Today, this is the way we define the vacuum: It is the…

““Today, this is the way we define the vacuum: It is the remnant left over in a volume out of which we have removed everything that can possibly be taken out. The existence of electromagnetic zero-point radiation implies that this vacuum still contains photons, the carrier particles of the electromagnetic field. The photons fluctuate into electron-positron pairs; that means that their fields also don't vanish in the vacuum. And so it goes for all the fields of all the other particles in our particle zoo-of the quarks and antiquarks, of the neutrino and the antineutrino, of the muons and the antimuons, of the W and Z bosons, of the gluons. All of them swirl around in this emptiest of all spaces, in all imaginable configurations, and for the briefest of time only. Recall, the larger the mass of any of these excitations, the shorter its lifetime, the smaller the volume in which it is concentrated. The excitations of the vacuum come and go; they oscillate, they fluctuate. It is not easy to conjure up imagery vivid enough to do justice to the fluctuating vacuum. But we know that all these fields filling the vacuum must, on the average, vanish; the only quantity that remains immutable is the energy, the zero-point energy. The average charge of a vacuum is zero, and so is its average momentum. Any calculation we might attempt to determine the energy of the vacuum must fail; the contributions from some particles and fields will tend to positive infinity, of others to negative infinity. Summing up such indeterminate quantities does not give us a finite answer.””

— Henning Genz