Black Hole, An Unsolved Mystery

Representative Image Credits-Bronzwaer/Davelaar/Moscibrodzka/Falcke/Radboud University src:eso

The universe is a realm of various galaxies, concealing in it the unsolved mysteries of its creation and evolution of life. One among them is the Black Hole. According to scientists a black hole, as the name suggests, is a cavity in space which sucks in all matter from the surrounding and condenses it at its center. It is one of the most controversial and highly researched mystery that the Universe has.

What is a Black Hole?

A Black Hole, as stated above, is the giant hungry monster which keeps on sucking matter from the surrounding to satiate its hunger. A black hole has a very strong gravitational pull which is capable of tearing matter apart. The gravitational pull is so strong that no matter, not even light, can escape the boundaries of the black hole. The scientists have not been able to practically see a black hole due to lack of a proper telescope(diameter that of the diameter of the earth). But their position in the space is inferred from the stars orbiting them and by the interaction with electromagnetic radiations.

Formation of a Black Hole-

The formation of a black hole is directly related to the death of a star. When a star collapses at its center, it gives birth to a black hole.

A star is formed of plasma, held together by its own gravity. A star survives by deriving all its energy from the nuclear reactions occurring in the core of the star. The star exists as long as the thermodynamic pressure created by the nuclear reactions equals the gravitational pull of the core. When their is no fuel left for the star to produce energy then its center is unable to produce sufficient pressure and the gravity overpowers it. The collapse induces a blast and throws all elements of the star in the space with a velocity of about 30,000 km/s while a black hole is created at the center. This leads to the formation of a Supernova, a bright spot in the universe with brightness greater than the rest of the galaxy it is present in. 

Structure of a Black Hole-

Structure of a Black Hole src:eso

A black hole has an outermost region known as Event Horizon. This is the last point upto which matter can escape. Although this point does not really exist due to continuous change in the outer layer boundary by the emission of radiations(Hawking radiation), it would be better to call the outer boundary to be Apparent Horizon. Any matter which safe until it crosses the event horizon. The event horizon is also the place around which the space-time deforms. This causes a major change in the time lapse of the matter near it to that very far away from the black hole.

The center of the black hole is known as Singularity. This is the place where all matter is condensed down to a single point. This is the place of the black hole which has an infinitely large mass in an infinitely small one dimensional space.

The radius of a black hole is known as Schwarzschild Radius. The radius of a black hole decides its mass and size. The radius is given by the expression-

R=GM/c²

A black hole when sucks in matter from stars or any celestial body moving nearby, all the matter gets distributed along and around the event horizon in form of a circular disk(in case of a rotating black hole). This disk is known as Accretion Disk. It has variable speeds all over its surface. The part of the disk closer to the hole has extremely high speeds ranging from 90-70% of the speed of light. As we move away from the horizon the speed slows down but still is too fast for a body to stay. The matter close to the horizon keeps on heating up due to extension and finally loses all its gravitational energy in the form of electromagnetic radiations, visible through telescopes.

Properties of a Black Hole-

A black hole has various physical properties like- angular momentum, mass and charge. Two black holes having all of these three properties same have been said to be indistinguishable from each other, though no one has a proof of this. Black holes generally spin around their axis and hence contain angular momentum. But some black holes do not spin so they only have two measurable physical properties. Any object falling into a black hole gets all its information distributed on the horizon and lost there forever.

The black holes are also charged. This is inferred from the fact that they repel similar charged matter. The scientists say that they can change the charge on the black hole hence convert its property of taking matter in. But it would require a very large oppositely charged matter to be thrown inside the black hole.

Time Dilation Around a Black Hole-

Representative Image

The black hole has a very interesting property apart from the basic property of sucking in matter. Around the event horizon of the black hole the space-time disturbs and the time passes in a much slower way for an object revolving the black hole. 

Let us consider two astronauts, one moving towards the black hole and other too far from the hole. The astronaut when approaches too close to the black hole then it will be attracted by the gravitational pull. This force is so strong that it pulls the astronaut at a large speed. The astronaut will keep on speeding and elongating due to the difference in the gravitational force at the two ends pointing towards and away from the hole till it reaches the event horizon. On reaching the horizon the astronaut will encircle the horizon with further extension of his body and also rise in the temperature to the end that it starts emitting red light. While to the second astronaut this will look like the time has freezed for the first astronaut. The falling astronaut will experience no such changes in time and it would be moving normally for him according to his watch. This effect is known as Time Dilation effect. Due to this effect if a person orbits the massive black hole for about 5 minutes then it would be equivalent to about 10-20 years(depending on the power of the black hole) on earth. This effect is easily seen on the stars orbiting the black hole. The stars when move close to the hole experience slow passage of time and when they are away from it they tend to experience time passage as normal. Hence they have two different time zones.

Other Theories About Black Hole-

Combination of Black and White Holes

There are some theories also associated with the black hole. The scientists believe that since the black holes suck in matter there must be white holes present which throw out matter. And these two would be working simultaneously, one sucking in matter from one side and the other throwing it out from the other end. These holes are considered to be present as a pair with a common singularity. One side of the singularity is a black hole while the other side has a white hole. If one would fall into the black hole then it would be sucked in with a strong gravitational pull and squeezed up to a point till the singularity and then after crossing it it will be pushed away with a large force till the event horizon of the white hole. Although no one would survive to notice this happening after falling in it.

Therefore, they can be considered to be portals for entering other galaxies or universe. But once they are there, no one would be able to return back and tell what they experienced.