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Free-Falling Atoms Offer New Test of Gravity's Strength

Technique promises a sharper view of the weakest force
gravity test



COURTESY OF MARK KASEVICH
A new technique may allow researchers to get a closer bead on the intrinsic strength of gravity, which is so feeble compared with other forces that its signature is easily drowned out in the laboratory. Physicists have built a sensor that kicks atoms into free fall so as to detect subtle quantum changes that precisely reveal gravity's strength, referred to as G.

Although such experiments have come into vogue in recent years as a way of testing theories that suppose gravity leaks into large but unseen extra dimensions of space, the true value of measuring G is probably much more prosaic, says experimental physicist James Faller of the University of Colorado, who was not involved in the study. "It's sort of the intellectual push-ups of measurement science," Faller says. By learning to probe gravity, he says, physicists may become better able to measure other effects that have greater scientific or technological value.

The new measurement is still about 100 times less precise than some other techniques for measuring gravity's strength, such as suspending a mass by a slender fiber. But the researchers say that if they can hone its precision, the test could add weight, so to speak, to the reliability of the other methods' results. "If you start getting the same answer as [other groups], then you kind of have the feeling you're starting to understand nature a little better than you have before," says Mark Kasevich of Stanford University, lead author of the new report.

Any two objects exert a gravitational pull on each other that depends on their mass, the distance between them and the so-called gravitational constant, G. If G changed at all, that would mean the strength of gravity had changed but, so far, the constant has indeed proved unwavering no matter the distance between objects.

To get a new handle on G, Kasevich and his colleagues used a technique called atom interferometry. They placed an ultracold vapor of cesium atoms inside a vacuum chamber below a 540-kilogram lead stack, which exerted a strong gravitational tug, and then struck the vapor with a laser beam. Kicked by the laser, the atoms flew upward then fell back down like a fountain.

The team adjusted the laser so that it also put the atoms into a quantum state, or superposition, such that the fountain reached two different heights at the same time. The precise difference in those two heights depends on the strength of gravity pulling down on the atoms.

Researchers cannot measure that difference directly--in fact, it would cease to exist if measured, thanks to quantum strangeness. But they can take advantage of the fact that the atoms on the two trajectories interfere with each other, much like light or sound waves can.

When the atoms finished their descent, the group measured their probability of being in one of two states, which--crucially--depended on the amount of interference they had experienced.

To control for various sources of error such as Earth's gravity and vibrations in the room, they performed simultaneous measurements in another vacuum chamber that was placed above the lead stack mass, and they moved the lead up or down between tests.

The resulting value for G should be accurate to within a few tenths of a percent, the group reports in this week's Science.

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