Einstein’s theory of gravity—the general theory of relativity—is based on the universality of free fall, which specifies that all objects accelerate identically in an external gravitational field.
Now, an international team of scientists has confirmed the cornerstones of the universality of free fall, suggesting 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).
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.
For the study, The measurements were recorded by a collaborative team from The University of Manchester, Paris Observatory—PSL, the French CNRS and LPC2E (Orléans, France), and the Max Planck Institute for Radio Astronomy.
The observations of Pulsar J0337+1715 shows that it orbits two white-dwarf stars which have a much weaker gravity field.
The universality of free fall principle states that two bodies dropped in a gravitational field experience the same acceleration independently of their composition. First demonstrated by Galileo, the principal 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.
Dr. Guillaume Voisin from The University of Manchester, who led the research, said: “The pulsar emits a beam of radio waves which sweeps across space. At each turn, this creates a flash of radio light, which is recorded with high accuracy by Nançay’s radio telescope. As the pulsar moves on its orbit, the light arrival time at Earth is shifted. It is the accurate measurement and mathematical modeling, down to nanosecond accuracy, of these times of arrival that allows scientists to infer with exquisite precision the motion of the star.”
“Above all, it is the unique configuration of that system, akin to the Earth-Moon-Sun system with the presence of a second companion (playing the role of the Sun) towards which the two other stars’ fall’ (orbit) that has allowed to perform a stellar version of Galileo’s famous experiment from Pisa’s tower. Two bodies of different compositions fall with the same acceleration in the gravitational field of a third one.”
“The pulsar emits a beam of radio waves which sweeps across space. At each turn this creates a flash of radio light, which is recorded with high accuracy by Nançay’s radio telescope. As the pulsar moves on its orbit, the light arrival time at Earth is shifted. It is the accurate measurement and mathematical modeling, down to nanosecond accuracy, of these times of arrival that allows scientists to infer with exquisite precision the motion of the star.”
The team has demonstrated that the extreme gravitational 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. This result is the most accurate confirmation that the universality of free fall is valid even in the presence of an object in which mass is largely due to its own gravity field, thus supporting further Einstein’s theory of general relativity.
- G. Voisin et al. An improved test of the strong equivalence principle with the pulsar in a triple star system. DOI:10.1051/0004-6361/202038104