In quantum mechanics, given ψ, we calculate the probability density in a given space by (mod[ψ]^2)*dx*dy*dz (as given in any standard textbook). But my suggetion is that it should be (mod[ψ]^2)*dx*dy*dz*dt where dt is the differential time element. Its seems to me to be more fundamental. Then why dont we do that? Thanks Curious undergrad!