Marrone and his team are hoping to take the first picture of a black hole in 2015 (Image: Jorge Vieira)

Images of a black hole could test general relativity as well as prove they exist, says astronomer Dan Marrone

A black hole, by definition, is black. So how are you going to take a picture of one?

If you look right at the black hole it should look quite dark, as very little light escapes. But just around the edge of it you see a bright ring, which is due to the photons that barely missed going into the black hole and skimmed around the edge of it a couple of times. This light is what we think we will be able to detect with the Event Horizon Telescope (EHT).

The EHT is a “whole Earth telescope”. How does it work?

In radio astronomy, to get a higher resolution than you can from a single telescope, you record signals from many telescopes around the world and multiply them together with a special computer. It is as if you have a single telescope almost the size of the Earth.


Which black holes are you targeting?

Sagittarius A*, which is the supermassive black hole at the centre of our galaxy, and the black hole at the centre of M87, the biggest galaxy in the Virgo cluster of galaxies. With a telescope the size of the Earth and at the frequencies we are observing, we can just make out black holes of this size.

Every image of a black hole so far has been an artist’s impression. Will the real thing match expectations?

The question of creating an image from what we measure is a tricky one. We will most likely represent it as a false colour image, using colours to represent how bright the light is. This image will not be as pretty as an artist’s impression. The galaxy blurs the light between us and the black hole, so there are a lot of sharp features we can’t possibly see. But any image we get shouldn’t disappoint – we are looking at something no one has ever seen before.

What about capturing a moving image – “black hole, the movie” as it were?

We can, if there is something orbiting the black hole, as we expect there will be. If there is gas orbiting before it falls into the black hole, this takes between 4 and 27 minutes, depending on the spin of the black hole. If we look for several days and see changes in the structure, we can represent that as a movie as well.

What are you hoping to learn from this image?

Just being able to take a picture of a black hole, and show this shadow that we expect to be there because the light is not escaping, will be important. Beyond that we have a lot to learn about the structure of our galaxy’s black hole, and what happens to a black hole when it is being starved of material, as Sagittarius A* appears to be.

We also expect to be able to test general relativity, which tells us that the ring of light around the edge of the back hole needs to be perfectly circular. If general relativity is breaking in this very strong field regime, where gravity is at the limits of its power, then this ring of light won’t be perfectly circular.