  We consider finite-energy solutions to the defocusing nonlinear wave equation
in two dimensional space. We prove that almost all energy moves to the infinity
at almost the light speed as time tends to infinity. In addition, the
inward/outward part of energy gradually vanishes as time tends to
positive/negative infinity. These behaviours resemble those of free waves. We
also prove some decay estimates of the solutions if the initial data decay at a
certain rate as the spatial variable tends to infinity. As an application, we
prove a couple of scattering results for solutions whose initial data are in a
weighted energy space. Our assumption on decay rate of initial data is weaker
than previous known scattering results.
