DEGENERATE METRIC PHASE BOUNDARIES

The structure of boundaries between degenerate and non-degenerate solu tions of Ashtekar's canonical reformulation of Einstein's equations is studied. Several examples are given of such 'phase boundaries' in whi ch the metric is degenerate on one side of a null hypersurface and non -degenerate on the other side. These include portions of flat space, S chwarzschild and plane-wave solutions joined to degenerate regions. In the last case, the wave collides with a planar phase boundary and con tinues on with the same curvature but degenerate triad, while the phas e boundary continues in the opposite direction. We conjecture that deg enerate phase boundaries are always null.