What is black hole in-universe?

1 Answer
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 = (2GM)/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*10^30Kg, GM=1.327*10^20, c=299,792,458 m \/ s. So, r_s=2,953m. 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.