Light and Shadow
A black hole is a region of space from which nothing, not even light, can escape. Despite the name, they are not empty but instead consist of a huge amount of matter packed densely into a small area, giving it an immense gravitational pull. There is a region of space beyond the black hole called the event horizon. This is a “point of no return”, beyond which it is impossible to escape the gravitational effects of the black hole. On April 10, 2019, the National Science Foundation issued a press release revealing the first ever photograph of a black hole. The photograph of black hole at the center of Messier 87 galaxy was made using the Event Horizon Telescope . In the released photograph, a supermassive black hole and its shadow can be seen. Following the release of the photograph, Katie Bouman was credited for developing the first visualization.
Prof Heino Falcke, of Radboud University in the Netherlands, who proposed the experiment, had the idea for the project when he was a PhD student in 1993. At the time, no-one thought it was possible. But he was the first to realise that a certain type of radio emission would be generated close to and all around the black hole, which would be powerful enough to be detected by telescopes on Earth. He also recalled reading a scientific paper from 1973 that suggested that because of their enormous gravity, black holes appear 2.5 times larger than they actually are. These two factors suddenly made the seemingly impossible, possible. After arguing his case for 20 years, Prof Falcke persuaded the European Research Council to fund the project. The National Science Foundation and agencies in East Asia then joined in to bankroll the project to the tune of more than 40m pounds.
No single telescope is powerful enough to image the black hole. So, in the biggest experiment of its kind, Prof Sheperd Doeleman of the Harvard-Smithsonian Centre for Astrophysics led a project to set up a network of eight linked telescopes. Together, they form the Event Horizon Telescope and can be thought of as a planet-sized array of dishes. Each is located high up at a variety of exotic sites, including on volcanoes in Hawaii and Mexico, mountains in Arizona and the Spanish Sierra Nevada, in the Atacama Desert of Chile, and in Antarctica. A team of 200 scientists pointed the networked telescopes towards M87 and scanned its heart over a period of ten days.
The information they gathered was too much to be sent across the internet. Instead, the data was stored on hundreds of hard drives that were flown to central processing centres in Boston, US, and Bonn, Germany, to assemble the information. Katie Bouman, a PhD student at MIT, developed an algorithm that pieced together the data from the EHT (Event Horizon Telescope). Without her contribution the project would not have been possible.
This presented an unprecedented computational challenge: the amount of data collected was so enormous that it had to be physically shipped to a central location, the MIT Haystack observatory, in the form of half a tonne of hard drives. Developing new, sophisticated algorithms was a crucial part of turning the EHT data into an image. These needed to not only combine the data but also filter out noise caused by factors like atmospheric humidity, which warps radio waves, and precisely synchronising the signals captured by the far-flung telescopes. While still studying at MIT, the computer scientist Katie Bouman came up with a new algorithm to stitch together data collected across the EHT network. Bouman went on to lead an elaborate series of tests aimed at ensuring that the EHT’s image was not the result of some form of technical glitch or fluke. At one stage, this involved the collaboration splitting into four separate teams which analysed the data independently until they were absolutely confident of their findings.
“We’re a melting pot of astronomers, physicists, mathematicians and engineers, and that’s what it took to achieve something once thought impossible,” said Bouman.
And finally on April 10, 2019 all the hardwork paid off. Human made history again and took an image of a black hole. One of the main results of the EHT black hole imaging project is a more direct calculation of a black hole’s mass than ever before. Using the EHT, scientists were able to directly observe and measure the radius of M87’s event horizon, or its Schwarzschild radius, and compute the black hole’s mass. That estimate was close to the one derived from a method that uses the motion of orbiting stars — thus validating it as a method of mass estimation. The size and shape of a black hole, which depend on its mass and spin, can be predicted from general relativity equations. General relativity predicts that this silhouette would be roughly circular, but other theories of gravity predict slightly different shapes. The image of M87 shows a circular silhouette, thus lending credibility to Einstein’s theory of general relativity near black holes.
