What a black hole ``is'' depends on what you think an accurate description of the Universe is. What a black hole ``does'' is not dependent on that.

Most people know there is a ``problem'' in physics: General Relativity and Quantum Mechanics don't work together. General Relativity, more or less, describes space-time as a ``fabric''. General Relativity works extremely well for describing objects that are large and either moving extremely fast and/or have very large masses. Quantum mechanics works extremely well for describing objects that are very small.

Now for black holes:

You may be familiar with the concept of

escape velocity; the speed you need to escape the gravitational pull of an object. If you are standing on the surface of the Earth, for example, you need to reach a speed of 11 200 m/s or more if you want to avoid being eventually pulled back by Earth's gravity.

Knowing about the escape velocity, one can easily calculate how massive and how large a spherical object needs to be before the surface escape velocity is equal to the speed of light; this is the

Schwarzschild radius (and is easily found by setting

*v*_{e} =

*c* in the escape velocity equation and rearranging to put

*r* on the left-hand side).

*EVERY* object (including you) has a Schwarzschild radius. However black holes are the

*ONLY* objects whose mass falls entirely

*within* that Schwarzschild radius.

So far this is pretty simple. The real question about ``what'' a black hole is depends on what happens

*inside* the Schwarzschild radius.

**In General Relativity** there is nothing to stop matter from being compressed infinitely, and it is impossible to exceed the speed of light. Therefore the large surface gravity of a black hole causes the matter making up that black hole to get squeezed down to a point.

In other words; once you have squeezed an object

*below* its Schwarzschild radius (by a supernova explosion perhaps, or just an impossibly strong anvil press in your lab, or with your own superman hands, or whatever) that object will have sufficient surface gravity to squeeze

*itself* smaller and smaller. (See the wiki on the

Schwarzschild metric for a more technical description.)

General Relativity has no limits on how small an object can be squeezed, so eventually this black hole will be a true singularity - an infinitely small object with a finite mass.

For objects falling into the black hole the Schwarzschild radius acts as the ``boundary'' or the ``event horizon'' of the black hole; as long as you stay outside this boundary you can still (theoretically) escape.

In General Relativity, gravity is described as ``curvature of space''. In this description a singularity can be pictured as an infinitely deep ``dimple'' on the ``fabric of space-time''.

For a

*true* Schwarzschild black hole, as described above, there is nothing ``one the other side''. There is no ``other side'' to go to. At the singularity all directions (not just those in

*space*, but also those in

*time*) are meaningless: it is kind of like trying to go ``North'' when you are already at the North pole.

**In Quantum Mechanics** a black hole cannot be properly described because Quantum Mechanics cannot describe gravity. However, Quantum Mechanics does have a lot of rules that prevent any kind of singularity from forming, so a black hole may have a finite size. Unfortunately a full Quantum description of what a black hole would look like is not possible at the present.

--------------------------

One problem with talking about black holes is that we are unable to observe what happens inside the event horizon. There is good evidence that black holes exist; but we have no way of telling if these event horizons contain true gravitational singularities, or some sort of Quantum-balanced

dark star. Without a Quantum theory of gravity we can speculate, but not rigourously define the limit where

gravity beats the Pauli exclusion principle.

--------------------------

In the context of General Relativity, it is also possible for the space-time ``dimple'' to be a space-time ``tunnel'' to somewhere else (another part of our Universe, or perhaps somewhere else entirely). A sufficiently narrow ``tunnel'' could still have an event horizon around the entrance, but not end in a true singularity.

There is no known mechanism for naturally forming this sort of black hole/wormhole however: a wormhole to a parallel Universe is theoretically possible according to General Relativity, but

*someone* still has to build it - a supernova remnant will

*not* turn into a wormhole.

--------------------------

I would also like to stress a subject which I think is often misunderstood:

*outside* of the Event Horizon black holes are normal, ``well behaved'' objects. If some super-villain somehow managed to spontaneously convert the Moon into a black hole we would

*not* all be sucked to our doom. The black-hole Moon would still orbit the Earth like normal, the tides would still behave normally. Obviously we wouldn't be able to

*see* the black-hole Moon anymore, and attempting to land on it would be

*extremely* ill-advised, but other than that there would be no danger.