By Emily Levesque, University of Washington
In the 1960s, a group at Caltech led by Kip Thorne had begun exploring the possibility of detecting gravitational waves. By the mid-1980s, teams at MIT and Caltech had joined forces and begun developing what would ultimately become the Laser Interferometer Gravitational-Wave Observatory, or, in short, LIGO.
What Are LIGO Detectors?
In a gravitational wave detector, these waves would produce a distinctive signal known as a chirp, because if we translate the frequencies of the gravitational waves into audible waves it sounds something like a chirp sound. It may sound silly but detecting that quick swooping note became a key goal of gravitational wave experiments.
LIGO are detectors with arms nearly two and a half miles long and mirrors hanging at the ends, painstakingly engineered to detect the tiny chirps and other signals of minute density waves in space-time.
Today, there are two LIGO detectors: one in Hanford, Washington, and the other in Livingston, Louisiana. As the name suggests, the LIGO detectors are interferometers, an instrument that combines multiple sources of light. The LIGO interferometers take the same basic principle—recombining light after it has traveled over a distance—to measure exactly how far the light itself has traveled and search for perturbations caused by gravitational waves.
Facing Many Challenges
A gravitational wave squeezing and stretching a LIGO detector will, at a fundamental level, cause the mirrors in the arms to move very slightly and reflect the laser shining down the arms at slightly different times. But, think about everything else that could cause the mirrors to move even a little bit. Earthquakes. Trucks. Falling trees. Rain. Wind. Even footsteps.
The real challenge with these detectors became not detecting all these other extraneous signals—referred to as noise—that could obscure, or drown out, or mimic the tiny chirps announcing the arrival of a real gravitational wave. Remember, we’re talking about detecting something a thousand times smaller than a proton.
Building something sensitive enough to detect subatomic-sized compressions of space-time, but robust enough to ride out an earthquake, sounds like an engineering nightmare. The mirrors alone presented enormous difficulties.
Each mirror is suspended on a four-link pendulum system of thin glass fibers to reduce the outside vibrations that get transmitted to the mirror. Glass was chosen, in part, because the molecular motion within these fibers is an improvement over what would be seen with metal wires, and this reduces the microscopic vibrations that get passed along to the mirror.
The mirrors hang inside enormous vacuum tubes, built so straight that they take the curvature of the Earth into account over their nearly-two-and-a-half-mile length. The tubes themselves are mounted on an underground slab that helps minimize the effects of wind and seismic activity.
Even the ground itself is a potential source of trouble. You can probably think for a few minutes and imagine a dozen other potential sources of noise: Logging operations in Louisiana; waves hitting the shore 200 miles away from the Washington detector.
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What Helped the Project Grow
To succeed, the LIGO project needed to grow dramatically, encompassing and organizing a team of thousands that spanned multiple continents. This crucial expansion happened under the leadership of Barry Barish, a Nebraska-born physicist who had led the High Energy Physics Group at Caltech for more than a decade before becoming LIGO’s director.
Barish’s leadership combined immense scientific talent with the equally vital capability of directing LIGO’s human and logistical machinery. Barish led LIGO through the thorny and challenging process of getting funding from the National Science Foundation, oversaw the construction and commissioning of the detectors in Washington and Louisiana, and corralled thousands of people into an international and smoothly functioning scientific collaboration.
Crucially, Barish proposed the idea of building LIGO in stages. The initial stage, he knew, would likely not be sensitive enough to detect gravitational waves, but would serve as a vital verification of whether the detectors would function properly. The second stage would represent a system-wide upgrade, painstakingly shifting LIGO’s operations to more advanced technology that would greatly increase the likelihood of detecting gravitational waves.
Excellent, but Fruitless
Skeptics were displeased with the lengthy timeline: construction began in 1994 and LIGO didn’t even begin gathering data—in its initial, less sensitive state—until 2002. It then ran for three years, working excellently, but not detecting a single gravitational wave.
While this was a notable success for the technology, some astronomers had come to see gravitational wave detection as a pipedream, with Barish’s team engaged in a careful but ultimately fruitless search for an undetectable phenomenon.
Barish served as LIGO’s director until 2006. That year kicked off another painstaking decade of work by what was now an enormous international team, upgrading and testing LIGO in preparation for the second, more advanced stage that would begin gathering gravitational wave data in September of 2015.
Common Questions about LIGO
In the 1960s, Kip Thorne led a group at Caltech trying to explore these mysterious waves. During the 1980s, Caltech and MIT teams joined forces and started developing the Laser Interferometer Gravitational-wave Observatory, or LIGO.
A four-link pendulum system of thin glass fibers is used to suspend each mirror in the LIGO. Glass fibers were chosen over metal wires because of the glass’s molecular motion, which helps reduce the microscopic vibration that gets passed to the suspended mirrors.
He is a Nebraska-born talented physicist. Barish was the High Energy Physics Group leader but later became the LIGO director. He also had the role of overseeing the construction of the detectors in Louisiana and Washington.