Solution
= Solution
For a stationary gas with constant phase, $\psi=a(x)$ is real and time independent. The field equation reduces to
$$
\boxed{a''+(1-a^2)a=0}.
$$
A positive constant solution obeys $d(1-d^2)=0$, so
$$
\boxed{d=1}.
$$
= Solution
For a stationary gas with constant phase, $\psi=a(x)$ is real and time independent. The field equation reduces to
$$
\boxed{a''+(1-a^2)a=0}.
$$
A positive constant solution obeys $d(1-d^2)=0$, so
$$
\boxed{d=1}.
$$