Do you know why time is relative to the observer? it has nothing to do with Descartes. It is a mathematical result of the speed of light being fixed; that is, no matter the speed an object is moving, if that object could measure the speed of a photon, it would measure the same speed for the photon as another object with half its speed would measure the speed of the same photon. Get it? It doesn't mean that what you can't see doesn't exist (although that would be a compelling argument against God).