May/June 2008

Catching Einstein's Waves

Continued from page 2

By Katherine Bourzac, SM '04

Over the next few years, Weiss developed prototypes for what would become LIGO. Researchers at Caltech, as well as funders at the National Science Foundation, got behind his plans before the MIT administration did; the Institute was reluctant to dedicate money to what seemed like a risky proposition. But once Weiss's collaboration with Caltech's Kip Thorne and Ronald Drever was in place and funding was secured, MIT got on board.

Today's LIGO is a set of huge interferometers, two of them with four-kilometer-long arms and a third with two-kilometer arms. More like an ear than an eye, LIGO will catch whatever waves it can "hear," no matter what direction they come from. But Weiss hopes the technology will come to serve as a new kind of telescope.

At the corner of each L is a mirror that splits a beam of laser light in two and sends one beam down each arm, through vacuum-sealed stainless-steel tubing, to a mirror at the end. The mirrors reflect the beams back to the corner, where they recombine to create an interference pattern of bright and dark spots. This pattern stays the same as long as nothing's moving. But if a gravitational wave passes through the interferometer, it subtly squeezes and stretches space, jostling the mirrors so that the pattern changes.

This sounds simple enough. But getting the noise out of the system is incredibly complex. "Everything else on the planet can move the mirrors more than gravitational waves can," Mavalvala says, with very little exaggeration. She ticks off a short list of phenomena that can disturb LIGO: tectonic-plate movements, ocean waves, road traffic, subways, "even just the activity of people moving around." Not even in the fabrication of microprocessors with very tiny features do things need to be held nearly as still as they do in LIGO's detectors. "Only a visionary could look at the technical requirements of LIGO and be undaunted," Bertschinger says.

LIGO scientists are analyzing data from the detectors' first full-scale run, from November 2005 through October 2007. The sheer volume of data--the detectors generated about a gigabyte a day--presents a huge computational challenge. Even more difficult is deciding how to determine when a gravitational wave has been detected beyond any reasonable doubt. Given Weber's legacy, Erik Katsavounidis says, "we want to be dead sure."

Researchers think LIGO's current technology should be able to detect violent cosmic events such as supernovas, as long as they aren't too far. "For our galaxy, we have good sensitivity," says ­Katsavounidis. Supernovas are rare, however; they're thought to occur within LIGO's range about once every 30 years. Until LIGO can see farther, he says, the best potential sources of detectable gravitational waves might be objects astrophysicists don't even know about yet.

The Next Generation
Using the largest ultrahigh-vacuum system on Earth, LIGO can detect mirror displacements as tiny as 10^-18 meters--a thousandth as big as the nucleus of the smallest atom. That sensitivity will double within about a year, thanks to improvements that Mavalvala likens to putting a turbo engine in a car. And the LIGO team is planning to replace the initial detectors altogether. Built for a project called Advanced LIGO, the new set of detectors will be 10 times as sensitive, increasing the volume of observable space a thousandfold.

In the LIGO team's brightly lit lab of hangarlike proportions on the west side of MIT's campus, Mavalvala ducks her baseball-hatted head under one of two 15-meter stainless-steel tubes. The tubes connect in an L shape to form a smaller-scale replica of the Louisiana and Washington interferometers. Nearby, an apparatus that appears to have been constructed from a giant Erector set sits on an elevated steel platform encased in plastic sheeting. Mavalvala explains how Advanced LIGO will make sure the platforms from which the mirrors are suspended stay very, very still.