Major gravity experiment recreated aboard a satellite

MICROSCOPE experiment
Scientists compared how two objects fell within a satellite in orbit 710 kilometers above Earth (as depicted in this illustration).

Galileo’s most famous experiment has taken a trip to outer space. The result shows Einstein was right — again. The testing confirms a major part Einstein’s theory of gravity. What’s more, it was about 10 times more precise than earlier such tests.

According to science lore, Galileo dropped two balls from the Leaning Tower of Pisa. In fact, it seems unlikely that Galileo actually carried out this experiment. But he supposedly was trying to show that those balls would fall with the same acceleration. Their change in speed due to gravity would be the same, no matter what they were made from or how much they weighed.

Scientists have now performed a similar experiment in a satellite. They dropped two hollow cylinders and measured them for more than 120 orbits. The cylinders were in free fall for about eight days’ worth of time.

This experiment was designed to test what’s known as the equivalence principle. It’s a foundation of Einstein’s gravity theory, the general theory of relativity. The equivalence principle states that two different ways of defining an object’s mass are actually the same.

Now stick with us here, because this can get a bit complicated. Mass is a way of quantifying how much “stuff” an object is made up of. One way to define mass is by how easily an object moves when pushed. That’s the kind of mass called “inertial mass.” An object with more inertial mass requires a more forceful push to increase its speed by a certain amount. A second way to define mass is by how strong a gravitational pull an object feels. That’s known as “gravitational mass.”

Einstein’s equivalence principle holds that this inertial mass is equal to the gravitational mass. And if that’s true, then two different objects should fall at the same rate (at least when they’re in a vacuum, where air resistance is eliminated). In a perfect vacuum, for instance, a feather will accelerate just as much as will a ball of iron.

In the satellite experiment, known as MICROSCOPE, the two…

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