Twice in 2022 the newsstands were filled with magazines and newspapers with pictures of an orange doughnut! And everybody says that this is a picture of a black hole. Is a doughnut the black hole after all? And orange? Can I understand what I see? Where is the black hole?
Usually, the advent of a new telescope with better and more modern means of detection leads to unprecedented discoveries. So it was with the Event Horizon Telescope. This is a telescope that “photographs” objects in the radio part of the electromagnetic spectrum. It consists of a large number of coupled radio telescopes. The coupling is such that it is as if we have a huge telescope with a diameter equal to the diameter of the earth. This is currently the best telescope we have now (in 2022) for the most accurate observation of a black hole.
Two well-known black holes were recently observed with the Event Horizon Telescope. One is located at the centre of a known “active” galaxy called M87. M87 is located at a distance of about 53 million light years from us. The other black hole photographed is the one at the centre of our own galaxy, which is in Sagittarius A. These observations, after appropriate processing, led to the orange doughnuts we saw in magazines and newspapers.
The central dark area in the photo is the “shadow” of the black hole, i.e. an area from which we do not receive light. It should be noted that the event horizon, i.e., the “boundary” of the black hole beyond which there is no return, is much smaller than the shadow. However, most photons in the region near the event horizon follow paths that eventually lead them into the black hole, so the region appears dark.
The next picture compares the photos of two black holes, the active galaxy M87 and the one in the centre of our galaxy in Sagittarius A (Srg A). For an easy comparison with understandable sizes the photo shows the extent of the orbit of the dwarf planet Pluto and the size of our Sun.
To illustrate the importance of this achievement, we can say that the accuracy of this photograph is equivalent to taking a picture from Earth of an astronaut’s palm on the surface of the Moon!