How Gravitational Waves Lead to Space-Time Ripples


Jennie McGrath:
How Gravitational Waves Lead to Space-Time Ripples
By Andrew Bridges
Pasadena Bureau Chief
posted: 12:46 pm ET
15 November 1999

Since they first proposed their existence 83 years ago, the power to detect extremely faint ripples in the fabric of space-time had long eluded scientists.

Not any more. The $371-million Laser Interferometry Gravitational-wave Observatory, known as LIGO, is a two-part instrument designed to detect such phenomena.

To detect gravitational waves, LIGOs two L-shaped detectors -- one in Livingston, Louisiana, the other in Hanford, Washington -- work in concert across the nearly 2,000 miles (3,000 kilometers) that separate them.

Gravitational radiation causes a strain on the fabric of space-time transverse to (i.e., extending across) the direction in which its waves are propagated. As they strike, say, the Earth, the waves will stretch the fabric in one direction, while along another, they compress it.

Each LIGO installation will capitalize on that effect by using its two 2.5-mile (4-kilometer) arms -- positioned at a 90-degree angle -- to measure how gravitational waves alternately stretch and compress the distances between two suspended masses.

However, to do so requires an ability to measure changes in distance on the order of one-one hundred millionth the diameter of a hydrogen atom.

"We are in the process of doing something that is amazing," said Peter Saulson, a physicist at Syracuse University. "It is a marvel of measurement technique.

The distances between the test masses -- fitted with fused silica mirrors and hung in enormous vacuum pipes encased in concrete -- will be measured with a laser beam split in two, each half then traveling the length of each arm multiple times.

By then recombining (or interfering) the separate beams, any change in the distance between either of the pairs of masses will throw them out of phase with each other, thus indicating -- if all goes well -- the form of the gravitational wave as it reaches the Earth. If no waves pass, the distances between the test masses will be the same.

To make sure, the two detectors are located far, far apart. That way, no local interference -- earthquakes, noise or fluctuations in the lasers -- can be mistaken for a gravitational wave.

"Having a signal at both is the trick," said Barry Barish of Caltech, LIGOs director. "We use two to make sure its something traveling at the speed of light and comes from the distant universe." Actual detection work at the two LIGO facilities will likely begin in early 2002.

Eventually, Caltech and MIT hope to upgrade the facility to make it even more sensitive. By that time, other gravitational-wave observatories in Italy, Great Britain and Japan should be up and running, allowing all five to work in unison. NASA and the European Space Agency have plans to launch a space-based observatory to detect the waves, perhaps by 2009.

The $465 million project would use three spacecraft flying in unison, each 3 million miles (5 million kilometers) from the other, said Karsten Danzmann of the Institut fuer Atom und Molekuelphysik at the University of Hannover in Germany.


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