Gravity: Making Waves

Waiting for Gravity at LIGO

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Taking a Reading
To scan for a passing gravitational wave, a laser generator near the vertex first emits light into a beam splitter. The splitter divides the beam into two identical beams that race down each arm. To maximize the travel distance, the two beams bounce 100 times between the far and near mirrors. They eventually return together to the corner station to be analyzed by a photodetector.

Operate LIGO!

Interactive

If no gravitational wave occurred, both arms will have remained identical in length. So the two beams reach the light sensor at the same exact moment. But if a gravitational wave hit, one arm would shrink, producing a difference in the arrival times of the two racing light signals. (The shorter arm’s light beam will have spent a mere .0000000000000000000000001 fewer seconds in transit, taking the term “photo finish” to a whole new level.) This difference means the wavelengths of the two returning light waves are now out of sync. The photodetector registers this interference (thus, interferometer) and alerts the scientists of a positive reading.

No Noise, Please!
If only it were so easy. Many, many types of competing vibrations, or noise, can jostle the test masses enough to mask the effect of a true gravitational wave.

Loggers felling trees nearby cause noise. The crash of ocean waves produce noise. “Even the motion of the atoms inside the mirrors are making the mirrors move,” says Gabriela González, a physicist at nearby Louisiana State University.

Scientists have taken painstaking precautions to reduce the impact of noise on LIGO. The mirrors are suspended on a single thin metal wire to reduce the effects of forces other than gravity. To dampen competing vibrations, investigators constantly adjust the mirrors with the ultraprecise equivalent of a car suspension system.

Still, how do you detect a gravitational wave and not a rabbit jumping nearby? “That’s the $300 million question,” laughs González. One way is by checking if a suspected wave coincided with a disturbance registered by other instruments on site, which look for changes in ground motion, magnetic field, power line voltage, and other aspects. Another way, González explains, is by double-checking results with LIGO’s twin—a complete duplicate facility constructed in a barren scrub desert in Hanford, Washington. At 3,030 km away, it’s distant enough that seismic and other disturbances won’t affect both observatories simultaneously.

So Have They Found Anything at All?
Nope. LIGO started making preliminary runs in 2002, but it still hasn’t noticed its first gravitational wave. Weiss says the instruments are not yet at the level of sensitivity they need to be to detect waves easily. To get to that level, the team spends their days “commissioning”: calibrating devices, finding and solving glitches, and analyzing noisy squiggles of initial data.

Expect the observatory to turn on for real in 2005. By 2010, next-generation equipment will be retrofitted into all existing LIGO facilities. This will improve the sensitivity by about 15 times to capture more, fainter, and different frequencies of gravitational signals.

Most astrophysicists feel that LIGO’s payoffs will be worth the incredible effort it has taken to construct and operate it. “Imagine life before Galileo pointed a telescope at the sky,” suggests Janna Levin, a Columbia University cosmologist. “There’s no question. It could be that big.”