So What?
If hearing news about LIGO makes you say "Oh that's neat," there may be more to the story you don't realize. Please allow me to go a little deeper on the effects of gravity on space-time.
What is gravity?
A Force?
Many of us think of gravity as a force that causes individual masses to be pulled together. This is essentially how Newton described it and how Newtonian physics allows for a limited understanding of the world around us. The more mass an object has, and the closer it is to another mass, the stronger the attraction between them.
A Curvature
Einstein broadened our understanding by redefining gravity as the curvature of space-time. In this line of thinking, gravity as a force doesn't exist. So instead of the magnitude of gravity being a function of the amount of mass an object has, it's actually the curvature of space-time that is a function of the mass of an object. The greater the mass, the greater the curvature, the greater the change in world-line of an object near the mass over time as compared to the outside universe. (yes, yes, wibbly-wobbly timey-wimey stuff). I don't want this post to go on forever so I'll try to skip over some of the explanations. At any rate, the reason we needed to move beyond Newtonian physics is because space-time is curved causing parallel lines to merge (just as two apples inside a box falling toward the earth will move closer together as they fall because they are falling toward the same fixed point). Newtonian gravity breaks down on a larger scale. The only constant, it seems, is the speed of light. So how does all this tie in to LIGO?
Space-Time
LIGO (Laser Interferometer Gravitational-Wave Observatory) measures the time it takes lasers to travel 2.5 miles in a vacuum and back again. Sounds pretty simple right? Well it pretty much is. The complexity comes in executing that simple concept on such a large scale while maintaining precision.
The Laser is split between two orthogonal 2.5 mile vacuum tubes. The two beams travel down the tubes and are reflected back again by mirrors. When they converge at the source of the split they cancel each other out because the lengths of the legs of the interferometer are set up so that the returning lasers are 180 degrees out of phase. The light left over after the convergence is measured by a photo-detector. As long as no light is detected, nothing has happened.
If there is light detected, something changed. Since the speed of light is a constant, the only variable would have to be the distance the laser has traveled. If the distance changes, then the two returning laser will no longer be 180 degrees out of phase and won't completely cancel each other out.
But how did the distance change?
This is the really cool part. Remember when we said that gravity was really the curvature of space-time? So when we say "gravity waves" we really mean ripples in space-time. The components of the interferometer are fixed points with relation to each other. The speed of light is fixed. It is the space-time of the vacuum in which the laser is traveling that is changing. LIGO measures the warping of space-time! Space-time is essentially the fabric of the universe. So essentially...
LIGO Detects Distortions in Reality