The principle that all objects accelerate identically, regardless of their own gravity, when falling in an external gravitational field has passed the most stringent test to date, in an analysis of a neutron star-white dwarf-white dwarf triple system. The findings - which support this key prediction of Einstein’s general theory of relativity - are reported in this week’s Nature.

Unlike alternative theories of gravity, general relativity is based on the premise that all falling objects accelerate identically. This should even apply to bodies like neutron stars that have their own strong gravitation field, a principle known as ‘strong equivalence’. So far, however, no tests of this concept have successfully explored the strong-field regime.

Anne Archibald and colleagues observed the motions of a binary star system containing a neutron star closely orbited by a white dwarf, which are, in turn, both orbited by another, distant white dwarf. This triple star system allows for an investigation of how the pull of the outer white dwarf influences both the inner dwarf and its companion neutron star, which has strong self-gravity. In alternative gravity theories, the space-time curvature associated with the neutron star would cause it to fall differently to the inner dwarf, distorting the inner orbit. The authors find, however, that there is an upper limit of just 2.6 millionths fractional difference between the accelerations of the pair - a finding that continues to support general relativity, improving on previous tests of the equivalence principle by a factor of a thousand.