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Background field method
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<p>In <a href="/facts/Theoretical_physics/Cs2Y12rQ">theoretical physics</a>, background field method is a useful procedure to calculate the <a href="/facts/Effective_action/slyeRgDh">effective action</a> of a <a href="/facts/Quantum_field_theory/VoewsNJV">quantum field theory</a> by expanding a quantum field around a classical "background" value <i>B</i>: </p> ϕ ( x ) = B ( x ) + η ( x ) {\displaystyle \phi (x)=B(x)+\eta (x)} . <p>After this is done, the Green's functions are evaluated as a function of the background. This approach has the advantage that the <a href="/facts/Gauge_invariance/fSW3gggv">gauge invariance</a> is manifestly preserved if the approach is applied to <a href="/facts/Gauge_theory/fSW3gggv">gauge theory</a>. </p>