Scientists have successfully tested a 400-year-old theory in orbit, confirming the equivalence principle tested by Galileo Galilei in the 16th century.
As reported in Science, the experiment took place about 700 kilometers (440 miles) above Earth's surface, on board a French satellite called MICROSCOPE (Micro-Satellite à traînée Compensée pour l'Observation du Principe d'Equivalence).
The satellite was launched on April 25, 2016, with the goal of testing the equivalence principle. This is the idea, later built upon in Einstein’s theory of general relativity, that objects of different masses are accelerated at the same speed by gravity, regardless of their mass.
The experiment was famously tested on board the Apollo 15 mission in 1971, when astronaut Dave Scott dropped a feather and a hammer on the Moon. In the absence of air, and thus air resistance, both hit the ground simultaneously.
The $240 million MICROSCOPE experiment was a bit different. Led by the French space agency CNES, it involved using two cylindrical masses each a few centimeters long. The satellite uses thrusters to fly around the cylinders, preventing them from touching the sides. Something similar was employed on an ESA mission called LISA Pathfinder.
In doing the experiment, the team made sure to remove all other forces that could be acting on the cylinders. These included things like aerodynamic forces from the outer reaches of the atmosphere and impacts from solar photons.
The cylinders were kept in constant free fall while orbiting Earth, kept in position inside the satellite by applying small voltages. As the satellite orbited Earth every 1.5 hours, a difference in the voltage applied to each would indicate that they were falling at different speeds. After 1,500 orbits, however, there was no such difference.
“The mission appears to have performed fantastically,” Clifford Will, a theoretical physicist at the University of Florida in Gainesville, told Science. A paper describing the findings is to be published in Physical Review Letters.
The team now plans to perform the experiment on another 900 orbits, further refining their results so far. At the moment, no difference in acceleration was found to be about one part in 100 trillion, or 1014, over 10 times better than anything we can do on Earth. They hope to improve this to 1015 with the subsequent orbits.