We believe in things we cannot see – God, say, or black holes – because we observe their effects on the things that we can see. Still, there is a little bit of Doubting Thomas in all of us. It would be nice to have a direct image of what a black hole actually looks like!

The Event Horizon Telescope (EHT) is designed to do just that. It is not one instrument, but a collection of radio telescopes spread around the world, observing the same object at the same time. Using a technique called Very Long Baseline Interferometry (VLBI), which compares the very tiny differences between different telescopes’ signals, a high-resolution picture emerges. The farther apart the telescopes are, the better the image’s resolution; the EHT telescopes span the width of Earth itself.

After nearly 25 years assembling the team of telescopes and refining the technique, this month the EHT began its first observations of the black hole at the centre of our galaxy. We cannot see the black hole itself; light cannot escape from its gravity. But a black hole’s size and shape can be measured by the shadow it casts against the radiation emitted when material falls into the hole.