Changes

==Nature of a Black Hole==

==Nature of a Black Hole==

The theoretical model of what we now call a black hole has evolved considerably over the centuries. The [[corpuscular theory of light]] held that light was made up of invisibly small particles, and that these particles moved along ballistic trajectories, like tiny bullets. In this framework, it was believed possible that a distant star could be so massive that light emitted from its surface would be dragged back down again. This theory was first advanced by John Michell, who wrote in 1783, "If the semi-diameter of a sphere of the same density as the Sun in the proportion of five hundred to one, and by supposing light to be attracted by the same force in proportion to its [mass] with other bodies, all light emitted from such a body would be made to return towards it, by its own proper gravity."<ref>http://www.aps.org/publications/apsnews/200911/physicshistory.cfm</ref>

The theoretical model of what we now call a black hole has evolved considerably over the centuries. The [[corpuscular theory of light]] held that light was made up of invisibly small particles, and that these particles moved along ballistic trajectories, like tiny bullets. In this framework, it was believed possible that a distant star could be so massive that light emitted from its surface would be dragged back down again. This theory was first advanced by John Michell, who wrote in 1783, "If the semi-diameter of a sphere of the same density as the Sun in the proportion of five hundred to one, and by supposing light to be attracted by the same force in proportion to its [mass] with other bodies, all light emitted from such a body would be made to return towards it, by its own proper gravity."<ref name="r1">http://www.aps.org/publications/apsnews/200911/physicshistory.cfm</ref>

Suggesting the same possibility independently, Pierre-Simon Laplace wrote in 1796, "It is therefore possible that the greatest luminous bodies in the universe are on this account invisible."<ref name="r1" />

Suggesting the same possibility independently, Pierre-Simon Laplace wrote in 1796, "It is therefore possible that the greatest luminous bodies in the universe are on this account invisible."<ref name="r1" />

===General Relativity===

===General Relativity===

As it happened, the question became unavoidable shortly after the publication in 1915 of Einstein's [[general theory of relativity]].  [[Schwartzchild]] solved the Einstein field equations in a way that describes the geometry of spacetime outside a spherically symmetric, uncharged, non-rotating distribution of mass. Well away from the center of this distribution of mass, the Schwarzchild solution closely matches the Newtonian model of a gravitational field; only close to the mass, where the curvature of spacetime is large, do significant differences between the two models appear. But if the diameter of the mass distribution is taken to be arbitrarily small, then the region of spacetime immediately surrounding the mass appears to take on extremely curious properties, properties so curious that many questioned whether they had any physical interpretation at all.  Therefore, Schwartzchild showed that black holes were possible under the theory of [[general relativity]].

As it happened, the question became unavoidable shortly after the publication in 1915 of Einstein's [[general theory of relativity]].  [[Schwarzschild]] solved the Einstein field equations in a way that describes the geometry of spacetime outside a spherically symmetric, uncharged, non-rotating distribution of mass. Well away from the center of this distribution of mass, the Schwarzschild solution closely matches the Newtonian model of a gravitational field; only close to the mass, where the curvature of spacetime is large, do significant differences between the two models appear. But if the diameter of the mass distribution is taken to be arbitrarily small, then the region of spacetime immediately surrounding the mass appears to take on extremely curious properties, properties so curious that many questioned whether they had any physical interpretation at all.  Therefore, Schwarzschild showed that black holes were possible under the theory of [[general relativity]].

[[File:Cygnusx1.jpg|right|200px|thumb|Jet-powered nebula formed from the accretion disk of the binary star Cygnus-X1]]

[[File:Cygnusx1.jpg|right|200px|thumb|Jet-powered nebula formed from the accretion disk of the binary star Cygnus-X1]]

However, neither Schwartzchild himself nor [[Albert Einstein]], who developed the theory of relativity, believed that black holes actually existed<ref>http://amazing-space.stsci.edu/resources/explorations/blackholes/lesson/whatisit/history.html</ref>.  Einstein even tried to re-work general relativity to render these singularities impossible.  However, Roger Penrose and Stephen Hawking proved the first of many Singularity Theorems, which states that singularities must form if certain conditions are present. This demonstrated that, rather than mathematical oddities, singularities are a fairly generic consequence of realistic solutions to relativity:  any mass with radius less than its Schwarzschild Radius is a black hole..  Since then, support for black holes among the scientific community has grown.

However, neither Schwarzschild himself nor [[Albert Einstein]], who developed the theory of relativity, believed that black holes actually existed<ref>http://amazing-space.stsci.edu/resources/explorations/blackholes/lesson/whatisit/history.html</ref>.  Einstein even tried to re-work general relativity to render these singularities impossible.  However, Roger Penrose and Stephen Hawking proved the first of many Singularity Theorems, which states that singularities must form if certain conditions are present. This demonstrated that, rather than mathematical oddities, singularities are a fairly generic consequence of realistic solutions to relativity:  any mass with radius less than its Schwarzschild Radius is a black hole..  Since then, support for black holes among the scientific community has grown.

==Nature of a Black Hole==

==Nature of a Black Hole==

Within a certain distance from an arbitrarily small distribution of mass — a distance now known as the [[Schwarzchild radius]] — the curvature of spacetime becomes so great that no paths leading away from the mass exist. That is to say, a test particle released inside the Schwarzchild radius will inevitably move in toward the mass, not because the force of gravity is great as in the Newtonian approximation, but because spacetime is curved to such an extent that ''no other directions exist.'' A particle within the Schwarzchild radius can no more move further from the central mass than it can go backwards in time. In fact, from the frame of reference of an infalling observer beyond the Schwarzchild radius, all directions that once pointed away from the central mass now point backwards in time. Once inside the Schwarzchild radius, further motion toward the central mass is as inevitable as further motion through time is for any other observer.

Within a certain distance from an arbitrarily small distribution of mass — a distance now known as the [[Schwarzschild radius]] — the curvature of spacetime becomes so great that no paths leading away from the mass exist. That is to say, a test particle released inside the Schwarzschild radius will inevitably move in toward the mass, not because the force of gravity is great as in the Newtonian approximation, but because spacetime is curved to such an extent that ''no other directions exist.'' A particle within the Schwarzschild radius can no more move further from the central mass than it can go backwards in time. In fact, from the frame of reference of an infalling observer beyond the Schwarzschild radius, all directions that once pointed away from the central mass now point backwards in time. Once inside the Schwarzschild radius, further motion toward the central mass is as inevitable as further motion through time is for any other observer.

For some time after the publication of the Schwarzchild solution, the validity of these results was hotly debated. In the solution's original coordinate frame, some terms in the equations ''diverged,'' or became infinite, at the Schwarzchild radius, leading physicists to wonder whether the results of the equations in that region had any valid physical interpretation. One proposed interpretation was that at the Schwarzchild radius, all time for the infalling observer would stop. This led to the use of the term "frozen stars;" it wasn't believed frozen stars were cold, but rather that they were literally frozen in time.

For some time after the publication of the Schwarzschild solution, the validity of these results was hotly debated. In the solution's original coordinate frame, some terms in the equations ''diverged,'' or became infinite, at the Schwarzschild radius, leading physicists to wonder whether the results of the equations in that region had any valid physical interpretation. One proposed interpretation was that at the Schwarzschild radius, all time for the infalling observer would stop. This led to the use of the term "frozen stars;" it wasn't believed frozen stars were cold, but rather that they were literally frozen in time.

Later refinement of the Schwarzchild solution demonstrated that the apparent infinities were merely an artifact of the coordinate frame chosen, and that an infalling observer would in fact notice no effects when passing beyond the Schwarzchild radius. But any attempt on the part of that infalling observer to communicate with the outside universe, say by sending a radio message, would be doomed to failure, as the radio waves would traverse geodesics through the severely curved spacetime and end up bent toward the central mass. From this, we can say that nothing that occurs within the Schwarzchild radius can ever affect events outside the Schwarzchild radius. This gives the Schwarzchild radius of a non-rotating black hole its other name: the [[event horizon]].

Later refinement of the Schwarzschild solution demonstrated that the apparent infinities were merely an artifact of the coordinate frame chosen, and that an infalling observer would in fact notice no effects when passing beyond the Schwarzschild radius. But any attempt on the part of that infalling observer to communicate with the outside universe, say by sending a radio message, would be doomed to failure, as the radio waves would traverse geodesics through the severely curved spacetime and end up bent toward the central mass. From this, we can say that nothing that occurs within the Schwarzschild radius can ever affect events outside the Schwarzschild radius. This gives the Schwarzschild radius of a non-rotating black hole its other name: the [[event horizon]].

===Inside the Event Horizon===

===Inside the Event Horizon===