When a gravitational wave travels through the Earth it causes tiny changes in the distance between the mirrors, which is observed as changes in how the light interferes. In LIGO and other observatories, laser beams are deflected between mirrors at the ends of long (several kilometres) evacuated pipes and then interfere with one another. The best developed technique for detecting gravitational waves, and the one used to discover them in 2015, relies on interferometry.

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