https://phys.org/news/2020-06-astrophysicists-cornerstone-einstein-theory-relativity.html An international collaboration of scientists has recorded the most accurate confirmation to date for one of the cornerstones of Einstein's theory of general relativity, 'the universality of free fall." The new research shows that the theory holds for strongly self-gravitating objects such as neutron stars. Using a radio telescope, scientists can very accurately observe the signal produced by pulsars, a type of neutron star and test the validity of Einstein's theory of gravity for these extreme objects. In particular, the team analyzed the signals from a pulsar named "PSR J0337+1715' recorded by the large radio telescope of Nançay, located in the heart of Sologne (France). The universality of free fall principle states that two bodies dropped in a gravitational field undergo the very same acceleration independently of their composition. This was first demonstrated by Galileo who famously would have dropped objects of different masses from the top of Pisa's tower to verify that they both reach the ground simultaneously. This principle is also at the heart of Einstein's theory of general relativity. However, some hints such as the inconsistency between quantum mechanics and general relativity, or the conundrum of the domination of dark matter and dark energy in the composition of the Universe, have led many physicists to believe that general relativity might not be, after all, the ultimate theory of gravity. The observations of Pulsar J0337+1715, which is a neutron star with a stellar core 1.44 times the mass of the Sun that has collapsed into a sphere of only 25km in diameter, shows that it orbits two white-dwarf stars which have a much weaker gravity field. The findings, published today in the journal Astronomy and Astrophysics, demonstrate the universality of free fall principle to be correct. more at link........................... the paper: https://www.aanda.org/articles/aa/abs/2020/06/aa38104-20/aa38104-20.html An improved test of the strong equivalence principle with the pulsar in a triple star system⋆ Abstract Context. The gravitational strong equivalence principle (SEP) is a cornerstone of the general theory of relativity (GR). Hence, testing the validity of SEP is of great importance when confronting GR, or its alternatives, with experimental data. Pulsars that are orbited by white dwarf companions provide an excellent laboratory, where the extreme difference in binding energy between neutron stars and white dwarfs allows for precision tests of the SEP via the technique of radio pulsar timing. Aims. To date, the best limit on the validity of SEP under strong-field conditions was obtained with a unique pulsar in a triple stellar system, PSR J0337+1715. We report here on an improvement of this test using an independent data set acquired over a period of 6 years with the Nançay radio telescope. The improvements arise from a uniformly sampled data set, a theoretical analysis, and a treatment that fixes some short-comings in the previously published results, leading to better precision and reliability of the test. Methods. In contrast to the previously published test, we use a different long-term timing data set, developed a new timing model and an independent numerical integration of the motion of the system, and determined the masses and orbital parameters with a different methodology that treats the parameter Δ, describing a possible strong-field SEP violation, identically to all other parameters. Results. We obtain a violation parameter Δ = ( + 0.5 ± 1.8) × 10−6 at 95% confidence level, which is compatible with and improves upon the previous study by 30%. This result is statistics-limited and avoids limitation by systematics as previously encountered. We find evidence for red noise in the pulsar spin frequency, which is responsible for up to 10% of the reported uncertainty. We use the improved limit on SEP violation to place constraints on a class of well-studied scalar-tensor theories, in particular we find ωBD > 140 000 for the Brans-Dicke parameter. The conservative limits presented here fully take into account current uncertainties in the equation for state of neutron-star matter.