Question: How big is the black hole and how do we know?

Pratik Samant answered on 17 Nov 2020: last edited 17 Nov 2020 1:45 pm

A black hole is in theory infinitesimally small, but in terms of quantifying the size, we often use something called the Schwarzschild radius.This is the radius of the black hole at which the event horizon occurs. The event horizon is the point after which nothing can escape a black hole, so the size of the event horizon (which has the shape of a sphere) is a good way to quantify the size of the black hole itself.

The formula for the Schwarzschild radius is r=2GM/c^2 where c is the speed of light, G is the gravitational constant [6.674×10^-11 N m^2/kg^2 (newton square meters per kilogram squared)], and M is the mass of the black hole. So you can see that the more massive a black hole is, the larger its Schwarzschild radius.