example in considering the hadron spin structure. See also Renormalization Virtual particles QED vacuum QCD vacuum Schwinger limit Schwinger effect Uehling potential Vacuum birefringence Remarks Notes References Further reading For a derivation of the vacuum polarization in QED, see section 7.5 of M.E. Peskin and D.V. Schroeder, An Introduction to Quantum Field Theory, Addison-Wesley, 1995. Gauge theories Quantum electrodynamics