Few people have hated the idea of black holes more than the man who helped establish their existence: Albert Einstein. His celebrated general theory of relativity in 1915 described how matter can cause space and time - "spacetime" - to curve, much the way a bowling ball will dimple the surface of a trampoline. Gravity is the effect of that warping of the geometry of the cosmos. Physicists put it this way: "Matter tells space and time to curve. Space and time tell matter to move."
Just months after Einstein's equations were published, a German scientist, Karl Schwarzschild, tweaked Einstein's equations and came up with a disconcerting conclusion. If an object was dense enough, he found, it would eventually punch through the universe's fabric, creating a bottomless pit in spacetime known as a "singularity."
These singularities - named "black holes" in the 1960s - are gravity wells so strong that, once you enter, you can't turn back. The gravitational field of a black hole is so intense that not even light can escape the "event horizon," the point of no return.
Einstein found the notion of a singularity so preposterous that he devoted an entire research paper to debunking it. But as time went on, more and more researchers found more clues that Schwarzschild's wild idea might be real.
The astrophysicist Subrahmanyan Chandrasekhar, an expert on stellar evolution, explained that some stars would collapse catastrophically after they died. Then physicists Robert Oppenheimer and Hartland Snyder showed how that collapse could continue until it formed the kind of singularity Schwarzschild had written about.
But stellar-mass black holes, the kind that form from collapsed stars, are not the only black holes in the universe.
In the 1960s and '70s, scientists proposed that some mysteriously bright objects they'd been calling quasars might actually be clouds of swirling, superheated material surrounding black holes hundreds of thousands of times more massive than the sun.
As these "supermassive" black holes slurp up gas and stardust, the theory goes, matter outside the event horizon speeds up, and some of it is flung back into the outer reaches of galaxies. X-rays and radio waves accompany these jets of ultrahot gas. Paradoxically, all this activity makes the black holes some of the brightest things in the universe - and obscures the darkness at their cores.
In 1974 researchers at the Green Bank Radio Telescope in West Virginia captured the powerful radio signal from the core of the Milky Way. This was the first observational evidence for the existence of Sagittarius A*, the black hole around which our entire galaxy spins.
Now astrophysicists believe that supermassive black holes lurk in the hearts of all large galaxies. One of the big unknowns is the chicken-and-egg question of what came first - the black hole or the galaxy?
In case you're wondering, black holes are not among the many things that humans should be worried about. They aren't rogue entities on the prowl, looking to devour nice planets. They're not cosmic vacuum cleaners. Still, you wouldn't want to cross the event horizon. That would go badly. If you fell feet-first, the gravitational tug on your feet would be greater than on your head and that tidal force would rip you apart. You'd be, as astrophysicists say, spaghettified.
Rainer Weiss, an emeritus professor of physics at M.I.T. and a Nobel Laureate for the detection of gravitational waves emanating from black hole collisions, explains:
"Eventually it will just tear you up. Eventually as you get in deeper it will tear up every molecule. Eventually as you go in further it will tear up every nucleus."
Eventually you would reach the "singularity." That's where, according the physics equations, the curvature of spacetime becomes infinite and all known laws of physics cease to apply.
"At that point, nobody knows what the hell to say about it," Weiss said.
To understand the singularity, Weiss said, "you need to know how to do the quantum theory of gravity."
Which is?
"We don't have a theory that is the quantum theory of gravity," he said.
Science writers often describe black holes as "monstrous," but they don't really have much personality at all. Indeed, anything that plunges into a black hole is eventually stripped of all identity other than mass, charge and spin. Protons, neutrons, electrons, big metallic atoms, whatever goes in becomes undifferentiated as it reaches the "singularity" at the centre of the black hole.
As for what a "singularity" is, no one really knows. You probably can't describe it with spatial coordinates, and neither is there such thing as time in a singularity. In a sense, there's no there there. And nothing really happens.
The Event Horizon Telescope has looked at two supermassive black holes: Sag A and the one in M87 (for Messier 87, named for the astronomer who catalogued galaxies in the night sky before anyone knew exactly what a galaxy was). M87 is a truly enormous elliptical galaxy in the constellation Virgo. Whereas the Milky Way's central black hole has a mass equal to 4.3 million suns (or "solar masses"), the black hole in M87 is more like 6 billion solar masses - billion with a b. M87 is remarkable for producing a dramatic jet of particles streaming some 5,000 light years into intergalactic space.
So the announcement could reveal an image of the Milky Way's supermassive black hole, or M87's, or both. The black hole may reveal itself in silhouette, or as a shadow of sorts.
Theories are great, but seeing is believing.
