In the canonical formulation of QFT (to which I've been exposed), it is always argued that only differences in energy are physically observable and so we can deal with the fact that the vacuum energy is infinite by redefining the vacuum such that its energy is zero and we subsequently measure all energies relative to it. Why is this the case? Why are only energy differences physically observable (neglecting gravity, as one does in ordinary QFT). Is it because the vacuum energy is spatially homogeneous (i.e. constant throughout 3D space), it is in a sense "normalised" such that, for intents and purposes, one can only measure a difference in energy relative to it?