Deep Field

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Deep Field
29 September 2026 2518 stories tracked

Glossary · Black Holes

Schwarzschild radius

How small you would have to squeeze something to make it a black hole.

Every mass has a radius at which its escape velocity would reach light speed: about 3 km for the Sun, 9 mm for Earth, and 30 microns for an adult human. Karl Schwarzschild derived it in 1916, in the trenches, months after Einstein published the field equations. It scales linearly with mass, which has the counter-intuitive consequence that the largest black holes are the least dense — a supermassive one can have an average density lower than water.

Related terms

Event horizon The boundary around a black hole past which nothing gets out — not a surface, but a point of no return. Accretion disc The flat, ferociously hot spiral of matter falling onto a compact object. Singularity The point where our description of gravity stops producing meaningful answers. Supermassive black hole Millions to billions of solar masses, one at the centre of essentially every large galaxy.

Questions

What is Schwarzschild radius?

How small you would have to squeeze something to make it a black hole.

Why does it matter?

Karl Schwarzschild derived it in 1916, in the trenches, months after Einstein published the field equations. It scales linearly with mass, which has the counter-intuitive consequence that the largest black holes are the least dense — a supermassive one can have an average density lower than water.

Written and computed by . Corrections and errors to hello@cantinalabs.dev.