What's actually going on in that cryptic black hole photo?

"We have now seen the unseeable."

These were the words spoken by astrophysicist Avery Broderick on Wednesday morning, one of some 200 scientists of the Event Horizon Telescope collaboration who captured humanity's first image of a black hole — a zone in space so gravitationally powerful that whatever light falls in can't possibly escape. "Black holes are gravity run amok," said Broderick.

Yet, we now have a picture of "the unseeable."

More precisely, we can see a prominent ring of super-heated gas around the very edge of a black hole. This final boundary between space and the black hole, the point of no return, is called the "event horizon." 

"What this brings to the table is the event horizon," astrophysicist Erin Macdonald, who had no role in the project, said in an interview. "It's the last possible point that we could see until you're so close to the black hole that nothing can escape."

So, although the black hole is itself invisible — a black mass that consumes light — we can see exactly where this giant spherical object begins, and where it lies. 

This particular black hole is "supermassive." It's located some 54 million light years away at the center of the galaxy us Earthlings call Messier 87, or M87. This black hole contains the mass of 6.5 billion suns, and is 38 billion kilometers across. 

And its picture is scientifically priceless.  

Before today, astrophysicists often called black holes "putative black holes," meaning something that's supposed or presumed to be a black hole. Not anymore. 

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"We learned that we don’t have to say 'putative black hole'," said Misty Bentz, an astrophysicist who researches black holes at Georgia State University. "We don’t have to couch it in those terms." 

"The fact that we see the evidence of the event horizon means the thing we call a black hole actually exits in nature," added Bentz, who had no role in the collaboration.  

"That’s something we didn’t know before today."

The event horizon

Without a clear view of the event horizon, we wouldn't have an image depicting a conspicuous, circular (though somewhat blurry) ring around the black hole. But what is an event horizon, actually?

The center of many, but not all, galaxies swirl with lots of matter, specifically gas. Inevitably, this hot gas rotates around the great black hole. This is the nature of gravity, somewhat similar to the way the moon orbits around a more massive Earth, explained Dong Lai, an astronomer and black hole expert at Cornell University. Gases get condensed as they speed around the black hole, and all this friction creates heat. "It's like moving your hands together in the wintertime," said Lai, who was also not involved in the project.

Image: NASA / CXC / M.Weiss

The hottest, most squeezed, and compressed gas lies just at the edge of the event horizon. "Whenever you squeeze something down you end up heating it up — and hot gas glows," said Bentz, noting that the brightest gases will soon fall into the black hole. "It's the last little scream of the gas until we can't hear it scream anymore."

The fervent scream of light gets devoured. "You don't want to be too close to a black hole," said Lai. 

As long as there's a rotating disk of super-heated gas around a black hole, there will be a circular boundary, showing where the event horizon ends, and the black hole begins. 

An invisible sphere surrounded by a donut 

From our far-off view of this great black hole, it might look like a bright, flat ring. But that's not exactly the case.

We're largely seeing the "face" of the event horizon, like the face of a coin, as opposed to the side or edge, explained Chris Fryer, an astrophysicist at Los Alamos National Laboratory who had no role in the collaboration. 

Yet from another view, we would see that the event horizon is not a flat disk with a big hole in the middle (where an enormous black hole lies). "It’s a donut sort of thing — but not a frisbee," said Lai. 

Still, we are viewing this black hole — and the event horizon around it — from an ideal angle. It's a bit like hovering above Earth and looking down onto the North Pole, said Bentz. This allows us to glimpse the ring around the rotating black hole, which scientists suspect is a great big sphere, like Earth. 

It's an invisible sphere surrounded by a donut of hot gas, if you will. 

It's not perfect

The picture of the glowing ring isn't perfect. It's fuzzy, but this is expected. 

The outer and inner edges of the ring aren't flawlessly defined because this light — emitted from the super-heated gas rotating around the black hole — has taken quite a journey through the cosmos before arriving at Earth's telescopes. (In total, the collaboration used eight radio telescopes positioned around the globe that, taken together, created an Earth-sized "virtual telescope").

First, the escaping light passed through the chaotic, swirling gases at the center of the M87 galaxy, which scattered and distorted some of the light, said Fryer. Eventually, the light journeyed through the Milky Way galaxy, where it got scattered once again when it passed through the bounties of dust and gas in our galaxy. Finally, this tortured light — some 54 million years old — traveled through our planet's thick atmosphere, again distorting this cosmic energy.

So when the light (emitted in radio wave form) from the event horizon finally reached us, it had been distorted. Consequently the image, even with correction and sophisticated computer modeling, isn't perfect. But it's pretty darn good. 

"I was surprised how good it looked knowing there were all these additional paths it had to travel," said Fryer. 

Black holes aren't evil

Black holes are often thought of as ominous things. But they're just part of the cosmos. There's almost certainly a supermassive black hole at the center of our galaxy, too. Though, scientists don't have a picture — yet.  

"We tend to anthropomorphize these things," said Bentz. "But really, black holes aren’t evil, mean or scary. They just… are."

And these powerful objects have fulfilled scientists' expectations. Albert Einstein's theory of relativity led scientists to predict that these objects exist in space, and that they were the remnants of dead, collapsed stars. If the collapsed mass was large and dense enough, a gravitational behemoth — a black hole — would form, astrophysicists theorized. 

But for decades upon decades, there was no definitive proof. Twenty-five years ago, as a graduate student, Fryer attended a meeting wherein scientists discussed the lack of definitive proof about the existence of black holes. There, some scientists still questioned if black holes existed at all. 

But that's changed. Especially as astronomers detected energy released when black holes interacted with or shredded other objects. To boot, now there's a picture.

"We went from not having direct proof to having images," said Fryer. "That change in our understanding is huge."

And this latest picture doesn't just confirm the existence of black holes. It gives tremendous weight to Einstein's greater, century-old theories of relativity and gravity. The theory also predicted the existence of gravitational waves, created by the collision of two black holes. Scientists detected these invisible waves in 2017.  

Astrophysicists, relying on the theory of relativity, anticipated that black holes would look like this very first image: an event horizon surrounding the black abyss. And that's what we see.  

"That’s what kind of gives me chills," said Macdonald.

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