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The Wikipedia page for apparent horizon is pretty sketchy and requires some GR knowledge. Is there any simple definition?

The black hole region of a spacetime is defined as a region where nothing can escape to infinity and an event horizon at a given time is the boundary of a connected region of space which is part of the black hole region. As you're after a simple answer I won't give a formal definition of a black hole or an event horizon, but they can be found in Wald.

The problem though is to know whether anything in a region of space can escape to infinity depends on precise knowledge of the future, also not all spacetimes have a suitable notion of infinity. Yet clearly even when we don't know the entire history of the spacetime or when there isn't a suitable notion of infinity, we can still identify objects that are functionally equivalent to black holes. The apparent horizon if you like is the spatial boundary of what we might consider to functionally be a black hole.

As nothing can escape a black hole, even light directed away from it is pulled back towards the singularity so we know that a black hole can cause even light directed outwards, relative to a point in space, to move inwards to that point. The apparent horizon is the boundary between where outwardly directed light moves outwards and where it moves inwards.

In Schwarzschild spacetime, or more generally Kerr-Newman spacetime (in standard coordinates) the apparent horizon and the event horizon coincide. However, more generally, the location of the apparent horizon depends on how you 'slice' spacetime (it is observer-dependent if you like - unlike the event horizon). Apparent horizons needn't be associated with (formally-defined) black holes, however as long as the spacetime has certain properties, they will indeed be associated with black holes and will lie at or inside the event horizon.

## Singularidade no Plural

In general relativity, an **event horizon** is a boundary in spacetime, most often an area surrounding a black hole, beyond which events cannot affect an outside observer. Light emitted from beyond the horizon can never reach the observer, and any object that approaches the horizon from the observer's side appears to slow down and never quite pass through the horizon, with its image becoming more and more redshifted as time elapses. The traveling object, however, experiences no strange effects and does, in fact, pass through the horizon in a finite amount of proper time.

More specific types of horizon include the related but distinct absolute and apparent horizons found around a black hole. Still other distinct notions include the Cauchy and Killing horizon the photon spheres and ergospheres of the Kerr solution particle and cosmological horizons relevant to cosmology and isolated and dynamical horizons important in current black hole research.

## 10 Replies to &ldquoUsing Gravity to Peer into the Most Violent Places in the Universe: Colliding Black Holes&rdquo

Here are two relevant (PDF) papers:

The ringing does not escape the event

horizon. The ringing is spacetime

curvature or waves which are emitted close to the event horizon. If you have two black holes in a close orbit

there is dynamical spacetime close to the two event horizons. By dynamical it

means the space in the spacetime is being twisted around in such a way that

these twists or distortions form waves that can escape the system. The intense gravity means that null

directions, or the paths taken by massless particles such as light or gravity

waves, remains close to the black holes and slowly peels away. These waves can be present near the black

hole after the merger. They reach “infinity”

some time later and carry information about the original black holes which

formed the merger. From the perspective

of an observer the data about the final state of the two initial black holes

appears later than the apparent merger.

imagine a different type of event: a rare linear collision of two somewhat equal masses. would not the mass/volume relation be distorted above the limit required to maintain collapse and give rise to an explosive relaxation?

The merger of two black holes has different appearances for an exterior observer and an interior observer who decides to enter. For the exterior observer the merger of two black hole horizons is similar to the merging to two pant legs leading up to the waist. If one thinks of the vertical direction as time the two horizons fuse into a single event horizon. For that exterior observer there will be gravity waves produced from the rapidly changing geometry of space in time. Once this observer sees a single black surface for the merged black hole there will continue to be gravity waves present which are due to the complicated spacetime configuration before the merger. This is what this group has computed. Now of course this will damp out with time and escape to infinity.

For the infalling observer who enters one of the black holes things are stranger. The observer who falls through a single black hole observes no change in the horizon, or what is the apparent horizon. As one gets closer to the black hole the event horizon appears as a black spherical surface that increases in size and becomes a nearly flat plane. This persists after the observer has passed the actual event horizon. This means there is still a region closer to the singularity that is causally disconnected from the observer. This persists up to the point the observer reaches the singularity.

What happens with a black hole merger according to the interior observer is a bit more complicated. First one has to take off the pair of pants and put on a skirt. The legs coming out of the skirt are the apparent horizons of the two black holes inside the event horizon. The skirt is the appearance of a new apparent event horizon according to an interior observer not too close to either of the two apparent horizons of the black hole. An observer not too close to either black hole apparent horizon witnesses the sudden appearance of a larger apparent horizon. An observer close to one of the apparent horizons in effect goes “up the skirt” and does not see it.

So an interior observer will see the large horizon of one black hole they are in and the other black hole horizon approaching, appearing as a black sphere. If the observer is not too close to the first apparent horizon then the two apparent horizons abruptly become one. If this observer is close to the apparent horizon of the black hole they entered then the two apparent horizons remain distinct up to the point they reach singularity.

The different between these two perspectives is that the exterior observer witnesses physics that is covariant, or not frame dependent. The interior observer witnesses events that depends upon the frame they are on. Depending on their frame they either witness the appearance of the “skirt,” the new apparent horizon, or they do not. This is a rather interesting development for it suggests some topology. However, the exterior world is such that observables are covariant and so anything actually inside the event horizon is not observable.

so a distant observer wouldn’t see much. no energy released.

Energy is a funny thing in general relativity. Energy is only defined properly if the symmetry of the spacetime is of a certain type.

In the end nothing that is observed passes across the event horizon. This solution here finds there is a delay of signal from before the coalescence of the black holes which gives data concerning the two black holes which made up the final black hole. So external observers who collect data about the final black hole shortly after the two input black holes coalesce can measure data concerning the initial two black holes. However, this data has been “time delayed” by the curvature of spacetime.

I’m fascinated by what kind of frame dragging goes on in such a system.

The math surrounding this phenomena has got to be staggering… I’d love to hear Leonard Susskind talk about this somewhere…

When two black holes near each other, doesn’t the gravity on facing sides cancel out? One black hole is pulling one way, the other the other way – there should be low-to-zero gravity between. I imagine that would get ugly as the compressed matter between the two releases – though it’s all in the event horizon so we’d never see it.

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## 10 things you should know about black holes

And what makes them different — or not so different— from everything else in the Universe.

**1.** **What is a black hole?**

A black hole’s defining property is its horizon, which is the boundary of a region from which nothing, not even light, can ever escape. If the disconnected region remains disconnected forever, we speak of an “event horizon”. If it’s only disconnected temporarily, we speak of an “apparent horizon”. But temporarily could still mean the region remains disconnected for much longer than the present age of the universe! If the black hole horizon is temporary but very long lived, the difference between the both cases cannot be observed.

**2.** **How large are black holes?**

You can think of the black hole horizon as a sphere, and its diameter is directly proportional to the mass of the black hole. So the more mass falls into the black hole, the larger the black hole becomes. Compared to stellar objects though, black holes are tiny because the mass has been compressed into a very small volume by enormous gravitational pressure. The radius of a black hole with the approximate mass of planet Earth, for example, is only a few millimeters. Compare that to the actual radius of Earth, which is about 10,000,000,000 times larger!

The radius of a black hole is called the Schwarzschild radius, after Karl Schwarzschild who first derived black holes as a solution to Einstein’s General Relativity.

**3.** **What happens at the horizon?**

Somebody crossing the horizon doesn’t notice anything different in their immediate surroundings. This is a direct consequence of Einstein’s equivalence principle, which implies that one cannot tell the difference between acceleration in flat space and a gravitational field that causes the curvature of space. However, an observer far away from the black hole who watches somebody fall in would notice that the person seems to move slower and slower the closer they get towards the horizon. It appears like this because time close by the black hole horizon runs much slower than far away from the horizon. But it only takes a finite amount of time for the infalling observer to cross over that event horizon, and find themselves inside that Schwarzschild radius.

What you would experience at the horizon depends on the tidal forces of the gravitational field. The tidal forces at the horizon are inversely proportional to the square of the mass of the black hole. This means the larger and more massive the black hole, the smaller the forces are. If the black hole is only massive enough, you might cross the horizon before you even noticed anything was happening. The effect of these tidal forces is that you would get stretched: the technical term physicists use is “spaghettification”.

In the early days of General Relativity it was believed that there was a singularity at the horizon, but this turned out to be wrong.

**4.** **What is inside a black hole?**

Nobody really knows, but it is almost certainly not a bookshelf! General relativity predicts that inside the black hole is a singularity, a place at where tidal forces become infinitely large, and that once you cross the horizon, you cannot avoid crashing into the singularity. Alas, General Relativity is not good to use in this region because we know that the theory breaks down. To be able to tell what is inside a black hole we would need a theory of quantum gravity. It is generally believed that this theory would replace the singularity with something else.

**5.** **How do black holes form?**

We presently know of four different ways black holes may form. The best understood one is stellar collapse. A sufficiently large star will form a black hole after its nuclear fusion runs dry because everything that can be fused has been fused. When the pressure generated by the fusion stops, the matter starts falling towards its own gravitational center, becoming increasingly dense. Eventually it is so dense that nothing could overcome the gravitational pull on the stars’ surface: a black hole has been created. These black holes are called ‘**solar mass black holes**’ and are the most common ones.

The next common type of black holes are ‘**supermassive black holes**’ that can be found in the centers of many galaxies and have masses about a billion times that of solar mass black holes. Exactly how they form still isn’t entirely clear. It is believed that they once started out as solar mass black holes that in the densely populated galactic centers ate up a lot of other stars and grew. However, they seem to be eating stuff faster than this simple idea suggests, and exactly how they manage this is still subject of research.

A more controversial idea are **primordial black holes**, that might have been formed at pretty much any mass by large density fluctuations in the early universe. While this is possible, it is difficult to find a model that produces them without producing too many of them.

Finally, there is the very speculative idea that **tiny black holes** with masses similar to that of the Higgs boson could form at the LHC. This only works if our universe has additional dimensions. So far, there has not been any observation that this might be the case.

**6.** **How do we know black holes exist?**

We have a lot of observational evidence for very compact objects with large masses that do not emit light. These objects reveal themselves by their gravitational pull, for example by affecting the motion of other stars or gas clouds around them. They also cause gravitational lensing. We furthermore know that these objects do not have a hard surface. One can tell this from observations because matter falling onto an object with a surface would cause more emission of particles than matter falling through a horizon. An upcoming experiment, the “Event Horizon Telescope” will be looking for another hallmark of black holes, their photosphere. This is basically an extreme gravitational lensing event.

**7.** **Why did Hawking say last year that black holes don’t exist?**

He meant he thinks black holes do not have an eternal event horizon but only a temporary apparent horizon (see 1). In a very strict, and not common, use of terminology, only an event horizon counts as black hole.

**8.** **How can black holes emit radiation?**

Black holes emit radiation by quantum effects. It is important to note that these are quantum effects of matter and not quantum effects of gravity. What happens is that the dynamical space-time of the collapsing black hole changes the notion of what a particle is. Like the passage of time that gets distorted nearby the black hole, the notion of particles too depends on the observer. In particular, while an observer falling into the black hole thinks he is falling in vacuum, the observer far away from the black hole thinks that it’s not vacuum but full of particles. It is the stretching of the space-time itself that causes this effect.

First discovered by Stephen Hawking, the radiation that black holes emit is called “Hawking radiation”. This radiation has temperature which is inversely proportional to the black hole’s mass: the smaller the black hole the hotter. For the stellar and supermassive black holes that we know of, the temperature is well below that of the CMB and cannot be observed.

**9.** **What is the information loss paradox?**

The information loss paradox is caused by the emission of Hawking radiation. This radiation is purely thermal which means it is random except for having a specific temperature. The radiation in particular does not contain any information about what formed the black hole. But while the black hole emits radiation, it loses mass and shrinks. Eventually, the black hole will be entirely converted into random radiation and the remaining radiation depends only on the mass of the black hole. It does not at all depend on the details of the matter that formed it, or whatever fell in later. Therefore, if one only knows the final state of the evaporation, one cannot tell what formed the black hole. Such a process is called “irreversible” — and the trouble is that there are no such processes in quantum mechanics.

Black hole evaporation is therefore inconsistent with quantum theory as we know it and something has to give. Somehow this inconsistency has to be removed. Most physicists believe that the solution is that the Hawking radiation somehow must contain information after all.

**10.** **What is Hawking’s recent proposal to solve the black hole information loss problem?**

The idea is that black holes have a way to store information which has so far been neglected. This information is stored on the black hole horizon and can cause tiny shifts of the particles in the Hawking radiation. In these tiny shifts there could be the information about the infalling matter. Exactly how this is supposed to work is presently entirely unclear. Scientists are waiting for a more detailed technical paper of Stephen Hawking, in collaboration with Malcom Perry and Andrew Strominger. The paper is rumored to appear in late September.

At this point in time, we are certain that black holes exist, we know where they are, how they form, and how they’ll eventually, on timescales of 10^67 years and up, cease to exist. But the details of where the information that went into them goes are still up for grabs, and that’s one of the problems unique to black holes among all objects in the Universe.

## Stephen Hawking debunks Albert Einstein's black hole theory

Eminent scientist Stephen Hawking has posted a new paper online that demolishes modern black hole theory.

The wheelchair-bound genius said that the idea of an event horizon, from which even light cannot escape, is flawed.

Hawking smashes the idea of a black hole by saying that instead of there being an inescapable event horizon, they should think of a far less total &ldquoapparent horizon&rdquo. And, at a stroke, he has contradicted Albert Einstein, as event horizons are just mathematically simple consequences of general theory of relativity that was put forward by Einstein, the Daily Express reported.

Hawking wrote in his paper, called &lsquoInformation Preservation and Weather Forecasting For Black Holes,&rsquo that event horizons&rsquo absence means that there are no black holes - in the sense of regimes from which light can&rsquot escape to infinity.

He suggested that light rays attempting to go away from the black hole&rsquos core is going to be held as though stuck on a treadmill, from which they can slowly shrink by spewing out radiation.

Hawking told leading science magazine Nature that there can be no escape from a black hole in classical theory, however, quantum theory enables energy and information to escape from a black hole.

The new grey hole theory is going to allow matter and energy to be held for a period of time before they are released back into space.

## What's the difference between the apparent horizon and event horizon of a black hole? - Astronomy

The problem of defining the gravitational entropy of a nonstationary black hole is considered in a simple model consisting of a spherical shell which collapses into a preexisting black hole. The second law of black-hole mechanics strongly suggests identifying one-quarter of the area of the event horizon as the gravitational entropy of the system. It is, however, impossible to accurately locate the position of the global event horizon using only local measurements. In order to maintain a local thermodynamics, it is suggested that the entropy of the black hole be identified with one-quarter the area of the apparent horizon. The difference between the event-horizon entropy (to the extent it can be determined) and the apparent-horizon entropy may then be interpreted as the gravitational entropy of the collapsing shell. The total (event-horizon) gravitational entropy evolves in a smooth (C 0 ) fashion, even in the presence of δ-functional shells of matter.

## Black Holes

**Black holes** are the most intriguing objects in the cosmos. It is broadly defined as the region in space-time which exhibits a strong gravitational pull , such that even light cannot escape.

**Black holes** were first predicted in the theory of General Relativity as sufficiently compact masses that can deform the fabric of space time.In this article we will understand each of these terms : black holes, **event horizon**, **Schwarzschild Radius** , **singularity**, **supermassive black holes**, gravitional **pull of black holes** in depth.

Now before jumping directly into the further technicalities of the black hole , lets brush up on some definitions like , “escape velocity” .

##### Escape velocity

##### Is the minimum speed required for an object to escape the gravitational influence of a massive body. Or its the minimum launching velocity for an object such that it never returns back. It is assumed that no extra energy is added along the way. A NASA rocket has fuel to continuously add energy, so it doesn’t need to be launched at the escape velocity. On the contrary , a ball if to be launched into space ,such that it never returns back needs to be launched at a specific velocity since there would be no fuel to provide energy through out its path.

Therefore , a ball or a cannon when launched with the escape velocity is assumed to travel towards infinity when finally the velocity becomes 0 and thus its kinetic and potential energy as well.

##### Equating E(at surface) = E( at ∞) we get

Now to calculate the escape velocity of earth we just nee to plug in the values of mass(M) , gravitational constant(G) , and radius(R) in the equation. And it comes out to be **11.6 km/sec or 11184 m/s.** Which means that if launch a cannon or a ball up into the air with a velocity of 11.6 km/sec that it would escape out the earths gravity and go out into space.

##### Why did we spent so much time knowing what escape velocity is?

It’s simply because , **Black Holes can be defined as any object whose escape velocity is greater than the speed of light.** And since we know nothing can travel more than the speed of light (including light itself) so nothing can escape from it.

Clearly the earth or in that case any other planets escape velocity is much much less than that of a black hole.

##### Before we proceed ahead i would want to clear this fact that , any object can in theory be transformed into a black hole (which includes you and I as well !!) , if we can somehow manage to shrink down the object to a particular dimension(which is very very small compared to its actual size) keeping the mass constant.

**Well , here’s how,**

**∴** If we reduce the radius (r) , then automatically the Velocity (v) will increase.

As we stated earlier that, in order to form a black hole , the escape velocity needs to be greater the speed of light. So if we can reduce “r” to a certain value such that velocity becomes ≥ the speed of light, then we can make the object a black hole ,ofcourse keeping the mass constant throughout the process.

##### So if we wanted to convert the earth into a black hole then,

Now lets assume we successfully did that ( Yes we do assume more bizarre things in Physics!) , then would the newly formed black hole would be able to sustain itself ?

Lets analyse it with an example.

suppose we keep a box of 1 kg on this newly formed black hole. Then the force that it would experience , would be

Now the question is how do we create a black hole that can sustain itself?

Lets see how this process takes place,

##### Formation of Black Holes

Stars as we know are nothing but huge glowing ball of fire, which is powered by nuclear fusion. We have discussed in details on how a star glows in our article on ‘The Standard Model of Particle Physics”.

A star continues to glow throughout its lifetime, because the Gravitational force of the star due to its own mass is counteracted by the radiation pressure due the nuclear reaction going on in its core.

This keeps the star stable and prevents the star from getting collapsed due to its own weight.

##### But at the end of star’s life , when all the hydrogen gets over and there is no more “fuel’ left to continue the fusion, the radiation pressure slowly goes away and the gravity starts to dominate.

Consequently the star starts to collapse on itself. Now the obvious question is whats going to finally stop this collapse? or will this collapse ever come to a stop?

The answer is, yes the collapse will stop at a point and there arises two cases :

##### The atomic forces stops the collapse

##### Or Pauli’s Exclusion principle comes into action, which states that no two electrons can be in the same state.

Sometimes when the star is big enough the gravity completely knocks of the Pauli’s Exclusion Principle and the protons and the electrons are squashed together to form neutrons, and we get whats known as the neutron star. But on the other hand when the star is much much bigger say a million times bigger than our sun , then the whole mass of the stars core collapses and a black hole is formed.

So black holes are the by-product of star, that has ended its journey.

But as we mentioned earlier, theoretically any mass can be converted into a black hole if we could shrink the object down to a much smaller scale , keeping its mass constant.

Now that we know how a black hole is formed and all its definitions, its time to familiarize ourselves with few terms related to a black hole.

##### Event Horizon –

**a boundary in spacetime** **through which matter and light can only pass inward towards the mass of the black hole.Nothing,** not even light, can escape from inside the event horizon. The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.The shape of the event horizon of a black hole is always approximately spherical.

##### Schwarzschild Radius –

**The boundary surrounding the singularity within which the escape velocity is greater than the speed of light(c) is called the Schwarzschild Radius.** It is a physical parameter that shows up in the Schwarzschild solution to Einstein’s field equations , corresponding to the radius defining the event horizon of a Schwarzschild black hole. It is a characteristic radius associated with every quantity of mass. The *Schwarzschild radius* was named after the German astronomer Karl Schwarzschild, who calculated this exact solution for the theory of general relativity in 1916.

The Schwarzschild radius is given as

##### What if you fell into a black hole? Would you die ? or survive as shown in the movie “Interstellar” ?

**Well unfortunately sooner or later, depending on the size of the black hole we do die**. However it is not the strong gravity that is main and direct cause of our death its the very high tidal forces , which arises due to the way gravity acts at different parts of our body. The difference in the gravitational attraction at the Schwarzschild Radius and say 6 feet above the Schwarzschild Radius will be huge. So If we fall feet first then, our head would experience much much less force than our feet does and the consequence is you are going to get stretched out of existence by a process called spaghettification. But you would keep accelerating towards it until you cross the Schwarzschild Radius and then finally collapse into the black hole.

Now interestingly , if someone were to see this entire fate of you falling into a black hole, he would observe an entirely different story. He would see that you, instead of accelerating towards the Schwarzschild Radius, would get slower and slower and never actually reaching the Schwarzschild Radius. So actually it is of a false hope, because he will think that you haven’t yet met your doom and there is still time to save you ! but the reality is you have long since crashed into the black hole and been completely annihilated.

The reason for this is that the gravitational forces here are so strong that light is having an enormous difficulty travelling away and consequently the observer simply sees this space craft apparently travelling and taking longer and longer and getting slower and slower to get to the Schwarzschild Radius but never crossing it.

There is a mathematical way to demonstrate it.

From special relativity there is an invariable quantity called the proper time. The point about special relativity is that all observers if they are measuring a particular distance or time,depending on their relative velocities they will measure different distances at different time. Nobody agrees on the measurement.

##### But there is a quantity called proper time, which everybody can agree on.

you can calculate proper by :

we know the energy of the photon is hv , where h = plank’s constant. v is the frequency of the light of which the photon is a part. E=hv can also be written as :

However this is not the whole story. There is literally a lot more to Hawking radiation and we will definitely study it in details in some future article( which would come really soon !! )

##### Black Holes are the most efficient way to convert Mass into Energy.

Black holes have an unreasonable efficiency . They are great at extracting energy from mass.

**This is weird , because we have seen that, nothing can escape a black hole once it is inside its schwarzschild radius.**

But the efficiency of a black hole comes from what stuff does while falling towards them. Anything that falls in a gravitational field speeds up gaining kinetic energy and when it crashes with something this kinetic energy is converted into heat. This heat can then radiate away as infrared radiation , slightly decreasing the mass of the object.

For example , when a meteor comes in the gravitational field of a planet , it gains kinetic energy and when it collides with the air in the atmosphere this kinetic energy gets converted into heat which is radiated away , and in this process the mass of the meteor decreases.

for planets and stars this conversion of mass into energy is pretty pathetic. An object falling through the earths atmosphere and crashing into it only converts 0.000001% of its mass into energy. . This is bad as any ordinary chemical reaction.

But black holes have something special going on for them. Black holes have incredible gravity and it completely bends the spacetime so much that any object that comes under its gravitational influence, accelerates so fast and gains so much kinetic energy that it almost converts almost 50% of its mass into energy. However if the object keeps falling past the event horizon, then all the energy will be stuck in the black hole.

**∴** Therefore the we can let black holes to convert mass into energy is by , letting the object to slowly spiral into the black hole, crashing into other stuffs on the way, heating up , radiating that energy away and thereby reducing its speed , slowing down more , spiraling to yet to a lower orbit. This process continues until it reaches the innermost possible orbit.

This is exactly what goes on in accretion disks around Black holes

##### All the matter in the accretion disk , which includes space debris mostly , is converted into energy. And for rotating black holes , this mass to energy conversion rate is around 42% . Which is way more than even nuclear reactions and any chemical reaction. All the energy gets shot out into space from the poles of the black holes.

##### Black Hole Information Paradox and its possible solution(MECOs)

Black Holes are engines of destruction that removes from our universe anything that crosses out the event horizon.

But mass and energy aren’t removed from existence , rather they add up to the mass of the black hole as we say earlier.

And we also know that this mass can escape as it gradually leaks away by Hawking Radiation, over long scales of time. This same Hawking Radiation maybe more destructive than the black hole itself. It may destroy the complete information, and also information is no way can escape out the black hole if it happens to cross its event horizon. This complete destruction of the information violates a major principle in physics i.e the ” Law of Conservation of Information”. Which cant be done at any cost.

So the only way it can be in the Hawking radiation (naively) is if it creates a copy of that information while it is inside. Having two copies of the information, one inside, one outside, also violates quantum theory and the ” Law of Conservation of Information”.

This was a major paradox that kept theoretical physicist puzzled till date.

However Indian physicist Dr. Abhas K.Mitra believes that the problem with the black hole information paradox lies in the problem itself. He tells us that maybe black holes aren’t that complicated as it seems.

In the year 2000, he wrote a long paper on ” Non-occurrence of trapped surfaces and Black Holes in spherical gravitational collapse” and attacked the problem from various sites. He came to the conclusion that:

- there cannot be no exact black holes
- no exact event horizon
- and no apparent horizon

Immediately before the formation of a black hole, the outer radiation pressure must counteract the inward pull of the gravity. So we have a quasi static state, which has the same size of a black hole. But this object is quasi static and is still contracting and radiating maybe at an infinitesimally slow rate and trying to attain the perfect black hole state.

But in doing so , in its journey it must radiate its entire mass energy. It has to become a zero mass black hole. He also proposed that such objects should be strongly magnetized and thus called it , “Magnetospheric Eternally Collapsing Object” or MECO.

This prediction of the MECO was verified by his American colleagues in 2006. In a famous Quasar, the central object appears more to be ultra-magnetized as MECOs rather than black holes, as predicted by him. Incidentally black holes themselves don’t have any magnetic field, they only have accretion disks which gives weak magnetic fields.Since then almost a 100 black holes have been found to have ultra strong magnetic field, that cannot be explained by present black hole paradigms.

In 2016 a NASA report on a quasar revealed that it was emitting corona, which is fire, and this was also predicted long before by Dr. Mitra as he said these MECOs were like ultra compact suns, which emit fire.

Dr. Mitra doesn’t deny the fact that there are no exact black holes or rather goes against General Relativity

Actually there is a thin line.

The mass of all objects in General Relativity is a result of an integration constant. This value is large for galaxies and entire solar systems, smaller for individual stars and much smaller for planets and moons. What Dr. Mitra has shown is, since the mass of the black hole resides in a point, its mass-energy is zero and zero mass energy in relativity doesn’t mean the absence of matter, it means that all positive energies are counteracted by the negative gravitational energy. This means that whatever we are thinking as event horizon being a sphere is nothing but a point.

##### So to sum it up, according to Dr Mitra , whatever we have called black holes for so long may might not be so. Rather they are quasi static black holes or MECOs.

And back to the information paradox , when there is not exact black hole, then nothing is trapped, hence there is no information paradox.

However there exists many other theories which we will discuss later.

##### CONCLUSION

We are left with no choice but to agree that indeed black holes are fascinating objects of the cosmos. They definitely are still not fully understood yet. Black holes are full of wonder and mystery which is yet to be discovered , and probably that’s why they fascinates me the most. Scientists are continuously researching on these amazing “space creatures” even though they are “invisible”.

Regardless of everything I believe that black holes might be key to understanding the nature of reality itself.

More articles on black hole will be coming soon.

please leave a comment below or ask any question you want to regarding the article “Black Holes”.

I would highly recommend few books that would really help you to know in depth about black holes and much more regarding the cosmos:

## Title: Hawking radiation and the boomerang behavior of massive modes near a horizon

We discuss the behavior of massive modes near a horizon based on a study of the dispersion relation and wave packet simulations of the Klein-Gordon equation. We point out an apparent paradox between two (in principle equivalent) pictures of black-hole evaporation through Hawking radiation. In the picture in which the evaporation is due to the emission of positive-energy modes, one immediately obtains a threshold for the emission of massive particles. In the picture in which the evaporation is due to the absorption of negative-energy modes, such a threshold apparently does not exist. We resolve this paradox by tracing the evolution of the positive-energy massive modes with an energy below the threshold. These are seen to be emitted and move away from the black-hole horizon, but they bounce back at a 'red horizon' and are reabsorbed by the black hole, thus compensating exactly for the difference between the two pictures. For astrophysical black holes, the consequences are curious but do not affect the terrestrial constraints on observing Hawking radiation. For analogue-gravity systems with massive modes, however, the consequences are crucial and rather surprising.

## Title: Pre-Hawking radiation cannot prevent the formation of apparent horizon

As an attempt to solve the black hole information loss paradox, recently there has been the suggestion that, due to semiclassical effects, a pre-Hawking radiation must exist during the gravitational collapse of matter, which in turn prevents the apparent horizon from forming. Assuming the pre-Hawking radiation does exist, here we argue the opposite. First we note that the stress energy tensor near the horizon for the pre-Hawking radiation is far too small to do anything to the motion of a collapsing shell. Thus the shell will always cross the apparent horizon within a finite proper time. Moreover, the amount of energy that can be radiated must be less than half of the total initial energy (if the particle starts at rest at infinity) before the shell becomes a null shell and cannot radiate any more without becoming tachyonic. Here, we conclude that for any gravitational collapsing process within Einstein gravity and semiclassical quantum field theory, the formation of the apparent horizon is inevitable. Pre-Hawking radiation is therefore not a valid solution to the information paradox.

## Black holes & Supermassive Black Holes

On the left, an optical image from the Digitized Sky Survey shows Cygnus X-1, outlined in a red box. Cygnus X-1 is located near large active regions of star formation in the Milky Way, as seen in this image that spans some 700 light years across. An artist's illustration on the right depicts what astronomers think is happening within the Cygnus X-1 system. Cygnus X-1 is a so-called stellar-mass black hole, a class of black holes that comes from the collapse of a massive star. New studies with data from Chandra and several other telescopes have determined the black hole's spin, mass, and distance with unprecedented accuracy.Black holes are amongst the universe’s family of anomalies that we’ve just recently begun to understand in the recent decades. The term black hole was laid to claim by John Wheeler in 1969, yet the theory dates back over 200 years. In 1783, John Michell wrote a paper declaring that a star that was massive enough would have a gravitational influence so strong that light would not be able to escape its surface. He also believed there were a number stars like this in the universe, but because light could not escape their gravity they would just be black voids in space. Michell also conjectured that even though we could not see the stars’ light we could feel their gravitational influence. It took 200 years before Michells theories could be put to the test, but it came.

Of course Re won the battle each and every day, to shine his rays onto the fertile lands surrounding the river Nile, bringing food and prosperity to the realm. It’s not surprising that the most important god of Egypt was the sun, source of all wealth. The Pharaohs didn’t take on the name and depiction of Re for no reason. The sun was the embodiment of life AND eternal life. But how eternal is the life of the sun really?

### A relatively small star

The expected life span of our sun is about 14 billion years. The sun is about one-third through that time, and can be compared to a human being in her late twenties, still full of strength and vigor.

In order to understand black holes, one must understand the life process of a star. Stars form when a large quantity of interstellar gas – mainly hydrogen atoms – begins to contract due to self-gravity. The colliding atoms begin heating up as they collide at greater rates and at high velocities. Eventually, the collapse gets so hot that the atoms no longer repel off of each other, but fuse together into helium atoms. This is called thermonuclear fusion. Eventually, the heat produced from these collisions counters the contraction of gravity and a star is formed. Stephen Hawking’s analogy works great: “It is a bit like a balloon – there is a balance between the pressure of the air inside, which is trying to make the balloon expand, and the tension in the rubber, which is trying to make the balloon smaller.” Inevitably, the star will run out of nuclear fuel and will no longer be able to melee with gravity. Thus, gravity wins the war and the star is doomed to collapse but it isn’t necessarily doomed to a collapse so severe it creates a black hole.

### How will the sun die eventually?

During the next billion years or so, the sun will become brighter by 10%. This will heat up our planet as a result of a severe greenhouse effect. All of the oceans on earth will vaporize and all life will be destroyed. After another 5.5 billion years the sun will burn up all of its hydrogen fuel located in the core, and then it will start using up the hydrogen from the layers surrounding the core.

This will cause the sun to swell like a big balloon. 2.5 billion years later the sun will have become about 100 times bigger than its present size. By this point it has swallowed Mercury, Venus and very probably the Earth in the process of expansion. At that moment we call the sun a Red Giant.

The sun’s exhaust gas, helium – generated through nuclear fusion – will serve as the sun’s new fuel, when it has devoured all of its hydrogen. The standard hydrogen core can reach temperatures as hot as 100 million degrees, while a helium core can reach up to 600 million degrees. The temperatures increase and the fuel runs out quicker. The transition from a G2 star (our sun) to a Red Giant is roughly 160 million years. On the cosmic scale that’s quite fast. The lifespan of a Red Giant is only 1 billion years, compared to our sun’s 10 billion years.

Once all the sun’s helium is consumed it will then eject enormous amounts of matter into space. After it ejects its surface layers, the sun will then cool down and contract to be an object with a very high density, but only a few thousand miles in radius. We call this object a White Dwarf. A teaspoon of white dwarf material would weigh five-and-a-half tons or more on Earth. Yet a white dwarf can contract no further its electrons resist further compression by exerting an outward pressure that counteracts gravity. This balance between gravity and outward pressure, called electron degeneracy pressure, is the reason why stars do not explode very soon after birth. Effectively the sun is now around its dying years.

### Shrinking Star

White dwarfs are very common objects in the universe.Most of them are very dim and invisible to our eye and telescopes. A very famous one is Sirius B. Astronomer W.Bessel was the first to suspect that Sirius had an invisible companion when he observed that the path of the star wobbled. In the 1920’s it was determined that Sirius B, the companion of Sirius, was a “white dwarf” star. The pull of its gravity caused Sirius’s wavy movement.

Here is an X-ray image of the Sirius star system located 8.6 light years from Earth. This image shows two sources and a spike-like pattern due to the support structure for the transmission grating. The bright source is Sirius B, a white dwarf star that has a surface temperature of about 25,000 degrees Celsius which produces very low energy X-rays.

The dim source at the position of Sirius A – a normal star more than twice as massive as the sun – may be due to ultraviolet radiation from Sirius A leaking through the filter on the detector. The picture was taken with the Chandra X-ray Observatory. Since its launch on July 23, 1999, the Chandra X-ray Observatory has been NASA’s flagship mission for X-ray astronomy, taking its place in the fleet of “Great Observatories.”

The picture to the bottom right shows the same star system, now through a ‘normal’ visible light telescope, to show exactly how small Sirius B is compared to Sirius A, which is about 1.6 times the size of our own sun, but 22 times the luminosity of our sun. Sirius B has a luminosity of 1/400 of our sun, making it very dim.

Next to these facts it was also discovered that Sirius B had another important trick up its sleeve: it was the first star of which the light showed a gravitational red-shift, making a nice piece of evidence to support Einstein’s theory of relativity. Einstein had predicted that photons (light particles) that meet a strong gravitational pull will lose energy. Thus, the light’s wavelength stretches so that their color will shift toward the red spectrum. Until that moment (in 1924) it had been very difficult to detect red-shifted light in low-mass stars such as our sun. You’re probably now saying, “Light particles and light waves! Which is it!”? We will discuss this effect of light shifting toward the red again when the black hole is being explained.

### A big star dies

Contrary to what you might think, a larger star burns out more quickly than a small star like our sun. The moment all of a star’s fuel is consumed, the big star will shed most of its mass into space – much like our own sun will do, but then with an incredible force, a stellar explosion which astronomers call a supernova. There are more spectacular explosions, called hypernovae, but scientists are still in doubt as to their cause. What happens before the bang of a supernova?

### We are stardust

Massive stars burn up hydrogen, which is converted to helium. They do that at tremendous rates: a star, 25 times the mass of our sun will live its life a thousand (!!) times faster. It will also burn a 100,000 times brighter. Because a massive star has more mass, gravity will build up pressure and temperature around the core, which will help to fuse the fuel into elements of increasing atomic weight. There are many of these processes going on in a star, and depending on the distance from the core, we will see different layers.

At the stars surface we would see hydrogen being fused to helium, somewhat deeper there would be a layer where helium was fused into carbon and oxygen, carbon would be fused into neon and magnesium and so on. At the stars deepest point, where it is really hot (8 billion degrees Kelvin), iron is created by fusing silicon. The creation of this iron core takes place in about a week.

Once the iron core is formed it is no longer possible to produce more energy just by compressing it to start a new fusion reaction. Gravity is indifferent to this and will go on compressing the core, raising temperatures to about 10 billion degrees Kelvin.

At this temperature the photons split the iron nuclei into protons and neutrons. They don’t do that quietly: in a tenth of a second a 12,000 km iron core collapses into a neutron star of about 20 km in diameter. The outer layers of the star are suddenly without support, and they now collapse and bounce on the dense, incompressible neutron core, resulting in the instant release of a huge amount of gravitational potential energy. Boom!!

As you see, during its lifetime and especially toward the end the sun is the creator of all elements we find on earth and in ourselves. Truly we are stardust, the remains of a dead star, which once burned brightly in the heavens.

### Neutron Star

A star that exceeds 1.4 solar masses, and is limited to 3 solar masses, after its supernova will collapse further than a white dwarf into a very dense star called a neutron star.

A neutron star is nothing more than an incredibly dense core made of just neutrons. Its mass is packed in a volume roughly 10^14 times smaller than our sun and has a mass density around 10^14 times higher than the sun it is so dense that a teaspoon would weigh 100 million tons. A neutron star less than 3 solar masses will not contract any further, because the neutrons will resist the inward push of gravity, just like the white dwarf’s electrons do.

This is now called neutron degeneracy pressure. When the neutron star’s mass far exceeds 3 solar masses (no-one exactly knows the precise critical point) there is a good chance that the process of inward gravity exceeds that of the neutrons’ resistance. The core of the neutron star collapses further and then there’s no more stopping the ongoing process, the star infinitely collapsing a black hole is formed.

### Black Hole: the making of:

What exactly IS a black hole? A black hole is a region in space-time that has a gravitational field so strong that the escape velocity is faster than the speed of light.

This means nothing can escape its clutches, not even light. When the core of a massive neutron star collapses, the inward gravity prevailing over the neutron degeneracy pressure, the process will go on and on, until we reach a point in which all matter of the star if being compressed into a point of infinite density.

### The tale of the black hole has the following chapters:

-A singularity

-The Schwarzschild radius

-The event horizon

-The apparent horizon

### The Singularity

The singularity lies at the heart of the black hole. This is where all matter has been crushed to an infinitely small point of infinite density, where space-time has an infinite curvature. The laws of physics break down at the singularity it is really a point where space and time as we know them cease to exist. Astrophysicists say the big bang started as a singularity.

### The Schwarzschild radius

The German astrophysicist Karl Schwarzschild used the equations in Einstein’s theory of relativity to determine the radius for a given mass at which matter would collapse into a singularity. An example: A black hole with a mass of about 10 of our suns will have a radius of only 30 (!!) kilometers (19 miles). Thus, the radius between the singularity and the event horizon is called the Schwarzschild radius.

### The event horizon

The event horizon is what some would call “the point of no return.” Beyond this unseen border the escape velocity for the black hole is greater than that of the speed of light, meaning light would have to travel faster than its constant velocity of 300,000km/h in order to escape. The event horizon is a static state at some point. The event horizon coincides at some point in time with the apparent horizon.

### The apparent horizon

The collapsing dying star will show an “apparent” event horizon forming all of a sudden. This horizon moves out like a balloon expanding until it coincides with the event horizon of the black hole (see diagram). This horizon – during its existence – will separate trapped light rays from the light rays that can still move away. Some of these rays can be trapped later when more matter or energy falls into the hole, increasing the gravity inside.

### Apparent versus Event Horizon

Even before the star meets its final doom, the event horizon forms at the centre, balloons out and breaks through the star’s surface at the very moment it shrinks through the critical circumference. At this point in time, the apparent and event horizons merge as one: the horizon. The distinction between apparent horizon and event horizon may seem subtle, even obscure. Nevertheless the difference becomes important in computer simulations of how black holes form and evolve. Beyond the event horizon, nothing, not even light, can escape. So the event horizon acts as a kind of “surface” or “skin” beyond which we can venture but cannot see.

### How can we see a black hole?

As more and more research is compiled we find more evidence to support black holes. The black hole itself can not be directly observed for reasons aforementioned. There are several ways we can observe a black hole. One of the most obvious is observing the effects it has on surrounding celestial bodies. Obviously if we see a star orbiting around an invisible mass, we conclude it is a black hole of an X amount of mass.

If gas from a nearby star is “sucked” into the black hole, the gas will begin orbiting the event horizon, accelerating to velocities near the speed of light and heating up to many millions of degrees. We then will be able to detect the radiation. Another way is through Hawking radiation, where virtual particle and anti-particle pairs are created outside of the event horizon. The two will immediately collide and obliterate themselves, releasing gamma radiation. However, there are times when one of the pair is pulled in beyond the even horizon. The particle pulled into the black hole will then have a negative mass-energy and the one released will have a positive mass-energy, thus being detected as radiation.