May/June 2008

Catching Einstein's Waves

Continued from page 3

By Katherine Bourzac, SM '04

When the apparatus is running, accelerometers on the platforms detect motion, and motors correct for it by moving the platform in the opposite direction. Each mirror will hang from its platform on a wire holding metal and glass weights. The resulting pendulum has a natural frequency lower than that of the gravitational waves. When the platform is shaken rapidly by seismic noise, the pendulums will buffer the motion of the mirrors, ensuring that the tiny motions due to gravitational waves will not be masked.

These enhancements will improve LIGO's sensitivity to low-­frequency gravitational waves. But when measuring displacements far smaller than an atom, researchers must also contend with different sources of noise that limit sensitivity in other ranges. In the intermediate range, LIGO is limited by thermal motion: atoms at temperatures above absolute zero jostle around. Thus, the metal atoms in the wires suspending LIGO's mirrors introduce noise into the system. Advanced LIGO will use specially made fibers of bare glass, a less "lossy" material: the atoms move less, and less of their motion is transferred to the mirror.

At higher frequencies, light's quantum properties are the problem. "When you make a measurement with light," Mavalvala says, "you have to deal with the noise properties of the light itself." Greater laser power means a better signal-to-noise ratio: Advanced LIGO's laser will have 20 times the power of the current one.

With these improvements, "we should see something once a week," says David Shoemaker, SM '80, the MIT senior research scientist who leads Advanced LIGO. "If we see nothing, there's something wrong with general relativity."

When a gravitational wave is first detected, "everyone's going to have a raucous party," says Scott Hughes. "Then after the hangovers are over, we're going to say, 'Okay, now what do we do?'"

By creating computer models of objects like black holes, Hughes is trying to figure out how to use gravitational waves to do ­astronomy. Since no light escapes from black holes, physicists have seen them only indirectly--say, by detecting x-rays that stars emit when they're pulled inside one. But when black holes "eat" something, says Weiss, "they let out a very satisfied burp"--a gravitational wave. "Given the way that general relativity describes things, given the detectors as we've designed them," says Hughes, "how precisely can I do things like measure the mass and spin of a black hole?"

The physicists asking such questions are stepping out into the unknown, and they can't predict everything they might learn. But, says Bertschinger, "I want MIT to be part of that era--to participate in the feast of science to come."