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 3 of 5)
Among gravitational wave scientists, auditory analogies abound, and the analogies are apt. Gravitational waves are expected to arrive here at all frequencies and from all directions in space. Just as following a single conversation in a large cocktail party requires a listener to reject the background chatter and the clinking of glass and silver, the challenge of locating an individual source of a gravitational wave requires determining direction with two “ears,” plus filtering out noise, including noise at the same frequency as the desired signal.
The oldest way of trying to detect gravitational waves is literally listening for them, using solid aluminum bars that function essentially as giant tuning forks. As you can make a tuning fork held near your mouth hum faintly if you sing at its resonant frequency, acoustic bar detectors are expected to hum when struck by a gravitational wave of the bar’s resonant frequency. One of the largest is ALLEGRO, which stands for A Louisiana Low-temperature Experiment and GRavitational wave Observatory. Designed and built by Louisiana State University’s William O. Hamilton (professor emeritus of physics) and Warren Johnson, it is a huge aluminum tank on the ground floor of the Physics and Astronomy Building on the campus in Baton Rouge. The tank, its vacuum pump periodically emitting a stream of ticking burbles, is surrounded with all manner of pipes, hoses, and other structures. Its external cylinder is basically a giant Thermos bottle, insulating a smaller chamber cooled to the temperature of liquid nitrogen (77 degrees Kelvin); that first chamber in turn insulates a second, chilled to the temperature of liquid helium, a mere 4.2 degrees K above absolute zero (the lowest temperature possible in nature).
Deep inside the frigid, dark inner chamber, in as rarefied a vacuum as is possible to produce in the atmosphere, is a two-and-a-half-ton solid cylinder of aluminum alloy suspended by a single titanium-alloy wire so that it hangs in exact balance. The bar’s size, the properties of its aluminum, and the precision with which it was manufactured all contribute to its sensitivity and bandwidth: It is able to “hear” in two narrow bands near the resonant frequency of 900 Hertz. Should a gravitational wave of those frequencies pass through the bar, the wave should set the bar to ringing.
Theorists have predicted that certain classes of supernovae could produce gravitational waves at a frequency within ALLEGRO’s bandwidth. Because the tone would be so extraordinarily faint (and inaudible anyway in a vacuum), delicate accelerometers are affixed to the bar’s ends to sense minute accelerations of the bar produced by the vibration. The vacuum eliminates air molecules, whose bouncing off the metal surface might otherwise damp the faint vibration, and the extreme cold quiets the thermal jiggling of the bar’s own molecules.
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.