  A hypersurface formed of two null sheets, or "light fronts", swept out by the
future null normal geodesics emerging from a common spacelike 2-disk can serve
as a Cauchy surface for a region of spacetime. Already in the 1960s free
(unconstrained) initial data for general relativity were found for such
hypersurfaces. Here an expression is obtained for the symplectic 2-form of
vacuum general relativity in terms of such free data. This can be done, even
though variations of the geometry do not in general preserve the nullness of
the initial hypersurface, because of the diffeomorphism gauge invariance of
general relativity. The present expression for the symplectic 2-form has been
used previously to calculate the Poisson brackets of the free data.
