= Relative weak phase condition for time reversal
{title2=$\operatorname{Im}(g_Ag_V^*)=0$}
If a <Fermi interaction> contains vector and axial operators with the same <time-reversal symmetry> component signs, <antiunitarity> conjugates their coupling constants without exchanging the two operators. After removing a common field phase, invariance requires $g_A/g_V$ real when $g_V\ne0$. Equivalently $\operatorname{Im}(g_Ag_V^*)=0$. Fixed intrinsic phases can additionally constrain the common phase; the relative condition is independent of that choice. Adding a Hermitian-conjugate interaction guarantees a <Hermitian operator> Hamiltonian, but does not guarantee <time-reversal symmetry>.
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