How Was The Black Hole Photographed?

Apr 15, 2019 By Deepa Gopal
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Why was a shadowy hole encircled by a ring of fire one of the most shared pictures on the internet last week? 

It was the first picture of a black hole in the galaxy M87 that is 55 million light years from our Earth! Until now, the only images of black holes had been drawings by artists based on what scientists have been telling us.

According to Katie Bouman, the MIT scientist who played a role, "it required the amazing talent of a team of scientists from around the globe and years of hard work to develop the instrument, data processing, imaging methods, and analysis techniques that were necessary to pull off this seemingly impossible feat."

To understand black holes, read our earlier articles (in the Related Section). In this article, we look at how scientists managed this amazing feat.

What Is The EHT?

EHT or Event Horizon Telescope, is a network of eight ground radio telescopes scattered across the globe. These large telescopes are located on volcanoes in Hawaii and Mexico, on mountaintops in Arizona and Spain, in Chile's Atacama desert, and in Antarctica.

The M87 black hole is the largest black hole visible from our Earth, with a weight that is 6.5 billion times that of our sun! It is also brighter than the black hole in our Milky Way galaxy, the Sagittarius A. A black hole's brightness depends on its accretion disk -- a collection of gases and space dust that orbit it, and from where this matter is sucked into the black hole itself. Because of the intense gravitational force with which the gases are sucked in, the accretion discs radiate heat and X-rays, and this can be detected by radio telescopes.

Even though the ground telescopes are not connected, they are all synchronized to a very precise atomic clock. The best time to record the radio signals is in late March and early April when the weather is favorable in all the countries that the telescopes are located in.

The telescopes use a technology known as VLBI (or Very-Long-Baseline Interferometry). It is based on a principle that two telescopes in two parts of the world will record the same signal slightly differently, and this difference can be related to the distance between the telescopes. So, by continuously capturing the signals, one can see variations in distance with time, and this can tell us a lot about the incoming signal. 

Generating A Picture

Data collected from these telescopes is typically very large and of the order of five petabytes (one petabyte is one million gigabytes!). It is transferred the old-fashioned way in hard disk drives to the MIT Haystack Observatory in the U.S and the Max Planck Institute for Radio Astronomy in Germany. 

The data is then loaded into special supercomputers, and this is where scientists like Katie Bouman come in. Since the data is very sparse as not every photon of light reaches the telescopes, scientists have to develop algorithms that fill in the blanks. It is like trying to put together a puzzle where 90% of the pieces are missing! The algorithms have to decide which results make sense and which are unlikely. 

Based on data collected in 2017, four different teams worked separately on algorithms to generate an image. These images were then blended together to create a final picture of the black hole. This picture also shows how solving complex problems is a collaborative effot involving many dedicated young men and women!

Sources: Eventhorizontelescope.org, Science News, Fivethirtyeight, Space.com, Earthsky.org