Relativistic radio-cavity enthalpy ($H_{\mathrm{cav}}=\gamma pV/(\gamma -1)\simeq 4pV$)

For an approximately pressure-balanced cavity containing relativistic fluid with $\gamma =4/3$, internal energy plus displacement work is $4pV$. Dividing by an estimated inflation or buoyancy age gives a mechanical-power diagnostic. Projection, pressure gradients, the equation of state, age estimates and additional shock energy affect the inference; cavity power is not identical to locally dissipated heat.