This feature-length article was originally written for the September-October 2017 issue of Popular Astronomy magazine.
Through a modest 4-inch telescope on a clear night, you might be able to see the rings of Saturn and the moons of Jupiter. Through a meter-wide telescope, you might just be able to make out the characteristic blips in starlight that signify the presence of extrasolar planets. But to see the distant supermassive black hole in the heart of our galaxy, you’d need something much, much bigger – in fact, you’d need a telescope the size of a planet.
Enter the Event Horizon Telescope. A network of linked radio telescopes located all over the world, the Event Horizon Telescope (EHT) is the largest radio telescope ever constructed. By bringing together telescopes in Europe, North America, South America and even the South Pole, the result is a composite radio telescope almost as big as our entire planet. Its intended goal is no less grand than the scale of its construction: its aim is to observe the centre of our galaxy with unprecedented resolution and detail, image the supermassive black hole that lurks there and put the General Theory of Relativity through some of its most strenuous tests yet.
As seen from Earth, the black hole lies in the constellation Sagittarius, giving rise to the name that astronomers use for it, Sagittarius A*. But it’s not a case of pointing a big enough telescope at the sky and immediately seeing it – not only is it immensely far away, but in the way lie huge clouds of interstellar dust and gas. They prevent visible light from being able to reach us from the heart of the galaxy, so optical telescopes are out. Instead, astronomers turn to radio telescopes.
Radio telescopes are useful because long-wavelength radiation isn’t as badly affected by dust and gas as short-wavelength radiation is. Think of our own sky, for example: the relatively short-wavelength blue light from our Sun is scattered all across the sky by particles in our atmosphere, making our sky appear blue. The longer-wavelength red and orange light from our Sun is not so badly affected – if it were, our sky might appear purple instead of blue!
The same principle applies on a galactic scale: clouds of dust and gas that are completely opaque to visible light can appear transparent to the much longer-wavelength radio waves. This means that radio waves are the tool of choice if astronomers want to see into the dusty, cloudy regions in the heart of our galaxy.
Radio astronomy already has an illustrious history, having previously been used to study high energy objects like quasars and pulsars, map the surfaces of planets and nearby stars and there have even been earlier attempts to peer into the heart of our galaxy in search of the supermassive black hole. The latest effort using the Event Horizon Telescope, though, is the most ambitious attempt yet.
The earlier observations have allowed astronomers to infer the existence of the supermassive black hole from the erratic movement of stars near the galactic centre. So far though, no one has actually seen the black hole itself. As massive as the black hole is – thought to be millions of times the mass of our Sun, and tens of millions of kilometres in width – it’s also around 26,000 light years away, meaning that despite its immense size, it’s incredibly small when viewed from Earth. To image it in any detail, we’ll need a telescope larger and more powerful than all those which have come before.
When it comes to telescopes, what they can see is almost entirely determined by their size. Essentially, the wider the lens, the more detail they can pick out from the sky.
This is as true for radio telescopes as it is for optical telescopes. A big advantage with radio telescopes is that we don’t need to go constructing complex mirrors or lenses, but instead just need a comparatively simple metal dish, of the sort you might receive satellite TV on. Even so, there’s a limit to how large a single dish we can make – even the UK’s own 76m-wide Lovell Telescope at Jodrell Bank, the third-largest steerable radio telescope in the world, isn’t capable of high enough resolution to make out our closest supermassive black hole.
Instead of building bigger and bigger dishes, radio astronomers got a bit clever and came up with the idea of combining multiple different telescopes together into vast arrays. This technique is known as Very Long Baseline Interferometry (VLBI). It’s a common practice in radio astronomy, but is usually applied to arrays of relatively small telescopes within the same geographic area. For example, the Atacama Large Millimetre Array (ALMA) in Chile that makes up part of the EHT is itself a network of 66 smaller radio telescopes, each 12m in diameter, linked using this technique.
The net result of this is that all of the telescopes in the array come together and act like a single telescope the size of the array itself. In the case of ALMA, all of the telescopes are located within the same part of the Atacama Desert, but with modern-day communications technology it’s now possible to synchronise telescope arrays all over the world, allowing them to come together and form one truly global radio telescope – and that’s exactly what the Event Horizon Telescope is.
“VLBI links radio dishes around the world to form a single Earth-sized telescope,” explains project director Dr Shep Doeleman, “and does so by recording data at each dish then combining these recordings many weeks later to form an image.”
Synchronising this vast collection of telescopes is a huge technological challenge. Because the radio waves arrive at each telescope at slightly different times, for the data to be useful the team must record the time of each signal at each telescope incredibly precisely so that the data from all of them can be combined and analysed. Atomic clocks, accurate to around one second per million years, are used to tag the arrival times of the signals for later comparison.
“It creates a culture of hyper-vigilance,” Dr Doeleman told Popular Astronomy, “where people at each of the radio dishes check all the equipment and then check it again because forming this global array requires everything to be working perfectly.”
The main objective of the Event Horizon Telescope is that which gives it its name – to image the event horizon of the black hole in the heart of our galaxy. The event horizon is the point of no return, the shadowy halo that surrounds a black hole beyond which nothing can come back from. Rather than just being pure black, astronomers expect the event horizon to be surrounded by a ring of glowing, superheated material due to dust and gas heating up as it spirals in towards the singularity. This is what astronomers hope to see.
The environment around a black hole is one of the most extreme high-energy environments in the universe, with incredible gravitational forces unlike those found anywhere else. By being able to study and observe this environment, researchers hope to be able to put the General Theory of Relativity to the test in ways not possible elsewhere, perhaps even finding ways in which it breaks down that may pave the way to more complete future theories of gravity and spacetime. But before getting to that stage, they need to complete the first step – imaging the black hole itself for the first time.
After over a decade of planning, collaboration and intricately connecting radio telescopes all over the globe, astronomers might just have done it. In April 2017, over five nights of observing, the team have finally recorded the data that they hope will contain the first image of a black hole.
“I am really excited about getting the first glimpses of what the observations show: the moment of finally getting access to an object that’s designed by nature to be invisible,” says Dr Doeleman. “There are no guarantees here – even if the EHT works perfectly, we can’t know for sure what we will see – and really that is part of the attraction and mystery that has motivated us to work for so long on the project.”
We won’t know for sure for some months yet – collating the data, analysing it and generating the image from it could take until early next year. In fact, so much data was gathered from this observing run that it’s not possible to send it electronically. Instead, physical copies of the data on hard drives must be flown to the collation sites (an MIT computing facility near Boston, USA and the Max Planck Institute for radio astronomy near Bonn, Germany) for processing and analysis. There’s an additional complication in that the data from the South Pole site can’t be flown out until October, so we have to wait at least until then before all of the data can be brought together.
After all of that, astronomers should know by early 2018 whether they have been successful in imaging the event horizon of a black hole for the first time. If so, then the result is more than just a photograph – it’s the beginning of a new era for radio astronomy and the ability to investigate the physics of enigmatic black holes like never before.
My thanks to Dr Doeleman for responding to my request for interview: you can keep up with the latest news from him and his team here. Also, thanks to Hayden Goodfellow (a tutorial student of mine, long ago…) for his help with some background material that didn’t make it into the final article. I tried to match the images in this post to those used in the print article, but I had to use a different image of ALMA for copyright reasons. The featured image for this post (visible in some browsers) is a NASA image.