Massless scalar field
= Massless scalar field
{title2=$m=0,\quad\Box\phi=0$}
A free real <scalar field> with zero <mass> has the displayed wave equation in <Minkowski spacetime>. In curved spacetime its dynamics also depend on curvature coupling: a <minimally coupled massless scalar field> and a <conformally coupled scalar field> need not have the same cosmological modes.