After all, why is detecting light behind a black hole important?

The world of astronomy has come alive once again in recent days! On July 28th, it was published in the magazine nature, one of the most prestigious scientific journals in the world, a new confirmation of Albert Einstein’s theory of General Relativity: the first detection of light behind a black hole! To understand why this is important and exciting, we need to understand a little bit about how gravity works and what black holes really are.

Since Einstein published his famous theory a little over 100 years ago, in 1916, physicists and astronomers have come to understand gravity in a very different way than their classical conception, the one derived from the studies and work of Isaac Newton. According to Newtonian mechanics, gravity is an attractive force existing between any two (or more bodies) that have mass, its intensity being directly proportional to the mass of these bodies, but inversely proportional to the square of the distance separating them. This concept of gravity is even the most common for the general public. However, since the publication of the theory of general relativity (which is also a theory of gravitation) and successive confirmations of its predictions, it is well known that gravity is the manifestation of the curvature of spacetime. Is it a little confusing? It doesn’t matter, let’s take it easy.

Simplified representation of spacetime distortion by a star and a planet.Source:  Techque

Today, it is understood that space and time are no longer two separate and inseparable physical entities. Instead, they were found to be part of the same aspect of nature and to be strongly linked. Now we talk about spacetime! This component is highly sensitive to the presence of matter and, for simplicity, we can think of it as a stretched fabric mesh. If we deposit a bowling ball in its center, the fabric will “feel its presence” and suffer a deformation: the more “heavier” the ball, the greater the deformation caused in the fabric. In a more general scenario, this is a valid analogy to gravity. In the universe, the greater the mass of the celestial body (planet, star, galaxy…) the greater the deformation in space-time around it. Any other celestial body that passes or is close to this region will feel such deformation and, depending on its speed, will enter the body’s orbit. In simple terms, this is the case with the Sun and the planets of the Solar System, with the Earth-Moon system and all the instances where one body orbits another in the universe.

Representation of the functioning of a gravitational lensRepresentation of the functioning of a gravitational lensSource:  NASA, ESA & L. Calcada

Depending on how massive the body is, even light will feel the spacetime deformation around it and will deflect, that is, the light will change its original trajectory and will be curved according to the intensity of the deformation ( or, better said, according to the intensity of the gravitational field). This phenomenon is known as gravitational lensing and is quite common in the universe, especially in gigantic structures such as galaxy clusters.

Distorted light from a galaxy surrounding two distant galaxies creates the impression of a smiling face in space.Distorted light from a galaxy surrounding two distant galaxies creates the impression of a smiling face in space.Source:  NASA/ESA/Hubble

Black holes – a class of celestial object adored by professional astronomers, amateurs and science fiction fans alike – are regions in space that take spacetime warping to an extreme: they are so massive and the warping is so intense that neither not even light can escape its influence. Also, for this reason it is called a black hole, since, as the light that arrives at it never leaves, we cannot see it directly, only its close effects.

In recent years, several important discoveries and observations have been made related to black holes. On the last Wednesday of July (28), astrophysicists from Stanford University, California, United States, reported in an article the first detection of light behind a black hole. In observations of X-rays produced in the accretion disk of a gigantic black hole located at the center of a galaxy 800 million light-years away from us (the Zwicky 18 galaxy I), a series of X-ray flashes and of smaller flashes and of a different color from the flashes. But if all the light that goes into a black hole doesn’t come out, we shouldn’t be seeing anything, right?

Representation of X-ray flashes detected in the black hole, evidence of the distortion of light and magnetic fields around a black hole.Representation of X-ray flashes detected in the black hole, evidence of the distortion of light and magnetic fields around a black hole.Source:  Dan Wilkins/Stanford News

Correct in parts. This is precisely one of the indirect effects we were talking about. The reason why the detection could be done is because the deformation of space-time caused by the black hole, in addition to bending the light, also distorts the surrounding magnetic fields, making the light flashes consistent with the idea of ​​rays. -X reflected behind the black hole.

While the distortion of light in regions near black holes is nothing new, the nature of this detection is unprecedented and adds another piece to the puzzle of the universe. Furthermore, it presents one more evidence in favor of the already well-established theory of general relativity, further strengthening the paths followed in our scientific journey.

Nicolas Oliveira, has a degree in Physics and a Master in Astrophysics. He is a doctoral candidate at the National Observatory, where he researches orphan stars in galaxy clusters. He has experience in Teaching Physics and Astronomy, with research in Extragalactic Astrophysics and Cosmology. It acts as a scientific communicator and disseminator, seeking the popularization and democratization of science

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