Universality of free fall from the orbital motion of a pulsar in a stellar triple system

Einstein's theory of gravity-the general theory of relativity(1)-is based on the universality of free fall, which specifies that all objects accelerate identically in an external gravitational field. In contrast to almost all alternative theories of gravity(2), the strong equivalence principle of general relativity requires universality of free fall to apply even to bodies with strong self-gravity. Direct tests of this principle using Solar System bodies(3,4) are limited by the weak self-gravity of the bodies, and tests using pulsar-white-dwarf binaries(5,6) have been limited by the weak gravitational pull of the Milky Way. PSR J0337 + 1715 is a hierarchical system of three stars (a stellar triple system) in which a binary consisting of a millisecond radio pulsar and a white dwarf in a 1.6-day orbit is itself in a 327-day orbit with another white dwarf. This system permits a test that compares how the gravitational pull of the outer white dwarf affects the pulsar, which has strong self-gravity, and the inner white dwarf. Here we report that the accelerations of the pulsar and its nearby white-dwarf companion differ fractionally by no more than 2.6 x 10(-6). For a rough comparison, our limit on the strong-field Nordtvedt parameter, which measures violation of the universality of free fall, is a factor of ten smaller than that obtained from (weak-field) Solar System tests(3,4) and a factor of almost a thousand smaller than that obtained from other strong-field tests(5,6).