Science Sunday: Introducing Advanced LIGO, the Future of Gravitational Astronomy
On Tuesday, the LIGO Laboratory announced the completion of the Advanced LIGO detector array, which may finally be able to detect elusive gravitational waves.
Gravitational waves are a consequence of Einstein’s General Relativity. They operate similar to other kinds of waves, and are thought to carry gravitational energy. Even though all objects with mass generate gravitational waves, only very large objects, like stars and black holes, produce waves large enough to be detected. Some of the best candidates for detecting these waves are binary systems composed of white dwarfs, neutron stars, or black holes. Being able to detect gravitational waves could tell us a lot about star systems, distant galaxies, the early universe, and would go a long way toward developing a unified theory of quantum gravity.
To detect a gravitational wave, we measure its effect on other objects. The gravitational wave bends spacetime: it stretches it in one direction and compresses it in another. By measuring the rate of stretching and compressing, scientists can determine all sorts of things about the source of the waves. The problem is that the change is minuscule; the total amount of stretching and compressing amounts to less than a billionth of a centimeter in length, or thousands of times smaller than the size of an atom.
Enter LIGO: The Laser Interferometer Gravitational-Wave Observatory, two gigantic detectors built in Livingston, Louisiana and the Hanford site, just outside of Richland, Washington. Each is kilometers long, and designed specifically to observe the tiny changes required to gather gravitational wave data. To do this, they employ a 120-year-old technique, interferometry, in the largest way the world has ever seen.
In its most basic form, an interferometer consists of a laser, a beam splitter, two mirrors, and a detector. As the laser passes through the beam splitter, it (unsurprisingly) splits the beam into two identical beams, travelling perpendicular to each other. Each beam bounces off a mirror and is reflected back toward the beam splitter. The splitter combines them, and they both hit the detector, where the interesting bit happens. If both laser paths are the same length, then the two split beams just cancel each other out. But if the path lengths are a tiny bit different, then the two beams don’t completely cancel, producing an interference pattern at the detector. This provides a way to measure the tiny distortions created by gravitational waves.
LIGO takes the simple idea of an interferometer and cranks it up to 11. The mirrors it uses weigh 40 kg each, and are some of the most polished mirrors in existence. These mirrors are suspended from vibration isolators, which ensure that the mirrors stay completely still. These vibration isolators were developed specifically for use in gravitational wave detectors, and are the most advanced in the world. Before striking one of the mirrors, LIGO’s laser travels down 4 kilometers of pipe, all of which has been pumped to a vacuum (the largest sustained ultra-high vacuum in the world) to avoid any scattering of the light. The whole apparatus cost $365 million, and is the largest project ever funded by the NSF.
LIGO ran for eight years, starting in 2002. Halfway through, it got an upgrade, called Enhanced LIGO, that further improved its already impressive specs. But it still wasn’t enough. Even with the best instruments in the world, LIGO wasn’t sensitive enough to detect the miniscule distortions created by gravitational waves. But the LIGO scientists weren’t giving up. So beginning in 2011, both LIGO facilities in Livingston and Hanford have been receiving massive upgrades that would improve their sensitivity by up to ten times. Dubbed Advanced LIGO, this project represents our best hope for detecting gravitational waves. The upgrades and construction have just been completed on Tuesday, and the first results are expected to be released sometime this fall, so stay tuned. If there are any gravitational waves out there, Advanced LIGO will find them.
Featured image courtesy NASA.