Local-density sound speed of a condensate
= Local-density sound speed of a condensate
{title2=$c(\mathbf r)=\sqrt{[\mu-V(\mathbf r)]/m}$}
In occupied bulk regions where density-gradient <energy> is negligible, the stationary relation $g\rho=\mu-V$ gives <speed of sound> $c=\sqrt{g\rho/m}$. Empty regions do not have an imaginary physical <speed of sound>; the bulk condensate formula is simply inapplicable there.