Seeing the unseeable

Astronomers are peering ever closer to a black hole’s invisible heart

As bodies move through the Universe, they emit gravitational waves that stretch and squeeze anything they pass through by a tiny amount. When two black holes merge, they create immense gravitational radiation, which can be picked up by gravitational-wave detectors on Earth. These detectors have two 4km-long (2.5-mile) arms. Light travels down each arm and is reflected back at the end, before both beams are picked up by the detector. By comparing the two light beams using a technique called interferometry, the detector can see if they’ve been stretched or squeezed by passing gravitational waves. This allows astronomers to reveal the properties of the black holes that created the waves. In 2019, the first direct images of a black hole were taken by the Event Horizon Telescope (EHT). This is…

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BBC Sky at Night United Kingdom (Digital) - 1 Issue, April 2025

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