View source for Vacuum (quantum electrodynamic)

===Electromagnetic properties===
As a result of quantization, the quantum electrodynamic vacuum can be considered as a dielectric medium, and is capable of [[vacuum polarization]].<ref name=Weisskopf/><ref name=Zeidler/> In particular, the [[Coulomb's law|force law between charged particles]] is affected.<ref name=Schroeder/><ref name=Schweber/> The electrical permittivity of quantum electrodynamic vacuum can be calculated, and it differs slightly from the simple [[electric constant|ε<sub>0</sub>]] of the [[Vacuum (classical)|classical vacuum]]. Likewise, its permeability can be calculated and differs slightly from [[magnetic constant|μ<sub>0</sub>]]. This medium is a dielectric with relative dielectric constant > 1, and is diamagnetic, with relative magnetic permeability < 1.<ref name= Donoghue/> Under some extreme circumstances (for example, in the very high fields found in the exterior regions of [[pulsars]]<ref name=Meszaros/>), the quantum electrodynamic vacuum is thought to exhibit nonlinearity in the fields.<ref name=Hartemann/> Many of electromagnetic effects of the vacuum are small, and only recently have experiments been designed to enable the observation of nonlinear effects.<ref name=nonlinear/> Calculations also indicate birefringence and dichroism at high fields.<ref name=Heyl/>