Despite all this, Werner Israel had the idea that a black hole is actually a very simple thing. Up until he published this theory in 1967, it was believed that the only truly simple things in Nature were elementary particles like electrons or neutrons. Virtually everything in Nature is much more complicated and cannot be described so easily. Rocks have jagged cracks. Planets have rugged mountains.
Stars have complex magnetic fields.
Israel used mathematical techniques to show that black holes are the simplest big objects in the universe. Like an electron, they can be completely described by only their mass, spin and charge. Israel is currently working on several projects involving the internal geometric structure of black holes. He wants to know what goes on inside a black hole. You need another approach because relativity theory does not handle infinity very well, and a lot of things become infinite inside a black hole.