The groundbreaking picture of a black hole that graced most newspaper front pages on 11th April must be one of the most deceptive scientific images ever. That’s not to impute any intentional deceit to the international team of 200 or so astronomers who created it—it’s just that nature conspires here to produce something that looks archetypally, almost simplistically black-holeish, but which in reality is a mind-bendingly complex sight.
What we got was a bright yellow-orange blob with a black void in the middle: a cosmic doughnut in which the voracious gravitational field of the light-gobbling entity in the centre had apparently chomped out a perfect circular gap. If you ever imagined staring through a telescope at a black hole, you might have thought this is just what you’d find. But you’re not seeing what you might think you are, partly for reasons that are deeply strange.
The reason for all the excitement is that this is the first time a black hole has ever been imaged directly by any telescope. Black holes are stars—or in this case, vast conglomerates of many stars—that have collapsed under their own gravity. Albert Einstein’s theory of general relativity, published in 1916, predicts that in some circumstances this collapse becomes a runaway process that continues until the objects shrinks to an infinitely small point, called a singularity, where its mass becomes infinitely dense. The gravitational field created by such a dense object is so strong that, within a certain distance (called the event horizon), nothing can escape from being sucked in—not even light. What, according to Einstein’s theory, is really happening here is that the unthinkably dense mass warps time and space itself, so that there are simply no paths for the light inside the event horizon that can lead back out. This is the key to the oddness of the new image: what it’s really showing is not simply a sphere that absorbs all the light falling on it, but a place where time stops and space is curved in on itself.
Black holes were thought at first to be too absurd to be believed. But from the 1960s, when research on general relativity came back into fashion in physics, it became gradually accepted that singularities really could exist in nature, and that in fact the universe is likely to be full of black holes. As well as star-sized ones, formed when a large star burns itself out and can then no longer support its own mass against the inward tug of gravity, astrophysicists concluded that immense black holes with masses millions or billions of times greater were likely to exist at the centre of galaxies like ours. An object in the centre of our galaxy (the Milky Way) called Sagittarius A* is thought to be such a “supermassive” black hole.
Although black holes themselves emit no light, virtually by definition, their presence can be inferred in various ways. The supermassive black hole at the galactic centre can be deduced from the way stars close to it seem to move, as if pulled by the gravity of some unseen object. What’s more, black holes draw nearby gas and dust inwards, creating a disk-shaped halo of hot material that swirls around rather like a vortex of water running down the plughole. This so-called “accretion disk” gets heated to extreme temperatures and emits radiation, primarily as X-rays and gamma rays, which can be seen by telescopes sensitive to those wavelengths. And spinning supermassive black holes at galactic centres eject huge jets of hot gas from their poles, some of which have been seen by astronomers.
The black hole in the new picture sits at the centre of the galaxy M87, which is 55m light years away. It has a mass of 6.5bn suns, and even the dark void in the centre is wider than our entire solar system. It was imaged by an international collaboration using the Event Horizon Telescope (EHT), which is in fact a network of eight radio telescopes across the globe, interconnected and synchronised with high-precision atomic clocks to create what is effectively a telescope the size of the planet. Only a device this big could zoom in on an object this size when it is so very far away. The EHT was set up specifically to see this black hole in M87 and the one in Sagittarius A*, which is much closer but also almost a thousand times smaller. An image of that black hole at the centre of our galaxy is anticipated soon.
The temptation is to interpret the EHT image as a straightforward telescopic view of the hot accretion disk, seen from above, with the black hole carving out a hole in the middle. But it isn’t quite that.
For a start, this is not visible light we’re being shown but radiation at the border of the radio and microwave regions of the spectrum, the white-yellow-red scale showing how intense it is. And the boundary of the light and dark regions doesn’t correspond to the event horizon, which is smaller than that—there is a space between the inner edge of the accretion disk and the event horizon within which any matter is quickly sucked into the hole. What’s more, this is a composite image assembled from all the data collected by the array of eight individual telescopes. The amount of information used to put the image together is itself astronomical: this is probably the most data-intense scientific image ever made. The EHT array collected over its five viewing nights in April 2017 more data than the Large Hadron Collider particle accelerator at CERN does in a year, stored on half a ton of hard drives. Assembling all this data into a single image is a tour de force of computing, and uses an algorithm devised largely by graduate student Katie Bouman, while she was at the Massachusetts Institute of Technology, and Andrew Chael of Harvard. The informal mobile-phone snap of Bouman taken by a friend as the image first took shape on her laptop has already become iconic—she looks as though she has just done something naughtily transgressive.
But the most striking aspect of the image is the doughnut shape. We aren’t simply fortunate enough to be, on Earth, in just the right position to see the disk face on. It is actually at an oblique slant. The reason it looks like a circular ring is that this is pretty much how it would appear from most viewing angles, because the light from the rear of the disk, behind the black hole, is pulled up, as if coming from above the hole, by the curved spacetime that the hole itself creates.
Even if we saw the black hole fully side-on, the accretion disk wouldn’t just be a line-like object but would still look like a kind of halo, albeit with a sombrero-like shape: an image much like that used for the movie Interstellar, which was based on expert advice from Nobel laureate astrophysicist Kip Thorne.
Even the fact that the ring is brighter on one side than the other is no mere artifact of the imaging but is a real effect, due to the fact that the material in the accretion disk is swinging towards us at close to the speed of light on one side while receding on the other. That all this matches the predictions made by general relativity shows once again that Einstein’s theory is spot on.
You could, then, regard the black hole image as a kind of distortion of the “real sight,” a bit like a picture pulled about by a strangely shaped lens. But in this case the lens is spacetime itself. Which is to say: there is no other “what it would really look like,” free from such distortions—because there is no meaningful definition of reality outside of the geometric confines of spacetime. Light, in this view, is not “bent” at all: it travels in straight lines all along, but it is the meaning of “straight” itself that is altered by the black hole’s gravity. This is why the picture of the black hole is spookily weird. Once you know what you’re truly looking at, you realise that you’re seeing not stuff suspended in space and time but stuff reshaping space and time. It is visual proof that the fabric of reality can be warped. Bouman is right: this picture really is transgressive.