On February 11, 2016, scientists from the Laser Interferometer Gravitational Wave Observatory (LIGO) in Washington State and Louisiana announced that they had detected a gravitational wave for the first time in human history. The wave originated 1.3 billion light years away, when two black holes, each 30 times the mass of our sun, collided and morphed into one. The invisible—but very real and violent—collision of mass sent a gravitational wave rippling through the fabric of spacetime, reaching Earth over a billion years later.
Albert Einstein famously predicted the existence of these waves back in 1916, when he published his theory of general relativity. But they had never been observed—until last year. The achievement is so profound, some have compared it to when Galileo looked through a telescope for the first time.
These ripples in spacetime are so infinitesimal by the time they get to us, humans had never before been able to detect them until recently. But thanks to an astoundingly sensitive instrument that could've been plucked from the pages of science fiction, and the optimistic guidance of lead engineer David Shoemaker, physicists were finally able to prove Einstein right. One hundred years later.
Decades of Preparation
In 1979, a young David Shoemaker embarked on a 35-year journey through spacetime that would eventually lead him to the bleeding edge of astrophysics. After having been a student and a technical instructor at MIT for the better part of a decade, he began studying gravitational waves under the tutelage of physicist Rainer Weiss.
Weiss, a pioneer of the gravitational waves field, was someone who loved "picking people up like myself who were wondering what the hell they were doing with their lives and giving them a direction" Shoemaker recalled to Motherboard in a Skype interview.
While fascinated by the waves themselves, Shoemaker concerned himself more with creating the machine that could find them. "I really am an instrument builder and designer so I was responsible for pulling together the instrument and making it function," he said. In our initial emails, he even modestly appended one message: "Be forewarned: I build detectors and am an amateur in the domains of general relativity and astrophysics."
"I build detectors and am an amateur in the domains of general relativity and astrophysics."
The LIGO project, based on a concept conceived by Weiss, kicked off in 1989 with about three dozen scientists and engineers involved. The first instruments they developed lacked the sensitivity required to detect the elusive waves. But in 2004, the National Science Foundation (NSF) graced the LIGO crew with the much needed funding to build a more sensitive LIGO detector. With the NSF's sword tap on each shoulder, Shoemaker pushed off into new technological territory in designing and building the new Advanced LIGO detector—that would eventually find a gravitational wave.
"I took on leadership of Advanced LIGO," said Shoemaker, "and from 2004 to 2015 I just made that happen—with a huge team of people of course—but I had the luxury, and enjoyment, and stress of leading that effort." After a pause he added with a sheepish smile that "The instruments worked pretty well, so we're happy."
A couple months after the announcement of the first wave detection last February, the LIGO team reported that it had detected a second.
How The Detector Works
There are two LIGO observatories, 3,000 km apart, in Hanford, Washington, and Livingston, Louisiana. Both are enormous facilities with arms stretching 4-kilometers (about 2 and a half miles) across the ground. The LIGO Hanford facility itself contains 5 buildings over 5 miles connected by vacuum tubes.
The two work in semi-tandem and function like this: Inside a vacuum tube, a highly pure and powerful laser is shot through a partially-reflective mirror made of fused silica glass. The mirror splits the laser into two beams that flow perpendicular to each other. Those beams travel a couple kilometers in each direction before they hit fully reflective, 40-kilogram mirrors that send them back from whence they came, towards the giant mirror in the center. Once there, they recombine and shoot off into a detector.
Gravitational waves' effect on spacetime is to "stretch it and squeeze it," explained Shoemaker. "Specifically, it makes a wave," he said, and "stretches space first along one axis, and then along another axis, making ellipsoids in different directions." Over Skype, he made an oval with his hands, and flattened it first horizontally, then vertically.
When the wave passes through the split laser, it shortens one of the beams and extends the other, and vice versa. In the recombined beam, this change in length makes a little wave pattern, almost like a footprint, indicating the wave's presence. The change in beams that revealed the first gravitational wave last year was 1,000th the diameter of a proton particle.
If each observatory detects a wave event within 10 milliseconds of each other—the time it takes for the wave to travel 3,000 km between them at the speed of light—then they know they've got something.
The Future of LIGO
Shoemaker acknowledges that "times are changing" when it comes to government funding of scientific endeavors, but he remains hopeful for the future. "There's already a great deal of progress that we can make in this field with just a continuing maintenance fund for our instruments," he said, adding "and we believe that will be possible."
That continued maintenance could help make detections of gravitational waves a regular occurrence. And as scientists detect more waves in the future, they'll get better at reading the clues contained within them. "You can pull out all kinds of details about the inner death throes of neutron stars, and supernova and so forth and so on," said Shoemaker.
Despite funding limitations, The LIGO crew "do have visions," according to Shoemaker. They plan on teaming up with VIRGO, their European counterpart in Italy, that is nearly built and ready go. There's also an instrument construction in Japan, and a third LIGO detector that will go to India. "Over the next five years or so we'll see the network grow from just LIGO to this worldwide network which will allow even better precision pointing of the overall instrument," he said.
Through it all, Shoemaker never once thought that they wouldn't find a gravitational wave. "It was clear to me when we got started that it was going to be decades and decades until we were likely to succeed," he said.
"There never was a time when it looked like we had a technical problem that would be insurmountable."
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