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Galileo's famous gravity experiment holds up, even with individual atoms

According to legend, Galileo dropped weights from the Leaning Tower of Pisa, proving that gravity drops objects of different masses with the same acceleration. In recent years, researchers have tried to replicate this evidence in a way that the Italian scientist probably never imagined falling atoms.

A new study describes the most sensitive atom drop test to date and shows that Galileo's gravity experiment still holds, even for individual atoms. Two different types of atoms had the same acceleration at about one part per trillion, or 0.00 billion, according to physicists in an article published in Physical Review Letters.

Compared to a previous atom drop test, the new research is a thousand times more sensitive. “It represents a leap forward,” says physicist Guglielmo Tino of the University of Florence, who did not participate in the new study.

The researchers compared rubidium atoms of two different isotopes, atoms that contain different numbers of neutrons in their nuclei. The team dropped clouds of these atoms about 8.6 meters high into a vacuum tube. As the atoms went up and down, both varieties accelerated at essentially the same rate, the researchers found.

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By reaffirming Galileo's gravity experiment, the result defends the principle of equivalence, a foundation of Albert Einstein's theory of gravity, general relativity. That principle states that the inertial mass of an object, which determines how much it accelerates when force is applied, is equivalent to its gravitational mass, which determines the force felt by a gravitational force. The result: the acceleration of an object under gravity does not depend on its mass or composition.

So far, the principle of equivalence has withstood all tests. But atoms, which are subject to the strange laws of quantum mechanics, could reveal their weaknesses. “When you do the atom test … you’re testing the equivalence principle and stressing it out in new ways,” says Stanford University physicist Mark Kasevich.

Kasevich and colleagues studied tiny particles using atom interferometry, which takes advantage of quantum mechanics to make extremely accurate measurements. During the flight of atoms, scientists placed atoms in a state called quantum overlap, in which particles do not have a definite location. Instead, each atom existed in an overlap of two locations, separated by up to seven centimeters. When the two places of the atoms reunited, the atoms interfered with each other in a way that accurately revealed their relative acceleration.

Many scientists think that the principle of equivalence will eventually falter. “We have reasonable expectations that our current theories … are not the end of history,” says physicist Magdalena Zych of the University of Queensland in Brisbane, Australia, who did not participate in the research. This is because quantum mechanics – the branch of physics that describes the counterintuitive physics of the very small – does not fit well with general relativity, leading scientists to look for a theory of quantum gravity that can unite these ideas. Many scientists suspect that the new theory will violate the principle of equivalence by an amount too small to be detected with the tests conducted so far.

But physicists hope to improve such atom-based tests in the future, for example by performing them in space, where objects can fall freely for long periods of time. A test of principle of equivalence in space has already been performed with metal cylinders, but not yet with atoms (SN: 12/4/17).

Therefore, there is still a chance to prove that Galileo is wrong.

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