Black holes are supposed to be the darkest regions in the entire universe. And yet, when scientists announced last week that they had, for the first time ever, been able to capture a photograph of a black hole, the image they unveiled was anything but dark. It appeared bright orange and doughnut-shaped in what became one of the most widely circulated images in the last one week. When light cannot escape a black hole, how was the photograph achieved, and what makes the achievement important?
What the image shows
The main subject of the photograph, a black hole located 55 million light-years from Earth, at the centre of a galaxy named Messier 87, was confined to the small and dark central core of the doughnut shape in the image, identifiable only because of the bright surroundings it was enclosed within. This was the only way that a black hole could have been photographed — by capturing the entire area surrounding it. The black hole itself does not emit or radiate light, or any other electromagnetic waves that can be detected by instruments built by human beings. But the area just outside the boundary of the black hole — referred to as event horizon — which has vast amounts of gas, clouds and plasma swirling violently, emit all kinds of radiations, including even visible light.
The outside of the black hole was not easy to be photographed either. The black hole in question had a diameter of 1.5 light-days, or about 40 billion kilometres. The ring outside a black hole usually has 4 to 5 times greater expanse. But the very vast distance from Earth meant that recording anything better than a point-size picture was physically not possible with available instruments. Scientists had calculated that a greater resolution picture, like the one they finally were able to capture, required a telescope whose antenna was as large as Earth itself.
Why it matters
Scientists have been using computer-simulated images of black holes for several years to study these regions. For the first time, they have an actual image. While they appear quite similar, scientists will now start looking closely at the actual image to see whether it differs from the computer-simulated images in the details, and whether these differences could be explained by instrumentation, observation or other errors. This can provide a test for existing theories of the universe, and lead to a better understanding of black holes and the nature of the universe itself.
Choosing the black hole
There was an alternative to photographing the black hole in the M87 galaxy — trying to photograph a black hole that was much nearer. There are thousands, possibly millions, of black holes much nearer to Earth, but not every black hole could be a candidate for being photographed. Scientists were looking for a particular size of black hole, large enough to be captured by instruments available on Earth. The black hole in the M87 galaxy is about 6 billion times the size of the Sun, and one of the biggest ones known. There is no black hole of comparable size nearer to Earth.
There was a candidate nonetheless, in our own Milky Way galaxy. The Sagittarius A* black hole, at the centre of the Milky Way, is about 4.3 million times the size of the Sun, and only 25,000 light-years from Earth. It is about 2,000 times nearer to Earth compared to the one in the M87 galaxy, but also about 1,500 times smaller. In scale, therefore, the two candidate black holes offered similar opportunities to be photographed.
Setting up the telescope
An Earth-size telescope was not something that could be made available. So, scientists had to devise ingenious new methods to overcome the limitations of their instruments. They decided to use eight of the biggest and most sophisticated radio telescopes in the world, and linked them with a technique that could make them act like a virtual Earth-sized telescope. The telescopes made simultaneous recordings of the radiations coming in from the black hole region. Each of the telescopes was fitted with atomic clocks so that their recordings could later be matched with extreme precision.
The individual telescopes each collected the radiation coming in from the black hole region. But because of limitations of size, they all had only very limited information about the black hole. Matching the data recorded by each of these telescopes at exact moments in time gave the scientists some more information, but nothing could be done about the huge amount of information that could not be captured by these telescopes.
Building image from data
It is here that scientists took the help of supercomputers to recreate the full image of the black hole with the limited information that the telescopes had captured. Rebuilding entire pictures with limited data is not unusual. The compression techniques that we use to reduce the sizes of music, image or video files on our computers work on similar principles. We throw away a lot of information while reducing the size, but the computer is still able to recreate the music or video, though with some loss of quality.
Of course, the challenge for the scientists working on the black hole image was more complicated than the techniques used for compression of files. They had a huge amount of amount of data to deal with, and yet extremely limited information directly obtained from the radiation. Not surprisingly, therefore, they had to write entirely new algorithms, using groundbreaking approaches, to regenerate the image.
As a result, a large number of pixels on that photo presented to the world could have been generated by the computer. But they were generated using the information in the pixels that were the result of direct observation of the telescopes, rather than being produced from mathematical models, as happens in a computer-simulated images.
It took two years for some of the world’s fastest supercomputers to process the huge amount of data and recreate the image of the black hole in the M87 galaxy. A photograph of the Sagittarius A* black hole is yet to be released, apparently because the image is not yet ready.