Galileo’s famous gravity experiment holds up, even with atoms

According to legend, Galileo dropped weights off of the Leaning Tower of Pisa, showing that gravity causes objects of different masses to fall with the same acceleration. In recent years, researchers have taken to replicating this test in a way that the Italian scientist probably never envisioned — by dropping atoms.

A new study describes the most sensitive atom-drop test so far and shows that Galileo’s gravity experiment still holds up — even for individual atoms. Two different types of atoms had the same acceleration within about a part per trillion, or 0.0000000001 percent, physicists report in a paper in press in Physical Review Letters.

Compared with a previous atom-drop test, the new research is a thousand times as sensitive. “It represents a leap forward,” says physicist Guglielmo Tino of the University of Florence, who was not involved with the new study.

Researchers compared rubidium atoms of two different isotopes, atoms that contain different numbers of neutrons in their nuclei. The team launched clouds of these atoms about 8.6 meters high in a tube under vacuum. As the atoms rose and fell, both varieties accelerated at essentially the same rate, the researchers found.

In confirming Galileo’s gravity experiment yet again, the result upholds the equivalence principle, a foundation of Albert Einstein’s theory of gravity, general relativity. That principle states that an object’s inertial mass, which determines how much it accelerates when force is applied, is equivalent to its gravitational mass, which determines how strong a gravitational force it feels. The upshot: An object’s acceleration under gravity doesn’t depend on its mass or composition.

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

Kasevich and colleagues studied the tiny particles using atom interferometry, which takes advantage of quantum mechanics to make extremely precise measurements. During the atoms’ flight, the scientists put the atoms in a state called a quantum superposition, in which particles don’t have one definite location. Instead, each atom existed in a superposition of two locations, separated by up to seven centimeters. When the atoms’ two locations were brought back together, the atoms interfered with themselves in a way that precisely revealed their relative acceleration.

Many scientists think that the equivalence principle will eventually falter. “We have reasonable expectations that our current theories … are not the end of the story,” says physicist Magdalena Zych of the University of Queensland in Brisbane, Australia, who was not involved with the research. That’s because quantum mechanics — the branch of physics that describes the counterintuitive physics of the very small — doesn’t mesh well with general relativity, leading scientists on a hunt for a theory of

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Physicists keep trying to break the rules of gravity but this supermassive black hole just said ‘no’


A new test of Albert Einstein‘s theory of general relativity has proved the iconic physicist right again — this time by re-analyzing the famous first-ever picture of a black hole , which was released in April 2019.

That image of the supermassive black hole at the center of galaxy M87 was the first direct observation of a black hole’s shadow — the imprint of the event horizon, a sphere around the black hole’s singularity from which no light can escape. Einstein’s theory predicts the size of the event horizon based on the mass of the black hole; and in April 2019, it was already clear that the shadow fits general relativity’s prediction pretty well. 

But now, using a new technique to analyze the image, the researchers who made the picture showed just how well the shadow fits the theory. The answer: 500 times better than any test of relativity done in our solar system. That result, in turn, puts tighter limits on any theory that would seek to reconcile general relativity, which describes the behavior of massive celestial objects, with quantum mechanics, which predicts the behavior of very small things. 

General relativity’s great accomplishment was to describe how gravity operates in the universe: how it pulls objects toward each other; how it warps space-time; and how it forms black holes. To test general relativity, scientists use the theory to predict how gravity will act in a certain situation. Then, they observe what actually happens. If the prediction matches the observation, general relativity has passed its test.

But no test is perfect. Watch how the sun’s gravity tugs Mercury along its orbit, and you can measure general relativity in action. But telescopes can’t measure the movement of Mercury down to the nanometer. And other forces — the tug of Jupiter’s gravity, and Earth’s gravity and the force solar wind, to name just a few — impact Mercury’s movement in ways that are difficult to separate from the effects of relativity. So the result of every test is an approximation and Einstein’s theory is only proven more or less.

Related: 8 ways you can see Einstein’s Theory of Relativity in real life

The size of that uncertainty — the “more or less” factor — is important. When scientists test general relativity over and over, they are putting constraints on Einstein’s idea. The reason this work is important is that even though general relativity keeps passing tests, physicists do expect it to eventually fail.

General relativity must be incomplete, physicists believe, because it contradicts quantum mechanics. Physicists believe that discrepancy signals the presence in our universe of some larger, all-encompassing mechanism describing both gravity and the quantum world that they have yet to uncover. Looking for cracks in relativity, they hope, might turn up clues to help them find that complete theory.”We expect a complete theory of gravity to be different from general relativity, but there are many ways one can modify it,” University of Arizona astrophysicist Dimitrios Psaltis said

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