When Stars Collide

Enter Einstein's grand construct of gravitational wonders, and do not attempt to adjust your television set.

Astronomers observed the ripples in space-time caused by gravitational waves from two black holes colliding. (Artist's impression: LIGO)
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Who cares?

Physicists do. An unambiguous sign of gravitational waves would confirm the speed and characteristics of such waves as predicted by Einstein’s general theory of relativity, which undergirds all modern physics of the very massive and the very fast.

And astronomers do. Gravitational waves carry information about extreme astronomical processes now unknowable any other way.

“All the light we see from an [exploding] star is just from individual atoms in its outer layers,” explains Lee Samuel Finn, director of the Center for Gravitational Wave Physics at Pennsylvania State University in State College. “We can’t peer into its thermonuclear engine. But gravitational waves come from its bulk matter, traveling through the outer layers without scattering, extinction, or reddening, letting us directly see the collapse of the stellar core.”

Gravitational disturbances, like light and sound, move in waves with characteristics like frequency, wavelength, and strength that can vary over time. In fact, one type of detector is trying to convert gravitational vibrations into ordinary sound.

Hum a Few Bars
“We’re like deaf people, watching other people’s lips move and trees fall. We suspect there is sound, but we have never heard it, and can only guess how to build something that can detect its vibrations,” explains Michael E. Zucker, a gravitational wave physicist who splits his time between the Massachusetts Institute of Technology and a gravitational wave detector outside Baton Rouge, Louisiana.

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.

About Trudy E. Bell

Trudy E. Bell, M.A. has been an editor for Scientific American, senior editor for IEEE Spectrum magazine, and senior writer for the University of California High-Performance AstroComputing Center. She is the author of a dozen books and more than 500 articles, 19 of which have won journalism prizes, including the 2006 David N. Schramm Award of the American Astronomical Society (won in part for her Air & Space/Smithsonian article “When Stars Collide.”) Reach her at trudyebell.com or t.e.bell@ieee.org.

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