How an Earth-sized telescope will 'see' a supermassive black hole

Jake Port explains how a global network of radio telescopes will image, for the first time, the event horizon of a black hole.

The centre of the Milky Way galaxy, with the supermassive black hole Sagittarius A* (Sgr A*) in the middle, is the first target for the Event Horizon Telescope.
X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI

Around the world, observatories gaze at the sky. But what if you could combine these to make a single high-resolution image – and examine the event horizon of the supermassive black hole at the centre of the Milky Way galaxy while you're at it?

This is the idea behind the Event Horizon Telescope, a virtual telescope so big it spans continents and hemispheres thanks to an imaging technique called interferometry.

It works as different telescopes scattered across the globe record data on the same subject, which is then combined and processed by a supercomputer. This fills in the gaps to produce a final image.

For instance, astronomers can use this technique to take information from two telescopes 100 kilometres apart, which creates an image similar to that taken by a single telescope 100 kilometres wide.

The accuracy gets better with more telescopes and the greater their vertical and horizontal separation.

Interferometry is used at sites such as the Atacama Large Millimeter Array in Chile, which comprises 66 moveable antenna dishes.

Now astronomers are thinking bigger. Much, much bigger.

By combining radio telescope observatories in Antarctica, Greenland, Chile, Hawaii and a number of other locations scattered across the globe, astronomers plan to image the event horizon of the supermassive black hole at the centre of our Milky Way galaxy.

Black holes are the great consumers of the universe. Their event horizon is the point of no return – not even light can escape once over the boundary.

Astronomers suspect that in the centre of each big galaxy lies a supermassive black hole. The one hosted by the Milky Way is thought to be around 4.5 million times the mass of the sun.

Despite its size, it and other black holes (supermassive or not) are currently impossible to see directly. Astronomers must instead observe the effects they have on their surrounds, such as the motion of stars.

So why radio telescopes over other types? Unlike optical light, radio waves can penetrate clouds of dust and other material between Earth and the target.

Supermassive black holes, being in the centre of a galaxy, are surrounded by plenty of dust and gas.

And specific wavelengths of light work better in different situations. Speaking to the BBC, University of Arizona astrophysicist Feryal Özel explained: "We've run upwards of a million simulations, for many different configurations of what that gas might look like. And in all cases, we think that the 1.3-millimetre wavelength is the right choice to see down to the event horizon."

The Event Horizon Telescope won’t just reveal what the event horizon of a black hole looks like, but may also test Einstein’s general theory of relativity, which describes gravitation.

This theory has been tested by measuring the distortion of light by large astronomical bodies such as the sun. But black holes are gravity powerhouses. Will the theory stand up in this extreme setting?

Contrib jakeport.jpg?ixlib=rails 2.1
Jake Port contributes to the Cosmos explainer series.