Exact solutions to Einstein's equations in the (2+2) Hamiltonian reduction

This paper illustrates a new method of finding exact solutions to the Einstein's equations obtained by the Hamiltonian reduction based in the (2+2) formalism. In this Hamiltonian reduction, the Einstein's equations are written in the privileged spacetime coordinates, where the area element of the 2-dimensional cross-section of an out-going null hypersurface is used as a physical time, and spatial coordinates are chosen as suitable functions on the phase space of gravitational fields. The Hamiltonian constraint is solved to define a non-vanishing gravitational Hamiltonian in terms of conformal two metric and its conjugate momentum, which dictates the evolutions of the conformal two metric and its conjugate in the physical time. The momentum constraints are also solved to express local momentum densities of gravitational fields in the canonical form. Explicitly, we find two exact solutions to the Einstein's equations in the privileged coordinates using ansatz, and show that they are just the Einstein-Rosen wave and the Schwarzschild solution by making suitable coordinate transformations to the standard coordinates.