The fundamental principle of general relativity has just been strictly tested on a satellite

The fundamental principle of general relativity has just been strictly tested on a satellite

Using a specially designed satellite, an international team of scientists measured the acceleration of pairs of objects in free fall in Earth orbit. This is the first experiment in space to test the principle of low equivalence.

Einstein formulated this in 1907 as the basis for the general theory of relativity, making the "inertial" and "gravity" masses equivalent. Galileo is believed to have tested this principle from the top of the Pisana Tower in Italy, like astronaut David Scott, by dropping a hammer and a pen on the surface of the Moon in 1971.

Since then, many tests have been carried out on the accuracy of the principle of equivalence; ground experiments culminated in the early 2000s when it was shown that the accelerations of the two falling objects were identical to the accuracy of several to 12. A weak principle of equivalence was also tested using the movement of the Moon and the Earth around the Sun to improve the accuracy of measurements. The idea of testing the equivalence principle in the space laboratory was put forward in the 1970s with a view to achieving greater accuracy.

Observing the principle of equivalence is indeed contrary to intuition and illustrates that gravity is a strange and mysterious force.

This is why CNES developed the MICROSCOPE satellite in the 2000s. launched in 2016, the satellite was orbiting the Earth for two years at an altitude of 710 km, accumulating five months of scientific data in free fall. The experiment was free from many of the systemic uncertainties inherent in ground measurements, such as the noise of seismic fluctuations or the variation of the gravity field caused by nearby mountains.

During the experiment, two titanium and platinum coaxial cylinders were placed in a free fall in the Earth's gravitational field; they were held in equilibrium by electrostatic forces that corrected the tiny disturbances on the satellite; any deviation in these adjusting forces, a measure commonly known as Etwös, would indicate that the two cylinders were falling at slightly different speeds and therefore the principle of equivalence was violated. The measurements were carried out using supersensitive differential electrostatic accelerometers developed by ONERA and on board the satellite.

The first results published in 2017 showed that there were no discrepancies between measurements with accuracy of about 10 to 14. The latest results from the team confirmed that the accelerations of the two cylinders were not different by more than one part of 10 to 15.

An experiment that could lead to new physical theories

This result is important because it imposes the most severe limitations to date on the extent to which any violation of the principle of equivalence can occur.

Physicists hope that in time these highly accurate experiments will detect irregularities that may lead to new physical theories to explain dark matter or dark energy." said Giles Metiris, a scientist from the Lazour Coast Observatory and co-author of the study.

Many theories in cosmology predict the existence of interactions that may affect the principle of equivalence on different scales of the universe. For example, some theories designed to explain dark energy suggest that the principle of equivalence may be violated in orbit around the Earth. The next generation of experiments, such as STE-QUEST and MICROSCOPE 2, are expected to reach a level of accuracy of about 10-17 and further extend the boundaries of these theories.

However, the results of MICROSCOPE are likely to remain the most precise restrictions on the principle of equivalence for some time: "", said Manuel Rodríguez, a scientist from ONERA and a member of the MICROSCOPE team.