Hi, In classical and quantum physics and even in special relativity, shifting the energy of a system by a constant (i.e. resetting the zero point) changes nothing in the dynamics and is not observable. In quantum field theory, we even have to shift by an infinite (but "constant") energy to get reasonable results (normal order). However, if you do something like that to the stress-energy-tensor, the solution of the Einstein field equations and hence the dynamics of the universe will be different. In fact, Einstein's cosmological constant is such a shift. So how do we know where to place the zero-level of the energy we need to use? For example, where is the zero-level of the energy of the vibrational modes of a hydrogen molecule?