News from the scientific world

How did we shoot the picture of a black hole?

I am sure everybody already know about the topic of this new post, but today I would like to talk about the first picture of a black hole, the photo described as the picture of the century. But first things first:

What exactly is a black hole?

A black hole is a physical entity with a mass so dense (or compact) that produces a gravity force so strong that nothing around it can escape, not even light. The reason why its gravity is so strong is because matter has been squeezed into an incredibly small space. This can happen when a star is dying, then this kind of black holes are called “stellar” black holes and their mass can be up to 20 times more than the mass of our Sun. There are two more different types of black holes: the “supermassive” ones, with masses of more than 1 million suns together, which scientists have proof to be in every large galaxy, as the Sagittarius A in the center of (our) Milky Way, believed to be formed at the same time as the galaxy they are in; and “small” black holes, that can be as small as just one atom, teeny tiny but with the mass of a mountain, and these are believed to have been formed when the universe began.

Since light can not get out of it, black holes are actually invisible: we can not see black holes because they do not reflect light. However, some telescopes have special features that allow scientists to “see” these mysterious and fascinating physical entities: since we can not directly see the black holes, we observe the light and objects around them. We are able to observe the stars that are very close to the black holes, that act differently than other stars. Indeed, strange things happen around black holes to do with quantum physics and space time, thus affecting the gravity of stars and gasses orbiting around it.

Then, how did we shoot a picture of it?

The picture of the century has been captured thanks to the Event Horizon Telescope project, a global collaboration of over 200 scientists, using radio telescopes around the world, from Hawaii to the South Pole. All these telescopes were recruited and connected, so that they could work as a single telescope with the size of the Earth, and they collected more than 4000 Terabytes of data to “take a picture” of the M87 black hole in April 2017. This black hole is 50 million light years away from us, in a galaxy called Messier 87.

From eventhorizontelescope.org : “Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations of the center of the galaxy M87. The image shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more massive than the Sun. This long-sought image provides the strongest evidence to date for the existence of supermassive black holes and opens a new window onto the study of black holes, their event horizons, and gravity.” Credit: Event Horizon Telescope Collaboration

The picture we all have seen by now (see picture above) is not a single snap, but it is the product of a process called interferometry, that combines the observations from different telescopes in one single picture. Four different teams of scientists independently analysed the massive amount of data collected by these radio telescopes around the world, using different algorithms and verifying them on different models. They built a very large library of simulation models, from which they obtained “synthetic/simulated” pictures produced on the basis of mathematical models that could be seen in reality according to the different scenarios available. They then compared the measurements observed with the ones contained in the library and, since the pictures produced by each team of scientists were very similar, the observations were robust and the final picture we can look at today, 2 years after the “photo” was shot, is the most accurate possible, almost indistinguishable from the simulations produced before.

One of the main results of this observation is a more direct measurement of the black hole mass, very close to the estimations made via the observations of the orbiting stars around it. Moreover, it could be now possible to start properly “observe” black holes, first to verify all the theories previously formulated and then to study astrophysical phenomena that only now we started to observe but we currently can not understand.

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