= Vacuum polarization
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= Vacuum polarisation
{synonym}
Vacuum polarization is the modification of a gauge field's propagation by quantum fluctuations of charged fields. In <quantum electrodynamics>, charged-particle loop contributions change the <photon propagator>; their proper two-point insertion is <photon vacuum polarization>.
<Lorentz invariance> and the <Ward identity> constrain the tensor structure. For an invariant vacuum it is transverse and can be written $\Pi^{\mu\nu}(q)=(q^2g^{\mu\nu}-q^\mu q^\nu)\Pi(q^2)$, up to the stated sign convention. This momentum-dependent response contributes to the scale dependence of the effective electromagnetic coupling. The calculation describes a field correlation, rather than a literal medium of separately detectable virtual pairs. A mass term for the photon is not generated merely by renormalizing a gauge-invariant vacuum polarization.
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