What is a black hole?

Black holes are objects with an intense gravitational pull so strong that not even beams of light, the fastest things in the universe, can escape. This makes them impossible to see directly, and so astronomers have had to use a variety of clever techniques to confirm that they actually exist.

Over the past few years, spectacular observations using gravitational wave detectors and vast telescope arrays have given us pretty good reasons to believe that black holes are lurking out there in the darkness. We’ve even caught one on camera in 2019, when we finally took a direct picture of the “event horizon” that marks the point of no escape from a black hole. But why do we care?

Black holes are cast-iron predictions of general relativity, Einstein’s peerless theory of gravity, and yet they stretch it to breaking point. General relativity says that matter warps space and time; black holes are simply very dense agglomerations of matter. But simple it isn’t. General relativity’s equations fail catastrophically at a black hole’s centre, known as its singularity, where the warping of space-time simply goes off the scale.

Even Einstein thought that these cosmic objects were too absurd to be real. Though we can’t see them, we infer their presence from their influence on nearby matter as they suck in gas and dust and stars, the contortions of which produce awesome light shows. In 2015, when we detected gravitational waves for the first time, the observed ripples in space-time matched the predicted signal from two black holes spiralling into one another and merging.

The event horizon is where the problems start. To get down to the nitty-gritty of what happens there, you need to bring quantum theory into the picture – but quantum theory and general relativity famously don’t agree on anything. General relativity says that when matter falls into a black hole, information is destroyed, but quantum mechanics says firmly it can’t be. A unified theory requires us to somehow reconcile the two, probably by reimagining space-time as only an approximate thing. String theory offers one way, and might turn what we think are black holes into “fuzzballs” with no singularity and no event horizon – dense, star-like objects that essentially amount to a tangled ball of space-time string.

Whatever they are, we think that black holes are really rather common. Space is pockmarked with ones formed when over-massive stars collapse and die: our galaxy alone holds perhaps 100 million of these smaller ones. Most galaxies also have a humongous one at their centre. We have a supermassive black hole at the heart of our Milky Way that packs over 4 million solar masses into a region and would fit inside the orbit of Mercury. Richard Webb