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

Anne M Archibald; Nina V Gusinskaia; Jason W T Hessels; Adam T Deller; David L Kaplan; Duncan R Lorimer; Ryan S Lynch; Scott M Ransom; Ingrid H Stairs

doi:10.1038/s41586-018-0265-1

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

Nature. 2018 Jul;559(7712):73-76. doi: 10.1038/s41586-018-0265-1. Epub 2018 Jul 4.

Authors

Anne M Archibald^{1

2}, Nina V Gusinskaia³, Jason W T Hessels^{3

4}, Adam T Deller^{5

6}, David L Kaplan⁷, Duncan R Lorimer^{8

9}, Ryan S Lynch^{9

10}, Scott M Ransom¹¹, Ingrid H Stairs¹²

Affiliations

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, The Netherlands. archibald@astron.nl.
² Netherlands Institute for Radio Astronomy (ASTRON), Dwingeloo, The Netherlands. archibald@astron.nl.
³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, The Netherlands.
⁴ Netherlands Institute for Radio Astronomy (ASTRON), Dwingeloo, The Netherlands.
⁵ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria, Australia.
⁶ The Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav), Hawthorn, Victoria, Australia.
⁷ Department of Physics, University of Wisconsin-Milwaukee, Milwaukee, WI, USA.
⁸ Department of Physics and Astronomy, West Virginia University, Morgantown, WV, USA.
⁹ Center for Gravitational Waves and Cosmology, Morgantown, WV, USA.
¹⁰ Green Bank Observatory, Green Bank, WV, USA.
¹¹ National Radio Astronomy Observatory, Charlottesville, VA, USA.
¹² Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada.

PMID: 29973733
DOI: 10.1038/s41586-018-0265-1

Abstract

Einstein's theory of gravity-the general theory of relativity¹-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 gravity², 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 × 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.