LIGO doesn’t just detect gravitational waves. It makes them, too

WASHINGTON, D.C.—The Laser Interferometer Gravitational-Wave Observatory (LIGO) is not only the most sensitive detector of ripples in spacetime. It also happens to be the world's best producer of gravitational waves, a team of physicists now calculates. Although these waves are far too feeble to detect directly, the researchers say, the radiation in principle could be used to try to detect weird quantum mechanical effects among large objects.

"When we optimize LIGO for detection, we also optimize it for emission" of gravitational waves, says Belinda Pang, a physicist at the California Institute of Technology (Caltech) in Pasadena, who presented the analysis here last week at a meeting of the American Physical Society.

Gravitational waves literally stretch space itself. In 1915, Albert Einstein explained that gravity arises when massive objects such as Earth warp spacetime, bending the otherwise straight trajectories of free-falling objects. Einstein also predicted that certain swirling configurations of mass would radiate gravitational waves. The 1000 physicists working with LIGO have twice detected such waves emanating from a pair of massive black holes spiraling into each other.

LIGO relies on exquisitely sensitive twin detectors in Hanford, Washington, and Livingston, Louisiana. Each detector essentially consists of a pair of crossed, 4-kilometer-long rulers. To detect the stretching of space, researchers use laser light to compare the lengths of the two rulers on a near-constant basis. Light waves bounce back and forth between 40-kilogram mirrors at either end of each arm, and researchers compare the arms' lengths using an optical technique called interferometry, in which the light waves from the two arms are combined to see how they reinforce or cancel each other. Gravitational waves are so feeble that to detect one, physicists must compare the lengths of the two arms to within 1/10,000 the width of a single proton.

But the fact that LIGO is so sensitive to the stretching of spacetime implies that it is also exceedingly efficient at generating ripples. To prove it, Pang and her colleagues developed a quantum mechanical model of how the stretching of space affects or “couples” to light waves bouncing back and forth in one of LIGO’s arms.

To make their measurements as sensitive as possible, LIGO physicists have to ensure that the positions of the peaks and troughs in each light wave—its so-called phase—remain steady and stable. But quantum uncertainty then requires that the size of the wave, known as its amplitude, must be less certain. The unavoidable amplitude fluctuations then give random impulses or "kicks" to the mirrors, and the mirrors' motion generates tiny ripples in spacetime, Pang says. Of course, LIGO doesn't generate large gravitational waves—you could probably make bigger ones yourself by whirling bowling balls around—but it does so with optimal efficiency.

The result isn't surprising, says Fan Zhang, a physicist at Beijing Normal University. "The fundamental thing about a detector is that it couples to gravitational waves," he says. "When you have coupling it's going to go both ways."

Although far too feeble to detect directly, the waves could still be used to probe quantum effects among macroscopic objects, Pang says. Quantum mechanics says that a vanishingly small object such as an electron can literally be in two places in once. Many physicists suspect that it might just be possible to coax a macroscopic object, such as one of LIGO's mirrors, into a similar state of quantum motion.

That delicate state wouldn't last long, as interactions with the outside world would make it "decohere" and put it in one place or another. However, one could imagine measuring the rate at which such a state decoheres to see whether it matches the rate expected from the radiation of gravitational waves, Pang says. Some theorists have suggested that gravity plays a special role in squashing quantum states among macroscopic objects.

"It's an interesting idea, but experimentally it's very challenging," says Yiqui Ma, a physicist at Caltech and one of Pang's collaborators. To see these quantum effects of gravitational radiation, researchers would have to suppress every other source of decoherence, he says. Pang acknowledges the point. "It's unbelievably difficult," she says. "But if you want to do it, what we're saying is that LIGO is the best place to do it."