Simulation



A black hole is a star that has collapsed into a tiny point known as a singularity. It is so dense that it sucks in everything near it, including light. Black holes can be seen via the death throes of the matter being sucked in. Although it becomes invisible past a certain point, an accretion disk, which is visible, develops as the matter swirls toward the black hole. The collapsed star is so dense that nothing can escape its gravitational pull, not even light.

A black hole is 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.

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. 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 Chandrasekhar limit, would collapse since there would be nothing that could stop the collapse. His arguments were opposed by Arthur Eddington, who believed that something would inevitably stop the collapse.

In 1939, Robert Oppenheimer and H. Snyder predicted that massive stars could undergo a dramatic gravitational collapse. Black holes could in principle be formed in nature. Such objects for a while were called frozen stars since the collapse would be observed to rapidly slow down and become heavily reddened near the Schwarzschild radius. However, these hypothetical objects were not the topic of much interest until the late 1960s. Most physicists believed that they were a peculiar feature of the highly symmetric solution found by Schwarzschild, and that objects collapsing in nature would not form black holes.

Interest in black holes was rekindled in 1967, due to theoretical and experimental progress. Stephen Hawking and Roger Penrose proved that black holes are a generic feature in Einstein's theory of gravity, and cannot