It would take a telescope as big as a planet to see the supermassive black hole at the centre of the Milky Way. But a team of scientists think they know how to do it

At the heart of our galaxy, a vast black hole is devouring matter from the dust clouds that surround it. Little by little, expanses of interstellar material are being swallowed up by this voracious galactic carnivore that, in the process, has reached a mass that is 4m times that of our sun.

The Milky Way’s great black hole is 25,000 light years distant, surrounded by dense clusters of stars, shrouded by interstellar dust and, like all other black holes, incapable of emitting light.

Yet scientists believe they will soon be able to take a photograph of this interstellar behemoth – an extraordinarily ambitious feat that will involve the creation of a radio telescope that has the effective size of our entire planet and whose operation will involve scientists from four continents.

“It is going to be very, very hard to take this photograph but we think we now have the technological capability to do it,” says Manchester University astronomer Tom Muxlow, based at the Jodrell Bank observatory in Cheshire.

“To be precise, we are not going to take a direct photograph of the black hole at our galaxy’s heart. We are actually going to take a picture of its shadow. It will be an image of its silhouette sliding against the background glow of radiation of the heart of the Milky Way. That photograph will reveal the contours of a black hole for the first time.”

It’s like a telescope that allows you to read a newspaper headline on the moon while standing on Earth Tom Muxlow, astronomer

Our galaxy’s great black hole is also known as Sagittarius A*, because it lies in the constellation Sagittarius and the data collection that will be used to create its image is set to take place in April. However, it will probably require a further six months of work to put together the observations made by all of the Event Horizon Telescope project’s component telescopes, which include instruments at the south pole and in the Andes, Hawaii and Europe.

The resulting image, say scientists, could look very much like the one created by director Christopher Nolan for the film Interstellar. Working with US astrophysicist Kip Thorne, Nolan went to considerable pains to develop something that looked like a “realistic” black hole. Gargantua, as it is named in the film, is depicted as a round black patch that hangs menacingly in the sky with swirling, luminous strands of matter pouring into it.

These strands of matter are known as an accretion disc. “In fact, the accretion disc around the black hole in our galaxy’s core is likely to be much thicker, geometrically, than the one in Interstellar, and so look somewhat different,” says Thorne. Nevertheless, most astronomers believe the film’s black hole is a good representation of what might be seen when the Event Horizon Telescope does its work.

A black hole is a region of space where matter has collapsed in on itself and become compressed into an incredibly small region. Its gravitational pull is so great nothing can escape from it – not even light. The point of no return, the boundary at which a black hole’s gravitational pull becomes so great nothing can emerge, is known as an event horizon.

“The event horizon is a surface in space-time, and if you go beyond that then you cannot get out again,” says Robert Laing, of the European Southern Observatory, a partner in the project. “Not even light can get out.”

The fact that light cannot escape black holes makes them tricky to observe, to say the least. However, we know they exist because they affect nearby dust clouds, stars and galaxies. As discs of material swirl around black holes they become extremely hot and give off electromagnetic radiaiton that can be detected in telescopes. “That radiation will provide the background against which we hope to see the shadow of the black hole at our galaxy’s heart,” adds Muxlow.

Facebook Twitter Pinterest US astrophysicist Kip Thorne helped design the black hole for the film Interstellar. Photograph: Ricardo DeAratanha/LA Times via Getty Images

Astronomers study black holes for several reasons. They are crucial to our attempts to understand the formation of galaxies, for example, and the EHT should provide vital observations for this work. However, its main purpose is simply to test general relativity. Einstein’s great theory has stood up well to scientific scrutiny over the last century – the recent discovery of gravitational waves, predicted by general relativity, being a good example. Black holes are also predicted by Einstein’s work and although astronomers have gleaned enough information to be sure they exist, their exact structures and shapes are unclear. The Event Horizon Telescope should put that right.

“We want to see whether the idea of a black hole having an event horizon is actually right and whether the quantitative predictions of what its shadow should look like are correct,” says Laing. “If general relativity is wrong in some way, you should eventually be able to see deviations from its predictions in the shape of the shadow and behaviour of our galaxy’s great black hole.”

In other words, if Einstein’s equations break down anywhere, they are most likely to do so at the edge of a black hole, where the fabric of space-time is being stretched more severely than any other place in the cosmos. As Laing says: “It’s the ultimate test.”

For example, if the shadow is precisely circular, this would indicate our galaxy’s black hole is not rotating. However, most predictions suggest that it should be spinning – which would produce a disc that has a dent.

The production of this evidence will strain the ingenuity and technological expertise of astronomers to their limits. Vast amounts of data, collected from observatories across the planet, will have to be combined to create a single image, an international collaboration that is being led by Shep Doeleman at the Harvard Smithsonian Center for Astrophysics.

“We are going to take advantage of the fact that all the gas and dust that is trying madly to fall into the black hole heats up to billions of degrees and the black hole casts a shadow against that intense light,” Doeleman said in a recent interview. “With the Event Horizon Telescope we capture light at different points on the Earth’s surface because our telescopes will be watching the same black hole at the same time. We freeze that light. We record it on hard-disk drives and then fly them back to a central computing cluster.” The computer will then create the image of the black hole.

In the UK, our astronomers’ main involvement in the project is channelled through our membership of the European Southern Observatory, which runs an array of 66 radio telescopes in the Andes called Alma (the Atacama Large Millimeter/submillimeter Array). This is one of the main instruments involved in the Event Horizon Telescope.

“To see through the dust and other material that lies between us and the centre of our galaxy, we have to use radiation that is about a millimetre in wavelength,” says Professor Tim O’Brien, another Jodrell Bank astronomer. “That compares with the wavelength of the light that we detect in our eyes, which is hundreds of times shorter in wavelength.”

Facebook Twitter Pinterest The Milky Way, at whose centre lies the supermassive black hole Sagittarius A*. Photograph: Property of Chad Powell/Getty Images/Flickr RF

This difference has a crucial consequence. Studying radiation with longer wavelengths makes it easier to peer through the dusty hearts of galaxies, including our Milky Way. However, to detect and study such radiation, astronomers need instruments that have far bigger collecting dishes than optical telescopes require. For an instrument designed to study millimetre-length radiation, you will need a telescope that is hundreds of times larger than a normal optical telescope, which gathers radiation of a much shorter wavelength. “In fact, if you want to observe, in detail, an object that is so distant and so obscured by dust as the black hole at the galaxy’s centre, you will have to design one that is as big as an entire planet,” says O’Brien.

Building a planet-sized telescope suggests all sorts of practical difficulties. Fortunately, there are ways round the problem. By combining the observations of a number of telescopes from different parts of the world, it is possible to create a machine that has equivalent gathering power to an Earth-sized device. The technique is known as very long baseline interferometry or VLBI and, in this case, it will create an instrument of unprecedented observing power. “The Event Horizon Telescope is the equivalent of a telescope that would allow you to read a newspaper headline on the moon while standing on the Earth,” says Muxlow.

The Event Horizon Telescope has not been designed solely to study our Milky Way’s black hole, however. Astronomers have other targets for it to observe. In particular, they plan to use it to try to take images of an even more remote object: a super-giant, elliptical galaxy in the constellation Virgo known as the M87 galaxy. It is 53m light years from the Earth and it also has a black hole at its heart.

“M87’s black hole is much larger than our galaxy’s but it is much further away, so it is going to be just as hard to study as the one that is inside the Milky Way,” said Laing. “However, the M87 black hole is much more active than our black hole. It is sucking in matter from surrounding space and ejecting it again in a spectacular jet, while our black hole is fairly quiescent at present. It will be very useful to compare the two black holes. black holes.”

Facebook Twitter Pinterest Some of the antennas of the Atacama Large Millimeter/submillimeter Array (Alma) in the Atacama desert, Chile. Photograph: Alamy

Just when we will get a chance to see these shadowy images of black holes is a different matter. Data from the different observatories that make up the Event Horizon Telescope will fill dozens of hard drives, the equivalent of 10,000 laptops’ worth of information. Shipping these from the South Pole Telescope, the project’s remotest instrument, is likely to take weeks, if not months. Then the data has to be combined on computers. “I think it will take at least nine months after we take our observations before we compile our first images,” says Muxlow.

Other problems that could affect the telescope’s cross-galactic photo bid in April include the weather, or to be more precise the levels of water vapour in the atmosphere. Water vapour plays havoc with observations made at millimetre wavelengths. Hence the placing of the Alma observatory high in the Andes – Atacama is one of the world’s driest places. Similarly, the south pole has a desert climate, almost never receiving any precipitation. These aid the EHT’s observing prowess but can occasionally be disrupted by weather that brings in unexpected clouds of water vapour. “We remain hopeful we will get our image in the next year, nevertheless,” says Laing.