Astronomers don’t usually jump out of bed when they receive alerts in the middle of the night that, somewhere far away, two black holes have smacked into each other and sent shock waves coursing through the universe. These days, the detection of colliding black holes verges on routine, and astronomers know what to do: Go back to sleep.
They will rouse in the case of a merger between neutron stars—leftover cores of giant stars that ran out of fuel and imploded—so that they can summon telescopes to scan the sky in search of a burst of light. But with a pair of black holes, there’s not much to see. Usually.
Astronomers announced today that they have spotted what might be a spark from the collision of two faraway black holes. If confirmed, the discovery would mark the first time that astronomers have captured light produced by the joining of the darkest objects in the universe.
But black holes don’t emit light; they trap it. Entire stars can become shredded and swallowed during an unlucky encounter with a black hole. How could any encounter between two black holes produce the kind of light our eyes can see?
It happened last May—from our perspective, at least, because it takes many millions of years for the aftermath of such events to reach Earth. The gravitational waves made themselves known, as usual, at the observatories in the United States and Italy specifically designed to detect them. At about the same time, an observatory in California, programmed to spot luminous objects in the sky, was making its nightly rounds. It caught a bright flare in the darkness. Astronomers traced the flash to the center of a distant galaxy, where a supermassive black hole lies, surrounded by a glowing disk of swirling gas and dust.