Physical vacuum, in quantum field theory — the lowest energy state of the quantised fields, characterised by the absence of any real particles. All the quantum numbers of the physical vacuum (momentum, electric charge and others) are zero. The possibility of virtual processes in the physical vacuum nevertheless leads to a number of specific effects in the interaction of real particles with the vacuum (see Level shift, Quantum field theory). The concept of the “physical vacuum” is one of the fundamental ones, in the sense that its properties determine the properties of all the remaining states, since any of them can be obtained from the vacuum state by the action of particle creation operators. In a number of cases, for example when symmetry is spontaneously broken, the vacuum state turns out not to be unique (that is, it is degenerate) — there is a continuous spectrum of such states, differing from one another in the number of so-called Goldstone bosons.