Black Holes

A black hole is an object with a concentration of mass great enough that the force of gravity prevents anything from escaping from it except through quantum tunneling behavior. The gravitational field is so strong that the escape velocity near it exceeds the speed of light. This implies that nothing, not even light, can escape its gravity, hence the word "black." The term "black hole" is widespread, even though it does not refer to a hole in the usual sense, but rather a region of space from which nothing can return. Theoretically, black holes can have any size, from microscopic to near the size of the observable universe.

Black holes are predicted by general relativity. According to classical general relativity, neither matter nor information can flow from the interior of a black hole to an outside observer. For example, one cannot bring out any of its mass, or receive a reflection back by shining a light source such as a flashlight, or retrieve any information about the material that has entered the black hole. Quantum mechanical effects may allow matter and energy to radiate from black holes; however, it is thought that the nature of the radiation does not depend on what has fallen into the black hole in the past.

The existence of black holes in the universe is well supported by astronomical observation, particularly from studying supernovae and X-ray emissions from active galactic nuclei.

Contents [showhide]

1 History

2 Qualitative physics

2.1 The event horizon

2.2 The singularity

2.3 Entering a black hole

2.4 Rotating black holes

2.5 Entropy and Hawking radiation

3 Reality of black holes

3.1 Formation

3.2 Observation

3.3 Have we found them?

4 Micro black holes

5 Mathematical physics

6 Recent discoveries

7 Alternate models

8 Related topics

9 External links

10 Further reading

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History

The concept of a body so massive that not even light could escape from it was put forward by the English geologist John Michell in a 1783 paper sent to the Royal Society. At that time, the Newtonian theory of gravity and the concept of escape velocity were well known. Michell computed that a body 500 times the radius of the Sun and of the same density would have at its surface an escape velocity equal to the speed of light, and therefore would be invisible. In his words:

If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae (inertial mass), with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.

Although he thought it unlikely, Michell considered the possibility that many such objects that cannot be seen might be present in the cosmos.

In 1796, the French mathematician Pierre-Simon Laplace promoted the same idea in the first and second edition of his book Exposition du Systeme du Monde. It disappeared in later editions. The whole idea gained little attention in the 19th century, since light was thought to be a massless wave, not influenced by gravity.

In 1915, Einstein developed the theory of gravity called General Relativity. Earlier he had shown that gravity does influence light. A few months later, Karl Schwarzschild gave the solution for the gravitational field of a point mass, showing that something we now call a black hole could theoretically exist. The Schwarzschild radius is now known to be the radius of a non-rotating black hole, but was not well understood at that time. Schwarzschild himself thought it not to be physical.

In the 1920s, Subrahmanyan Chandrasekhar argued that special relativity demonstrated that a non-radiating body above a certain mass, now known as the