Modern Physics

Black holes

Black holes are celestial bodies formed by an enormous amount of matter , in a space smaller than a single atom , thanks to an intense gravitational attraction , so that not even light escapes from its interior. Black holes were theoretically predicted as a mere solution of the equations of general relativity , however, recent observations have allowed us to discover a lot about these amazing celestial bodies.

What do we know about black holes?

Black holes are the result of some celestial body that has undergone a major gravitational collapse . These celestial bodies can have very different sizes, ranging from a few meters to kilometers, and their masses can be millions of times greater than that of the Sun.

Black holes distort space with their immense gravity , this causes light traveling in their vicinity to be deflected towards them, giving rise to phenomena such as gravitational lensing .

Contrary to what many think, black holes do not suck up all the matter around them, for this to happen, a minimum distance known as the event horizon is required If any piece of matter or even a ray of light enters the event horizon of a black hole, they will be attracted, by an intense gravitational field, towards the center of the black hole, in a region known as a singularity

Theoretically it is not possible to observe anything beyond the event horizon of a black hole , since the escape velocity of this region is higher than the speed of light itself , so what makes the detection of a black hole possible is the presence of accretion disks Accretion disks are formed from gases absorbed by the black hole, when accelerated, the gas particles emit detectable radiation until they enter the event horizon.

To say that nothing escapes a black hole is wrong. According to studies by English physicist Stephen Hawking , we know that black holes emit thermal radiation due to quantum effects. This radiation became known as Hawking radiation , which we will discuss later.

Where do black holes come from?

Black holes appear to be the final stage in the life of supermassive stars . When they reach their last billions of years of life, all the fuel of the supermassive stars becomes scarce, in this way, the gravitational forces tend to compress it more and more.

During this compaction, known as gravitational collapse, the star’s core starts to heat up more and more, thus starting to fuse increasingly heavier elements, such as carbon and iron . At this point, the star’s core becomes unstable and undergoes a violent expansion, flinging all of the star’s outer matter at speeds close to the speed of light , this process is known as a supernova .

After the supernova has occurred, there are two possibilities. The first, when the star is not so massive, it results in a small, super-dense, bright and extremely hot core a few kilometers in diameter and of incredibly high density , in other words, the death of an intermediate-mass star can give rise to a neutron star . At the other extreme, if the star is supermassive, gravitational collapse continues until all the matter in the stellar core is confined to an extremely small region, thus creating a black hole .

Discovery of black holes

Although they had already been imagined in the 18th century, black holes were predicted by the general theory of relativity only in 1916, by Karl Schwarzschild, however, the first direct observation of a black hole was only possible in the year 2019, thanks to an experiment of international collaboration known as the Event Horizon Telescope.

Although the first direct observation of a black hole was so recent, physicists already considered the existence of these celestial bodies long ago, due to indirect observations. Understand some of the evidence that suggested the existence of black holes:

  • On several occasions, telescopes were able to capture images of stars and even planets orbiting “empty” regions of space, without any brightness.
  • In some images captured by telescopes, it is possible to observe the image of the same star several times, this happens because black holes deform space, deflecting the direction of light rays, as a lens would do, that’s why we give the name to this phenomenon of gravitational lens.
  • In the year 2015, the LIGO gravitational wave observatory was able to detect the existence of gravitational waves based on a merger event between two supermassive holes. On this extremely rare occasion, singularities of masses equal to 36 suns and 29 suns collided, resulting in gravitational disturbances that could be detected even here on Earth , billions of light years away .

Stephen Hawking and Black Holes

Stephen Hawking was a British physicist of great importance to our current understanding of the Universe and, specifically, of black holes. According to the theoretical postulates left by him, black holes should be able to emit radiation, even if this contradicts the very concept of a black holeSuch radiation refers to the thermal radiation that escapes from the black hole, since this body is not at absolute zero temperature , being, therefore, liable to emit radiation due to quantum effects that arise at the boundary between space and the horizon of black hole events.

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