What exactly are gravity waves and how are they measured? — AY, Wayne, PA
Gravity waves are deformations of space/time that propagate through space at the speed of light. While many motions of matter and energy are thought to emit gravity waves, those waves are normally extraordinarily weak. The only sources of detectable gravity waves are probably collapsing and colliding stars. Careful studies of the dynamics of binary star systems have shown that they also emit reasonably strong gravity waves, but those waves haven’t been detected directly.
The two classes of gravity wave detectors currently in development or operation are large cryogenic bar detectors and laser interferometric detectors. A cryogenic bar detector tries to observe gravity waves by looking for vibrational excitations of huge metal bars. When a strong gravity wave passes through one of these bars, it should excite various vibrations in the bar that can be detected by sensitive motion sensors. A laser interferometric detector tries to observe gravity waves by looking at distance changes in the arms of a laser interferometer—a huge mirror system with laser beams bouncing back and forth within it. When a strong gravity wave passes through the mirror system, it should change the spacings of the mirrors enough to cause variations in the optical characteristics of the interferometer (for more info, see www.ligo.caltech.edu). So far, no gravity waves have been observed definitively.