Pulsar timing with the Nançay Radio Telescope confirms the universality of free fall

In June, researchers published the most accurate confirmation to date of one of the cornerstones of Einstein’s theory of general relativity: the universality of free fall.

The team, including scientists from Paris Observatory (part of PSL – Paris Science and Lettres University), the French CNRS and its Space & Environment Physics and Chemistry Laboratory (LPC2E - Laboratoire de Physique et de Chimie de l’Environnement et de l’Espace), The University of Manchester and Max Planck Institute for Radioastronomy, analysed pulsar signals recorded by the Nançay Radio Telescope (NRT) in central France. Nançay Observatory is home to two SKA pathfinders: the low-frequency telescope NenuFAR, and a station of the international LOFAR telescope.

Pulsar PSR J0337+1715 is a neutron star orbiting with two white-dwarf stars which have a much weaker gravity field. Spinning with a period of only 3ms, it emits a beam of radio waves which, like a galactic beacon, sweeps across space. Each turn creates a flash of radio light.

As the pulsar moves on its orbit, the light arrival time at Earth is shifted. NRT’s state-of-the-art timing instrumentation and mathematical modelling – down to a nanosecond accuracy – of these arrival times allow scientists to infer with exquisite precision the star’s motion.

The triple system, composed of pulsar PSR J0337+1715 and two white dwarfs, one nearby and the other further away.

Credit G. Voisin CC-BY-SA 4.0

The unique configuration of the system, in which the white dwarfs “fall” (orbit) towards the pulsar allowed a stellar version of Galileo’s famous experiment from Pisa’s tower to be performed: two bodies of different compositions fall with the same acceleration in the gravitational field of a third one (the Earth for Galileo, the pulsar in this case).

“The team demonstrated that the extreme gravity field of the pulsar cannot differ by more than 1.8 part per million (with a confidence level of 95%) from the prediction of general relativity,” explains Dr Gilles Theureau of Paris Observatory, Nançay Observatory and LPC2E. “This is the most accurate confirmation that the universality of free fall is valid even in the presence of an object whose mass is largely due to its own gravity field.”

The SKA telescopes plan to test Einstein’s theory even further, providing strongfield tests of gravity using very massive objects such as neutron stars and black holes.

Nançay Radio Telescope has observed the pulsar intensively since its discovery in 2012. Nançay Observatory is home to two SKA pathfinders: the low-frequency telescope NenuFAR, and a station of the international LOFAR telescope.

Credit: Letourneur and Nançay Observatory

By Maison SKA-France