Two coupled real Klein-Gordon fields
= Two coupled real Klein-Gordon fields
{title2=$m_\pm^2=m^2\mp g$}
For interaction <density> $g\phi_1\phi_2$, the combinations $\phi_\pm=\phi_1\pm\phi_2$ diagonalize the equations into independent <Klein-Gordon equations> with the displayed <mass> squares. When $0<g<m^2$, all Fourier <frequencies> are real and nonzero. <Initial data> in one original field generally excite both normal combinations.