Dead stars could help astronomers detect subtle ripples in spacetime: gravitational waves would disrupt pulsar's radio emissions.

DENVER -- A bunch of dead stars could serve as ready-made recorders for gravitational waves--subtle ripples in spacetime that if discovered would be the crowning achievement of Einstein's theory of general relativity, astronomers propose. Researchers have invested hundreds of millions of dollars to perfect sensitive devices on the ground and launch even more sophisticated experiments in space to detect this cosmic symphony.

The new search technique would instead rely on radio waves generated like clockwork by millisecond pulsars--the collapsed remnants of massive stars that spin about once every one to 10 milliseconds. The speed at which these pulsars rotate enables researchers to measure the timing of the radio waves' arrival at Earth with high accuracy.

Measuring arrival time is critical, says Frederick Jenet of the University of Texas at Brownsville and Texas Southmost College, who presented the proposal on May 2 at an American Physical Society meeting. Colleague Andrea Lommen of Franklin & Marshall College in Lancaster, Pa., reported additional details on May 5.

A gravitational wave passing by the pulsar would warp spacetime, altering the arrival time of the pulsar's radio waves ever so slightly. First the pulses would appear a little earlier, then a little later. The changes in timing would depend on the phase and direction of propagation of the gravitational wave, as well as its distance from the pulsar, Jenet notes.

Low-frequency gravitational waves, produced by the merger of supermassive black holes, would generate the biggest changes in arrival time and therefore be easiest to detect. To detect a general background of such waves, astronomers would need to monitor 20 of the millisecond pulsars for five to 10 years, with the arrival time of the radio waves determined to an accuracy of 100 nanoseconds, Jenet estimates. Recording individual gravitational waves produced when black holes merge would require five pulsars with radio wave arrival times known to 10 nanoseconds, he adds.

"We currently have about 20 millisecond pulsars, but only five of these can be timed to the needed precision," he notes.

The idea of using pulsars to search for gravitational waves dates back to the late 1970s, Jenet says. "Personally, the romantic notion of building a galactic-scale gravitational wave observatory using exotic stars as part of the instrument itself seems quite appealing," he says.

Using pulsars to search for gravitational waves "is not a crazy idea but a well-established area of research," says Tom Prince of Caltech. The increased sensitivity of radio telescopes and the discovery of additional millisecond pulsars have made the idea practical.

Scientists are currently using several large-scale gravitational wave detectors on Earth, including the twin LIGO detectors. Prince leads an effort to launch a detector called LISA into space sometime in the next decade. LISA will look for high-frequency gravitational waves, while the pulsar method would look for low-frequency waves.

Still, Jenet says, the pulsar method is available now, without the need for new telescopes.