I've been wondering two things lately. Why did Einstein make the assumption that the Ricci tensor is 0 in empty space. Is there a physical/mathematical reason? I know later he set it equal to another tensor...which leads to all the cosmological constant business, but I'm just curious why he favored 0 initially. What was his motivation? The other thing I don't understand is, what sorts of restrictions does setting the Ricci tensor place on the form of the metric? Any insights would be greatly appreciated.