The image released by the Event Horizon Telescope collaboration in 2019 showed a ring of orange light surrounding a dark centre — the shadow of the supermassive black hole at the centre of the galaxy M87, 55 million light-years away, its mass equivalent to 6.5 billion suns. It was the first direct image of a black hole’s event horizon, the boundary past which nothing, including light, can escape.
The image was blurry. It was also extraordinary. It confirmed, in the most direct way possible, that the objects predicted by general relativity a century earlier were real, and that the mathematics that described them was correct at scales that would have seemed like fantasy to Einstein.
What we’ve learned since
Subsequent work by the collaboration has produced a higher-resolution image of M87’s black hole and a first image of Sagittarius A*, the black hole at the centre of our own galaxy — smaller and more chaotic than M87’s, flickering on timescales of minutes because its accretion disk is less stable.
More broadly, the past five years have produced a clearer picture of the relationship between supermassive black holes and the galaxies that contain them. The mass of a galaxy’s central black hole correlates remarkably tightly with the mass of its central bulge — a correlation so precise that it cannot be coincidence. Something about the process of galaxy formation connects the scale of the central black hole to the scale of the structure around it.
The feedback problem
The leading explanation involves what astrophysicists call feedback. As material falls into a supermassive black hole, some of it is converted into energy and ejected as jets — streams of plasma moving at nearly the speed of light, extending sometimes for millions of light-years. These jets heat the surrounding gas, suppress star formation, and regulate the galaxy’s growth.
The black hole, in other words, is not merely a passive feature of the galaxy. It is an active participant in the galaxy’s history — pumping energy into its environment, shaping the conditions under which stars can form, governing its own fuel supply by altering the medium it feeds on.
We spent a century asking whether black holes were real. We are now asking what they are for.
