 |
Business | Industries | Finance | Tax |
| Index: > A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|
|||||
| First Prev [ 1 2 3 4 5 6 ] Next Last |
An artist's impression of a black hole with a closely orbiting companion star that exceeds its roche limit. Infalling matter forms an accretion disk, with some of the matter being ejected in highly energetic polar jets.
A black hole is a concentration of mass with a gravitational field so strong that the escape velocity from nearby points 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 the theory does not refer to any hole in the usual sense, but rather a region of space from which nothing can return.
Black holes are predicted by general relativity. According to classical general relativity, no matter or information can flow from the interior of a black hole to an outside observer (e.g., 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), although quantum mechanics may allow deviations from this strict rule. The existence of black holes in the universe is well supported both theoretically and by astronomical observation.
The concept of a body so massive that not even light could escape from it was put forward by the English geologistA geologist is a contributor to the science of geology. Geologists study the physical structure and processes of the Earth. Their undergraduate training typically includes significant coursework in chemistry, physics, mathematics and possibly biology, in John MichellJohn Michell ( 1724 April 29, 1793) was an English natural philosopher and geologist, whose work was rediscovered in the 1970s. It appeared that John Michell was the first to consider a heavenly object massive enough to prevent light from escaping it (the in a 1783Events February 3 American Revolutionary War: Spain recognizes United States independence. February 4 American Revolutionary War: Great Britain formally declares that it will cease hostilities with the United States of America. May 18 Saint John, New Brun paper sent to the Royal SocietyThe Royal Society of London is claimed to be the oldest learned society still in existence and was founded in 1660. The Royal Irish Academy, founded in 1782, is also closely affiliated with it. The Royal Society of Edinburgh (founded 1783) is a separate S. At that time, the NewtonianKneller's portrait of 1689. Sir Isaac Newton ( December 25, 1642 March 20, 1727 by the Julian calendar then in use; or January 4, 1643 March 31, 1727 by the Gregorian calendar) was an English physicist, mathematician, astronomer, philosopher, and alchemis theory of gravity and the concept of escape velocity were well known. Michell computed that a body 500 times the radiusThe word radius ( Latin for "wheel spoke"; plural radii pronounced ray dee-eye has several meanings in English: In classical geometry, a radius of a circle or sphere is any line segment with one endpoint on the circle (i. the circular boundary) and the ot of the sun and of the same densityFor other meanings of density, see density (disambiguation Density (symbol: rho Greek: rho) is a measure of mass per unit of volume. The higher an object's density, the higher its mass per volume. The average density of an object equals its total mass div would have at its surface an escape velocity equal to the speed of light, and therefore would be invisible. In his words:
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 theoretical interest until the late 1960s.
Interest in collapsed objects was rekindled in 1967 with the discovery of pulsars. Shortly thereafter, the use of the expression "black hole" was coined by theoretical physicist John Wheeler [1]. Prior to that time, the term black star was used occasionally. The term appears in an early episode of Star Trek, and was still used occasionally after 1967. This is because some people found the term "black hole" obscene when translated into French or Russian, for example.
| Politics | Business | Finance |
 |
 |
 |
|