Gravitational energy seen by quasilocal observers

There have been many attempts to define quasilocal energy/mass for a spacelike 2-surface in a spacetime by the Hamilton-Jacobi method. The main difficulty in this approach is the subtle choice of the background configuration to be subtracted from the physical Hamiltonian. Quasilocal mass should be positive for general surfaces, but on the other hand should be zero for surfaces in the flat spacetime. In this paper, we survey the work in a series of papers [6, 25-27] in which a new notion of quasilocal mass/energy-momentum is proposed and investigated. In particular, the notion of energy observed by a 'quasilocal observer' will be discussed.