Vacuum (quantum electrodynamic)

This article is about Vacuum (quantum electrodynamic). For other uses of the term Vacuum, please see Vacuum (disambiguation).

(PD) Image: John R. Brews
Feynman diagram for photon-photon scattering in QED vacuum.

The term quantum electrodynamic vacuum, or QED vacuum, 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

Virtual particles

It is sometimes attempted to provide an intuitive picture of virtual particles based upon the Heisenberg energy-time uncertainty principle:

$\Delta E\Delta t\geq \hbar \ ,$

(with ΔE and Δt energy and time variations, and ℏ the Planck constant divided by 2π) arguing along the lines that the short lifetime of virtual particles allows the "borrowing" of large energies from the vacuum and thus permits particle generation for short times.

This interpretation of the energy-time uncertainty relation is not universally accepted, however. One issue is the use of an uncertainty relation limiting measurement accuracy as though a time uncertainty Δt determines a "budget" for borrowing energy ΔE. Another issue is the meaning of "time" in this relation, because energy and time (unlike position q and momentum p, for example) do not satisfy a canonical commutation relation (such as [q, p] = iℏ) . Various schemes have been advanced to construct an observable that has some kind of time interpretation, and yet does satisfy a canonical commutation relation with energy. The very many approaches to the energy-time uncertainty principle are a long and continuing subject.

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.

The standard approach to the quantization of the electromagnetic field begins by introducing a vector potential A and a scalar potential V to represent the basic electromagnetic electric field E and magnetic field B using the relations: ${\begin{aligned}\mathbf {B} &=\mathbf {\nabla \times A} ,\\\mathbf {E} &=-\mathbf {A-\nabla } V\ .\end{aligned}}$ The vector potential is not completely determined by these relations, leaving open a so-called gauge freedom. Resolving this ambiguity using the Coulomb gauge leads to a description of the electromagnetic fields in the absence of charges in terms of the vector potential and the momentum field Π, given by: $\mathbf {\Pi } =\varepsilon _{0}{\frac {\partial }{\partial t}}\mathbf {A} \ ,$ where ε0 is the electric constant of the SI units. Quantization is achieved by insisting that the momentum field and the vector potential do not commute. That is, the equal-time commutator is: $\left[\Pi _{i}(\mathbf {r} ,\ t),\ A_{j}(\mathbf {r'} ,\ t)\right]=-i\hbar \delta _{ij}\delta (\mathbf {r-r'} )\ ,$ where r, r′ are spatial locations, ℏ is Planck's constant over 2π, δij is the Kronecker delta and δ(r−r′) is the Dirac delta function. Quantization can be achieved without introducing the vector potential, in terms of the underlying fields themselves: $\left[{\hat {E}}_{k}({\boldsymbol {r}}),\ {\hat {B}}_{k'}({\boldsymbol {r}}')\right]=-\epsilon _{kk'm}{\frac {i\hbar }{\varepsilon _{0}}}\ {\frac {\partial }{\partial x_{m}}}\delta ({\boldsymbol {r-r'}})\ ,$ where the caret '^' denotes a Schrõdinger time-independent field operator, and εijk is the antisymmetric Levi-Civita tensor.

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 excited 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. Many of electromagnetic effects of the vacuum are small, and only recently have experiments been designed to enable the observation of nonlinear effects. Calculations also indicate birefringence and dichroism at high fields.

Attainability

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

“One reason [a vacuum is not empty] is that the walls of a vacuum chamber emit light in the form of black-body radiation...If this soup of photons is in thermodynamic equilibrium with the walls, it can be said to have a particular temperature, as well as a pressure. Another reason that perfect vacuum is impossible is the Heisenberg uncertainty principle which states that no particles can ever have an exact position ...Each atom exists as a probability function of space, which has a certain nonzero value everywhere in a given volume. ...More fundamentally, quantum mechanics predicts ...a correction to the energy called the zero-point energy [that] consists of energies of virtual particles that have a brief existence. This is called vacuum fluctuation.” –Luciano Boi, Creating the physical world ex nihilo? p. 55

Because of virtual particles, a perfect vacuum cannot be realized. That situation leaves open the question of attainability of a quantum electrodynamic vacuum or QED vacuum. Predictions of QED vacuum such as spontaneous emission, the Casimir effect and the Lamb shift have been experimentally verified, suggesting QED vacuum is a good model for a high quality realizable vacuum. There are competing theoretical models for vacuum, however, for example quantum chromodynamic vacuum and the vacuum of quantum gravity. It remains an open question whether further refinements in experimental technique ultimately will support another model for realizable vacuum.