LIGO Hanford Observatory

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One of the two American laser interferometers whose simultaneous response constitutes a gravitational-wave detection, situated on the Hanford Site in eastern Washington State. Each arm is four kilometres long and holds a laser beam in an ultra-high vacuum; a passing gravitational wave changes the relative arm lengths by a fraction of the diameter of a proton, and the instrument reads that as a shift in the interference pattern where the beams recombine. A single such instrument cannot claim a detection, because seismic, thermal and human disturbances produce transients constantly; what makes a signal credible is a matching waveform appearing at a second, distant detector within the light travel time between them. In the 14 September 2015 event the signal reached Livingston first and Hanford about seven milliseconds later, which is both a consistency check and the crudest possible measurement of where the source was.