ALLEGRO has been listening for gravitational waves almost continuously since 1991. Because no scientist would believe any pulse to be a real gravitational wave unless it were registered nearly simultaneously by another detector of at least equal sensitivity, ALLEGRO has been collaborating with four other acoustic bar detectors in the United States and Europe.
So far, no pulse has been definitively proven to be due to a gravitational wave, but neither Hamilton nor Johnson is discouraged, primarily because astronomers now realize that the higher frequencies are likely to come from comparatively low-mass and infrequent astronomical events within our own galaxy. One hoped-for signal is a crescendoing and rising-pitch glissando from pairs of nearby neutron stars locked in an inward death spiral until they abruptly coalesce into a stellar-mass black hole, giving off one urgent accelerating chirp. That final death chirp is calculated to be brief, lasting maybe two minutes at most as it rises through ALLEGRO’s narrow range of resonant frequencies. Says Johnson: “The gravitational chirp of this in-spiral event, if it were converted to sound waves, would sound like a big, low-pitched bird.” The waves could be quite weak, depending on distance, but statistical calculations show that each year about a dozen pairs of neutron stars coalesce into black holes within “shouting” distance of Earth. Rarer still—maybe only three times a century in our galaxy—would be the scream of a massive star ending its life in a catastrophic supernova explosion. So, counting on luck as much as attention to detail, Hamilton and Johnson and ALLEGRO keep a patient vigil.
The Light Fantastic
Mirrors and lasers are the heart of a wholly different type of gravitational wave detector, which this fall will begin to record data at full sensitivity. This is the Laser Interferometer Gravitational-wave Observatory, or LIGO (pronounced LYE-go), its twin L-shaped detectors separated by more than 1,800 miles: one in the forests of Livingston, Louisiana, and the other in the desert of Hanford, Washington.
“LIGO is the biggest hole in the atmosphere ever built,” quips LIGO-Livingston director Mike Zucker. Each LIGO facility consists of a pair of vacuum chambers, their ends meeting at right angles. Each chamber is monumental, measuring four feet in diameter and two and a half miles long.
LIGO does not listen for gravitational waves in the same way the acoustic bar detectors do. Its purpose is to directly measure the degree to which passing gravitational waves momentarily deform space-time itself. “General relativity predicts [a passing wave] will lengthen one arm and compress the other,” says Rainer Weiss, emeritus physics professor at MIT. So if the two distant LIGO sites independently detected a tell-tale pattern of deflections nearly simultaneously, scientists would feel confident that they had observed a gravitational wave pass through Earth—and that, moreover, its measured behavior matched Einstein’s prediction.
But what a measurement! The deflection of space-time is so minuscule that over the 2.5-mile lengths of LIGO’s perpendicular arms—the arms at each site usefully if unoriginally dubbed X and Y—the scientists are preparing to measure deflections amounting to 10-16 centimeter, a thousandth the diameter of a sub-atomic neutron or proton.
Such a precise measurement presses science and engineering to the ragged edge of the possible. “Half of our technology is devoted to being able to detect a signal. The other half is devoted to identifying and eliminating sources of noise,” Zucker says.
To detect a signal, LIGO operates with elegant simplicity: At the junction of the arms, the input beam of an infrared laser strikes a beam-splitter—essentially a half-reflective mirror—which directs half the beam down the length of vacuum in each arm. At the end of each arm, a mirror reflects the laser light back to the apex, where (after some 100 reflections back and forth) both split beams are recombined. Now here’s the clever trick. The lengths of the arms are very slightly different, so the recombining laser beams will interfere destructively: The crests of the light waves in the laser beam returning from its trip down the X axis will cancel the troughs of the light waves returning from the Y axis. Thus, in the absence of gravitational waves, no light should reach the ultimate photo-detector. But should a passing gravitational wave distort space-time as Einstein predicted—and thus alter the relative lengths of LIGO’s perpendicular X and Y arms—the recombining beams should interfere constructively: Light wave crests should fall on crests, troughs on troughs, light should shine on the ultimate photo-detector, and physicists the world over should dance.
Problem is, the living world is replete with sources of noise, most of which could distort the lengths of LIGO’s arms by degrees far greater than the anticipated signal.
Daytime-warming expansions and nighttime-cooling contractions cause tiny but measurable differences in the detector, as do the pounding of ocean waves on distant beaches, the hum from 60-Hertz power lines, and the thumping from tree farms right around the Livingston LIGO site, where mighty growling machines chop soft pines for paper. Thus the mirrors within the LIGO arms are suspended as pendulums from a heroic arrangement of springs and masses that damp seismic vibrations; recently, hydraulic actuators and electronic controls were added to actively counter seismic disturbances.