  This article considers the quasi-local energy in reference to a general
static spacetime. We follow the approach developed by the authors in [19, 20,
7, 9] and define the quasi-local energy as a difference of surface
Hamiltonians, which are derived from the Einstein-Hilbert action. The new
quasi-local energy provides an effective gauge independent measurement of how
far a spacetime deviates away from the reference static spacetime on a finitely
extended region.
