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Message: <blockquote data-quote="imhotep" data-source="post: 28166410" data-attributes="member: 562115">The MICROSCOPE satellite experiment has tested the equivalence principle with an unprecedented level of precision.We all know that two unequal masses fall together in a vacuum. Galileo tested this equivalence principle from the top of the Leaning Tower of Pisa in Italy, and so did the astronaut David Scott by dropping a hammer and a falcon feather at the surface of the Moon in 1971. But the scientists need it proved to an extreme degree of precision. The new results by the space-borne MICROSCOPE mission have done just this. The experiment verified that two masses of titanium and platinum aboard a satellite orbiting Earth fall exactly in the same way to a precision of 1 part per 10^15.This confirmation places the strictest bounds yet on alternatives to general relativity, which is currently our best model of gravity. The finding is the most stringent test yet of the equivalence principle, a key tenet of Einstein’s theory of general relativity. The principle holds to about one part in a thousand trillion, researchers reported on September 14, 2022.The equivalence principle was first discussed in the early 17th century by Kepler and Galileo, then later in Newton’s theory of gravity. That theory states that the gravitational force between two massive bodies is proportional to the product of their gravitational masses. The gravitational mass is a feature of massive bodies, playing the same role for gravitation as the electric charge for charged objects. But this is not the only mass in Newton’s theories: the second law of motion states that the inertial mass relates a body’s acceleration to the force acting on that body. Hence, massive bodies have two masses potentially—the gravitational and the inertial—but Newton assumed that they are the same. As a consequence, the motion of a massive body in the gravitational field of Earth (or of other large objects) should be independent of its mass.This extraordinary prediction was first tested by Newton at the 1-part-per-1000 level using pendulums and employing their period of oscillation. Balls of different masses attached to a pendulum should oscillate with the same frequency, a property commonly used in standing clocks. The first improvement on this measurement was obtained in the 19th century by Friedrich Bessel who confirmed the equivalence with an accuracy of 1 part per 100,000. More accurate measurements appeared at the end of the 19th century with the work of physicist Loránd Eötvös, who used a torsion pendulum to confirm the equivalence principle to an accuracy of 1 part per 10^9. In honor of this groundbreaking experiment, the ratio between the acceleration difference of two massive bodies and their averaged acceleration is called the Eötvös parameter, η. The equivalence principle posits that η is zero.The MICROSCOPE experiment—built by the French National Center for Space Studies (CNES)—is the next step in the quest for the perfect confirmation of the equivalence principle. Launched in 2016, the satellite orbited Earth for two years at an altitude of 710 km. Being in space, the experiment freed itself from many of the systematic uncertainties inherent to Earth-bound measurements, such as the noise from seismic vibrations or the gravitational-field variations caused by nearby mountains. In the experiment, two coaxial cylinders of titanium and platinum were placed in free fall in Earth’s gravitational field. The cylinders were held in place by electrostatic forces that corrected for tiny perturbations on the satellite. The researchers looked for deviations in those correction forces, which would have signaled that the two cylinders were falling at slightly different rates and that the equivalence was violated. In 2017, MICROSCOPE published their first results, showing no sign of a violation at the level of η≤10^-14. The new publication by the MICROSCOPE team confirms the earlier result, reaching the mission’s projected sensitivity of η≤10^-15The next generation of proposed experiments such as MICROSCOPE 2 should reach a level of precision of η≤10^−17 and once more push theories to their limits.Read the CNES Press Release...<a href="https://presse.cnes.fr/en/final-results-microscope-mission-achieve-record-levels-precision" target="_blank">https://presse.cnes.fr/en/final-results-microscope-mission-achieve-record-levels-precision</a></blockquote>

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