Key Einstein principle survives quantum test

Equivalence of gravity, acceleration applies even for atoms in superposition of energy states

warping space

EINSTEIN ENDURES  According to Einstein’s general theory of relativity, massive objects warp space (illustrated above), producing gravitational attraction. Scientists don’t understand how general relativity interfaces with quantum mechanics, but a pillar of general relativity, the equivalence principle, has withstood a new quantum test.


Particles with mind-bending quantum properties still follow a standard gravitational rule, at least as far as scientists can tell.

The equivalence principle — one of the central tenets of Einstein’s theory of gravity — survived a quantum test, scientists report online April 7 at

In Einstein’s gravity theory — the general theory of relativity — gravity and acceleration are two sides of the same coin. According to the equivalence principle, the gravitational mass of an object, which determines the strength of gravity’s pull, is the same as its inertial mass, which determines how much an object accelerates when given a push (SN: 10/17/15, p. 16). As a result, two objects dropped on Earth’s surface should accelerate at the same rate (neglecting air resistance), even if they have different masses or are made of different materials.

One of the first reported tests of the equivalence principle — well before it was understood in the framework of general relativity — was Galileo’s apocryphal experiment in which he is said to have dropped weights from the Leaning Tower of Pisa. Scientists have since adapted that test to smaller scales, swapping out the weights for atoms. In the new study, physicists went a step further, putting atoms into a quantum superposition, a kind of limbo in which an atom does not have a definite energy but occupies a combination of two energy levels.

Manipulating rubidium atoms with lasers, scientists led by researchers from Italy gave the atoms an upward kick and observed how gravity tugged them down. To compare the acceleration of normal atoms with those in a superposition, the scientists split the atoms into two clouds, put atoms in one cloud into a superposition, and measured how the clouds interacted. These clouds of atoms behave like waves, interfering similarly to merging water waves. The resulting ripples depend on the gravitational acceleration felt by the atoms.

The scientists then compared the result of this test to one where both clouds were in a normal energy state. Gravity, the researchers concluded, pulled on atoms in a superposition at the same rate as the others — at least to the level of sensitivity the scientists were able to probe, within 5 parts in 100 million.

Quantum tests of the equivalence principle explore the murky realm where quantum mechanics and general relativity meet. The two theories don’t play well with one another. Scientists are currently struggling to unify the pair into one theory of quantum gravity, and some candidate theories predict that the equivalence principle breaks down at the quantum level.

The test “is a new way of confronting gravity with quantum physics,” says theoretical physicist Robert Mann of the University of Waterloo in Canada. “Any way that we can do that tells us something about how to put together gravity with quantum physics,” even if the test finds no violation, he says.

Guglielmo Tino, a study coauthor and physicist at the University of Florence, declined to comment on the work due to the policies of the journal where the paper has been accepted.

Scientists have previously tested the equivalence principle in atoms, comparing gravity’s effects on different types of atoms, for example. Because such tests deal with tiny particles, they also explore the nebulous territory between quantum physics and general relativity. But the new test is the first to study superposition, one of the weirdest properties of quantum mechanics.

“It’s a beautiful demonstration of the versatility of these quantum tests,” says physicist Ernst Rasel of Leibniz Universität Hannover in Germany.

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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