While black hole theory states that most black holes cannot be directly observed for verification, it asserts that these objects can be inductively concluded from observation of phenomena which are associated with matter "falling into" them, such as emission of electromagnetic radiation. Other inductive conclusions are drawn from objects, such as stars, that appear to be in orbit around space where there is no visible matter and from accretion disks and jets formed from these disks.

Accretion disks, jets, and orbital motions are found not only around black holes, but are common astrophysically around other objects such as neutron stars, and the physics of materials near these non-black hole objects is largely but not completely identical to the physics of materials around black holes. 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.

What leads to the conclusion that the object being observed is a black hole, is that the mass of the central object is such that general relativity states that the central object is a black hole. There have been some occasional papers which argue that some process will stop the collapse, and that the object being observed is not an actual black hole. However, these papers generally require new and untested physics.

There is some observational evidence that the object at the center of observed accretion disks are black holes. Accretion disks around low-mass objects will often experience flare ups, which have been interpreted as being caused when material block by magnetic fields suddenly falls on the surface of the compact object. These flare ups are not observed around high mass compact objects, and the standard explanation is that high mass compact objects are black holes with no surface onto which matter can be suddenly dumped.

Table of contents

1 Overview 2 Theoretical consequences 3 Observational evidence 4 Black hole formation 5 Related topics 6 External links

Overview

Black holes are believed to form from the gravitational collapse of astronomical objects containing two or more solar masses. Astronomical observations suggest that the centers of most galaxies, including our own Milky Way, contain supermassive black holes containing millions to billions of solar masses.

Black holes are predictions of Einstein's theory of general relativity. In particular, they occur in the Schwarzschild metric, one of the earliest and simplest solutions to Einstein's equations, found by Karl Schwarzschild in 1915. This solution describes the curvature of spacetime in the vicinity of a static and spherically symmetric object.

According to Schwarzschild's solution, a gravitating object will collapse into a black hole if its radius is smaller than a characteristic distance, known as the Schwarzschild radius. Below this radius, spacetime is so strongly curved that any light ray emitted in this region, regardless of the direction in which it is emitted, will travel towards the center of the system. Because relativity forbids anything from travelling faster than light, anything below the Schwarzschild radius - including the constituent particles of the gravitating object - will collapse into the center. A gravitational singularity, a region of theoretically infinite density, forms at this point. Because not even light can escape from within the Schwarzschild radius, a classical black hole would truly appear black.

The Schwarzschild radius is given by

The mean density inside the Schwarzschild radius decreases as the mass of the black hole increases, so while an earth mass black hole would have a density of 2 × 10³⁰ kg/m³, a supermassive black hole of 10⁹ solar masses has a density of around 20 kg/m³, less than water! The mean density is given by

More general black holes are also predicted by other solutions to Einstein's equations, such as the Kerr metric for a rotating black hole, which possesses a ring singularity, and the Reissner-Nordstrøm metric for charged black holes. The generalization of the Schwarzschild radius is known as the event horizon.

Theoretical consequences

Black holes demonstrate some counter-intuitive properties of general relativity. Consider a hapless astronaut falling radially towards the center of a Schwarzschild black hole. The closer she comes to the event horizon, the longer the photons she emits take to escape to infinity. A distant observer will see her descent slowing as she approaches the event horizon, which she never appears to reach. However, in her own frame of reference, the astronaut crosses the event horizon and reaches the singularity in a finite amount of time.

Black holes produce other interesting results when applied in unison with other physical theories. A commonly stated proposition is that "black holes have no hair", meaning they have no observable external characteristics that can be used to determine what they are like inside. Black holes have only three measurable characteristics: mass, angular momentum, and electric charge, and can be completely specified by these three parameters.

The entropy of black holes is a fascinating subject, and an area of active research. In 1971, Hawking showed that the total event horizon area of any collection of classical black holes can never decrease. This sounds remarkably similar to the Second Law of Thermodynamics, with area playing the role of entropy. Therefore, Bekenstein proposed that the entropy of a black hole really is proportionate to its horizon area. In 1975, Hawking applied quantum field theory to a semi-classical curved spacetime and discovered that black holes can emit thermal radiation, known as Hawking radiation. This allowed him to calculate the entropy, which indeed was proportionate to the area, validating Bekenstein's hypothesis. It was later discovered 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 proposal of the holographic principle.

Observational evidence

There is now a great deal of observational evidence for the existence of two types of black holes:

• Those with masses of a typical star (4-15 times the mass of our Sun), and
• Those with masses perhaps 1% that of a typical galaxy (supermassive black holes).

Additionally, there is some evidence for intermediate-mass black holes (IMBHs), those with masses of a few thousand times of the Sun. These black holes may be responsible for the formation of supermassive black holes.

A third proposed type of black hole, primordial black holes, have not been observed.

In the case of both a stellar size and supermassive black hole, matter can be drawn in, producing an accretion disk and large amounts of X-rays. As gas falls into a black hole, frictional heating causes large amounts of energy to be released. 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 energy.

This efficient radiation mechanism is essential in order to produce that amount of energy that is observed in active galactic nuclei such as quasars, and its introduction in the 1970's 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 1980's 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 there is no more, and the process shuts off. This picture also nicely explains why there are no nearby quasars.

They may be involved in gamma ray bursters, although observations of GRB's in association with supernova have reduced the possibility of a link.

Black hole formation

Close to solar mass black holes are created by the gravitational collapse of massive stars. When a star exhausts its nuclear fuel, the equilibrium between gravitation and radiation pressure is disturbed, and it collapses. If the mass of the star is greater than about 3 times the mass of the sun, the collapse cannot be stopped, and a black hole is created. (See stellar evolution.)

Instead of collapsing on themselves, black holes might also be created by compression of matter by extreme external pressure. Such black holes are called primordial black holes. The enormous pressures necessary for creating primordial black holes are thought to have existed in the very early stages of the universe. These black holes can have masses smaller than that of the sun.

The formation of supermassive black holes is currently matter of very active research. Though the mechanism of formation is still not clear, there is increasing evidence that the growth of the black hole is intimately related to the growth of the spheroidal component (elliptical galaxy, or bulge of a spiral galaxy) in which it lives.

Related topics

• timeline of black hole physics
• string theory
• white hole
• neutron star
• supermassive black hole
• Gravastar

• Jilian's Guide to Black Holes
• Supermassive Black Holes
• Schwarzschild Geometry