What is a black hole and evidence

Think of science's most momentous developments in the 20th century — Einstein’s theory of relativity, quantum physics, finding evidence of black holes. If you trace the chain of discoveries that led to these breakthroughs back far enough, you’ll end up with the Italian astronomer and physicist Galileo Galilei. Theoretical physicist Carlo Rovelli says we can learn a lot from Galileo today. He explains how 400 years ago, the renowned inventor was discovering new facts about the Universe to understand ourselves better — and so are we.