  We argue that finite-region observables in quantum gravity are best
approached in terms of boundary data on null hypersurfaces. This has
far-reaching effects on the basic notions of classical and quantum mechanics,
such as Hamiltonians and canonical conjugates. Such radical properties are not
unexpected in finite-region quantum gravity. We are thus motivated to
reformulate field theory in terms of null boundary data. As a starting point,
we consider the on-shell action functional for classical field theory in finite
null-bounded regions. Closed-form results are obtained for free scalars and for
Maxwell fields. The action of classical gravity is also discussed, to the
extent possible without solving the field equations. These action functionals
exhibit non-locality and, in special cases, a "holographic" reduction of the
degrees of freedom. Also, they cannot be used to define global charges. Whereas
for ordinary field theory these are just artifacts of a restrictive formalism,
in quantum gravity they are expected to be genuine features. This further
supports a connection between quantum gravity and null-boundary observables. In
our treatment of the GR action, we identify a universal imaginary term that
reproduces the Bekenstein entropy formula.
