 Hello, and welcome to General Relativity Part 2. In Part 1, we covered the equivalence principle, non-Euclidean geometry, and the Einstein field equations. Einstein had come up with a totally different way of looking at gravity. But was it a difference that made a difference? Or did his equations predict different physical phenomena than Newton's did? You may recall, if you've seen how small is it video book, that the theory for quantum mechanics and the standard model was developed to explain experimental evidence. But with General Relativity, there was no experimental evidence. The theory came first. So Einstein came up with three tests to demonstrate that his theory did indeed predict more accurately phenomenon like the orbit of Mercury, the bending of light, and gravitational redshift. We'll cover all three of these, and frame-dragging a more recent test around how rotating masses can twist and move space around it will end by building a black hole where all of these concepts come together. We'll use Gargantua, the black hole in the movie Interstellar. But first, Mercury.