Information Paradox

What is information and what is a Paradox?

  We now know that a black hole's grasp is very strong, and once it catches something, it never lets go of it, nor does it leave anything behind, not even information. So, we will dig deep into it in this article. So, what is this information, or here we will consider physical information? Physical information is a type of information which refers to information about a portion of a physical universe, or in general, we can say that information is that thing which describes an uncertain something. Now we know what we understand by information, so now let us know what a paradox is. Simply saying "paradox" is a contradictory statement.

Continuing our real question 

    Coming back to our real question, what is an information paradox? The paradox says that as nothing can deal with the grasp of a blackhole, then some piece of information once taken in by a blackhole can never be retrieved back, but scientists relating to quantum mechanics say that information can neither be created nor be destroyed, which contradicts the above statement, which gives rise to the term we know as paradox. So, let us take an example to understand the information paradox. Imagine you wrote a whole big book with a lot of information about a project you are working on. Ultimately, this information which you wrote in the book will be inherent in the molecular makeup of the pages and the ink upon them. There is no information without physical manifestation in this world. Now just imagine that your book catches fire and burns down the whole book. Now what for you and me? All the information about your project is lost. But that's from your perspective; from the perspective of physics, particularly quantum physics, things aren't going to be so bad.

     As a physical object or a system, the book is described by a mathematical object called a wave function. As it burns, changing its shape and consistency, the wave function changes accordingly. At the end, the wave function describes the ashes that remain. No information is lost in the process; it is simply transformed from the hard book into ashes. In theory, you can run the evolution of the wave function backwards and recover all the original information from the wave function about the ashes. But this is all on paper only. This can’t be shown in real life. This theory will only sound good in its own limited way. But now imagine your book has been squeezed to a very very small size, keeping the density and mass constant. At some point, your book will collapse into a black hole and the gravitational force will be extremely strong. As the size decreases, the gravitational pull increases. Now we are dealing with the black hole that contains all the information about your ongoing project. Now it's just a simple black hole and it has no relation to your information in real life. You just have to assume that it has your information within it.

 According to great scientist Stephen Hawking, particles and anti-particles pop out in this universe to annihilate each other and to cancel out their effects. But at the event horizon, the particle-antiparticle pair separates, and the member of the pair closest to the event horizon falls into the black hole while the other one escapes. That means the black hole evaporates and is continuously emitting radiation, but the emission of radiation does not depend on what is fed to the black hole. That means all the information that was inherited by the book containing the information about the project is lost for ever. So that is the information paradox in which Hawking says that the black hole eats all the information while quantum mechanics says information can never be lost.

 Stephen Hawking’s Bet….

    This was the theory proposed by Stephen Hawking, and he was so sure about his theory that he and Kip Thorne had a bet about this paradox with John Preskill. Although the bet was just a fun bet, the reward was their pride, and they decided that the loser would reward the winner with an encyclopedia of the winner’s choice. After 7 years of bets, Hawking conceded his bet and said that he now believes that black holes also transmit information after reading the reports of physicist Juan Maldacena, and as promised, he rewarded John Preskill with an encyclopedia of his interest, and that was Total Baseball, The Ultimate Baseball Encyclopedia. But still, John Preskill never concealed the bet and refused it in front of the jury, so this bet is still an unsolved mystery and another paradox remains a paradox.

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A brief introduction to time travel

What is time ?

      Earlier, before Einstein, "time" was considered to be a constant quantity, but Einstein said that time is an illusion, it is relative, and it is different for different observers, speed, and space. He considered time as a free dimension, a fourth dimension. If we consider 3 dimensions, then they are length, width, and height. All three together define space, and the fourth dimension is time, which defines the direction to it, which is specifically only in the forward direction. So, all together, is the four-dimensional fabric known as space-time. According to Albert Einstein’s theory of relativity, time slows down or moves faster as we approach the speed of light. When a four-dimensional piece of fabric is introduced with a piece of mass, then it produces a curve or dimple or groove in it. This curve, or bending, allows the object to move in the curve. This curve represents gravity.

    Gravity plays a vital role in time. Time is inversely proportional to gravity, i.e., the stronger the gravity, the slower the time will move, and the weaker the gravity, the faster the time. So, when you stand for a minute, your head will be nearly 9.5 femtoseconds younger than your feet, as gravity is a tiny bit weaker on your head as compared to your feet. As a result, the clocks on the satellites are calibrated so that they correspond to the time on Earth. From all this, we could say gravity can slow down or move fast.

Paradoxes related to time travel

Bootstrap paradox:

     According to this, an object, person, or piece of information can be stuck in an endless loop in which its existence and origin are not possible to define. Consider this: you are working when suddenly the doorbell rings. When you open the door, you receive a mysterious packet that contains a book that explains all the possible ways by which you can travel through time, i.e., an ideal way to create a time machine. After understanding all these concepts, you start to build the time machine. It took you almost 15 years to build this. At last, after these endless trials, you can now travel back to the past or to the future. Now, what you do is grab that same book and travel back to the past on the same day when you received it, keeping the book on your doorstep. That means you were the one who gave the book to yourself. But then, from where the book originally came from, the book will now move in an endless loop, the origin of which is unknown.

                                                                                                                                                                                                                                               

  This all means that you take all the proofs and explanations of Newton's laws and go back to the past before Newton was even born and published the papers explaining these laws. At present time, Newton's laws exist and are known under the name Newton, but you published them. That means now you are Newton. This was an example of a person stuck in an endless loop.

   The example of information stuck in an endless loop can be explained by this simple example. Consider that you went to the past and explained to Albert Einstein the theory of General and Special Relativity, and then at present you learn that in your college that means the piece of information about General and Special Relativity is now stuck in an endless loop.  

Grandfather Paradox:

    It is a paradox about time travel in which inconsistencies appear due to changes in the past. In this paradox, you go to the past and make changes that affect your present. Assume you went to your past when your grandfather was young and you killed your grandfather. This resulted in your father or mother not being born. That means you are not born. That means you can’t go back to the past to kill your. As a result, your grandfather is still alive. That means you are born, you travel back in time, and this endless loop continues. Due to this paradox, many scientists have started to believe that time travel is not possible on logical grounds.

Grandfather Paradox

                            

Black holes and time travel

    Many scientists believe that black holes are the primary and major mode of time travel. The gravitational force in a black hole is very very strong. That means time travels very slowly near it. The stronger the gravitational force, the slower time moves. Assume you're in a spaceship on its way to a black hole, and your twin is on Earth. Now you rotate around the black hole for five years. As now, the gravitational force is strong and time moves slowly for you, your ageing will also slow down. When you return to earth in five years, your twin could be 10 years older than you. So, it can be a way of travelling through time using gravity.

What is a Wormhole and how it is possible to travel through time?

  A wormhole is a tunnel that connects two distinct points in space or can extend up to two universes through space-time curvature. In fact, it provides the shortest path between two points without even having to travel that distance.

Considering a wormhole, space can be visualized as a two-dimensional surface. In this case, a wormhole would appear as a hole in that surface, lead into a 3D tube (the inside surface of a cylinder), then re-emerge at another location on the 2D surface with a hole similar to the entrance. An actual wormhole would be analogous to this, but with the spatial dimensions raised by one. For example, instead of circular holes on a 2D-plane, the entry and exit points could be visualized as spheres in 3D space.

Wormhole connects two space time

  One of the theories suggests that at the end of a black hole at the singularity, the singularity of a white hole is connected, i.e., it’s a path where the entrance is from the black hole end and the exit is through the white hole end. But, it’s all just a theory, but that could be possible.

 Finally, we know that all these concepts are still on paper only. It is still not possible in our real world, but it will not be impossible in the future. We will be able to travel through time.

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A Miniature Black Hole on Earth

What we know so far ?

   Till now, we were studying black holes that were far away from us, that is, at a distance in light years, and it was impossible for us to reach near to them. We always studied them alone. Let’s study black holes together with our own planet, "Earth." Scientists believe that there are black holes, particularly stellar black holes, that are the size of an atom or less all around the Earth. These black holes are so small that they can only trap small atoms or molecules and force them to revolve around them as the electrons revolve around the protons. These mini black holes are known as Gravitational Equivalents of an Atom, or GEAs. These mini black holes seem to be formed at the beginning of the universe and, due to continuous cooling and heating, these black holes are formed.

What we are going to consider ?

    As we all know, there are many types of black holes according to their size. So we'll look at what happens if a black hole with a radius of about one centimeter collides with our planet. Now, black holes are super dense, which means our hypothetical assumed black hole’s mass will be almost six times the mass of the earth and its gravitational force will be a million to billion times greater than the earth. Our scientists attempted it in 2008 in the Large Hadron Collider under the supervision of Stephen Hawking, where small atoms were accelerated to collide and produce enough energy to create a miniature black hole. It was put forward that if this experiment goes successfully, very small black holes will be formed all around the globe. But this would have been the way to destruction created by humans, but as we are still alive, that means this experiment was unsuccessful as the collision of atoms used to radiate a huge amount of hawking radiation, meaning not a single black hole has been formed. The Large Hadron Collider requires a very large amount of energy to create a black hole, and in 2008 the energy was 1000 times less to create a small black hole.

How will it devastate our planet ?

    A black hole this small is also so strong that it can rip off the entire Earth. First, the small black hole on the earth's surface begins to more on the earth's surface will create a super strong gravitational force with a diameter of about 10 meters and will begin to pull everything around it. Because a dense object is present on Earth, it increases the gravitational pull of the Earth, which affects the orbit of the moon, causing the moon's circular orbit to become elliptical. This will cause a great deal of damage to the earth as the moon is the main cause of the lunar tides and the moon's appearance so close to the earth will affect the tides, which can destroy cities in a very short period of time. Slowly, everything on earth will rip off, and the miniature black hole will start to move towards the core of the black hole. It will take him less than an hour from when it starts to move towards the surface to reach the center. As soon as it reaches the centre of the earth, it will start to move towards the other end of the earth. At the other end of the earth, the black hole will now start rotating around the earth, absorbing all the mass of the earth. Many parts of the black hole will be absorbed, but the parts that remain will start to rotate around the black hole. The Earth will then be nothing more than a disc rotating around the black hole. The temperature of this disc will be very high, so high that it will match the temperature of the sun and will continuously radiate energy. So, will this be the end of humanity? Will such a small thing be able to destroy such brilliant minds on the planet?

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Collision: Destruction or Creation

   We still know how giant and strong these black holes are. This was all for about a single black hole, but now imagine what would happen if a black hole collided with another black hole of about the same mass. What will be the result of this destruction? Will it be a very huge big bang or the creation of a new beast? So, let’s find out what will happen.

Binary black holes.

     A binary black hole system is a pair of black holes revolving around a particular point that is its epicenter. So, is the collision of all the black holes the same? The answer is no, as when different sizes of black holes collide with each other, the resultant energy released is different. That is, the collision of two stellar black holes will result in a different amount of energy being released than the collision of two supermassive black holes. So, there are different types of collisions. One type of collision is the collision of two stellar black holes formed by the revolution of the remains of two heavily dense stars revolving around a particular point. Then there is the collision of two galaxies that results in the collision of two super massive black holes. So, all these black holes before colliding revolve around each other, and this system is coined as the binary black hole system.

 Process of collision.

    Imagine two black holes of nearly 80 solar masses each. That is the mass of 80 suns densely packed together into an area that is the size of Japan. Then what would happen is the formation of a black hole. Now, imagine two black holes, the second one similar to the first one we discussed earlier, approaching each other. What will happen? Earlier, when it was not observed, it was very difficult to locate a black hole precisely where it was and how far it was from us, because the black hole does not give any signs of its existence due to its terrible gravitational force. But when two black holes collide, an immense amount of energy in the form of gravitational waves is released in the space and time whose energy has been calculated by the use of general relativity. It is said that the collision of two black holes results in the strongest and largest production of gravitational waves. These are the only quantities that remain the same in all dimensions, so it is easy to detect them. As the gravitational waves are released, the orbit of a black hole decays, and the period of the orbit also decreases. This whole phenomenon is known as an inspiral. Once it happens, the back holes start to merge into each other, and finally a single black hole is formed. In this process, as two black holes combine, the total mass of the black hole is not the same as the addition of the masses of two black holes, as at the time of collision, a huge amount of energy is released in the form of gravitational waves. So some amount of mass is converted into energy in the form of gravitational waves and released.

So, what ones happened is….

    On the 14th of September 2015, approximately 1.4 billion light-years from Earth, two black holes spiraled around each other and, after some time, collided, creating waves in the fabric of space-time. These waves, known as gravitational waves, arrived at Earth and were observed and announced by Virgo and LIGO in February 2016. The readings given by LIGO were approximately and more accurately correct and were accurate to the general relativity prediction of two massive bodies spiraling inwards towards each other, having asses approximately equal to 36 and 29 solar masses, which finally emerged into a single black hole. The signal that was detected was named GW150914, which says, Gravitational waves noticed on September 14, 2015. It was the first time an observation of the merging or collision of black holes was observed and put forward with proof. This gave a clear response that the collision of stellar black holes still occurs in this age and era. Earlier, gravitational waves had only been inferred indirectly, via their effect on the timing of pulsars in binary star systems. The news of the first direct observation was spread around the world as a remarkable achievement for many reasons. Many efforts have been made in the last fifty-five years to show the existence of such waves, and the waves are so small that Albert Einstein himself never thought that they could ever be detected. But, this gave the boost to the scientists and researchers that they needed for a long time to discover many more things about them, and the day is not so far when we could actually go near this beast.

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Entropy of Black hole

Introducing thermodynamics to Black holes.

Entropy is the term which when we read about it leads us to the path towards thermodynamics. In the early 1800s, researchers and scientists started studying heat, temperature, and the behaviour of gases, which later evolved into thermodynamics. According to thermodynamics and the famous three laws of thermodynamics, it says that:

• The zeroth law states that if two bodies are each in thermal equilibrium with a third body, then the first two bodies are also in thermal equilibrium with each other.
• The first law states that the total energy of an isolated system always remains constant. It can only transform from one state to another but never be destroyed.
• The second law states that the change in the entropy of the entire universe can never be negative.
• The third law states that the entropy of a system at absolute zero is a well-defined constant.

So, considering all this above, many controversies and paradoxes arise when we try to apply them to black holes. As black holes have mass, rotation, and temperature, it is obvious for them to have entropy, so as the second law states that (the total energy of an isolated system always remains constant, it can only transform from one state to another but never be destroyed). The energy of a black hole should always remain constant, but if you could throw an object (with a considerable amount of entropy) into a black hole, the entropy would simply go away. It would vanish nowhere. In other words, the entropy of the system would get smaller and smaller, which would violate the second law of thermodynamics. Considering another situation is that the classical black hole has a temperature of absolute zero. This means you could take a bucket full of hot water and throw it into a black hole, which would essentially be cooling an object to absolute zero. It is a violation of the third law of thermodynamics.

Bekenstein-Hawking entropy :

 Bekenstein-Hawking entropy, also known as black hole entropy, is the amount of entropy that a black hole must have in order to obey thermodynamic laws as interpreted by observers outside the black hole. A black hole can be formed in many ways . After it settles down, space and time outside are described by only M and J. The radiation it emits is essentially thermal. It can’t depend on the information inside without violating causality or locality. 

There are several ways to justify the entropy of a black hole.

• Considering the loss of signal with a body outside the black hole, when a body enters into a black hole, it is the same as the loss of information, and in ordinary physics, entropy is the measure of the loss of information. Hence, entropy can be defined for a black hole.
• A black hole is usually formed from the collapse of matter under its own gravity or radiation. Both the terms which relate to the formation of a black hole, i.e., matter and radiation, are associated with entropy. However, the black hole’s matter inside is unknown to the observer outside the black hole. Thus, a thermodynamic layout of the collapse from that observer's point of view cannot be based on the entropy of that matter or radiation (the key roles in the formation of a black hole) because these are unobservable. Associating entropy with the black hole provides a handle on thermodynamics.

 

Formulation for a concrete formula for entropy…

There is a need for a concrete formulation to describe the entropy of a black hole, but from the above discussion it is clear that only the observable parameters can be considered for the formulation of black holes. So the major observable parameters were mass, angular momentum, and electric charge. So, taking the area theorem into consideration, all these parameters come into the same combination as that which represents the surface area of a black hole. The area theorem states that the surface area of a black hole can’t decrease; it can only increase in black hole transformation. So, the final formulation provided as a solution to all these is

                                                   

Where,

             A represents the surface area of black hole.

             G Newton's gravity constant.

             h the Planck-Dirac constant (h/(2π)).

             c speed of light.

   For, Schwarzschild or spherically symmetric black hole the horizon's radius is 

So,                      

                                     A=16π(GM/c2)2

Hence, the considerable efforts to make all the parameters fit to make a defined formulation to find the entropy of black was possible, but still, much more research is going on to find more reasonable formulations in the present and future.

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Accretion Disc.

Discovering the nature around the black hole.

    One of the most important parts of a black hole by which we can actually locate or see a black hole is the accretion disc. So, what is this disc? The answer to this question is very simple. It is a disc formed by diffused matter around a heavy, dense, or massive central body, such as a black hole. So, now it is clear that the disc contains only diffused material, but then why does it glow so much? The reason is that, due to friction, the diffused material is pulled inward towards the central body, and under this immense pressure and friction, it gets compressed and its temperature increases to a very high level, a level so high that the diffused material appears to glow and it emits electromagnetic radiation, and the frequency of the waves depends on the mass of the central body.

What physics says about it,

    The physical definition of a black hole is when matter accreted has enough rotational and angular momentum to prevent it from falling inward into its own accretor. As black holes take everything inside them once it is trapped, there is no possibility That light can also escape its grasp, so like its name, it is pitch black, so it is very difficult to detect. But once the black hole is fed by its vicinity, it becomes the brightest in the cosmos.  There are many ways for black holes to light up their cosmic surroundings. Some of the black holes require very special circumstances, but one that is universal is that whenever matter falls into a black hole, there is the production of thermal radiation. Matter falling towards a central object under the influence of gravity gets accelerated to higher and higher speeds and travels faster and faster, gaining more and more kinetic energy. But once a particle of falling matter plunges into an accretion disc – and possibly earlier – the particle’s motion is disturbed. So, due to these frequent collisions, there is no specified orbit of rotation.

 The entire motion is chaotic; none of the particles follow a definite path. However, this is typical of an accretion disc. As the particles fall inwards, motion becomes chaotic, and matter in the accretion disc is heated to very high temperatures, the temperature of which is far beyond our imagination. The maximum temperature in an accretion disc around a super massive black hole, which is about a hundred times the mass of our sun, will be around one million kelvin, and for the disc around a stellar black hole, it can be up to a factor of a hundred higher than the super massive one. By comparison, the temperature in the core of our sun amounts to about 15 million Kelvin, so by this we can get an idea of how high the temperature of the accretion disc can be. In physics, wherever there is a reference to heat, there is always a word about thermal electromagnetic radiation. Everyone emits thermal radiation in some way because, according to the law, the heat in a body cannot remain constant if it is to be released or exchanged with the environment over its entire lifetime. Only a body with an absolute zero temperature would not, but such bodies do not exist in this world. As the temperature of the body increases, so does the energy emitted in the form of radiation. The temperature of an accretion disc around a black hole is high enough for the disc matter to emit large amounts of highly energetic X-rays.

Concluding it with the help of quasars

    These accretion discs are also known as quasars. Quasars are the oldest known bodies in the universe and (with the exception of gamma ray bursts) the most distant objects we can see until now, as well as the brightest and most massive, outshining trillions of stars. So, all that we see in the accretion discs are small quasars which collide with each other, releasing an enormous amount of heat and energy. Therefore, the black holes would have been hard to detect and our research into them would not have been to its level where it is now if there had been no accretion disc around the black hole. They are the source of light or vision for us to see it.

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Singularity

Concept Of Singularity:

     So, when we read about black holes, the most common question is, "What is a gravitational singularity?" Then the answer to this most frequently asked question is very simple. The term singularity means the single center point of black holes, which is one-dimensional and contains a huge amount of mass in that infinitely small point, where the density and gravitation become infinite. It is a region where the spacetime curvature becomes infinite. It is predicted that any star reaching beyond its point of limit or a certain point in its mass would exert a gravitational force so strong that it would collapse under its own gravity. For a non-revolving black hole, this region takes the shape of a single point, and for a revolving black hole, it is smudged out to form a ring singularity that lies in the plane of rotation. In both aspects, the singular region has zero volume. It can also be shown that the singular area contains all the mass of the black hole solution. The singular area can thus be thought of as having infinite density.

Is there only one type of singularity ?

  Singularities are of many types, arising from different characteristics of the theories from which they emerge, such as according to different shapes. As a result, the singularity can be classified as conical or curved. So let’s see both one by one.

   Conical Singularity

     A conical singularity occurs when there is a point in space where the limit of every general invariant quantity is finite. Due to this, space-time is not smooth at the point of limit. So, it looks like a world where all space-time surrounds the conical tip that is the point of singularity. The Schwarzschild Black is a very good example of this type. An example of such a conical singularity is a cosmic string, a type of hypothetical one-dimensional point that is believed to have formed during the early Universe.

Curvature Singularity

     What happens is that while solving the equations of general relativity or of gravitational equations, we come up with a function that defines the distance between each pair of elements of a set, which is also called metric points tending to infinity. So sometimes it happens that these points are completely smooth, which is found by the continuous derivative of the function.

In order to test whether there is a singularity at a certain point, one must check whether at this point the general covariance quantity becomes finite. Such quantities are the same in every coordinate system, so these infinities will not "go away" with a change of coordinates. A Curvature Singularity is the best example of a black hole. At the centre of a black hole, space-time becomes a one-dimensional point, which contains a huge mass densely packed into a point. As a result, gravity becomes infinite, space-time curves infinitely, and the laws of physics as we know them cease to function.

Another type of singularity is a naked singularity, which is one that is not hidden behind an event horizon. In this case, what actually transpires within a black hole would be visible. Such a singularity would theoretically be what existed prior to the concept very famously known as the Big Bang. The major, or essential word here, is theoretical, as it remains a mystery what these objects would look like. Singularities play a major part in the existence of singularities. The singularity of space-time can also be defined as an indefinite or incomplete path which does not have any end or beyond which we don’t know what exists.

But does it really exist ?

    In the real universe, no black hole has singularities. In general, singularities are the non-physical mathematical results of some imperfect physical theory. When scientists and researchers talk about black hole singularities, they are talking about the errors that appear in the current theories and not about objects that actually exist. A Singularity can be seen in this light, so what exactly is it? It is that point where the mass is infinite at an infinitely small point, but in our real world, the term infinite does not exist. Whenever an infinity term comes up in a theory, it is simply a reason that your theory is too simple to handle extreme cases. So let us consider a case. Assume that we are continuously supplying heat to a particular glass plate. According to theories, what should happen is that the glass plate must heat up to infinity, but in reality, the case is different. The glass plate will melt after reaching a particular temperature, which is its melting point. So, this might show that the singularity term is just on the pages.

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The Event Horizon.

So, what is the event horizon…?

       Black holes are still one of the mysterious things that exist and attract everything inside them that crosses the boundary. This boundary is known as the "event horizon. An event horizon, a term associated with a black hole, is a point from which the gravitational attraction is so strong that even light can't escape through it. An object which crosses this boundary or approaches towards it is observed to be moving slower and slower and never appears to pass through the event horizon. The object appears stretched and, due to redshift, appears redder as it moves towards the black hole. This point is also called a point of no return, as from here if an object wants to return, its escape velocity needs to be greater than the speed of light. That is, it needs to travel against the pull of a black hole in the opposite direction at a speed greater than the speed of light.

What does the past say...?

  So, in 1784, the Newtonian theory of gravitation and the particle theory of light were dominant, and they proposed that if the required escape speed was greater than the speed of light, light originating inside or from such a distance of super massive objects could escape temporarily but would return. But, at the same time, John Michell proposed that near super massive objects, the pull of gravitation would increase to such an extent that not even light could escape. After a long time, in 1958, David Finkelstein, using General Relativity, proposed that the boundary, i.e., the event horizon, is the final boundary of a black hole, beyond which nothing can escape the attraction of black holes.

The future and past for the event horizon….

Black holes are generally final in nature; that is, we need to know all the future space-time of a black hole to identify its correct and current location, which is practically impossible. Generally, the model suggested earlier for black holes has

It does not tell us that black holes rotate and have a fixed point singularity, but now the present situation is different and proposes that the black hole has a spin. Because of this, the singularity does not remain as a point, but now it is a collection of compact discs. Because of this  the event horizons of rotating black holes appear uneven and also squashed at the poles and bulging at their equators. As presented by today's situation, a rotating black hole is surrounded by a region of space-time in which it is impossible to stand erect in the same position, called the ergo sphere. This is due to a process known as frame-dragging, which says that any rotating mass tends to drag slightly along the space-time surrounding it. But still, space-time in it is technically pulled a bit faster than the speed of light.

The flow of time in the event horizon.

Due to extreme gravitational forces, it has some effect on time. Time inside the boundary of the event horizon slows down as gravitation is inversely proportional to time. This phenomenon is known as gravitational time dilation, relative to observers outside the field. So, for a person inside the event horizon and a person outside it, time moves differently. The person inside the event horizon is never seen to be moving by the person outside because the time difference is huge. So, the denser the black hole is, the greater the pull of gravitational attraction and the slower time moves. As shown in the film Interstellar, the main character discovers different planets around a black hole, and the time spent on each planet differs significantly from the time spent on Earth.

The final conclusion to sum up.

  Some virtual particles exist at the boundary of the event horizon. These particles are continuously divided. That means half of the particles are drawn inside the black hole and half of them move out of the black hole and become real particles. The particles that move inside the black hole have negative energy, which combines with the positive energy of the black hole, resulting in the loss of energy. This process is slow at the start, but as the size of the black hole becomes smaller, the process catches up in speed, but still, with respect to time, this process is very slow.

  Schwarzschild's radius is the radius to which an object needs to be squeezed, keeping its mass constant so that it collapses under its own gravitational field. As an example, to convert the earth into a black hole, we need to squeeze it to the size of a marble with an inch diameter. The radius which it defines is the radius of the event horizon from the singularity to the boundary. The objects far away from the event horizon move according to their own space-time, but as they approach the event horizon, the black hole sucks up all the space-time and the motion of the object is inevitable the same as time. It can now move only in one direction, and that is inside the black hole. But the observer never sees it cross the event horizon because as we get closer to the black hole, the gravitational force increases and time slows down, so it takes an infinite amount of time for the object to cross the event horizon.

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Types of Black Holes

As observed by scientists and astronomers, all black holes are not exactly the same. Black holes are completely classified by only three conditions: mass, rotation, and charge.

Classification black holes according to rotation and charge :

1. Schwarzschild Black Holes
   
   

Non-rotating black holes are called Schwarzschild black holes. These black holes don't have rotating cores and have two main properties: a singularity and an event horizon. These holes don't have any electrical charge. It is characterized solely by its mass.

2. Kerr's Black Holes:

Rotating black holes are termed "Kerr black holes. These black holes rotate because the object that collided into the black hole was originally rotating. 

They have four main properties: 

• A Singularity 
• An Event horizon
• The Ergosphere
• The Static limit. 

There is no presence of electrical charge in these holes.

3. Black Holes with Charges:

There are two types: A charged and non-rotating black hole is called a Reissner-Nordstrom Black Hole.

When a charged, rotating black hole is known as a Kerr-Newman Black Hole.

• According to the classification by mass, there are 3 types:

1. Stellar mass black holes.
2. Intermediate Mass Black Holes
3. Supermassive Black Holes.

Stellar Black holes - small but strong

   These types of black holes are comparatively smaller in size than the other two. Their sizes range from a few 5-6 solar masses to a few hundred times the solar mass. So, when a star with a core twice to three times the size of our sun burns completely to iron, its energy production ceases and it collapses into a stellar black hole. These black holes are also called Kerr black holes as the rotation of the original massive star is conserved during the time of collapse and it contains very little electric charge. These black holes are mostly uncharged and rotate along their own central axis.

Finding a black hole is very hard since the radiation emitted cannot escape the gravitational pull of it. But the way in which the scientists found them is through an X-ray binary system. When the gases from the star nearby to it or acting as a companion to it are sucked into it, x rays are produced by these gases, which heat up to millions of degrees. So far, nearly 20 x-ray binary systems with a stellar black hole have been discovered so far.The nearest stellar black hole is V616 Monocerotis, which is nearly 3000 light-years away from us and nearly 10–14 times as massive as our sun.

Intermediate black holes - stuck in the middle

    The size of these black holes varies from 100 solar masses to nearly a hundred thousand times the mass of the sun. These black holes are certainly larger than stellar black holes, but not as large as supermassive black holes.

The true surety of finding these black holes is still a mystery, but many intermediate-mass black holes are found in our galaxy and nearby due to the accretion disc and gas cloud spectral. The strongest result which shows that these black holes exist is the low luminous active galactic nuclei that are the centre of the galaxy and have a comparatively higher luminosity, which is certainly not exhibited by a star. The origin of these types of black holes is determined by these three ways.

They were formed at the time of the Big Bang, so they are primordial black holes. Secondly, by the merging of stellar black holes and other smaller objects together. The third way is through the collision of massive stars in a dense stellar cluster.

 

Supermassive black holes - Giant-Sized 

     These are the largest types of black holes, being the size of a few thousand to millions of times the size of our sun. These black holes are very large compared to the other two types of black holes. Because of being so large, the density of these black holes is less than the stellar and intermediate-mass types of black holes. As we know, the Schwarzschild’s radius is proportional to mass, so for spherical objects, the density of objects is inversely proportional to the square of mass. So, the larger the black hole, the lower the average density. The pull of the supermassive black hole is also weaker than the other two black holes inside the event horizon.

     The origin of these types of black holes is still an open mystery, but still, as long as there is a black hole at the centre of every galaxy, it continuously grows by feeding the matter around it or by merging with other black holes. The active galactic centre of galaxies is where most of the supermassive black holes exist that continuously engulf the matter and are the source of the extreme luminosity at the center.

The supermassive black hole at the centre of our Milky Way galaxy is Sagittarius A*. Its diameter is said to be 44 million km and it is about 25,640 light-years from Earth.

Still, there are many more things to know about black holes, which we will discover slowly one by one.

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