“Imagine squeezing all the space out of an atom. If you did that to all the atoms in all the people in the world, you could fit the entire human race in the volume of a sugar cube. This is because matter is incredibly, mind-bogglingly empty. An atom is like a miniature Solar System, with a tiny nucleus orbited by distant electrons. Physicists refer to the maximum compressed mass of an object (e.g. the entire human race) by a number called the Schwarzschild radius. Once mass collapses below this point, light cannot escape and the object is no longer visible and becomes a black hole. The Schwarzschild radius was named after the German astronomer Karl Schwarzschild who calculated this exact solution for the theory of general relativity in 1915. Amazingly, Schwarzschild made these discoveries while simultaneously serving as a German army lieutenant on the Russian front during the first world war.”