In the 16th century, Galileo Galilei promulgated the equivalence principle. It took scientists over 400 years and $240 million to prove during an orbital experiment what Galilei has told.
The equivalence principle is one of the physics concepts that deal with gravitational equivalence and inertial masses. This principle was first elaborated by Galileo Galilei, with more than 400 years ago, when he has been able to express, upon an experiment, that the acceleration of a mass under the force of the gravity doesn’t depend on the amount of the mass. More recently, Einstein has brought some improvements to the Galilei’s theory.
In short, the theory says that objects are gravitationally accelerated in the same measures, regardless of their masses.
Moreover, in 1971 during the Apollo 15, astronaut David Scott has proven Galilei’s theory as right. He dropped one feather and one hammer and observed that both the objects hit the Moon’s surface at the same time, in the absence of atmosphere, thus in the absence of air resistance.
Last year, in 2016, a French satellite named MICROSCOPE was launched to 450 miles above the Earth in order to prove the equivalence principle theory in orbit. The French Space Agency used in this experiment two cylinders of different masses. The satellite was designed to use thrusters to keep the cylinders away from touching the sides of the space they were kept inside for observation. The two cylinders were left in free fall inside the satellite as it was orbiting the Earth.
After 1500 orbits, no differences in the falling speed of the two cylinders were detected.
The MICROSCOPE satellite experiment cost $240 million just to prove a theory that was already proven. However, the experiment is unique and its measurements were considered 10 times better than anything that can be done on Earth. The scientists plan for another 900 orbits, hoping they will refine the experiment’s results.