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The first-ever black hole image, explained in 500 words

How do you take a photo of a black hole? It’s simple: Just make a virtual telescope the size of the Earth.

Joss Fong/Vox

The supermassive black hole at the center of the Messier 87 galaxy is a beast, with a mass of 6.5 billion suns. The force of its gravity propels jets of plasma, moving at near the speed of light, that are some 4,000 light years long.

But as immense and powerful as this black hole is, when viewed from Earth, it is extremely small. Compared to the full moon, the shadow cast by the M87 black hole is 46.5 million times smaller.

Taking a photo of this black hole is like taking a photo of a quarter in Los Angeles all the way from Washington, DC. To see such a small object, researchers needed a telescope the size of the Earth.

So the researchers behind the Event Horizon Telescope, which released the first image of the black hole at the center of M87 Wednesday, connected eight telescopes around the world — from Antarctica to Greenland — to create a virtual telescope the size of the Earth.

Here’s how they did it.

Think about a simple mirror telescope. In it, the curved surface of a mirror reflects light back to a central point, where an image comes into focus. The EHT does a similar thing. “When the EHT sites are synchronized, their recordings can later be perfectly aligned in the same way that the mirror aligns the optical light,” the National Science Foundation explains in a video.

The math involved in stitching together an image from these recordings is similar to what an MRI scanner or a CAT scan does when mapping the inside your body, Dimitrios Psaltis, an astrophysicist at the University of Arizona and one of the lead scientists on the effort, says. “A CAT scan takes X-ray pictures of all around your head and then uses mathematics to unravel it and see what is inside your head.”

The Event Horizon Telescope is an expanding global network of radio telescopes that transform the Earth into one giant radio telescope.

The EHT did a similar thing with the black hole, using algorithms to project an image of it.

The more perspectives — i.e. slices — of the black hole it can capture, the clearer the image. Luckily, the rotation of the Earth gives each telescope in the array an opportunity to collect data at a slightly different angle. But it took extremely precise atomic clocks — precise to a fraction of a trillionth of a second — at each of the observatory sites to ensure all the data would line up and the resulting image would be clear.

Part of the reason this announcement was years in the making is because the data files were too big — this data was measured in petabytes (one petabyte is a million gigabytes) — to be transferred digitally. The hard drives had to be flown from the observatories to get processed. And the data set in Antarctica was inaccessible for months due to harsh winter conditions.

Psaltis called the image “the eureka place. I’m sure many other times in human history, people saw something for the first time ... and what you see, you cannot unsee. This is it for the rest of humanity, for the rest of human history.”