What is a black hole?


A black hole deforms the fabric of space-time. Illustration by Daniela Guzman Angel

Somewhere lurking in the shadows of the universe are regions of space-time where gravity is so strong that nothing can escape. These dots are called black holes, the garbage can of space, and there are potentially hundreds of millions of them in the Milky Way galaxy alone.

Black holes were first emitted by scientists at the 18e century, but it was not until two centuries later that their presence could be explained by science. Although we have been able to observe them directly, we still do not fully understand how they behave.

Directing physics

In 1915, Albert Einstein expanded on his previously published work on special relativity, which brought together the three dimensions of space and one dimension of time in a single geometric model: space-time.

In this new work – known as general relativity – he argued that a sufficiently massive body would distort spacetime, like a bowling ball sitting on a trampoline, and any lighter body that interacted with this deformation would feel the effects of gravity. He also realized that the more mass a body has, the more it distorts space-time. This helps explain how black holes, which are incredibly dense, trap lighter objects – including photons – within them.

How are black holes formed?

Stellar black holes are usually formed through the collapse of a massive star. If the star isn’t massive enough, it will collapse into a white dwarf or neutron star instead. Although the lower limit of the mass that becomes a black hole is not fully understood, it is theorized to be somewhere around triple the mass of the Sun.

Stellar fusion involves the combining of two atoms to form a larger atom with the release of energy.

A star’s collapse occurs when it runs out of fuel, which is created by a process called nuclear fusion. In its simplest form, fusion occurs when a fusion of two atoms, such as hydrogen, in the nucleus of a star results in the formation of a new atom, helium.

Every time two atoms merge, a huge amount of energy is released, which radiates outward in the form of heat, light and thermal pressure. Throughout a star’s life, its outward thermal pressure is balanced with its gravity, keeping the star stable.

Fusion will continue to make elements heavier and heavier: helium melts to make carbon, which melts to make oxygen, which makes neon, which makes silicon. As the elements increase in size, the amount of energy produced by fusion decreases, threatening the equilibrium by reducing the thermal pressure pushing the gravity away from the star.

Finally, the star’s fusion will produce iron, a reaction that consumes energy instead of releasing it. As the iron melting consumes more and more energy from the star, the relative force of gravity inward increases rapidly, causing the star to collapse violently.

What makes a black hole?
The event horizon and the accretion disk

The event horizon is the black hole’s point of no return – anything passing through it would have to move faster than the speed of light to escape the black hole’s gravity. Mathematically, this boundary is physically defined by a relationship between the mass of the black hole, the Newtonian gravitational constant, and the speed of light.

It is important to note here that this is only true if the black hole does not have an electrical charge or angular momentum. This area is also characterized by the accretion disc, an area completely separate from the event horizon.

The accretion disc is shown in the photos and rendered as a luminous halo of matter orbiting the black hole. This material is heated to tens of millions of degrees, causing it to emit different wavelengths of electromagnetic radiation, including visible light.

As this superheated mass orbit, it may appear to an observer on Earth as unbalanced, uneven in brightness due to the flow of matter entering the disk. Another reason for the uneven brightness of the accretion disk is the Relativistic Doppler Effect – the side in orbit towards the observer will be perceived as brighter.

Time dilation

Einstein’s general theory of relativity states that increasing gravity slows down time. This phenomenon is called time dilation, and as an object approaches the event horizon, it will begin to sense the immense gravity of the black hole and the ensuing time dilation.

It will appear to an outside observer red shift, as the wavelengths of light reflected from the object lengthen under the influence of black hole gravity.

For this outside observer, the object will also move slower and slower towards the black hole, eventually disappearing from view when it finally crosses the event horizon. However, the object itself will experience time passing at a normal rate.

Render of a black hole
Black holes are regions of space-time characterized by a luminous halo
Gravitational forces

Black holes are known for the incredible amount of gravity associated with them, and the effects of this gravity on the mass approaching the black hole are quite interesting. If an observer fell towards a black hole with his feet first, he would feel stronger gravitational effects at his feet than at his head, much like we do on Earth. However, since the gravity of a black hole is so strong, this gradient will have bewildering effects.

As the observer gets closer to the black hole, he will undergo a process aptly called spaghetti, where the difference in gravity between their feet and their heads would stretch the viewer lengthwise toward the black hole.

The singularity

A singularity is an area inside a black hole of zero volume and infinite density. Once the mass reaches the singularity, the science of what comes next is unclear, but theoretically it is crushed to infinite density and added to the overall mass of the black hole.

It is important to note that singularities are not necessarily a defining characteristic of black holes, and their location and characteristics depend on the black hole itself. For example, in a black hole with no rotation and no electric charge, the singularity will be at the center of the black hole. However, when the black hole is rotating, the singularity will be ring-shaped (a ring singularity or ringularity, if you will). In different theories of gravity that are not based on the theory of general relativity, there can technically even exist black holes without singularities!

What’s new in black hole research?

In 2016, scientists directly detected the large-scale remnants of two black holes – 29 and 36 times the mass of the Sun – merging together. The collision released gravitational waves, or ripples, into the fabric of space-time. These waves were proposed by Albert Einstein in 1916 following his theory of general relativity. But a century after they were proposed, physicists at the Laser Interferometric Gravitational Wave Observatory detected them, confirming their existence.

In 2019, the first photograph of a black hole was captured by the Event Horizon telescope. Every preexisting image of a black hole was a render or illustration, not an actual image. Capturing a black hole is incredibly difficult given that they appear relatively small due to their distance from Earth. Light signatures detectable from the accretion disk are frequently obscured by clouds of interstellar dust or other astronomical bodies. Instead, astronomers at eight observatories around the world, from Arizona to Antarctica, collected radio wave signatures from a black hole located across the universe and compiled them into a single image.

In 2020, the Nobel Prize in Physics was awarded to three scientists who have studied black holes. Roger Penrose received his share of the prize by confirming that black holes are a consequence of general relativity using complex mathematics. While Einstein originally believed that black holes were theoretical, Penrose’s work proved that they could, in fact, exist in the natural world.

Reinhard Genzel and Andrea Ghez shared the other half of the prize for their work confirming the presence of the supermassive black hole at the center of the Milky Way. Since the 1990s, these astronomers and their teams have been tracking the orbits of stars around the center of our galaxy, and deduced that something invisible and incredibly dense was to blame: a black hole the size of our own. solar system, yet four million times the mass of the Sun.

Black holes are mysterious anomalies, where the most basic theories of physics are pushed to their limits. As our ability to detect and study them becomes more and more advanced, we come closer and closer to their existence.

The Editors of Advanced Science News would like to thank Professor Maria Montserrat Jaurez, associate professor of mathematics and physics at Arkansas State University Campus Querétaro, for his contributions to this article.

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