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Black holes are objects so dense that not even light can escape their gravity, which are believed to form from the gravitational collapse of astronomical objects with two or more solar masses. Astronomical observations suggest that the center of most galaxies, including our own Milky Way, contain supermassive black holes with millions to billions of solar masses.

Black holes are predictions of Einstein's theory of general relativity. The simplest static and spherically symmetric solution to Einstein's equations was found by Karl Schwarzschild in 1915. The Schwarzschild metric describes the curvature of spacetime in the vicinity of a nonrotating spherical mass.

The Schwarzschild metric predicts that a gravitating object will collapse into a black hole if its radius is smaller than a characteristic distance called the Schwarzschild radius, which is proprotionate to the object's mass. Below the Schwarzschild radius, spacetime is so strongly curved that any light ray emitted in this region will travel towards the center of the system, regardless of the direction in which it is emitted. Because relativity forbids anything from travelling faster than light, anything below the Schwarzschild radius - including the gravitating object itself - will collapse into the center point, where a gravitational singularity forms. Because not even light can escape from within the Schwarzschild radius of a classical black holes would truly appear black.

More general black holes can be described by other solutions to Einstein's equations, such as the Kerr metric for a rotating black hole, which possesses a ring singularity. 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. Thus, a distant observer will see her descent slowing as she approaches the event horizon, which she never reaches in a finite amount of time. 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 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 of a typical galaxy. This evidence comes not from seeing the black holes directly, but by observing the behavior of stars and other material near them.

In the case of a stellar size black hole, matter can be drawn in from a companion star, producing an accretion disk and large amounts of X-rays.

Galaxy-mass black holes with 10 to 100 billion solar masses were found in Active Galactic Nuclei (AGN), using radio and X-ray astronomy. 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.

Black holes are also the leading candidates for energetic astronomical objects such as quasars and gamma ray bursts.

See also:

• Timeline of black hole physics
• string theory
• white hole

External Link

• Supermassive Black Holes