Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, The Netherlands.
Netherlands Institute for Radio Astronomy (ASTRON), Dwingeloo, The Netherlands.
Nature. 2018 Jul;559(7712):73-76. doi: 10.1038/s41586-018-0265-1. Epub 2018 Jul 4.
Einstein's theory of gravity-the general theory of relativity-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, 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 are limited by the weak self-gravity of the bodies, and tests using pulsar-white-dwarf binaries 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 × 10. 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 and a factor of almost a thousand smaller than that obtained from other strong-field tests.
爱因斯坦的引力理论——广义相对论——基于自由落体的普遍性，它规定所有物体在外部引力场中以相同的方式加速。与几乎所有其他引力理论相反，广义相对论的强等效原理要求自由落体的普遍性甚至适用于具有强自引力的物体。使用太阳系天体对该原理进行的直接测试受到天体弱自引力的限制，而使用脉冲星-白矮双星的测试则受到银河系弱引力的限制。PSR J0337+1715 是一个由三颗恒星组成的等级系统（恒星三重系统），其中由一颗毫秒脉冲星和一颗处于 1.6 天轨道上的白矮星组成的双星本身处于与另一颗白矮星的 327 天轨道上。该系统允许进行一项测试，比较外部白矮星的引力对具有强自引力的脉冲星和内部白矮星的影响。在这里，我们报告说，脉冲星及其附近白矮星伴星的加速度差异不超过 2.6×10。作为一个大致的比较，我们对强场 Nordtvedt 参数的限制，该参数用于衡量自由落体普遍性的违反，比从（弱场）太阳系测试获得的限制小一个数量级，比从其他强场测试获得的限制小近一千倍。
