How to detect gravitational waves? Gravitational waves just have a little interaction with other particles. How can we detect such little influence on mass?
 A: Quoting Caltech's article about how LIGO works:
LIGO will detect the ripples in space-time by using a device called a laser interferometer, in which the time it takes light to travel between suspended mirrors is measured with high precision using controlled laser light. Two mirrors hang very far apart, forming an "arm" of the interferometer, and two more mirrors make a second arm perpendicular to the first arm. When viewed from above, the two arms form an L shape. Laser light enters the arms through a beam splitter located at the corner of the L, dividing the light between the arms. The light is allowed to bounce back and forth between the mirrors many times before it returns to the beam splitter. If the two arms have identical lengths, then interference between the light beams returning to the beam splitter will direct all of the light back toward the laser. But if there is any difference between the lengths of the two arms, some light will travel to where it can be recorded by a photodetector.
The space-time ripples cause the distance measured by a light beam to change as the gravitational wave passes by, causing the amount of light falling on the photodetector to vary. The photodetector then produces a signal telling how the light falling on it changes over time. The laser interferometer is like a microphone that converts gravitational waves into electrical signals. Two interferometers of this kind are being built for LIGO — one near Richland, Washington, and the other near Baton Rouge, Louisiana. LIGO must have two widely separated detectors, operated in unison, in order to rule out false signals and confirm that a gravitational wave has passed through Earth.

