Question: What would you say your greatest discovery is?

Stephen Hawking answered on 27 Nov 2018:

I would say my greatest discovery was Hawking Radiation. Basically, Black Holes aren’t entirely black!

Hawking radiation is a patchwork combination of General-Relativity and Quantum mechanics. The idea is that space cannot be entirely empty, small quantum fluctuation cause particles and anti-particle pairs to pop up randomly. If they pop up close to a black hole some of the particles are sucked up while the partner particle doesn’t! The universe demands that the now un-partnered particle be compensated, the only way this can happen is if the black hole gives up some mass. Black Holes radiate!

This sparked a lot of debates in black hole theory. The biggest and still unanswered being, do black holes conserve information. Quantum physics states that in a given system no information is lost or gained, but if a quantum system passes through a Black Hole that information is removed. We lose information! The currently accepted idea is Holographic theory. This is the idea that the quantum information is stored in the event horizon of the Black Hole in a 2D plane. Projecting 3D information in 2D is like a holograph. This in itself has led to even more debates (see the Black Hole firewall problem)!

Great Question!