1 Moon, Sun and many others gravity objects exist in a vacuum 2 One postulate of SRT says that the speed of photon in the vacuum ( !!! ) is constant c=1. 3 Then on the one hand, as a peasant, I think that photon must move in the vacuum, but on the other hand physicists talk me the photon moves in the negative 4D Minkowski space = spacetime. And nobody explain me what spacetime really is. Then it seems to me that I find myself in a church or synagogue. And I never could understand the priest or rabbi even they are scientists with PhD in their pockets. =.