The MICROSCOPE mission has delivered its latest results and has confirmed the equivalence principle with unprecedented accuracy of 10-15. These results show that bodies fall in a vacuum with the same acceleration regardless of their composition or mass, meaning that the principle of equivalence remains unwavering today, to mark yet another victory for the Theory of General Relativity as proposed by Albert Einstein more than a century ago.
In 2017, the first results of the CNES MICROSCOPE satellite, equipped with ONERA accelerometers, improved the accuracy of the equivalence principle (or universality of free fall) to a level that had placed it as a world reference. Thanks to the first data available at that time, these results were obtained by the Géoazur laboratory (CNRS/OCA/UCA/IRD) and ONERA in cooperation with CNES and in partnership with the scientific working group (CNRS, IHES, Imperial College, University of Bremen, DLR, University of Delft, IGN). They earned four members of the MICROSCOPE team the Prix Servant Award from the French Academy of Sciences in 2019. Since 2017, 15 times more measurements was accumulated until the time of the satellite being deorbited in October 2018. The scientific team analysed all the data and managed to push back the test limits even further by doing 10 times better than in 2017. By comparing the free fall accelerations of two bodies of different compositions, the MICROSCOPE teams were able to demonstrate that their relative free-fall deviation is less than 10-15.
About the principle of equivalence
According to Einstein’s theory, the universe is represented by a four-dimensional space-time, and gravitation results from matter bending space-time itself. General Relativity has thus made it possible to explain the hitherto insoluble anomaly of Mercury's orbit, to predict phenomena as surprising as gravitational lenses, black holes or gravitational waves. Nevertheless, a fundamental question remains: why does General Relativity seem incompatible with quantum field theory, which faithfully describes the world of particles and the infinitely small? The search for a universal theory encompassing gravitation and quantum physics is the Holy Grail of physicists. Most candidate theories predict a violation of the founding principle of General Relativity: the equivalence between gravitation and acceleration.
Testing the equivalence principle amounts to testing the foundation of all theories of gravitational and more generally alternative theories to relativity. With its results, MICROSCOPE is pushing the boundaries by bringing new constraints to these new theories at a level of precision such that it will certainly take a very long time to improve on.
MICROSCOPE (MICROSatellite à trainée Compensée pour l’Observation du Principe d’Équivalence - Compensated Drag MICROSatellite for the Observation of the Principle of Equivalence) is a mission of CNES, carried out in partnership with ONERA, OCA, ESA, DLR, ZARM (microgravity laboratory of the University of Bremen) and PTB (German Physics and Metrology Institute). With its accelerometer-controlled microthrusters, the satellite is able to achieve ultra-fine control of its orbit and compensate for the residual atmospheric drag, at levels never before managed in low Earth orbit. ONERA’s T-SAGE instrument is right at the core of this perfect free-fall laboratory, ensconced in a thermal cocoon which stability is better than one millionth of a degree. The instrument is a differential accelerometer. It measures the position of its test masses, in free-fall around the Earth, with an atomic scale accuracy.
MICROSCOPE was launched on 25 April 2016 and decommissioned on 15 October 2018. Scientific measurements and fine characterisations of the instrument and satellite have made it possible to compare the “free fall” of two different materials, platinum and titanium, over the course of 1642 revolutions around the Earth - i.e.: 73 million km - equivalent to half the Earth-Sun distance.
The culmination of many years of effort, this French experiment represents a shining light in the landscape of fundamental physics and a challenge accomplished for engineers and scientists, who have succeeded in pushing the limits of equivalence principle testing accuracy ever further. These results were obtained by ONERA and OCA’s scientific teams with the contribution of CNES and the collaboration of European laboratories. This analysis has been published in two prestigious physics journals: Classical and Quantum Gravity (IOP Publishing) and Physical Review Letters (American Physical Society).
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