Vacuum (quantum electrodynamic)

The term quantum electrodynamic vacuum, or QED, refers to the ground state of the electromagnetic field, which is subject to fluctuations about a dormant zero average-field condition: Here is a description of the quantum vacuum: "“The quantum theory asserts that a vacuum, even the most perfect vacuum devoid of any matter, is not really empty. Rather the quantum vacuum can be depicted as a sea of continuously appearing and disappearing [pairs of] particles that manifest themselves in the apparent jostling of particles that is quite distinct form their thermal motions. These particles are ‘virtual’, as opposed to real, particles. ...At any given instant, the vacuum is full of such virtual pairs, which leave their signature behind, by affecting the energy levels of atoms.” -Joseph Silk On the shores of the unknown, p. 62"

Quantization of the fields
The Heisenberg uncertainty principle does not allow a particle to exist in a state in which the particle is simultaneously at a fixed location, say the origin of coordinates, and has also zero momentum. Instead the particle has a range of momentum and spread in location attributable to quantum fluctuations; if confined, it has a zero-point energy.

An uncertainty principle applies to all quantum mechanical operators that do not commute. In particular, it applies also to the electromagnetic field. A digression follows to flesh out the role of commutators for the electromagnetic field. Because of the non-commutation of field variables, the variances of the fields cannot be zero, although their averages are zero. The electromagnetic field has therefore a zero-point energy, and a lowest quantum state. The interaction of an excited atom with this lowest quantum state of the electromagnetic field is what leads to spontaneous emission, the transition of an exited atom to a state of lower energy by emission of a photon even when no external perturbation of the atom is present.

Electromagnetic properties
As a result of quantization, the quantum electrodynamic vacuum can be considered as a dielectric medium, and is capable of vacuum polarization. In particular, the force law between charged particles is affected. The electrical permittivity of quantum electrodynamic vacuum can be calculated, and it differs slightly from the simple ε0 of the classical vacuum. Likewise, its permeability can be calculated and differs slightly from μ0. This medium is a dielectric with relative dielectric constant > 1, and is diamagnetic, with relative magnetic permeability < 1. Under some extreme circumstances (for example, in the very high fields found in the exterior regions of pulsars ), the quantum electrodynamic vacuum is thought to exhibit nonlinearity in the fields. Calculations also indicate birefringence and dichroism at high fields.

Attainability
A perfect vacuum is itself only realizable in principle. It is an idealization, like absolute zero for temperature, that can be approached, but never actually realized: