How the Event Horizon Telescope takes pictures of black holes

How do you photograph something that soaks up all light?
John Loeffler

The Event Horizon Telescope collaboration recently made a major announcement about the Milky Way that had space nerds everywhere on the edge of their seats. Back in 2019, it held a similarly coordinated set of press conferences around the world to reveal the first-ever image of a black hole.

The incredible effort was the work of over 100 scientists and engineers from around the world who innovated a solution to a seemingly impossible challenge: to use radio telescopes to take a snapshot of M87*, the supermassive black hole in the heart of the galaxy M87. After this groundbreaking effort, the work has continued in the years since that first release, refining the image they had already taken to reveal the magnetic field lines around M87* (pronounced M87-star), and there is a wealth of data that they are probing for more details of the colossal and enigmatic structure.

But how did they do it in the first place? What kind of effort was required to take a picture of something that gives off no light? And how did that initial work lay a foundation for what is coming this week?

What is the Event Horizon Telescope collaboration?

The Event Horizon Telescope collaboration is the organized effort of more than 100 astronomers, engineers, and scientists from around the world, who use a variety of tools, resources, and expertise to image the outermost visible edge of a black hole, known as the event horizon.

This isn’t just the work of astronomers and researchers in astrophysics, but also data and computer scientists who have to stitch together more than a dozen streams of overlapping data that together form the image we see.

Why taking a picture of a black hole is so hard

It might seem obvious that a black hole would be hard to see because it doesn’t itself give off any light, and that isn’t wrong. But a black hole isn’t always invisible, and there are several ways we can “see” them.

For many years now, we’ve been able to observe the gravitational effect that a black hole has on its surrounding space. Often, this is through examining the orbits of other stars in orbit around the black hole, where those orbits can’t be explained by the presence of other, visible, bodies. If a star appears to orbit a point in space without a star or other visible body, then there is a very high likelihood that we are looking at a star orbiting a black hole. This is something that is seen a lot in the central regions of galaxies, where stars tend to be in very high concentration.

Another way to find a black hole is to look for an accretion disk around the black hole. If a black hole is actively consuming material, like from a companion star, the material forms into a flattened disk around it from its angular momentum around the black hole. As the material moves closer to the black hole’s event horizon — the distance from the black hole’s central singularity where the speed needed to escape from the black hole’s gravity exceeds the speed of light — that material orbits the black hole at larger fractions of the speed of light.

Whatever this material was before, by the time it is in the accretion disk, it has been turned into a hot ionized plasma that releases an enormous amount of radiation as it accelerates in the disk. This radiation is able to escape before being sucked into the black hole, and since light cannot come out from the event horizon itself, amidst this extremely radio-bright radiation you can see a total void or shadow in the center, with the light from the accretion disk behind it being bent by the intense gravity around the black hole, forming a kind of halo around it.

You would think this would make black holes easy to spot then, but there are two major challenges that have made it so hard to actually image them. The first is that the radiation being blasted out of the accretion disk is among the brightest radiation in the universe. Active supermassive black holes in the center of galaxies can vastly outshine the light of the entire galaxy itself, and so you have a situation akin to staring into the Sun with a naked eye and trying to see the sunspots.

What’s more, the black holes themselves are tiny in terms of actual dimensions. If you shrank the diameter of the Sun without changing its mass to the point where a black hole forms, it would only be about four miles wide. Most stellar-mass black holes are about 9 to 18 miles across and pack up to 100 solar masses of material in them. Even the largest supermassive black holes, which can have 10 or 20 billion solar masses, have diameters that can easily fit inside our solar system, and those can be millions of light-years away from us.

So going back to the analogy of our Sun, spotting a black hole is like looking at the Sun with the naked eye and trying to see a dark sunspot the size of a city. All of this taken together is what makes imaging a black hole so incredibly difficult, and why EHT’s accomplishment was so astounding. So how did they do it?

How a black hole image is taken

How the Event Horizon Telescope takes pictures of black holes
Source: Event Horizon Telescope collaboration

The amazing thing about the universe is that light never just disappears, outside of a black hole. Light also cannot spontaneously appear where it wasn’t before, and if that light hits our retinas or instruments, we can see it. By using lenses, we can focus the light from the most distant stars and galaxies in the universe and expand the resulting image into something we can see.

And since radio waves and X-rays are just as much light as the frequencies of the visible spectrum, our sensors and telescopes have everything they need to see the shadow of the event horizon of a black hole. The challenge is to construct a lens large enough to focus the light they receive into a visible image.

In the case of radio telescopy, the antenna’s dish acts as the lens, to reflect radio light in a way that focuses the image. However, when it comes to seeing the shadow of the event horizon of Sagittarius A* (Sgr. A*), the Milky Way’s supermassive black hole, the black hole itself isn’t all that large. It has a diameter of around 27 million miles, which is not that much less than the distance between the Sun and the mean orbit of Mercury.

It’s also just over 25,600 light-years away from us, and its incredible distance makes it appear even smaller. In order to capture an image of something so small from so far away, you would need an absolutely enormous lens to focus that minuscule amount of light into something we could see; specifically, you would need a radio antenna as wide as the Earth’s diameter itself.

Clearly, no such radio antenna can be built, so that would seem to be the end of the story, but that’s where the EHT comes in. We might not be able to build an Earth-sized radio telescope, but we have radio telescopes all across the world, and if we were to turn them all to the same radio source and record data at the same time, then you would get more than two dozen streams of data that are nearly identical. 

That nearly part is essential because the difference in those streams of data is perhaps more important than the data itself. We are able to map the distances between all of these radio telescopes and mathematically work out how the distance between two points on Earth’s surface should affect the differences in resulting data streams. That difference can then be algorithmically corrected to turn a network of radio telescopes into a single, Earth-sized virtual telescope that has the resolution necessary to zoom in on the shadow of the event horizon of a black hole.

So, in April 2017, The EHT radio telescope array turned its sensors toward Sgr A* and M87*, which despite being at vastly different distances and sizes from us look almost the same apparent size when seen from Earth, and recorded data for several days. The amount of data collected was so voluminous that it couldn’t be transmitted over the internet, the physical hard drives the data was stored on had to be physically shipped to a central lab where they could all be processed and stitched together.

This meant that it would be months before all of the data could be shipped where it needed to go, especially from one station in Antarctica which took nearly a year to ship back to the processing lab in the United States and Germany.

They got there nonetheless and thanks to an algorithm primarily developed by then-graduate student Katie Bouman, the world got its first look at the shadow of M87*’s event horizon. Sgr A*, however, has proven itself to be much more elusive. There’s evidence that Sgr A* is severely tilted magnetically, with one of its magnetic poles pointing almost dead-on in the direction of Earth. If so, it could be even harder to see since it would be shooting out a relativistic jet of highly charged and radio-bright particles directly at EHT’s virtual telescope, making it taking to describing a firefighter while they’re actively shooting you in the face with a firehose.

This absolutely raises the stakes for whatever the EHT researchers have found, and is part of the reason why this week's announcement is so exciting. The setup for the announcement, with simultaneous press conferences around the world, is the same structure used to announce the first image of M87*, and it’s being teased as an announcement about the Milky Way, so not only might we finally be able to see our galaxy’s beating heart, we might also find out if it is as weird and exotic as it seems.

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