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Chapter 3.3 LIGO

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Several countries plan to construct instruments that eventually will work together to detect and locate the sources of gravitational waves arriving from the distant cosmos. The U.S. effort, sponsored by the National Science Foundation, is called LIGO, which stands for Laser Interferometer Gravitational-Wave Observatory.

Here are but some of the goals that the U.S. instruments are being designed to accomplish:

1. Prove by direct measurement that gravitational waves  exist.
2. Ascertain whether gravitational waves propagate at the speed of light, as postulated by Einstein.
3. Verify that gravitational waves cause the predicted displacements in matter they travel through.
4. Confirm the presence in the universe of black holes and study their dynamics and evolution.
5. Observe cosmic cataclysms, from supernovae to coalescing black holes to the Big Bang.

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Catching the Waves

The problem with detecting gravitational waves is that they are extremely weak by the time they reach us. For example, according to calculations with Einstein Field Equations, if two black holes with the mass of 10 suns coalesced 1 billion light years away, the resulting gravitational waves reaching Earth would displace the oceans by only 10 times the diameter of an atomic nucleus!
Evidently, detecting gravitational waves poses a formidable engineering challenge. The answer may lie with a technique called laser interferometry.

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How Does Laser Interferometry Work?

In essence, the system employed by LIGO consists of suspended weights that are free to move horizontally. A passing gravitational wave would change the distance between the weights, first in one arm, then in the other arm, which is arranged at a right angle to the first. This distance is measured by a laser beam that is split between the two arms, multiplied many times by passing back and forth between the mirrored surfaces of the hanging weights, then recombined at a photodetector.
Normally the split laser beams optically "interfere" when combined, cancelling each other out. But if the length of one arm changes even minutely, the recombined beams will produce an interference pattern. The pattern's characteristics should reveal unique information about the passing gravitational wave. Relying initially on computed gravitational wave signatures, researchers will eventually build a catalog of characteristic wave shapes to differentiate between several types of events thought to emit detectable gravitational waves.

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LIGO Prototype

The U.S. team, comprised of researchers from the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT), has developed a prototype device that can detect when a test weight 40 meters away moves almost imperceptibly -- many times less than the diameter of a strand of hair.

Schematic of the full-scale instrument

Detecting gravitational waves entails measuring tiny displacements. The world's most sensitive laser interferometers are now being built and are expected be operational by 1999. Two matching U.S. installations will be constructed at relatively remote, seismically 'quiet' sites: at U.S. Department of Energy facilities at Hanford, Washington and Livingston, Louisiana.

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Artist's View

Each LIGO facility will consist of a 4 foot diameter vacuum pipe arranged as an L with 2- or 4-kilometer (1.2- or 2.4-mile) arms. Test weights fitted with mirrors will hang in a building at the vertex of the L and at the end of each of its arms.

This central building will also house the lasers and control equipment. The world's largest vacuum system (9,000 cubic meters at each site) will prevent stray gas molecules from deflecting the laser beams as they travel back and forth thousands of times through the pipes separating the mirrored surfaces.

Why 2 U.S. Locations?

Having two LIGO facilities separated by 2000 miles will decrease the likelihood of erroneous readings that might result from shifting of the equipment, noise, or other local disturbances. If a genuine signal is detected at one facility, it should simultaneously appear at the other facility

However, to pinpoint the origin in space of gravitational waves, and to extract all the information they carry, requires that a third instrument be built. In a joint project, France and Italy are planning to construct just such an in- terferometer, called VIRGO, in northern Italy. Scientific teams in the U.K. and Germany, and in Japan and Australia, are planning to build similar devices. It's hoped that early in the next century, LIGO will become part of an collaborative network of gravitational wave observatories that span the globe.

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