Gravity: Making Waves
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:
- Get $365 million from the National Science Foundation.
- Find some very flat land, like in Louisiana.
- Chop two 4 km long roads in an L shape through a swampy, snake-infested forest.
- Accounting for the Earth’s curvature, pave the roads to within a few millimeters of absolutely straight.
- Top each road with a 4 km long, 1.2 m wide steel tube.
- Fit the tubes with precision instruments that have taken six years to design and construct.
- Wait for a gravitational wave to hit. Could be in moments. Could be in decades. Adjust instruments in the meantime.
- 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.
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.
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.
