General relativity is based on the premise that all objects accelerate identically in an external gravitational field. This applies even to bodies like neutron stars, which have their own strong gravitational field. Tests that have been done so far were unable to explore the strong-field regime. Anne Archibald and colleagues measured the motions of a tight binary neutron-star–white-dwarf system (1.6-day orbit) that is orbited by another white dwarf (327-day orbit). They find that the accelerations of the neutron star and white dwarf differ fractionally by no more than a few parts per million.
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