What is black hole in-universe?

1 Answer
Phillip E.
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.

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