LIGO Livingston Observatory

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The second American laser interferometer, in Livingston Parish, Louisiana, with the same four-kilometre arm length as Hanford and roughly three thousand kilometres away from it. The separation is the point: it sets the maximum possible arrival-time difference between the two sites at about ten milliseconds, so a coincident signal with a plausible delay is far harder to produce by chance than a single-detector transient. Livingston registered the first confirmed gravitational-wave signal a few milliseconds before Hanford on 14 September 2015. Its local environment is noisier than Hanford's - the site sits in a working forestry landscape - and much of the engineering effort at both observatories goes into isolating mirrors from ground motion by many orders of magnitude.