The LIGO–Virgo–KAGRA collaboration has announced that the search for gravitational waves will resume in May. The next observational run – the project’s fourth – was meant to start last year but was postponed due to a series of engineering delays resulting from the COVID-19 pandemic. The run will be the longest to date, operating for 18 months.

Gravitational-wave detectors are L-shaped interferometers with arms several kilometres long. Laser beams are sent down each arm and then bounce off mirrors, called test masses. The beams are then recombined at the centre of the interferometer producing an interference pattern that cancels out when perfectly aligned. The instruments are thus sensitive to minute changes in length caused by any passing gravitational waves.

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