When Stars Collide
Enter Einstein's grand construct of gravitational wonders, and do not attempt to adjust your television set.
- By Trudy E. Bell
- Air & Space magazine, September 2005
Dana Berry/NASA GSFC
(Page 4 of 5)
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.
The twin LIGO detectors are sensitive to a wide range of frequencies, bracketing those detectable by the highly tuned ALLEGRO and other acoustic bar detectors: “from about 50 Hertz—an octave above the lowest note on a piano—to 10,000 Hertz, about that of the squeak of a mouse,” says Weiss. And the LIGO detectors are not alone. Somewhat smaller versions are operating in Germany, Italy, and Japan. In addition to searching for signals from supernovae, astronomers hope they can capture the entire glissando accelerating up to the death chirp of binary neutron stars coalescing into black holes. LIGO is so sensitive, in fact, that eventually it should detect supernova explosions, in-spiraling neutron stars, and black holes swallowing gases (and burping) all the way out to the Virgo Cluster, some 45 million light-years away. “We’re already within tasting distance of this!” Weiss exclaims.
Gravitational astronomers’ dearest hopes, however, lie on drawing boards. NASA and the European Space Agency are planning the Laser Interferometer Space Antenna, a constellation of three spacecraft that will orbit the sun in formation, 20 degrees behind Earth. When completed and launched in 2014, LISA will be the largest spaceborne instrument ever built.