  A hypersurface composed of two null sheets, or "light fronts", swept out by
the two congruences of future null normal geodesics emerging from a spacelike
2-disk can serve as a Cauchy surface for a region of spacetime. Already in the
1960s free (unconstrained) initial data for vacuum general relativity were
found for hypersurfaces of this type. Here the Poisson brackets of such free
initial data are calculated from the Hilbert action. The brackets obtained can
form the starting point for a constraint free canonical quantization of general
relativity and may be relevant to holographic entropy bounds for vacuum
gravity. Several of the results of the present work have been presented in
abbreviated form in the letter [Rei08].
