Huge detectors will seek gravity waves

April 07, 1991|By Luther Young

After decades of listening for gravity waves with detectors that weren't sensitive enough, physicists are planning to build a device that could finally confirm the existence of the elusive ripples in space.

The $211 million Laser Interferometer Gravitational Wave Observatory, or LIGO, involves construction of two state-of-the-art detectors -- at U.S. locations at least 1,500 miles apart -- in huge L-shapes with arms more than 2 miles long.

Designed and operated by scientists from the California Institute of Technology and the Massachusetts Institute of Technology, the observatory could expand knowledge of the cosmos as radically as X-ray and radio astronomy when those observational tools were born.

"LIGO will open an entirely new window on the universe, and it will be fascinating to take a look through it," said Rochus E. Vogt, director of the project and professor of physics at Caltech.

Gravitational waves -- predicted by physicist Albert Einstein in his theory of general relativity -- are a totally different phenomenon from the electromagnetic radiation that has provided our portrait of the universe through telescopes and huge radio dishes.

They are thought to emanate at the speed of light from all objects as they whirl, vibrate, collapse, collide or explode, but they are so weak that only those waves generated by extremely massive, violent events such as supernovas are theoretically detectable on Earth.

And detection of gravity waves is so tantalizing because -- unlike visible light and other familiar forms of electromagnetic radiation -- they escape from the obscuring violence at the heart of those events without being absorbed or scattered or changed.

So they carry unique information from across the universe and back in time, allowing astronomers to peer deep, to search for the existence of black holes and reveal never-seen details in the collapsing cores of stars or at the dust-shrouded center of our galaxy.

On March 11, LIGO officials announced that 18 sites in 17 states had met such requirements as terrain flatness and seismic stability. Maryland did not apply, and the only East Coast sites proposed are in New York, West Virginia and South Carolina.

President Bush's proposed 1992 budget allocates $23.5 million for initial construction work, but the project won't proceed until Congress approves funding.

"We want to be ready to begin construction early in 1992," Dr. Vogt said. The complex observatory -- one of the biggest, most expensive science projects of the decade -- is expected to take at least five years to complete.

The LIGO technology represents the best efforts yet of physicists to measure the incredibly faint effects caused by gravitational waves as they pass through objects on Earth.

Each L-shaped detector has hollow arms 2 1/2 miles long and 4 feet in diameter. Maintained at a strong vacuum, the tubes contain heavy mirrors at both ends suspended from wires to isolate them from vibration.

With the passage of a gravity wave, the distance between the mirrors in one arm should decrease slightly and increase slightly in the other. And that tiny difference can

measured by comparing laser beams bouncing back and forth between the mirror pairs.

How tiny? "One-one-hundred-millionth the diameter of a hydrogen atom. Until recently, the technology didn't exist to measure it," Dr. Vogt said.

A scale model of LIGO at Caltech is currently the largest gravitational wave detector in the world, with arms 130 feet long. It can note "the footsteps of a fly," but it's not sensitive enough to detect the waves, he said. "We need the big observatory to do the job."

And they need two of the facilities, separated by 1,500 to 3,000 miles, to operate in tandem and permit the factoring out of any local disturbances such as earth tremors or even automobile traffic that can give false readings.

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