Works in Progress

Was Einstein right again? An enormous observatory in the swampland looks for gravitational waves

By Karen Wright|Tuesday, May 01, 2001


In southern Louisiana, scientists building a brand-new kind of cosmic observatory have adapted an old Cajun credo to promote their venture. Laissez les bonnes ondes rouler, say the souvenir coffee mugs: Let the good waves roll. The waves in question aren't those in the nearby Gulf of Mexico. They are gravitational waves, ripples in the fabric of space so subtle that no one, so far, has been able to detect them.

If the 5-mile-long Laser Interferometer Gravitational-Wave Observatory (LIGO) in Livingston does, astronomers will get a map of an unseen sky, physicists may get a look at the earliest universe, and the genius of Albert Einstein will be vindicated once more. But first, the observatory's crew will have to iron out the formidable technical problems that crop up whenever big machines hunt very small game.

"The fundamentals are simple," says Jonathan Kern, an optics expert at the observatory. "The devil is in the details."

Laser beams that traverse LIGO's lengthy tunnels (below) could reveal infinitesimal ripples in space.
Laser beams that traverse LIGO's lengthy tunnels could reveal infinitesimal ripples in space.
Photograph courtesy of Caltech
The fundamentals are described in Einstein's theory of general relativity, which explains how gravity interacts with matter, space, and time. Einstein thought that all matter exerts some degree of gravitational attraction by bending the space around it, much as a bowling ball warps the plane of a trampoline. Really dense, compact objects such as black holes really warp space, and if those objects are in violent motion, they create traveling oscillations that alternately stretch and then squeeze space as they undulate across the cosmos. Thus, Einstein proposed that gravity waves would stream forth at the speed of light from cataclysmic events such as colliding black holes and exploding stars.

In theory, physicists have long known how to detect the waves. In practice, the quest is complicated by the fact that gravity waves are very weak. The cosmic catastrophes that spawn gravity waves happen nowhere near our planet (which is just as well, for such crises are inhospitable to life). And like the ripples from a stone thrown into a pond, gravity waves become fainter as they move away from their source. By the time they reach Earth from events tens or hundreds of millions of light-years away, gravity waves disturb the local geometry by only one part in a billion trillion— far less than the diameter of an atom's nucleus.

The Louisiana observatory uses minute fluctuations in beams of laser light to detect the pulsing of gravity waves. The light from a single infrared laser is split into two beams and sent down dual vacuum chambers that reach at a right angle 21/2 miles into the flat pine forest. At the end of each arm is a mirror suspended by fine wire and polished to optical perfection. The twin laser beams bounce off these mirrors and zip back to the intersection of the L, where they recombine to make a pattern that depends on the positions of the mirrors. Because the peaks and troughs of the incoming light waves interfere with one another to make the pattern, the technique is known as interferometry.

A passing gravity wave will alter the positions of the mirrors as it squeezes and stretches the space between them, shifting the interference pattern created by the returning laser beams. The length of the arms limits to some extent the range of frequencies the observatory will be able to detect. But within that range, scientists should be able to notice distortions in space less than one one-thousandth the diameter of an atom's nucleus.

Unfortunately, the observatory's instruments are also sensitive to terrestrial noise that jars the mirrors far more than any gravity wave might. To minimize these tremors, the optics are supported on stacks of steel and rubber that act as shock absorbers. The whole optical assembly is put in a vacuum system that helps eliminate acoustic noise, dust, and thermal effects. The observatory also boasts some of the world's smoothest optical surfaces and purest lasers.

The trick now is in the tuning. Scientists and engineers are tweaking an elaborate suite of electronic sensors and servo controls that continuously adjust the mirrors' positions to compensate for terrestrial perturbations. When these mechanisms are working right, the interference pattern will be completely stabilized, so that any shift at all in the pattern will signal the passing of a gravity wave. Just to be sure the signal isn't the result of local noise, another, nearly identical observatory, almost 2,000 miles away in Hanford, Washington, will double-check the data. Both facilities should be fully tuned and ready to begin formal observations by next January.
Technicians install optics in one of the observatory's tunnels.
Technicians install optics in one of the observatory's tunnels.
Photograph courtesy of Caltech
Based on a tally of suspected sources of gravity waves, experts think the first generation of the observatory's optics may pick up several episodes a year. As the optics improve, they may capture fainter signals at a wider range of frequencies, and observations might eventually come in by the hour. Skeptics in the physics community have called these figures hubris; they claim that the Livingston and Hanford facilities, which cost more than $300 million, were built too soon, before their optical components were refined enough to handle the job. And when signals do start coming in, there's bound to be some disagreement about what they mean and whether they represent gravitational pulses, agrees the observatory's director Barry Barish.

"It's very difficult to tell when you're successful, because it's so hard to make measurements," he says.

Meanwhile, more gravity-wave detectors are under construction in Japan, Italy, and Germany, and a space-based detector may be online by 2010. With that network, astronomers could determine the direction a gravity wave is moving and trace its path back to its source. No one's sure what might turn up in LIGO's sights. Astronomers expect to see the gravity-wave signatures of supernovas, merging black holes, spinning neutron stars, and neutron stars colliding with one another or with black holes. With more advanced optics, they might someday detect the gravitational spasms that accompanied the birth of the universe. Launched 10-43 seconds after the Big Bang, that primordial gravity-wave background would give astronomers their earliest view of the cosmos and theorists a rare chance to test the predictions of string theory, the concept that attempts to describe the fundamental constituents of the four known forces.

Yet some scientists are more tantalized by the sources they can't predict. Recent evidence suggests that as much as 95 percent of the matter in the universe consists of some exotic substance that neither emits, reflects, nor absorbs any kind of light. It can't be detected by existing telescopes, and its presence is inferred only by its gravitational effects on galaxies. Barish thinks that gravity waves might help map this so-called dark matter for the first time.

"Over and over in the history of astronomy, a new instrument finds things we never expected to see," says physicist Rainer Weiss of the Massachusetts Institute of Technology, who helped orchestrate the campaign to build the observatory. Weiss compares the birth of gravity-wave astronomy to the advent of radio and X-ray astronomy in the last century, which gave physicists new and surprising views of the universe. "We'll have all sorts of crazy signals. And you'd be a damned fool if you didn't look for things you weren't expecting, because that's probably what you're going to see first."










Read Einstein's Unfinished Symphony: Listening to the Sounds of Space-Time, Marcia Bartusiak, Joseph Henry Press, 2000. Also, visit the LIGO overview page at www.ligo-la.caltech.edu/Posters/index.html.


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