= Light cone gauge
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In gauge field theory its standard form is $A^+=n_\mu A^\mu=0$ for a fixed null direction $n$. A light-cone gauge uses <light-cone coordinates> to fix a longitudinal variable, often choosing $x^+$ as the evolution parameter for a <relativistic particle phase-space action>. Related field gauges set selected minus components to zero when a field has the required <gauge invariance>. A massive field without that gauge freedom must instead eliminate dependent components through its equations of motion.
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