Einstein and Galileo Theories Tested in Space And Prove Everything Always Falls at the Same Rate

The MICROSCOPE satellite in orbit around Earth, in an artist's rendition. CNES/Virtual-IT, 2017

Trying to prove Albert Einstein wrong is one of scientists' favorite hobbies, but it usually doesn't meet with a whole lot of success. That's still the case when you try to take general relativity to space, according to a new paper published in the journal Physical Review Letters.

The paper reports on early results from the MICROSCOPE satellite, a French space mission that was launched in 2016 to test what scientists call the equivalence principle, an offshoot of Einstein's general theory of relativity that harkens back to stories of Renaissance scientist Galileo Galilei flinging things off an Italian tower to see if they hit the ground at the same time. The equivalence principle says that no matter what objects are made of, they should fall at the same rate when exposed to the same gravity. It's the sort of thing most people never think twice about but that keeps physicists up at night.

MICROSCOPE is a super high-tech version of Galileo's same basic experiment: It contains two nested glorified cans, one made of titanium and one of a platinum alloy. Ever since the satellite settled into orbit, those cylinders have been essentially falling around Earth, and scientists have been monitoring them for any tiny discrepancies in their speed.

And the new paper reports that after 120 loops around Earth, the speed of the two cylinders has been just two-trillionths of a percent different, essentially zero. MICROSCOPE is still collecting data and these are only the first results, so the scientists behind the project are still hoping to make those results more conclusive.

But the early results are still good news for the equivalence principle—although not necessarily what the scientists were hoping for. That gets at the heart of why scientists are so interested in equivalence in the first place: The big hole in physics between very very small things and the sorts of things we can actually interact with.

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Very small things are governed by the principles of quantum mechanics, which generally works pretty well. And larger things are governed by the principles of general relativity and all the physics that comes with it. But scientists haven't figured out a way to get the two systems to talk to each other and work together.

Some of their possible solutions require miniscule differences in equivalence to get the two theories to align—hence the MICROSCOPE project, which keeps orbiting until next year.