The historic photo of a black hole

by Prof. André Astro 30 Aug

The year 2019 will be remembered forever in history. It was this year that the historic photo of one of the most intriguing astronomical objects in the universe, the black hole, was published. The photo not only consolidates international collaboration as an extremely powerful point for the advancement of the science and evolution of the human being as a community, but also once again empirically proves Einstein’s theories and predictions, made a hundred years ago.

What is a black hole?

Black holes are points in space which, being extremely dense, creates around itself a region with so absurd gravity that even light can not escape. There are basically two types: stellar black holes and supermassive.

A star like our sun, when in “life,” works by balancing forces. Nuclear fusion inside the star avoids the star’s own gravitational collapse due to its large mass. As our star approaches the time of its “death,” it will inflate, lose mass, and then cool down, forming what we call the White Dwarf. But stars with masses of at least ten times the mass of our sun will have another fate.

In the final stage of life these stars explode in a phenomenon known as a supernova, throwing in the space of its mass but leaving behind its dense stellar core. The nucleus, no longer having nuclear fusion to maintain its equilibrium, eventually collapses into itself. When this mass collapses into an infinitely small point, a black hole is born. Gravity is so intense, generated by this uniqueness, that within the distance known as the Schwarzschild ray (see figure below), nothing can escape. This region known as the black hole, by not letting the light escape, is also called the event horizon.

Supermassive black holes, which can be billions of times the size of our sun, inhabit the centers of most galaxies. Two of these cosmic monsters became study objects of the EHT (Event Horizon Telescope) project.

The EHT Project

“We exposed parts of the universe that we thought were invisible to us. We hope you are inspired by this. “Sheperd Doeleman, director of the EHT project.

The EHT project (Event Horizon Telescope) is an international collaborative initiative in which one of the goals is to deepen the study of black hole through observation. It is not a simple task. The telescopes we have are not large and accurate enough to accomplish this. However, the idea of EHT was to create a telescope as large as possible, in this case, the size of the earth itself. To this end, they have linked eight

radio telescopes around the world on four different continents. They installed high-precision atomic clocks (at least a billion times more accurate than a smartphone) to synchronize all receiving antennas to the EHT’s global network. To get a sense, the resolution of this group of radio telescopes is capable of photographing a melon on the moon’s surface. There were over two hundred researchers involved in the project, sixty institutions and a total of twenty countries.

Radio telescopes of the ALMA observatory in Atacama, Chile. Y. Beletsky (LCO) / ESO

Two supermassive black holes were being monitored. Sagittarius A * (Sgr A), the black hole that inhabits the center of our galaxy twenty-six thousand light years from Earth, is estimated to have a mass of approximately 2.6 billion solar masses. The second supermassive black hole is located in the center of the Vigo galaxy (Messier 87), about 6.5 billion times the mass of our sun and fifty-five million light-years away. Although Sagittarius A * is in the same galaxy as us, so it is much closer, we do not have an ideal view. Many cosmic objects and star dust stand between the earth and him. So the picture taken was of the cosmic monster of the galaxy Messier 87.

Brazil scored a point

Historic photo of the black hole, among other things, makes us remember and exalt a chapter of history for many unknown. The year 2019 marks the one hundred years of empirical proof of Einstein’s theories of relativity. On May 29, 1919, Sobral, a small town two hundred and forty kilometers from the capital Fortaleza in Ceará, was in perfect condition for observing a total solar eclipse.

In a joint effort between Brazilian and English scientists, Sobral’s solar eclipse proved the curvature of spacetime tissue by bodies of large mass. With the sunshine being temporarily blocked by the moon, it was possible to photograph the stars who were in the same direction as the sun. This meant that the rays of these stars necessarily passed through the supposed sun-distorted spacetime and were deflected. This deviation could be verified in the eclipse in Sobral. This not only revolutionized our scientific view about the concepts of space and time, but placed in the imagination of man the real possibility of extraordinary cosmic objects such as black hole.

Interpreting the black hole photo

At first glance one might think that the picture above is not that great, it looks like something out of focus. How can scientists study something through this? If something like that crossed your mind, let me guide your eyes and show what’s behind this picture.

To take a picture of something is necessary to capture the light that was emitted or reflected by the object we want to photograph. In the case of the black hole, the radiation emitted with the greatest ability to reach us is the radio wave, due to the high diffraction power, that is, the power to circumvent objects. The photons of light you see in the photo had to depart from the event horizon of a supermassive black hole, cross the accretion disk without being absorbed. After that, travel a distance of sixty thousand light years to leave the galaxy, and travel another 55 million years through intergalactic space to Earth.

Coming to Earth, these photons crossed the greatest danger of all, the Earth’s atmosphere, full of water vapor. For this reason, the radio telescopes used in the EHT project are at high altitude and in dry climates.

After the photons were collected by radio telescopes, 5 Petabytes of information were stored. By way of comparison, it is like putting together all the photos taken by 40,000 people over their entire lives. This information was sent by plane to a processing center, stored on more than half a ton of hard drives, because sending all this information over the Internet would be impracticable. A specific algorithm was produced, led by the PHD team Katherine Bouman, to unite all information from all radio telescopes. It took almost two years for the photo to render, forming the image you saw above.

The bright part represents the accretion disc, a region composed of gas and cosmic dust orbiting the black hole farther than the event horizon. This matter is at the temperature of millions of degrees Celsius and a speed significantly greater than the speed of light. In the innermost part of the photo, still in the illuminated part, is the so-called photon sphere, which delimits the maximum orbit of photons that will not be swallowed by the event horizon. This orbit is about 1.5 times the Schwarzschild radius.

The dark part gives us a sense of the size of the event horizon, as well as the curvature of spacetime around that horizon. In addition, matter around the black hole is moving at relativistic speeds. This means that the part of matter that is approaching Earth will be seen brighter than the part that is moving away from Earth.

The historic publication of the black hole photo extends the frontiers of human knowledge, opens up possibilities for studies that were previously impossible. Above all, it brings a teaching that we must always remember: without collaboration, there is no science.


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