When reading through this paper(http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px436/notes/lecture20.pdf), I have trouble understanding some parts of it, 1. Is the r in ρ=Rr a unit vector? 2. it shows x^2 + y^2 + z^2 + w^2 = ρ^2 + w^2 = R^2 but isnt ρ=Rr, thus isnt p^2 itself already R^2? 3. Why is w used for the time component? Is this w the angular frequency?