Essay: Waiting for Gravity at LIGO

Gravitational waves are energetic ripples of space traveling from massive objects in the cosmos. To measure one, do these things: 

  1.  Get $365 million from the National Science Foundation.
  2.  Find some very flat land, like in Louisiana.
  3.  Chop two 4 km long roads in an L shape through a swampy, snake-infested forest.
  4.  Accounting for the Earth’s curvature, pave the roads to within a few millimeters of absolutely straight.
  5.  Top each road with a 4 km long, 1.2 m wide steel tube. 
  6.  Fit the tubes with precision instruments that have taken six years to design and construct.
  7.  Wait for a gravitational wave to hit. Could be in moments. Could be in decades. Adjust instruments in the meantime.
  8.  When the wave finally hits, the instruments will detect the space of the steel tube expanding or contracting a distance of one-hundred-millionth the diameter of a hydrogen atom.  Just ignore any pesky sources of competing vibrations, like the logging across the street.
An aerial view of the Laser Interferometer Gravitational-wave Observatory in Livingston, Louisiana, known as LIGO. It looks like several connected buildings with flat roofs.

Common sense looks at this experiment and says: weird. And really, really hard. But the team at LIGO, the Laser Interferometer Gravitational-wave Observatory in Livingston, Louisiana, has so far managed to complete steps 1 through 7. While no gravitational waves have yet been found, the LIGO researchers are optimistic they’ll eventually get one reading every few days. The team even has their own upbeat Cajun slogan: “Laissez les bonnes ondes rouler!” (“Let the good waves roll!”)

Einstein predicted gravitational waves using General Relativity in 1916. These waves are emitted by any object undergoing rapid acceleration, but only gargantuan masses, like colliding black holes or exploding stars, produce waves LIGO can detect. As they travel, these ripples literally warp space; they shrink it in one direction and stretch it in another. The farther they roam, the fainter they get. By the time they reach Earth, their already tiny warp is barely measurable.

If LIGO regularly registers gravitational waves, it will more than vindicate Einstein. Analyzing gravitational information may allow astronomers to answer pressing questions about the cosmos’s biggest mysteries, among them black holes, dark matter, and the Big Bang. It may also reveal space objects entirely unknown to science.

Light and LIGO

“The concept of what we’re looking for is so important,” insists Rainer Weiss. “The fact that the effect is tiny is just our misfortune.” Weiss is a professor emeritus of physics at the Massachusetts Institute of Technology. MIT, along with the California Institute of Technology, launched the LIGO project in 1979 with funds from the National Science Foundation. The seemingly tireless Weiss is ostensibly LIGO’s granddaddy, having been a (not the, he insists) person who first conceived the “half-baked idea” of measuring infinitesimal warps of space using light beams traveling over long distances. This is precisely what an interferometer like LIGO does.

LIGO’s main attraction is its two 4 km long arms, labeled X and Y. Like the axes of a graph, the X arm is perpendicular to the Y arm. This orientation corresponds to the two directions in which a gravitational wave affects space. As a wave travels toward Earth (perpendicular to the arms, long the third dimension of a Z axis), it will shrink space along the X axis. For LIGO this means the space that is the X arm (and all the matter in it) will shorten a fraction. The space of the Y arm will, at the same moment, stretch in response to a gravitational wave. Then vice versa, again and again, many times per second.

Researcher in his lab, using his hands to explain a concept.
Rainer Weiss

Imagine LIGO were Manhattan, suggests Weiss: “Squeeze Manhattan from uptown to downtown, and expand it east to west. Bang-boom!” In a flash, a gravitational wave extends the distance from the East River to the Hudson. Traveling from river to river would take more time. It’s not that the landmarks themselves are moving. It’s that the distance between the landmarks is expanding. 

Researchers set up LIGO with its own landmarks, or “test masses,” between which the distance of space can be measured with traveling light. The test masses are a set of four highly polished mirrors. One is placed at the end of each arm, and the remaining two sit closer to the vertex.

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!

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 twina 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.”

Related Links

Laser Interferometer Gravitational-wave Observatory

National Public Radio: Hunting for Gravity Waves

Interference of light waves