Free-Falling Atoms Aboard China's Tiangong Recreate Galileo's Gravity Test

An experiment made famous by Galileo has been shrunk to the atomic scale and carried into orbit. The 17th-century Italian scientist is said, perhaps apocryphally, to have dropped weights off the Leaning Tower of Pisa to show that objects of different masses fall at the same rate. Researchers have now recreated that test with atoms falling aboard China's Tiangong space station, reporting August 28 in the journal Science Advances that two clouds of atoms fell with nearly identical accelerations.

The team measured the relative accelerations of two clouds of cold rubidium atoms. The atoms in each cloud had different masses, due to a difference in the number of neutrons in their atomic nuclei. Yet the accelerations of the two clouds matched with a precision of 0.05 thousandths of a percent.

The matching accelerations confirm that two different ways of defining mass are equivalent. Gravitational mass determines how an object responds to the pull of gravity, while inertial mass determines how much an object accelerates when pushed with a given force. If the two definitions are equivalent, the effect of mass cancels out of the equations, and objects of different masses fall at the same rate in a vacuum. This idea, called the weak equivalence principle, is a foundation of Albert Einstein's general theory of relativity, which describes gravity as the warping of spacetime.

Scientists have long tested the weak equivalence principle by dropping objects, including atoms, on Earth, but objects there can fall only so far, limiting the tests' precision. In orbit, free fall can continue indefinitely. Space-based tests using falling metal cylinders aboard a satellite have previously confirmed the weak equivalence principle.

Atoms, unlike larger objects, obey the rules of quantum physics, so it is worth checking whether they behave differently under the weak equivalence principle.