# What is a black hole?

##### 1 Answer
Dec 28, 2016

We don't actually know what a black hole is.

#### Explanation:

Black holes were predicted by the Schwarzschild solution to Einstein's General Theory of relativity. It defines a Schwarzschild radius ${r}_{s}$ for a body given by:

${r}_{s} = \frac{2 G M}{c} ^ 2$

Where G is the gravitational constant; M is the mass of the body and c is the speed of light.

The theory predicts that any body which is smaller than its Schwarzschild radius has a singularity at its Schwarzschild radius, called an event horizon and at it's centre. This is a black hole.

This means that the inside and outside of the black hole can never communicate. It is possible to enter a black hole never to return.

A blackhole occurs when the core of a star after a supernova explosion is too massive and so collapses in on itself, for a blackhole to be created the core must so massive that even Neutron Degeneracy pressure cannot support it.

The singularity inside the black hole presents a problem. It is a point of infinite density which everything entering the black hole must ultimately become part of. This means that General relativity can't describe the inside of a black hole and we need new theories to understand what is actually there. A theory of "Quantum Gravity" is needed to explain what happens inside a black hole.