On the energy of gravitational waves
  The energy of gravitational waves (GWs) is a fundamental problem in gravity
theory. The existing descriptions for the energy of GWs, such as the well-known
Isaacson energy-momentum tensor, suffer from several defects. Due to the
equivalence principle, the gravitational energy-momentum can only be defined
quasilocally, being associated with a closed spacelike 2-surface bounding a
region. We propose a new approach to derive the energy of GWs directly from the
quasilocal gravitational energy. Such an approach is natural and consistent
with the quasilocality of gravitational energy-momentum, and it is valid for
GWs with any wavelengths in any order of perturbations.
