Black Holes appear black or dark. If we cannot see them using light, how do we know they exist?

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Black Holes

Gargantua - Interstellar

According to our current understanding, a black hole is a region where particles nearby accelerate at a speed of light towards it, stating that the object had the strongest gravitational pull.

These things have the gravity to pull in even light! So they appear black or dark. 

If we cannot see black holes using light as an electromagnetic wave in the spectrum, how do we know they exist? The answer is by using other kinds of electromagnetic radiation, like X-rays or Infrared rays, we can see its influence on stars nearby like orbiting, or falling into them. The existence of black holes was predicted by German physicist Albert Einstein.

Parts of a black hole

A black hole has many parts:

1. Event Horizon - This is the boundary of the black hole, upon entering it will make you fall towards it at the speed of light (c). This means it will pull you into it and you cannot escape.
2. Accretion Disc - This is the disc of material in orbit around the black hole. This material orbiting will never fall into it but will orbit it endlessly.
3. Photon sphere - This part of the accretion disc is the closest to the event horizon. The gravity is so strong, that the matter gets ionized and travels at high speeds as photons. Since light travels through photons as a wave, light orbits this area of the black hole.
4. Particle Jets - The gravity of black holes sucks in the matter which travels in the form of a particle jet, these are funnel-shaped objects made by matter falling in the black hole.
5. Singularity—This is thought to be present in the black hole, a place where mass is zero but density is infinite, and all matter is squeezed into a single point.

 The singularity has mass = 0 but density = infinite. This breaks the laws of physics because no mass would mean no density. 

Birth of a black hole

According to the theory of relativity, an object bends the fabric of space-time. You can think of a ball on a rubber sheet. When a star bends the space-time, it distorts the space-time on its sides. When smaller objects come into the distorted space, they revolve around the star. This is how the earth revolves around the sun, as well as the moon revolves around the earth. When we compress an object, its 'dent' becomes deeper. As the object becomes smaller than an atom, it becomes a black hole. The distorted space now stretches light-years long. The black hole now has infinite density and a lot of mass. A black hole smaller than the Earth may have a mass of ~2.5 solar masses (a mass corresponding to our son's mass). But there is no star currently existing to form a supermassive black hole. This is still a mystery.

Black hole information paradox

Every object in the universe is recorded in subatomic particles as information. This information can never be created or destroyed. But as black holes suck objects, this forms the black hole information paradox. The information might be destroyed in the black hole. It might be mixed with other information too. It might be sending the information to a parallel universe. We don't know what's happening to the information in the black hole.

Black hole facts

Every galaxy has a supermassive black hole in its centre (~1,000,000,000 Solar masses). This makes the centre glow brightly. Thus, the galaxies are called Active Galaxies. The black hole in our galaxy, The Milky Way, is called Sagittarius A* (~4,000,000,000 solar masses). 

Messier 87 Black Hole (by Google)

This is the first image of a black hole captured by Katie Bouman using Einstein's algorithm. This is the quasar of a galaxy M87 (Messier 87). This is ~6,500,000,000 solar masses. The project is known as The Black Hole Imaging Project.

The largest known black hole is TON 618, a quasar 10 billion light-years away.

Death of a black hole

We thought that black holes were eternal. They would eventually eat up everything in the universe, including themselves. Stephen Hawking theorised his theory, which says particles and their corresponding antiparticles are continuously formed in the universe. These particles and antiparticles eventually will annihilate each other. Their mass is equal and proportional. If one falls into a black hole, its corresponding particle gets the mass of the black hole and flies out as radiation. Thus, the black hole is losing mass. This process gets faster as the black hole grows smaller and smaller and the black hole finally explodes and releases the mass they had eaten before.

Thank you for reading.

Aditya VN Kadiyala