Guide to “Photography” of Black Holes

  On the stage, getting married, drinking, taking selfies with drunk lions, taking pictures of food, taking pictures of beautiful scenery… Compared with ordinary people’s enthusiasm for taking pictures of “one machine in hand, I have the world”, scientists may be more keen to take pictures of mysterious black holes – despite the fact that for hundreds of years Never saw it for real. You may retort: ​​”I’ve seen a lot of black hole photos!” In fact, the black hole photos you’ve seen are most likely to be simulated images drawn by scientists and artists based on some data. Although astronomers believe that there are black holes in the center of almost all galaxies, it is really not that easy to take a picture of a black hole!
  We know the “model” of a black hole. Its powerful gravitational field will cause space-time distortion, and even light will be sucked in. It’s really dark, how can you shoot it? Don’t worry, there is a range near the black hole called the event horizon. Nothing inside the event horizon can escape the black hole’s gravity, but light outside the event horizon can still escape. Due to the huge gravitational effect of the black hole, the stars near the black hole will be attracted by the black hole, revolve around it, and will emit light and heat, forming a luminous “belt” called an accretion disk. And what we really want to photograph is the accretion disk outside the event horizon. Next, follow scientists to learn how to “photograph” a black hole!
Black Hole “Photo” Guide Step 1: Find the Device

  In fact, we have two best “models” for taking pictures of black holes: the massive and distant black hole at the center of the M87 galaxy, and the Sagittarius A* black hole at the center of the Milky Way. The former is 55 million light-years away from us, and the latter is the closest supermassive black hole to Earth, also about 26,000 light-years away from us. To photograph them is like standing on Earth and photographing a donut on the moon.
  To photograph the black hole at the center of the M87 galaxy, the diameter of the radio telescope must reach at least 8,000 kilometers, which is much longer than the radius of the earth. Is it necessary to build a telescope larger than the earth? Of course not, smart scientists always have a way: since one telescope cannot reach such a large aperture, can it be achieved by using multiple telescopes? For example, two radio telescopes separated by a certain distance form a network, and they are combined into a “hollowed out” reflecting telescope, and the distance between the two is equivalent to the diameter of the “hollowed out” telescope.
  According to this idea, scientists used very long baseline interferometry technology to combine the Chile-Atacama millimeter telescope wave array, the Hawaii-submillimeter wave telescope in the United States, the South Pole-SPT telescope wave array, the Arizona-SMT telescope in the United States, and the Spanish-millimeter telescope in the United States. Wave telescope, Hawaii-Maxwell telescope, Mexico-millimeter-wave telescope, Chile-Atacama Pathfinder Experiment Telescope These eight radio telescopes or arrays form a virtual, Earth-sized reflecting telescope, the event horizon telescope. This is the first experimental program designed to obtain images of black holes, or EHT (Event Horizon Telescope) for short.
Black Hole “Photo” Guide Step 2: Take a Photo

  In April 2017, the Event Horizon Telescope, which is enough to see a doughnut on the moon, was officially aimed at the black hole at the center of the M87 galaxy and the Sagittarius A* black hole at the center of the Milky Way. The eight radio telescopes are all located at high altitudes. Because they are distributed all over the world, scientists will set an alarm clock to observe black holes at a unified time. Of course, the alarm clock they set is not ordinary at all. It is the famous hydrogen atomic clock, and the error is only 1 second in hundreds of millions of years, which is enough to ensure the synchronization of observations.
  It only takes 1 second to press the shutter, and the EHT project’s shooting of the two black hole “models” lasted only 10 days. The really tough steps are yet to come.
Black Hole “Photographing” Guide Step 3: Develop Photos

  After 10 days of observation, the aggregated data is already huge, almost 3500TB! Such a large amount of memory cannot be transmitted by the network, and can only be transmitted through the hard disk. However, even using a 1TB special hard drive to transfer, it would require 3,500 hard drives, stacked 15 stories high. The huge workload of data processing can be imagined.
  Due to the no-fly period in Antarctica, the data waited for months before being shipped from Antarctica to the Massachusetts Institute of Technology in the United States and the Max-Planck Institute for Radio Astronomy in Germany. It took scientists two years to process the observational data of the black hole at the center of the M87 galaxy alone.
  Just as forensic painters painted criminals through descriptions of victims, multiple data processing teams began to “portrait” the black hole at the center of the M87 galaxy. Each team used supercomputers to synthesize and analyze data, compiled an unprecedented black hole simulation database, strictly compared with the observation results, and calculated and restored the true appearance of the “model” from different angles bit by bit, until thousands of The effect of the average of the photos, the black hole photo is really complete.
  This painting took 2 years. In 2019, a photo of the black hole at the center of the M87 galaxy was exposed. People were pleasantly surprised to find that it resembled a symmetrical doughnut. After another three long years of “flushing”, the first real image of the black hole at the center of our galaxy, or a direct image of the environment around the black hole, is finally presented. The dark black hole is surrounded by a ring of yellow luminous matter. If you look closely, it still looks like a doughnut, but it looks less symmetrical.
  You may be wondering why the Sagittarius A* black hole in the center of the Milky Way is closer to the earth, but the photo is “washed out” 3 years later than the black hole in the center of the distant M87 galaxy? And still so blurry? This is because when a radio telescope observes a black hole, there is a large amount of gas and dust interference, which causes the light to be scattered during observation, like clouds covering the sun, and it looks blurry. Moreover, it takes days to weeks for the gas in the accretion disk around the black hole to revolve around the black hole at the center of the M87 galaxy, but it only takes a few minutes to revolve around the much smaller Sagittarius A* black hole. This means that the brightness and pattern of the gas surrounding the Sagittarius A* black hole is also changing more rapidly, and taking a picture of it is like taking a clear picture of a puppy chasing its tail, and it is prone to ghosting , the “flushing” process is also more difficult.
  Despite the difficulties and time consuming, scientists finally photographed this supermassive black hole at the center of our galaxy, once again confirming Einstein’s general theory of relativity. Now, have you learned how to “photograph” a black hole? It is said that scientists also plan to shoot a “movie” of the black hole at the center of the Milky Way, let’s wait and see.