How to photograph a black hole thousands of light years away

In an article published in The Conversation, Derek Ward-Thompson, professor of astrophysics at Central Lancashire University, explains the process that enabled scientists to take a picture of Sagittarius A *, the black hole in the center of our galaxy.

Let’s put things in context first. The scientist explains to us that black holes were originally investigated as mathematical consequences of Einstein’s general theory of relativity. For many years they had only a theoretical existence, the result of a gravitational collapse that caused a galaxy to form.

One of the Event Horizon network telescopes. Credit: wikipediaCC BY-SA

If until now, most physicists were convinced that our galaxy revolved around a supermassive black hole, there was no concrete evidence that allowed having the heart network.

What is radio astronomy?

Derek Ward-Thompson is part of the team of more than 300 scientists who unveiled the first image of what’s lurking in the heart of our galaxy, and it’s not only a supermassive black hole, but it turns out to be in perfect harmony with Einstein’s predictions. Sagittarius A *, located about 26,000 light-years from our planet, poses no threat to Earth. Its apparent size in our night sky is similar to the size of a donut on the Moon.

To observe this, the researchers used what is called radio astronomy. They used a network of radio telescopes spread around the world, which together form a huge virtual telescope of 10,000 km in diameter. It is thanks to this worldwide collaboration that this surprising picture could be taken.

EHT Radio Telescope Network – Credit: ESO

However, you should know that it is not strictly a photograph. To fully understand what this is all about, you already need to understand what a radio telescope is. With a device of this type, one does not observe by placing one’s eye in an eyepiece, and one does not take pictures with CCD cameras or CMOS sensors of traditional cameras, like amateurs of astronomy and astrophotography.

Radio telescopes make it possible to image celestial bodies (this term is more appropriate) by capturing the radio waves they emit. In the case that interests us, the researchers observed the black hole using very long base interferometry (VLBI), that is, by accumulating the radio signals captured by 8 telescopes. To put it simply, by observing from 8 different locations, we can reproduce the observed object more faithfully by analyzing the differences between the received signals (results of interference that changes the signals during their journey to the telescopes). This results in a more defined image.

The principle of interferometry is also used in other areas, and was used, for example, in 2015 to prove the existence of gravitational waves.

In reality, it is impossible to observe a black hole, as gravity is by definition so strong that not even light can escape. It is therefore invisible, but we can nevertheless observe the deformation of its surroundings and form a luminous ring called the accretion disk, the yellow and orange part of the image.

To arrive at this result, the researchers observed the black hole for many hours over several nights. These multiple observations are similar to the use of a particularly long shutter speed in classical photography. The goal is to collect as much data as possible and then process and format it. Incidentally, this last process took some time as the researchers were unable to meet physically due to COVID-19.

Einstein was right

What most surprised the teams behind this picture is its resemblance to M87 *, the first black hole photographed by this team 3 years ago. A similarity which is explained by the fact that if M87 * is 1000 times larger than Sagittarius A *, but the latter is 100 times closer.

The black hole at the center of the M87 galaxy, more than 50 million light-years from us – Credit: ESO

Both confirm Einstein’s general theory of relativity, which shows that the scientist was right about a factor of 1,000 in the size scale. This further reinforces Einstein’s theory, which has held for more than a century now.

Researchers are preparing many other articles on this photo of Sagittarius A * because we are still far from having withdrawn all the knowledge that this historical image can offer us for humanity.

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