On 12 May 2022, the Event Horizon Telescope Collaboration published the first-ever image of Sagittarius A* for the eyes of the public. The rendition and publication of the photograph is a historical moment in the world of cosmology.
Most of the people in their late teens or early twenties today have probably grown up with a nominal or greater idea about what a black hole is. After all, the topic of cosmology and black holes in specific has been at the forefront of the amalgamation of modern human curiosity.
A black hole is a place in space where gravity is so intense, that even light can not get out. An incredibly large amount of matter, when squeezed into a tiny space, makes the gravity strong enough to form a black hole. Hence, logically, the mass of a black hole is also far beyond regular human comprehension. Beyond the event horizon, which is also known as the point of no return, there resides a singularity at the centre of a black hole. This singularity is a single-dimensional point in space that is infinitely dense.
Black Holes come in a plethora of sizes, ranging from microscopic to hyper-massive giants – larger than our solar system. Scientists assume that the regular ones are formed during the death of a star but it is still unclear how the giant ones could be formed.
Sagittarius A*, or Sgr A* when abbreviated, is the name of the black hole at the centre of our Milky Way galaxy. It is a part of the Sagittarius constellation and is approximately 26,000 light-years away from Earth.
How is a black hole photographed?
Without the realm of a question, photographing a black hole is not as simple as taking out a camera and snapping a picture. The first question to arise is how can we actually photograph a black hole if no lights escape from the subject? Since a black hole neither emits nor reflects light, it becomes impossible to view it. However, we need not view “it” to be exact. Due to the intense gravitational force and intense pressure, there’s always space dust and gas revolving around the black hole. This dust and gas revolve inwards at nearly the speed of light and friction heat it up. This heating emits energy in the form of radio waves. The Event Horizon Telescope (EHT) picks up these radio waves and registers them as a bright halo, gleaming in vibrant orange, around the black hole.
Another question might arise, which is why did it take so long to take another picture of a black hole? The first-ever photograph of a black hole was captured in 2019 and it featured M87*, a black hole which is 55 million light-years away from us and situated at the centre of a galaxy of the same name. The answer to this is that black holes are so far away that it is a feat in itself to just locate one, especially because they do not give off much light. We should note here that Sgr A* is a lot closer to Earth as compared to M87* but still, the latter was photographed first. This is because the stars and other objects surrounding Sagittarius A* create a very chaotic environment with a lot of light pollution. Because of this reason, it was actually significantly easier to photograph M87* as compared to Sgr A*. Even after that, a black hole like Sgr A* takes up as much space in the sky as would a tennis ball on the surface of the moon as viewed from Earth. To observe and capture the image of an object such as the ball on the moon, we would need a telescope with a mirror roughly the size of the entire Earth. Since this is not possible, we dealt with this issue with ingenuity.
This is where the Event Horizon Telescope comes in. Often referred to as “the Earth-sized telescope” or the “virtual telescope”, it is a feat of scientific innovation. We know that in a discussion about telescopes, size does matter. The bigger the telescope and the mirror, the more the capability of the device. The Hubble Space Telescope has a mirror that measures 2.4m and the Earth-based upcoming Extremely Large Telescope is supposed to boast a primary mirror which would be 39 metres in diameter. However, all of these pale in comparison to The EHT. By linking together 11 telescopes spread across the Earth’s surface, the EHT can emulate a mirror as big as the Earth’s surface. The way in which this virtual telescope works is by programming all the telescopes to observe the same astronomical object for several hours as the Earth is rotating. “At each telescope, the data are recorded on hard disks and are accurately time-tagged by precise atomic clocks” explained Thomas P. Krichbaum of Germany’s Max Planck Institute, at a press conference to announce the findings. According to his statement, the recorded data is then analysed and put together by supercomputers to create the final high-resolution image. This complicated nature of the process of photographing cosmological objects is the reason behind the 3-year gap between the publication of the two photographs of black holes.
What is the significance of this photograph?
The reason why scientists try to photograph astronomical objects such as the black hole is not merely blatant curiosity. Rather, visual imagery is concrete proof in support of what previously were just mathematical assumptions. Firstly, the image of Sgr A* confirms that there indeed is a supermassive black hole residing at the centre of our Milky Way galaxy. There is no denying its size after the photograph. Secondly, the size of the ring around the black hole lined up properly with the predictions made with the use of Einstein’s theory of General Relativity. EHT project scientist Geoffrey Bower from the Institute of Astronomy and Astrophysics, said, “These unprecedented observations have greatly improved our understanding of what happens at the very centre of our Galaxy, and offer new insights on how these giant black holes interact with their surroundings”.
What to expect in this realm of study in the foreseeable future?
As we discuss the EHT and its contribution to the world of cosmological studies, the telescope chain itself is going through several technical upgrades. The observations conducted in March of 2022 included a link of more telescopes than ever before.
Black Holes are a signifier of the extremes of what Einstein’s Theory of General Relativity can predict and the EHT looks forward to scouring them till we observe a point where the theory crumbles and falls apart.