The formula Stephen Hawking wanted on his gravestone

Stephen Hawking, the world-renowned physicist who died this week at 76 years old after living with ALS for most of his life, had decided a long time ago what loved ones should carve into his gravestone after his passing: an equation.

According to the Independent, the scientist wanted a formula known as Hawking’s equation to be written on his grave. It is his most important discovery, and it expresses the idea that black holes aren’t entirely black, but instead emit a glow that is known as Hawking radiation.

Hawking’s discovery, aided by his colleague Jacob Bekenstein, opened up new ways of understanding the universe and the way it changes over time. The S calculated by the formula is entropy, a measure of how much disorder is present in a black hole system.