# What is black hole in-universe?

Feb 14, 2018

A black hole is a region of space once something enters it can never leave, not even light.

#### Explanation:

Albert Einstein published his field equations of General Relativity in 1915. Soon after Karl Schwarzschild solved the field equations for the vacuum around a massive body.

The Schwarzschild solution has a singularity where time stops and nothing can escape. It is called the Schwarzschild radius ${r}_{s}$.

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

Where $g$ is the gravitational constant, $M$ is the mass of the body and $c$ is the speed of light.

This means that any mass which gets compressed to a radius below ${r}_{s}$ is effectively isolated from the rest of the universe. Nothing which enters this radius can escape, not even light. This is a black hole.

The Sun is too small to collapse into a black hole, but we can calculate ${r}_{s}$. The mass of the Sun $M = 1.989 \cdot {10}^{30} K g$, $G M = 1.327 \cdot {10}^{20}$, $c = 299 , 792 , 458 m \frac{\setminus}{s}$. So, ${r}_{s} = 2 , 953 m$. This is less than 3 kilometers!

So, Schwarzschild predicted black holes in 1915. More recently objects have been discovered which are so small and massive that they can only be black holes.