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LIGO: A New Window on the Universe

Almost all our current information about the cosmos is based on our ability to sense, pinpoint and measure electromagnetic radiation -- which until the early 1940s meant only visible light. Since then, radio telescopes, infrared telescopes, and X-ray telescopes have dramatically broadened our "vision." An even more dramatic chang e will occur once we are able to "see" the gravitational waves predicted by Einstein's General Theory of Relativity. In the next decade, a new generation of instruments should enable researchers to finally confirm that gravity waves really exist, pinpoint where they're coming from and, in doing so, open up a whole new observa tional window on the universe.

Larry Smarr, NCSA/Univ. of Illinois, on-camera
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QuickTime Movie (2.8 MB); Sound File (1.6 MB); Text

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 Foun dation, 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:

Catching the Waves

How Does Laser Interferometry Work?

From Prototype to Full-Scale Detectors

Why Two U.S. Locations?

What Will LIGO Observe?

Hunting for Gravitational Waves

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.

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 ang le 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.

LIGO/Caltech: Adapted with Permission:

JPEG Image (16K)

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 uniqu e 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 det ectable gravitational waves.

LIGO Prototype

Courtesy: LIGO/Caltech

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.
JPEG Image (33K)

From Prototypes to Full-Scale Detectors

Schematic of the full-scale instrument
Courtesy: LIGO/Caltech

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 facitilies at Hanford, Washington and Livingston, Louisiana.
JPEG Image (20K)

Artist's View

Courtesy: LIGO/Caltech

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.

Ed Seidel, NCSA/Univ. of Illinois, on-camera
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QuickTime Movie (1.3 MB); Sound File (860K); Text

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 gra vitational wave observatories that span the globe.

What Will LIGO Observe?

Gravitational waves triggered by cosmic events should cause specific displacements resulting in unique interference patterns. Converting these patterns into the more familiar squiggles of an oscilloscope, or "waveforms," will yield significant information about the source. However, because the first signals to be detected will likely be so weak, it may be difficult to discern true signals from "noise." In some cases, where the source of the gravitational wave is known through corroborating evidence -- electromagnetic radiation emitted by a supernova, for example -- researchers will be able to more easily confirm and pinpoint the event causing the gravit ational disturbance.

However, assigning observed waveforms to specific events will require overlaying them on templates that are now being created via supercomputer simulations of these same events.

One of the Grand Challenge goals of the research described in this exhibit is to build a catalog of predicted waveforms. Then, when scientists succeed in building detectors sensitive enough to detect the subtle oscillatio ns that reveal a passing gravitational wave, they'll be able to deduce the type of event that triggered it, and the properties of the object it came from.

Here are four waveforms computed on supercomputers at NCSA.

Distorted, Rotating, and Colliding Black Holes


The Larger Picture: Spiralling to Final Merger


Hunting for Gravitational Waves

There are no lack of candidates in Einstein's Relativistic Universe. Powerful gravitational waves are thought to be generated by non-spherical, large-scale vibrations of spacetime -- such as occur when a star explodes and its core collapses, two neutron stars collide, or two black holes spiral towards each othe r,then coalesce.

But will LIGO perform as required? According to its designers, the instrument's extreme sensitivity should suffice to pick out even the tiniest displacements from "noise." Over a fairly broad frequency range, LIGO's projected sensitivity should match th e calculated gravitational wave amplitudes for a variety of sources.

LIGO's Projected Sensitivity
diagram showing projected sensitivites of LIGO versus computed
strengths of gravitational waves for a variety of sources

LIGO/Caltech: Adapted with Permission:

In this diagram, the amplitude of the displacement measured by LIGO resulting from a passing gravitational wave is plotted against the wave's frequency in cycles per second or Hertz (Hz).
JPEG (22K)

The LIGO scientists are confident they'll be able to detect a variety of cataclysms, each taking place upwards of a hundred million light years away, including colliding neutron stars, coalescing black holes, and a collapsing star.

Larry Smarr, NCSA/Univ. of Illinois, on-camera
Movie/Sound Byte
QuickTime Movie (1.4 MB); Sound File (811K); Text

If you would like to find out more about gravitational wave detectors, visit LIGO's home page at Caltech or check out VIRGO's in Pisa, Italy. But please come back! There's a lot more ahead -- movies too!

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Copyright 1995, The Board of Trustees of the University of Illinois

David Curtis, NCSA. Last modified 8/29/95.