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Using a specially designed satellite, an international team of scientists measured the accelerations of pairs of free-falling objects in Earth orbit. This is the first experimental test in space of the weak equivalence principle. The team showed with unprecedented precision that two objects fall at the same speed in a vacuum.
The principle of weak equivalence (PEF) states that bodies of different compositions and/or masses fall identically into the same gravitational field (assuming there are no other outside influences, such as the resistance of the ‘air). Einstein formulated it in 1907 as the foundation of general relativity, equating “inertial” and “gravitational” masses. Galileo would have tested this principle from the top of the tower of Pisa in Italy, just like astronaut David Scott by dropping a hammer and a feather on the surface of the Moon in 1971.
Many PEF accuracy tests have been performed since; ground-based experiments peaked in the early 2000s, showing that the accelerations of two free-falling objects were identical within a few 10-12 close. The PEF was also tested via the movement of the Moon and the Earth around the Sun, to increase the precision of the measurements. The idea of testing PEF in a space laboratory was mooted in the 1970s, with the aim of achieving ever greater precision.
Probing the limits of general relativity
The PEF observation is truly counter-intuitive (common sense dictates that a heavier object will fall faster than a lighter one) and illustrates just how strange and mysterious force gravity is. To better understand this force — and by extension, some still unfathomable aspects of physics such as dark matter — physicists are trying to test the equality between inertial and gravitational masses with ever-higher precision. Finding the limit of the PEF — which amounts to highlighting a violation of general relativity — could potentially lead to new theories unifying quantum and classical physics.
This is why, in the 2000s, CNES (National Center for Space Studies), ONERA (National Office for Aerospace Studies and Research) and OCA (Observatory of the Côte d’Azur) developed the MICROSCOPE satellite. Launched in 2016, the satellite orbited the Earth for two years, at an altitude of 710 km, accumulating five months of scientific data in free fall. Performed in space, the experiment freed itself from many systematic uncertainties inherent in terrestrial measurements, such as the noise of seismic vibrations or the variations of the gravitational field caused by the nearby mountains.
The experiment consisted in placing two coaxial cylinders of titanium and platinum in free fall in the earth’s gravitational field; they were held in balance by electrostatic forces, which corrected tiny disturbances on the satellite. Any deviation in these correction forces—a measurement known as the Eötvös ratio—would have indicated that the two cylinders were falling at slightly different rates and therefore, the PEF was violated. The measurements were taken using ultra-sensitive differential electrostatic accelerometers designed by ONERA and on board the satellite.
The first results, published in 2017, revealed no discrepancy between the measurements, with an accuracy of around 10-14. The latest results obtained by the team confirmed that the accelerations of the two cylinders did not differ by more than one part in 1015 (i.e. one billionth) — which excludes any violation of the PEF up to this scale. ” In addition to its remarkable results, this experiment validated many concepts and identified areas for improvement. Given the challenges that fundamental physics still faces, this result may provide an incentive to go beyond this level of precision. write the researchers.
An experiment that could lead to new physical theories
This result is important because it sets the tightest constraints yet on the scale at which any violation of the PEF could occur. This first study also highlighted some improvements for future satellite experiments — including equipment upgrades, including replacing cables with contactless devices and reducing “crackles” in the satellite coating, which affected the measurements.
Physicists hope that eventually these precision experiments will uncover violations that can lead to new physical theories to explain dark matter or dark energy. ” We’re pretty sure there’s a violation at some level, but it’s hard to predict what that level is. said Gilles Métris, scientist at the Observatoire de la Côte d’Azur and co-author of the study.
Many theories of cosmology predict the existence of interactions that could affect the PEF at different scales in the Universe. For example, some theories constructed to explain dark energy envision that the PEF could be breached while orbiting the Earth. The next generation of experiments, such as STE-QUEST and MICROSCOPE 2, should achieve an accuracy level of the order of 10-17 and push the limits of these theories a little further.
However, MICROSCOPE results will likely remain the most accurate constraints on PEF for some time to come: “ For at least a decade or maybe two we see no improvement with a satellite experience », declared Manuel Rodrigues, scientist at ONERA and member of the MICROSCOPE team.