In extreme situations—when two black holes merge, a giant star explodes into a supernova or two neutron stars whip around each other in a close orbit—ripples can be created in the fabric of reality. These disturbances are called gravitational waves. In much the same way as accelerating electric charges produce electromagnetic waves, gravitational waves are produced by the acceleration of massive objects. And although predictions have been made about where and when gravitational waves appear—starting with Einstein's theory of General Relativity in 1916—they've never been detected directly.
That's the task of a mammoth scientific experiment, called LIGO—short for Laser Interferometer Gravitational-Wave Observatory. Gravitational waves are an exotic and complicated phenomenon, and detecting the location of these waves could help us understand the basic structure of reality—possibly leading to a unified theory that unites quantum behaviour with the effects of gravitation. This unification would result in a long-sought-after "theory of everything": a comprehensive set of equations describing the nature of reality in both the microscopic world of quantum mechanics and the macroscopic milieu of gravity.
At the very least, LIGO could offer a definitive answer to whether black holes exist—something physicists believe to be real, but also have no direct evidence for.
Like squeezing a balloon, gravitational waves compress spacetime and everything in it
The facility is essentially two 4 kilometer-long tunnels in a Pacific Northwest government research facility in Hanford, Washington. Designed by scientists at Caltech and MIT, LIGO collected scientific data from 2002 until 2010, but the whole thing was a bit of wash; not a single gravitational wave was detected. That's why, in 2010, construction began on a massive upgrade. The US National Science Foundation spent $205 million kitting out the detector with new optics and a high-tech stabilization system that scientists hope will help in their quest to finally nail down the detection of these elusive ripples. All this tech is crucial when looking for changes in space that amount to less than the diameter of a proton in size.
Although the upgrades will continue until 2017, an announcement was made in May that a test run of the new facility—dubbed Advanced LIGO—is set to take place this fall.
"Imagine a gravitational wave that's headed right towards your stomach," explained Mark Coles of the National Science Foundation and an advisor to the LIGO project. "The effect is to stretch space. First it would make you tall and thin, then short and fat. All of the things around you would be similarly acted upon. It would see a compression in one dimension as well as an elongation or a dilation in the other direction."
In other words, as the the first part of a gravitational wave travels through the cosmos, it compresses spacetime and everything in it in one direction. Like squeezing a balloon, spacetime becomes shorter in the horizontal axis and longer in the vertical one. As the next part of the wave travels through, the opposite happens.
In the LIGO apparatus, a beam of light is split into two, and sent down two tunnels set at right angles to each other. One beam travels down one arm of the detector, the other goes into the arm that lies at right angles to the first.
Now, imagine a gravitational wave ripples through the cosmos and passes through the detector. If the wave is oriented at right angles to LIGO, it will compress the very fabric of spacetime that is occupied by one arm of the detector, and stretch the spacetime in the other arm. This means one arm will actually get shorter than its sibling, and the other will get longer at the same time, changing the round trip time of the laser light bouncing around inside. This time change would be evidence of a gravitational wave.
In addition to possibly leading to a "theory of everything," confirming the existence of gravitational waves would provide a new way to view the majesty of the cosmos. Until now, our knowledge of the universe has come from observing light—or electromagnetic radiation—that comes from stars, or bounces off planets and other objects on the way to our eyes and telescopes. But only a small fraction of the cosmos emits or reflects light. By looking at gravitational waves, we might see the thumbprint that massive objects such black holes make in the fabric of reality. If it works, a whole new way of looking at the universe beckons.