The invisible manifests itself through the visible: so say many of the great works of philosophy, poetry, and religion. It’s also true in physics: we can’t see atoms or electrons directly and dark matter seems to be entirely transparent, yet this invisible stuff makes and shapes the universe as we know it.
Then there are black holes: though they are the most extreme gravitational powerhouses in the cosmos, they are invisible to our telescopes. Black holes are the unseen hand steering the evolution of galaxies, sometimes encouraging new star formation, sometimes throttling it. The material they send jetting away changes the chemistry of entire galaxies. When they take the form of quasars and blazars, black holes are some of the brightest single objects in the universe, visible billions of light-years away. The biggest supermassive black holes are billions of times as massive as the Sun. They are engines of creation and destruction that put the known laws of physics to their most extreme test. Yet, we can’t actually see them.
A simulation of superheated material circling the black hole at the center of the Milky Way. Credit: Scott C. Noble, The University of Tulsa
Black holes are a concentration of mass so dense that anything that gets too close—stars, planets, atoms, light—becomes trapped by the force of gravity. The point of no return is called the event horizon, and it forms a sort of imaginary shell around the black hole itself. But event horizons are very small: the event horizon of a supermassive black hole could fit comfortably inside the solar system (comfortably for the black hole, that is, not for us). That might sound big, but on cosmic scales, it’s tiny: the black hole at the center of the Milky Way spans just 10 billionths of a degree on the sky. (For comparison, the full Moon is about half a degree across, and the Hubble Space Telescope can see objects as small as 13 millionths of a degree.)
Both the size and nature of the event horizon make it difficult to observe black holes directly, though indirect observations abound. In fact, though black holes themselves are strictly invisible, their surrounding regions can be extremely bright. Many luminous astronomical objects produce so much light from such a small region of space that they can’t be anything other than black holes, even though our telescopes aren’t powerful enough to pick out the details. In addition, the stars at the center of the Milky Way loop close enough to show they’re orbiting an object millions of times the mass of the Sun, yet smaller than the solar system. No single object, other than a black hole, can be so small and yet so massive. Even though we know black holes are common throughout the universe—nearly every galaxy has at least one supermassive black hole in it, and thousands more smaller specimens—we haven’t confirmed that these objects have event horizons. Since event horizons are a fundamental prediction of general relativity (and make black…