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The Event Horizon Telescope: Taking a Picture of a Black Hole

There are few things quite as mysterious and frankly absolutely terrifying as black holes. A point in space where gravity is so strong it creates a deep gravity sink where even light cannot escape. If there was an end of the world it might look something like a black hole, although it is virtually impossible to see a black hole so that might not make sense – but I’m sure you get my drift. Virtually impossible, but not entirely, which brings us to the vast collaborative venture underway to photograph them – the Event Horizon Telescopes Project (EHT).   

By combining data from various very-long-baseline interferometry (VLBI) stations around Earth, astronomers are now able to produce an angular resolution which is enough to see and photograph a supermassive black hole. I’m well aware that for most, what I’ve just said makes very little sense right now, but keep watching and I promise it will all become clear. Well, as clear as general relativity and quantum field theory can be that is. 

Black Holes 

the black hole
The black hole by Event Horizon Telescope is licensed under CC-BY

It makes sense to start with black holes. Now, there are some for whom the thought of a steadily expanding universe with distances that the human brain can’t even begin to understand along with the notion that we might one day be wiped out by a giant meteor can be a bit overwhelming.

But trust me, that’s not even the scariest part. The general thinking behind the existence of black holes dates back to the 18th Century with a clergyman and philosopher by the name of John Michell but it wasn’t until 1916 that German physicist Karl Schwarzschild found the modern solution of general relativity that would characterize a black hole.

In the 1960s theoretical work began to show a generic prediction of general relativity and interest in black holes steadily grew, especially after the discovery of the first neutron star (the collapsed core of a supergiant star) in 1968. But our understanding of black holes has still crept forward at a slow pace. 

We now believe that black holes form when a supergiant star dies, leaving behind a small, dense remnant core. Theoretically, if the mass of said perished star was more than three times that of our own sun, the force of gravity overwhelms just about everything else and hey presto, you’ve got yourself a black hole. I know I’ve skipped over several pages of complex science there, but that’s the general idea. 

Now, here’s where things get a little uncomfortable. Black holes suck in everything around them including planets and stars and can even combine with other black holes to create a menacing supermassive black hole that might one day take over the universe. I may be slightly embellishing with this last phrase, but probably not by much.

The really unnerving part about black holes is that we don’t know to what extent they can grow and whether once the process has begun it’s simply an eternal expansion. But what we do know is that there are two vastly differing sizes of black holes. 

The “small” black holes are known as stellar masses and are thought to be around 10 to 24 times the size of our sun – really puts that word small in perspective doesn’t it. While you can’t see them with traditional telescopes, astronomers have on occasions become aware of them by witnessing the mass of passing planets or stars as it becomes ensnared by the black hole. To give you a very simplistic comparison, imagine the head of an invisible vacuum cleaner. If you were to turn it on and look very closely you would see tiny particles of dust moving towards it and eventually disappearing.   

The large black holes should be enough to keep anybody awake at night – in fact, large probably isn’t even enough of a word to describe them. These black holes can be millions, perhaps even billions, of times the size of our own sun. And considering we could fit 1.3 million Earths inside the sun, it would mean black holes could be over 1.3 quadrillion times the size of this rock that we call home. I’ll just leave that thought with you a moment. 

The final point before we move onto the telescope project that shares its name is the event horizon. Yes, this was the title of a mediocre at best sci-fi horror film from the 1990s but also refers to an object’s escape velocity. Remember how I said that black holes could pull other objects into it, well, the closer an object comes to the black hole the faster it will speed up, and the event horizon is the point of no return – essentially, it is when the escape velocity needed exceeds the speed of light. But here’s the really mind-blowing part. When an object moves towards an event horizon it begins to redden and dim. There isn’t any change happening to the object, but rather the intense gravity distorting the light coming from it. As it reaches the event horizon, the object simply disappears – it is as if it never existed.    

The Event Horizon Telescope Project 

The Event Horizon Telescope
The Event Horizon Telescope

OK, let’s move from the hypothetical apocalypse of the universe to something a little more down to Earth. The Event Horizon Telescope Project is a collective undertaking which includes 300 members and 60 institutions spread across 20 countries. It began back in 2009 with the expressed intention of finally capturing an image of a black hole.

Easier said than done, but after years and years of theoretical and technical developments even before the project officially got underway and 10 years after EHT began, we finally saw our first-ever image of a black hole. But I’m getting a little ahead of myself. 

The Array

What makes the EHT so unique is the complex array of telescopes that it uses. Right at the start of the video, I mentioned Very-long-baseline interferometry (VLBI) stations and this is probably as good a time as any to really dive into it. VLBI is essentially a collection of telescopes that collect signals from an astronomical radio source, such as a quasar (which typically has a supermassive black hole at its centre). 

All of the telescopes are carefully synced up using a local atomic clock and the distance between them is calculated using the time difference between the arrival of the radio signal at different telescopes. This allows the telescopes to essentially combine their power into one giant telescope with a size equal to the maximum separation between the telescopes. Not only is this how the EHT has managed to photograph black holes, but the system can even be used in reverse to perform earth rotation studies, incredibly precise map movements of tectonic plates as well as accurately measuring and understanding Earth’s geometric shape. 

But when it comes to photographing distant objects, it’s all about angular resolution. This is the smallest angle between close objects that can be seen clearly as being separate. If you imagine staring up at the sky, two stars might appear to be right next to each other, but in reality, they are millions of light-years apart. Roughly speaking, the larger the telescope the better the angular resolution is, but even the largest telescopes don’t have the angular resolution to observe a black hole. This is all done in arc minutes, arc seconds and microarcseconds – essentially these are ways of breaking the normal degrees into smaller sections. A circle can be divided into 360 degrees, but those degrees can then be divided down further into arc minutes, which are 1/60 a degree. Arc seconds and microarcseconds do exactly the same thing but get smaller and smaller 

The resolution of the human eye is around 60 arcsec of a degree in visible light. The Hubble telescope on the other hand with its 2.4-meter (7.8 ft) diameter is about 0.05 arcsec of a degree, which sounds great but would be hopeless in the hunt for black holes. The EMT array on the other hand has a resolution of a few dozen micro-arc-seconds, well over 1,000 times stronger than Hubble. 

The EMT array uses 8 main telescopes (although there are 66 individual telescopes used) scattered around the planet. These include:

  • Arizona Radio Observatory
  • Atacama Pathfinder EXperiment (APEX) located in northern Chile
  • IRAM 30-meter telescope in the Spanish Sierra Nevada mountains 
  • James Clerk Maxwell Telescope (JCMT) in Hawaii
  • Large Millimeter Telescope “Alfonso Serrano” (LMT) in Mexico
  • Submillimeter Array (SMA) in Hawaii
  • Atacama Large Millimeter/Submillimeter Array (ALMA) in northern Chile
  • South Pole Telescope (SPT) 

Three further telescopes are scheduled or have already joined the array

  • Greenland Telescope
  • IRAM NOEMA Observatory in the French Alps
  • Kitt Peak 12-meter Telescope (part of the Arizona Radio Observatory ARO)

But despite much of this work being done at the cutting edge of scientific understanding, there’s still some old fashioned leg work to be done. The data is considered so valuable that EHT doesn’t transfer it digitally, instead, hard drives containing the information gathered by the telescopes are transported via commercial freight aeroplanes to the MIT Haystack Observatory in Massachusetts and the Max Planck Institute for Radio Astronomy in Germany. There, all of the data can be cross-correlated and then analysed with the use of a grid computer (a large group of computers essentially working towards a common goal).  

Messier 87

On 10th April 2019, six simultaneous press conferences were held by the EHT project around the world. It hadn’t exactly been a secret what the large team had been working towards, but the announcement was seismic nonetheless. There were some on the team who had been working for that moment for nearly twenty years and for the first time in history, humans had a glimpse of a black hole.

The slightly fuzzy orange ring hovering in complete blackness might not be the most visually groundbreaking photo, but what it revealed was extraordinary. The image was of the supermassive black hole that lies at the centre of the supergiant elliptical galaxy Messier 87, which contains roughly 1 trillion stars in the constellation of Virgo.

What we can see in the image is a rotating disk of ionized gas surrounding the black hole, which is perpendicular to the relativistic jet emerging from it. This jet is the matter that is essentially ejected from the black hole, composed mostly of hot plasma containing electrons and positrons. Physicists estimate the jet itself continues for around 5,000 light-years and travels close to the speed of light. The rotating disk of gas is thought to be moving at around 1,000 km/s (621 miles per second) and spans a diameter of 3.7 trillion km (roughly 2.3 trillion miles)

To put that unimaginable number in a bit of a clearer perspective, Pluto is around 5.2 billion km (3.2 billion miles) from the sun – so, with some rudimentary maths, the diameter of the rotating disk of gas is over 700 times the distance between Pluto and the Sun. 

And there are plenty more fairly mind-destroying numbers. The calculated mass of the black hole was put at 6.5 billion solar masses – 6.5 billion times the mass of our sun which itself is around 333,000 times the weight of Earth. Ever wondered how much the Earth weighs? Well, I’ve got that too – 5.973 sextillion tonnes. I tried to figure this all out on the calculator but it basically just exploded. These are figures so far from the realm of the every day it’s difficult to even comprehend it all. They also calculated that the event horizon, remember the nightmarish point of no return where everything fades to black, is around 40 billion km (24 billion miles) in diameter.

3C 279

In April 2020, EHT released another extraordinary image, this of the optically violent variable quasar 3C 279 – and I’m sure you don’t need me to tell you how much I enjoy the phrase optically violent variable quasar. We don’t know a whole lot about 3C 279, apart from it lies about 5.5 billion light-years from Earth and we believe it went through a particularly turbulent period between 1987 and 1991 – but even today it continues to be one of the brightest and most variable sources in the gamma-ray sky. 

The image released by EHT was observed over a series of nights back in 2017 and was the first 20 microarcsecond resolution image. The pictures are again a little hazy but show what physicists say is a jet potentially moving at superluminal motion – which is a complex way of saying faster than the speed of light. Though they believe this could be down to emissions originating closer to the observer catching up with emission originating further from the observer, giving a somewhat distorted image of speed.

Sagittarius A*

The EHT team has already begun observing the next target, Sagittarius A* (you do need to pronounce the star here), a compact astronomical radio source at the Galactic Center of the Milky Way, where, you’ve guessed it, a supermassive black hole lies. That’s right, we have a black hole at the centre of our own galaxy, in fact, physicists now believe it’s probable that there is a black hole of some kind at the centre of most galaxies – which is a cheery thought. Scarily, it’s not actually the closest black hole to us, but it is our closest supermassive behemoth.   

Sgr A* has a mass of approximately 4.3 million times that of our sun, which sounds absolutely gigantic but is fairly small compared to other black holes, and can be found about 25,000 light-years away from the Earth. 

The World’s Telescope

As more telescopes are added to the EHT project and as technology improves, our understanding of these chilling abysses of space will undoubtedly grow. In an age of increased nationalism, it’s always nice to see a truly global project. There’s something slightly comforting knowing that there are teams that are working together to further the collective knowledge of humankind. 

The subject of black holes is one that isn’t often discussed openly and at length. Whether humans have really reached the point of both understanding and accepting the fact that there are floating holes in space that can suck in everything around them never to be seen again, is open for debate.  

And when we have so much to occupy our minds on our own floating rock in space, who can blame us. But there is something both thrilling and utterly horrifying about the existence of black holes. Especially when we consider the mind-bending question of where do they lead. When we think logically, anything entering a black hole would quickly be reduced to the tiniest atoms as gravity crushes it, but if we want to allow our mind to wander a little – and why the hell not – there are some fascinating theories out there.         

Could they, in fact, be wormholes leading to other dimensions, or other universes? Might they be portals to different time periods? These theories probably sound borderline crazy to some, but are they really crazier than the fact there are holes in space sometimes billions of times the size of our sun and we don’t know where they lead?

The Event Horizon Telescope Project is still only beginning to unravel the mysteries of black holes. Understandably, this is an area that some might find difficult to accept. The existence of supermassive black holes and their unfathomable size certainly puts a single human being, and even a single species, in perspective. Over the coming years, the EHT project aims to add further telescopes to the array, while also increasing aperture and sharpening the images through improved resolution. For those who dare to watch, this could get very interesting indeed.     

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