How the Event Horizon Telescope reveals black holes

How the Event Horizon Telescope reveals black holes ...

This week, the Event Horizon Telescope collaboration is getting ready to make a major announcement about the Milky Way that has space nerds everywhere on their seats. Back in 2019, the company organized a similarly coordinated set of press conferences around the globe to highlight the first-ever image of a black hole.

Over 100 scientists and engineers from around the world re-imagined a solution to a seemingly impossible task: to utilize radio telescopes to extract a snapshot of M87*, the supermassive black hole in the galaxy M87. After their development, their work has continued in the years since the first release, re-imagining the image they had already taken to reveal the magnetic field lines around M87* (pronounced M87-star), and there is a wealth of information that they are looking for more details.

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? How did that first work lays the foundation for what is coming this week?

What is the purpose of the Event Horizon Telescope collaboration?

The Event Horizon Telescope collaboration is organized by over 100 astronomers, engineers, and scientists from all over the world to use their individual tools, resources, and expertise to see the outermost visible edge of a black hole, known as the event horizon.

This is not just the work of astronomers and telescope researchers, but also data and computer scientists who must combine more than a dozen streams of overlap information that form the image we see.

Why is it so important to take a picture of a black hole?

It may seem obvious that a black hole is difficult to see because it doesnt itself give off any light, sucking it into itself instead, and that isnt wrong. However, a black hole isn''t always invisible, and there are many ways we can see them.

Weve been able to see the gravitational effects of a black hole on its surrounding space for years now. This is often in the form of other stars in orbit around it, where the stars'' orbits can''t be explained by what we can see. The same phenomenon occurs in the center areas of galaxies where stars are in great concentration.

Another strategy is to seek an accretion disk around the black hole. If a black hole is actively consuming material, the material forms into a flattened disk around it from its angular momentum around the black hole. As the material begins to progress closer to the black holes event horizon the exact distance from the black holes central singularity where the escape velocity from the black holes gravity exceeds the speed of light that the material orbits the black hole at greater fractions of the speed of light.

As it accelerates in the disk, whatever material it was before, it has been transformed into a hot ionized plasma that allows for the radiation to escape before being caught out of the event horizon itself. Amidst this extremely radio-bright radiation, you can see a total void or shadow in the center, which is being bent by the intense gravity around the black hole into a different halo.

You might think this would make them easier to see then, but there are two key issues that have forced it so difficult to see them. The first is that the radiation being thrown out of the accretion disk is among the most powerful radiation in the world. You are in a situation where you are effectively staring into the Sun with a naked eye and introspective efforts.

The majority of stellar-mass black holes, which can include 10 or 20 billion solar masses, have diameters that are easily accessible inside our solar system. These are thousands of light years away from us if you shrank the Sun''s diameter without changing its mass to the point where a black hole forms.

So, back to the same image of our Sun, observing a black hole is like looking at the Sun with the naked eye and seeing a dark sunspot the size of a city. All of this is what makes seeing a black hole so incredibly difficult, and why was it so remarkable. How did they do it?

How a black hole image is taken

The most astonishing thing about the universe is that light does not disappear outside of a black hole, and that light cannot spontaneously appear where it was before, and if that light strikes our retinas or instruments, we may see it. By using lenses, we can focus the light from the most distant stars and galaxies in the universe and transform it into something we can see.

As the wavelengths and X-rays of the visible spectrum differ from the frequencies of the 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 design a lens that is large enough to allow the light to be visible.

The lense is the antennas dish that focuses on radio light, but when it comes to seeing the shadow of the event horizon of Sagittarius A* (Sgr. A*), the Milky Ways supermassive black hole, the black hole itself isnt all that large. It has a diameter of 26 million miles, forming a correspondance between the Sun and the mean orbit of Mercury.

It is over 25,000 light-years away from us, so 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 a vast lens to focus that minimal amount of light onto something we could see; specifically, you would need a radio antenna as large as the Earths diameter itself.

There is no such radio antenna to be built, which means that the story might come to an end, but that''s where the EHT comes in. We may not be able to use an Earth-sized radio telescope, but we have radio telescopes everywhere in the world, and if we could turn them all to the same radio source and record data at the same time, then you would receive more than two dozen streams of data.

Because the difference in data streams is perhaps more important than the data itself. We can then map the distances between all of these radio telescopes and mathematically identify how the distance between two points on Earth''s surface should impact the differences in resulting data streams. This difference can then be algorithmically corrected to transform a network of radio telescopes into a single Earth-sized virtual telescope with the ability to zoom in on the event horizon of a black hole.

So, in April 2017, the EHT radio telescope array shifted its sensors towards Sgr A* and M87*, which despite being at quite different distances and sizes from us, looked almost identical at the same apparent size when measured from Earth. However, the amount of data collected was so vast that it had to be recorded for several days. The physical hard drives the data was then physically shipped to a central lab where they could all be processed and stitched together.

It would take months before all of the data could be delivered where it needed to go, mainly from one station in Antarctica, which took nearly a year to return to the processing facility in the United States and Germany.

Sgr A* has proved itself to be much more elusive, given that one of its magnetic poles would be shooting out a relativistic jet of highly charged and radio-bright particles directly at an EHTs virtual telescope, making it take to describe a firefighter while they''re not shooting you in the face with a firehose.

This is absolutely raising the stakes for whatever EHT researchers have discovered, and it is part of this week''s announcement so exciting. The concept for the M87* image, which is being discussed as a milky way, is so simple, so that we may finally see our galaxys beating heart, and that it may be even more bizarre and exotic.

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