A beam of laser light is split in two at a beam splitter and travels down the two perfectly sealed vacuum chambers that form the arms of the interferometer. After traveling 4 kilometers, the light bounces off mirrors at the end of the arms and returns to the beam splitter. At the beam splitter, the returning laser beams are recombined. What happens when the beams recombine depends on the distance they have traveled compared to each other. The LIGO team takes data by comparing the two beams' relative phase—how the peaks and valleys in the two waves of laser light line up. If the two waves are perfectly out of phase—if the peaks in one beam line up exactly with the valleys in the other—they cancel each other out—a phenomenon called "destructive interference." In its normal undisturbed state, the LIGO mirrors are aligned for perfect destructive interference. The team takes advantage of this effect to detect gravitational waves.