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Hunting for Gravity Waves
Observatory Seeks Signs of Einstein's Elusive Ripples
 Listen to David Kestenbaum's report.
LIGO's strategy is to split a laser beam down two long perpendicular tubes and look for differences caused by a gravity wave passing through the beams. Shown here, the starting point for the beams inside LIGO.
Photo: LIGO
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Einstein said matter pulls on the fabric of space-time around it. That warping is called gravity. He also said the movement of matter, such as the rotation of a planet, would send out ripples across space. Those ripples are called gravity waves.
Graphic: LIGO
Read more about gravity waves.
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An aerial view of the LIGO observatory in Louisiana.
Photo: LIGO
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Sept. 16, 2002 -- In the plains of eastern Washington state and in a Louisiana forest lie two peculiar L-shaped buildings. Their skinny arms stretch 4 kilometers across the land, and inside each arm, a flashlight-sized laser beam bounces back and forth. The facilities, called LIGO, are feeling for tremors in the fabric of space-time, called gravitational waves.
Einstein predicted the existence of "gravity waves" 86 years ago. He also predicted they would be too small to measure. So far, he's been right. LIGO, short for Laser Interferometer Gravitational-Wave Observatory, just finished taking its first round of data this week, and failed to detect any gravity waves. But, as NPR's David Kestenbaum reports, physicists are confident LIGO will reveal the waves.
Gravity waves are akin to sound waves. When a baby cries, sound waves spread through the air of the house, breaking the calm. Einstein predicted that just as air can vibrate, so can space and time. In fact, Einstein said that every time anything moved -- from the moon orbiting the Earth to one car bumping into another -- the fabric of space-time vibrates, sending out gravitational waves.
"The problem is that it's so hard to detect it that we have to go find something really incredibly violent to have a hope of detecting it," says Barry Barish, a California Institute of Technology physicist who works with the LIGO project.
One such event might be the collision of black holes. But even then, the resulting space quake would typically be so slight as to cause the distance between Earth and the nearest star to shiver by about the width of a hair. That's why no one has yet detected a gravity wave, says Kestenbaum.
LIGO's strategy is to split a laser beam down two long, steel vacuum tubes. A mirror at each end bounces the laser back, and machines measure any difference when the beams reunite.
A gravity wave passing over the Earth, says Barish, will slightly compress space in one tube and stretch it in the other. One mirror will move slightly forward, and the other slightly back. That tiny movement is what they're trying to measure.
For a two-week period that ended Tuesday, LIGO scientists bounced the laser up and down the tubes. The data turned up no evidence of gravity waves. The team says they will likely have to upgrade their equipment before it is truly sensitive enough to detect the first gravity wave.
Critics say the $296 million spent to build LIGO could have been better spent on telescopes. Advocates of the project maintain LIGO is a kind of telescope -- one that will someday be able to search the distant universe in an entirely new way.
In Depth
 Browse the NPR archives for more stories on investigations into space.
Other Resources
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Explore what scientists know about space and the universe at the Smithsonian's Universe Web site.
• LIGO is a joint project between the California Institute of Technology and the Massachusetts Institute of
Technology (MIT). Visit the project's main Web site.
• The LIGO Hanford facility in Washington.
• The LIGO Livingston Observatory in Louisiana.
• The LISA mission is a space-based attempt to detect gravity waves.
• NASA's Origins Education Forum, a gateway to the agency's exploration into the structure and evolution of the universe.
• Learn more about gravity waves at the LISA Web site.
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