system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what were Stephen Hawkings greatest discoveries?
gpt
"Were?" This made me think, 'Oh shit, he's dead,' and had to go check.

His branch is theoretical physics, which isn't really about 'discovery' as such. He and other theoretical physicists are all about bringing experimental and observed data from other branches of physics into the context of a wider framework. The supposed "theory of everything." Some of the 20th century's favourite physicists have been theoretical physicists; Hawking, Einstein, Sagan, deGrasse Tyson. Theoretical physicists work at the bleeding edge of what we understand about the universe.

Hawking's work is mainly in astrophysics and general relativity, though he has branched out into quantum physics and particle physics as well. His greatest work is probably his book *A Brief History of Time*, which brought theoretical physics into mainstream culture for probably the first time since Einstein. In the book, he explained phenomenon like the space-time continuum and the speed of light in a way that casual readers and high-school level science students could understand, as well as mathematically proving the existence of black holes and dark matter for the first time. He theorised that black holes do emit radiation, previously thought impossible because of their gravitational pull, then he proved it in both mathematical and layman's terms. As he demystified theoretical physics, he furthered it.

When he does eventually die, he won't be remembered for one stand-out discovery above all others, but for the body of his work as a whole. He has done as much to further the field of physics within his lifetime as Einstein, Newton, Copernicus, Aristotle...