holes are not so black after all, because they are radiating information. This needs careful explanation.

His idea is that "virtual particle" pairs are appearing just outside the event horizon, with one particle going into the black hole and the other radiating away what is known as "Hawking radiation." Now there is a vast amount of radiative activity around all observed black holes, otherwise they would not be observable, only discoverable by their gravitational effect on their luminous neighbors.

For supermassive black holes at the center of galaxies, all this activity is caused by the vast amounts of intergalactic material falling into the black hole. These active galactic nuclei were first detected as "quasars" or quasi-stellar sources. The largest are today called "blazers." They are the most powerful and brightest objects in the universe.

Hawking's second idea was that his radiation would be evaporating a black hole. This needs very careful examination. First, there is no evidence that black holes are evaporating any time soon. All observed black holes appear to be growing, fed by the intergalactic material they are accreting. Even that single virtual particle Hawking saw falling in (as its partner radiates away) does not reduce, it adds to the black hole mass. We can note that Hawking's pair production might be happening inside the event horizon a short distance. As long as the gravitational red shift is not enough to reduce the energy to zero, a weakened photon can pass through the event horizon.

The second law of thermodynamics suggests that even black holes that are currently unobservable are also growing and not evaporating. The intergalactic material is very sparse and very cold, warmed only by the 2.7K cosmic background radiation. But a black hole singularity is thought to be much, much colder, perhaps a millionth of a degree Kelvin. Heat flows from hot to cold, carrying the atoms of the intergalactic medium (perhaps as little as a few hydrogen atoms per cubic meter) into the black hole.

Hawking's third idea, and perhaps most important, was that information is being lost to the black hole. This is very likely correct

But multiple physicists claimed Hawking was wrong. Some made bets.

Susskind described this as the "black hole war" which, he said, was fought to "make the world safe for quantum mechanics." His argument was based on the argument that "information never dies." This is a "law of physics that may be even more fundamental than energy conservation. It's sometimes called reversibility, but let's just call it information conversation."².

As we saw, Roger Penrose and Stephen Hawking thought so. But their mathematical analysis of one-dimensional geodesic curves in general relativity said nothing about the distribution of collapsed matter around their singular point.

The mass of gravitationally collapsed objects ranges from a fraction of a solar mass to 100 billion solar masses. The smallest are terrestrial planets whose central temperatures never reached levels that support thermonuclear reactions in the smallest stars. Chemical forces support the planets against further gravitational collapse.

The smallest and dimmest stars are red dwarfs as little as .08 of a solar mass. They burn their hydrogen fuel so slowly they can live for billions of years. The largest stars are a thousand times more massive and are very short lived.

When our Sun consumes its core hydrogen, further collapse will begin to burn hydrogen in a shell around the depleted core, expanding the solar surface as a red giant star that over five billion years that swallows Mercury and Venus and approaches the orbit of Earth. A final collapse and explosion of the energy-genertaing core can blow off the rest of the Sun as a planetary nebula, leaving the core slowly cooling as a white dwarf. Further collapse of the white dwarf is prevented by the pressure of a degenerate electron gas.

Slightly larger stars explode as supernovae. Gravitational collapse of the core forces their electrons to combine with protons converting them to neutrons, whose degeneracy now provides further collapse to a black hole. The resulting neutron star may rotate rapidly and appear as a flashing pulsar.

So what about the matter in the largest black holes, which can contain many millions of solar masses? Neither electron nor neutron degeneracy can stop their cores from collapsing until all nucleons have combined into quarks. Extremely dense and high temperature quark matter would resemble the origin of the universe, though now contracting instead of expanding. Lots of little crunches.

They could be called quarkstars although their radiation could not escape beyond its distant event horizon. That radiation and the mass of quarks would likely occupy a far greater volume than the supposed infinitesimal point singularity, although from outside the event horizon their actual size coannever be observed.

In April 1988, Stephen Hawking gave a lecture at University of California, Berkeley entitled "Black Holes and Baby Universes" He asked what would happen to objects falling into a black hole. Then he suggested...

According to some recent work of mine, the answer is that they will go off into a little baby universe of their own. A small, self-contained universe branches off from our region of the universe. This baby universe may join on again to our region of space-time. If it does, it would appear to us to be another black hole that formed and then evaporated. Particles that fell into one black hole would appear as particles emitted by the other black hole, and vice versa.

Black Holes and Baby Universes: And Other Essays (pp. 113-114).

Notes

1. The Schwarzschild radius is the radius of a spherical boundary around a massive object; if the object's entire mass collapses within this radius, it becomes a black hole. This boundary, also known as the event horizon, is the point of no return, where the escape velocity equals the speed of light. The formula for the Schwarzschild radius is
   R = 2GM/c²
   where G is the gravitational constant, M the mass of the object, and c the speed of light.  
2. Susskind, The Black Hole War, p.87

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