Figure 16.5: LIGO gravitational wave detector at Hanford, Washington. LIGO stands for Laser Interferometer Gravitational (Wave) Observatory. The 4-km-long concrete covers of the beam pipes housing the arms of two interferometers can be seen stretching toward the horizon and to the right. The two ends of the interferometer illustrated in Figure 16.3 are at the ends of these pipes. The beam splitter is in the building housing the end station in the foreground. There is a similar facility in Livingston, Louisiana.