The PSR J0337 + 1715 is a pulsar 4,200 light years away, accompanied by 2 white dwarf stars. This triple star system proved to be the perfect test laboratory for Einstein’s General Theory of Relativity – in particular, the so-called universality of free fall (according to her, gravity attracts all objects with the same acceleration). Once again, it was proved that Einstein was right.

Discovered in 2007 in the Taurus constellation, the neutron star is 1.44 times the mass of the Sun and only 25 kilometers in diameter, rotating 366 times per second. One of the white dwarfs moves around it every 1.6 days and, orbiting this pair every 327 days, there is another dwarf star.

The proof of the universality of freefall would be to measure whether the gravitational attraction of the external white dwarf affects the pulsar (whose gravity is immense) as much as the tiny white dwarf in the internal orbit.

The international joint effort of radio astronomers and astrophysicists from the Jodrell Bank Center for Astrophysics (GB), the Max Planck Institute for Radio Astronomy (Germany) and the French observatories in Paris, Meuden and the Nançay Radio Observatory has managed to determine, with extreme precision, that the three bodies are subject to the same acceleration of gravity, even with such different masses.

Restrictions on alternative theories

“The precision with which we were able to prove the universality of free fall was one of the most rigorous tests of Einstein’s theory ever done – and it passed with honors. The results impose very strict restrictions on alternative theories of gravity, which compete with Einstein’s general relativity to explain the gravity and existence of dark energy in the Universe, “says the study’s leader, University of Manchester astrophysicist Guillaume Voisin.

The work concluded 8 years of measurements, through the Nançay Radio Observatory, of the arrival times of the radio pulses to Earth. “The pulsar emits a beam of radio waves, which creates a flash of light captured with nanosecond precision by the Nançay radio telescope. This record, similar observations made in 2018 and mathematical modeling allowed us to infer the movement of the star,” explained Voisin .

The universality of free fall – a unique characteristic of gravity in attracting all bodies with the same acceleration, regardless of their mass, density or even gravitational field – was first thought in the 16th century by Galileo Galilei, who tried to prove it when he started balls of different weights from the top of the Tower of Pisa.

Off-Earth Laboratories

An ongoing experiment, of the past 50 years, is the Lunar Laser Variation Test (LLR). In it, retroreflectors placed on the Moon reflect laser beams fired from the Earth’s surface. Measuring the intervals at which the beams return proved that the Earth and the Moon “fall” towards the Sun at the same speed.

This was confirmed by Microsatellite à traînée Compensée pour l’Observation du Principe d’Equivalence (acronym Microspore) in 2016, by measuring the acceleration of small objects inside.

In 2018, an international team completed a study similar to the one published now (one of its authors is the astrophysicist Duncan Lorimer, the same one involved in the research of the mysterious signal FRB 121102). The big question was whether Einstein’s principle applies to extremely massive bodies, and the pulsating PSR J0337 + 1715 continues to give the same answer: yes.

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