On Feb. 14, 1990, the Voyager 1 spacecraft looked back at our solar system and snapped the first-ever pictures of the planets from its perch at that time beyond Neptune. This past April, the Event Horizon Telescope (EHT) team unveiled humanity’s first image of a supermassive black hole –described as the Gates of Hell and the End of Spacetime– the picture of galaxy Messier 87’s central supermassive black hole. A monster the size of our solar system, and bigger, with the mass of six and a half billion suns, with a ring of gas—in hues of red, orange, and yellow—glowing around it, the shadow cast by the event horizon, predicted by Einstein’s theory of general relativity.
“We have seen what we thought was unseeable,” said Sheperd Doeleman, an astronomer at the Harvard-Smithsonian Center for Astrophysics, and director of the EHT effort to capture the ghostly image at the center of M87.
Black holes, paradoxically, the smallest objects in the known universe, have outsize effects on entire galaxies. But to see one you would need to build a telescope as large as the Earth, because the black hole that you’re looking at gives off copious radio waves. It’s emitting all the time.
Black holes are where the quantum world and the gravitational world come together, says Doeleman. “What’s inside is a singularity, where all the forces become unified because gravity finally is strong enough to compete with all the other forces—the strong, weak, and electromagnetic. But we can’t see the singularity. “The universe has cloaked it in the ultimate invisibility cloak. We don’t know what happens in there.”
In May, in a conversation with Doeleman, reports Brian Gallagher at Nautil.us, Chris Anderson, of TED, asked Doeleman to define a black hole, an object he described as “the central mystery of our age.“ Doeleman said that “over 100 years ago, Einstein came up with this geometric theory of gravity which deforms space-time. So, matter deforms space-time, and then space-time tells matter in turn how to move around it. And you can get enough matter into a small enough region that it punctures space-time, and that even light can’t escape, the force of gravity keeps even light inside.”
Until the capture of the April image of M87, “we had no idea what a black hole really looked like,” Doeleman explained. “The best we could do were simulations like this in supercomputers, but even here you see this ring of light, which is the orbit of photons. That’s where photons literally move around the black hole, and around that is this hot gas that’s drawn to the black hole, and it’s hot because of friction. All this gas is trying to get into a very small volume, so it heats up.”
“What you’re seeing,” Doeleman concluded “is that last orbit of photons. You’re seeing Einstein’s geometry laid bare.”
Erik Curiel who trained both as a philosopher and as a theoretical physicist and visiting scholar at Harvard’s Black Hole Initiative, found out when he asked theoretical and experimental physicists, mathematicians, and philosophers, “What is a black hole?” in a Nature Astronomy paper, titled “The many definitions of a black hole.”
Curiel writes: “the idea that nothing can escape the interior of a black hole once it enters makes implicit reference to all future time—the thing can never escape no matter how long it tries. Thus, in order to know the location of the event horizon in spacetime, one must know the entire structure of the spacetime, from start to finish, so to speak, and all the way out to infinity. As a consequence, no local measurements one can make can ever determine the location of an event horizon. That feature is already objectionable to many physicists on philosophical grounds: one cannot operationalize an event horizon in any standard sense of the term.
“Another disturbing property of the event horizon, arising from its global nature, is that it is prescient. Where I locate the horizon today depends on what I throw in it tomorrow—which future-directed possible paths of particles and light rays can escape to infinity starting today depends on where the horizon will be tomorrow, and so that information must already be accounted for today. Physicists find this feature even more troubling.”
Sean Gryb, a quantum gravity theorist at the Perimeter Institute, told Curiel, “The existence of [a classical event horizon] just doesn’t seem to be a verifiable hypothesis.”
Theoretical physicist Domenico Giulini, of Leibniz University Hannover, was skeptical that black holes were physical things at all. “It is tempting but conceptually problematic to think of black holes as objects in space, things that can move and be pushed around,” he said. “They are simply not quasi-localized lumps of any sort of ‘matter’ that occupies [spacetime] ‘points.’”
In contrast Ramesh Narayan, at the Harvard-Smithsonian Center for Astrophysics, are less fanciful. “A black hole is a compact body of mass greater than four solar masses—the physicists have shown us there is nothing else it can be.”
Chiara Mingarelli, a gravity-wave researcher at the Perimeter Institute: “It’s this interesting duality of them being really simple and really complicated. So they’re really simple. You can describe them by their mass and their spin, right? But what’s past the event horizon? What does the singularity actually look like? Is it actually this point of infinite curvature? It is some sort of quark soup? What does it look like?” She imagines that, as light travels toward the singularity, in all sorts of wonderful orbits, there must be fireworks. “I try to imagine it as like a water fountain, having light coming out and then falling back in on itself as it approaches the event horizon.”
“The gates of hell, the end of space and time.” That was how black holes were described at the press conference in Brussels where the first ever photograph of one was revealed to the excited audience on April 19th. And this black hole, a super-massive object at the center of the galaxy Messier 87 (M87 shown above), really is a monster, observed Ellie Mae O’Hagan for The Guardian. “Everything unfortunate enough to get too close to it falls in and never emerges again, including light itself. It’s the point at which every physical law of the known universe collapses. Perhaps it is the closest thing there is to hell: it is an abyss, a moment of oblivion.”
“Imagine taking a hammer, smashing a radio dish, and spreading the fragments all over the Earth,” Doeleman said recently while describing the project at the South by Southwest festival in Austin, Texas. “In reality, we did that by linking up telescopes on different continents and timing the data-taking perfectly.”
“It was phenomenal,” said Daniel Stern, a cosmologist who studies black hole formation at NASA’s Jet Propulsion Laboratory in La Cañada Flintridge, after he woke up at 6 a.m. to watch live as Doeleman unveiled the picture at a news conference in Washington, D.C.. “This will go down in the annals of astronomy as one of the greatest images ever taken.”