 The vacuum state \(|0\rangle\) is the energy ground state, i.e., the eigenstate of the energy operator with the lowest eigenvalue. It is a remarkable result in ordinary non-relativistic QM that the ground state energy of e.g., the harmonic oscillator is not zero in contrast to its analogue in classical mechanics. In addition to this, the relativistic vacuum of QFT has the even more striking feature that the expectation values for various quantities do not vanish, which prompts the question what it is that has these values or gives rise to them if the vacuum is taken to be the state with no particles present. If particles were the basic objects of QFT how can it be that there are physical phenomena even if nothing is there according to this very ontology? Eventually, studies of QFT in curved space-time indicate that the existence of a particle number operator might be a contingent property of the flat Minkowski space-time, because Poincaré symmetry is used to pick out a preferred representation of the canonical commutation relations which is equivalent to picking out a preferred vacuum state (see Wald 1994).