The Most Surprising Chapter in General Relativity

The first chapter is about the question whether the past still exists. It's really hard to explain without writing down equations, so I heavily rely on space-time diagrams. The stress energy tensor that we use in general relativity is considered to be a kind of an average. And now as you certainly know, Einstein's field equations are non-linear. So taking an average of the source isn't quite as trivial as it sounds.