Changes

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.

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 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]].

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 in fact can pass beyond the Schwarzchild radius.  In fact, the equations predicted he would notice no effects when doing so. 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===

What actually exists inside the event horizon of a black hole is a question physics has thus far been unable to answer. Some postulate that within the event horizon exists a point of zero (or nearly zero) volume but infinite energy density, a point sometimes referred to as a ''gravitational singularity,'' after the notion of a mathematical singularity in a field equation. Others suspect that infinite energy density is a physical impossibility, and that all the matter contained within a black hole is compressed into a [[degenerate matter|degenerate]] form, such as quark-degenerate matter. Since the presence of the event horizon surrounding a black hole makes it impossible to directly measure anything within, it is entirely possible that we may never know what the interior structure of a black hole is like.

What actually exists inside the event horizon of a black hole is a question physics is unable to answer. Some postulate that within the event horizon exists a point of zero (or nearly zero) volume but infinite energy density, a point sometimes referred to as a ''gravitational singularity,'' after the notion of a mathematical singularity (a term equaling infinity) in a field equation. Others suspect that infinite energy density is a physical impossibility, and that a black hole contains actual finitely-dense matter compressed into a [[degenerate matter|degenerate]] form, such as quark-degenerate matter. Since all black holes are surrounded by an event horizon which prevents any information or messages from leaving, all these theories are non-[[falsifiable]]; we can never be sure what the interior structure of a black hole is like.