A picture of a black hole is one of those great, self-negating concepts, like the sound of silence, the presence of absence or the lives of the dead. The nature of one refutes the other. But a picture of a black hole has arrived nonetheless — revealed Wednesday morning in simultaneous press conferences held in six different locations around the world.
At those events — in Washington, Brussels, Santiago, Shanghai, Taipei and Tokyo — astronomers gave humanity its first look at the black hole at the heart of the Messier 87 (M87) galaxy, nearly 54 million light years from Earth. With that, astrophysics opened one more tiny crack in the wall of secrets that is the universe:
“We are delighted to report to you that we have seen what was thought to be unseeable,” said Shep Doeleman, Harvard University senior research fellow and director of the Event Horizon Telescope (EHT), at the announcement at Washington’s National Press Club. “We now have visual evidence. We know that a black hole sits at the center of the M87 galaxy.” True to the nature of the science, the picture does not show the black hole itself. The defining feature of all black holes is that they are so dense, generating a gravity field so powerful, that nothing, not even electromagnetic energy — which, of course, includes visible light — can escape their pull. What the picture reveals instead is the black hole’s so-called event horizon, the swirl of gas and dust and stars and light itself, circling the gravitational drain, before they’re sucked inside never, ever to reemerge.
In April of 2017, a global web of eight radio telescopes located in six places (Chile, Mexico, Spain, Hawaii, Arizona and the Antarctic), the collective network that makes up the EHT, began surveying the Messier 87 black hole, as well as the black hole at the center of our own Milky Way galaxy. Seeing any sign at all of either formation was no small achievement.
“The light from the M87 black hole has to travel for 60,000 years through its own galaxy, then for 55 million years across interstellar space,” said Doeleman. “Then it had to make it through our atmosphere, where the greatest enemy of the photons is water vapor.”
For that reason, all of the telescopes are deployed in places where the air is generally dry and generally clear — though not always. Simple bad weather could stymie even the most sophisticated plans, but the EHT team caught a break.
“We were very lucky,” said University of Arizona astronomer Dan Marrone, of the EHT team. “Our first three days of observing were some of the best we’ve ever seen.”
The black hole at the center of our galaxy goes by the name Sagittarius A*. It has a mass equivalent to about 4.1 million of our suns. While that earns it the sobriquet “supermassive black hole” (more common black holes can be as small as five solar masses), it’s actually something of a pipsqueak as these things go. It measures perhaps 24 million miles across, or about a 50 billionth the size of the galaxy. Trying to take an image of that from the 26,000 light year distance at which the Earth sits from the center of the Milky Way is like trying to spot an orange on the surface of the moon—with the naked eye. (A picture of Sagittarius A* was not released Wednesday, but will be in the future.)