What a black hole looks like

Here’s what scientists think a black hole looks like

By Daniel Clery |

More than half a dozen scientific press conferences are set for 10 April, raising hopes that astronomers have for the first time imaged a black hole, objects with gravitational fields so strong that even light cannot escape. Although their existence is now almost universally accepted, mostly from the effect of their gravity on nearby objects, no one has actually seen one.

Black holes themselves are entirely dark and featureless. The giant ones at the centers of galaxies are also surprisingly small, despite containing millions or billions of times the mass of our sun. To make observing them yet more difficult, those giants are shrouded in clouds of dust and gas. But streams of superhot gas swirl around the holes, emanating radio waves about a millimeter in wavelength that can penetrate those clouds.

Two years ago, an international collaboration known as the Event Horizon Telescope (EHT) corralled time on eight different radio telescopes around the world to try to image the supermassive black hole at the center of our galaxy, known as Sagittarius A*, and another at the center of nearby galaxy M87. They used a technique known as interferometry to combine the output of the globally scattered instruments to produce images as if from a single dish as wide as Earth. A dish that large is needed to see the details of something that would fit easily within the orbit of Mercury and is 26,000 light-years away.

Their 5 nights of observing produced 4 petabytes of data. If that amount of data was music stored as MP3s, it would take 8000 years to play. The team has spent the past 2 years correlating, calibrating, and interpreting the data and they are now preparing to show us the results.

If the EHT has an image, it may reveal the shadow of the black hole’s event horizon, the point of no return for anything falling in toward the black hole, against a backdrop of the bright swirl of gas in orbit around it. The size of that shadow and the shape of the swirling gas, lensed by the hole’s gravity, will help confirm many theories about these enigmatic objects.

As we wait for this week’s announcement, Science spoke with someone who has spent much of his career imagining what black holes might look like. In February, in anticipation of the EHT results, Jean-Pierre Luminet of the Paris Observatory in Meudon, France, released an illustrated history of black hole imaging that records decades of progress from pen-and-ink drawings to supercomputer simulations and Hollywood movies. This interview has been edited for clarity and brevity.