  Within the t-J model of copper-oxides, the condensation energy can be related
to the change in the dynamical spin structure factor between the
superconducting and the normal states. By analyzing previous experimental data,
we show that the change associated with the resonant neutron scattering peak
found in $YBa_2Cu_3O_7$ can quantitatively account for the condensation energy.
We argue that this analysis suggests a microscopic mechanism for high Tc
superconductivity.
