A new test of conservation laws and Lorentz invariance in relativistic gravity

General relativity predicts that energy and momentum conservation laws hold and that preferred frames do not exist. The parametrized post-Newtonian formalism phenomenologically quantifies possible deviations from general relativity. The parametrized post-Newtonian parameter alpha(3) (which identically vanishes in general relativity) plays a dual role in that it is associated both with a violation of the momentum conservation law and with the existence of a preferred frame. By considering the effects of alpha(3) not equal 0 in certain binary pulsar systems, it is shown that \alpha(3)\ < 2.2 x 10(-20) (90% CL). This limit improves on previous results by several orders of magnitude, and shows that pulsar tests of alpha(3) rank (together with Hughes-Drever-type tests) among the most precise null experiments of physics.