What the Oxford English Dictionary Doesn't Tell You About Black Holes (complete guide)

Black holes are one of the most interesting objects present in the universe

Here is a complete guide to black holes, for you

Summary

What are blackholes?

It is a region in spacetime where the curvature of space tends to infinity

Will a blackhole disappear?

Yes a blackhole can disappear after sometime as it gets evaporated because of hawking radiation

By which mechanism hawking radiation takes away the energy of a blackhole?

Hawking radiation makes a blackhole loose energy because of combined effect of time dilation and fluctuations in quantum fields of space

When a blackhole dies what happens to it?

A blackhole ironically becomes brighter and brighter as it approaches it’s death, and ultimately dies in a burst of gamma rays

The information paradox of blackhole and one of its potential solution

It is unknown what happens to the information that goes inside a blackhole, a solution to this is that, information is present on the event horizon as a hologram

Spinning blackholes and ring singularity

Spinning blackholes are theorized to have ring shaped singularity which have a wormhole at its centre, leading to a whitehole at the other end

What will you see if you fall into a blackhole?

Light from the universe outside will compress and form a ring around your waist

What are black holes?

A Blackhole is a region in space-time whose gravity is so large that not even light cannot escape from it

It can also be said that

It is a region in space-time, escaping form which requires you to travel faster than light

It is actually, a piece of matter which is compressed so much that it deforms the geometry of space, to form a black hole

It can be thought of as a never-ending hole, once you fall into it, it’s hard to return

“Even light cannot return from it”

Do you know what this means?

Let’s explore

There are only 2 ways by which we can see an object

1- If they emit light (light source)

2- If they reflect light

If an object does none of these

Yes! You guessed it right, we won’t be able to see it

And this exactly how black holes are

1- They can’t emit light because for that light needs to travel faster than 3x10^8 meters/second (which according to relativity is the maximum speed limit)

2- They can’t light because when the light will strike it, it won’t come back

So this suggests we can’t see them

Then how do we know the actually exists

There are 3 reasons for

1- Mathematics suggests that

2- By observing their effect on nearby objects

3- By observing gravitational waves

4- By observing gravitational lensing

5- We have an image of black hole M87

Mathematics suggests the existence of black holes-

Short explanation-Mathematically, by using einstein's field equations the existence of black holes can be predicted

Long explanation-

One of the most prominent predictions of the general theory of relativity is that it predicts the existence of these ‘monsters‘

The math involved behind this is a little bit complicated

But

I’ll make sure to involve no math and make this explanation as simple as possible for you

You need to keep in mind a few things

1- Einstein's file is, the heart of general theory of relativity

2- These equations describe the curvature of spacetime with the matter, energy, or radiation present in that region of spacetime

These equations look are complicated

A scientist called Karl Schwarzschild derived an interesting conclusion from these equations

This is what he did

1- He derived something called Schwarzschild matrics from einstein's field equations

2- He found a number called Schwarzschild radius denoted by R_s

Now 3 possibilities are depending on the value of the radius of an object

a. If the radius of the object is greater than its Schwarzschild radius (R>R_S), in this case, the curvature of spacetime is flat

b. If the radius of an object is close to its Schwarzschild radius (R~R_s), in this case as well the spacetime curvature is flat

c. If the radius of an object is exactly equal to its Schwarzschild radius (R~R_s), this is the interesting one, where the curvature of space-time becomes infinite

In the last case, we say that the object has become a blackhole

NOTE- here the term “space-time” can be simply understood as space itself

INTERESTING FACT-

it is interesting to note that Einstein himself didn’t like that his theory predicted black holes

because black holes allow the existence of a singularity

A singularity in very simple terms is a physical infinity

So he even denied the existence of BHs

By observing their effect on nearby objects-

This is true that black holes can’t be observed because they don’t reflect light

But

What will happen, if it is destroying any matter near it?

We would then be able to see it’s the presence

Here’s how

1- Suppose a star is very close to a black hole

2- The star accidentally slips into the black hole

3- The star will not simply go inside

4- But some of its mass will revolve around the BH

5- While remaining mass will be thrown away at very high speed

6- The mass around the BH will star to revolve at very very high speed

7- The energy lost in friction amongst the particle will convert into heat and light

8- This will form a ring around the BH, called the accretion disk

9- This accretion disc can be observed from a distance

10- This is how a black hole can be identified

Here’s what happens later on

2 forces are acting on the material of the disk

1- The gravitation force (pulling in)

2- Frictional force (pulling in)

NOTE- the frictional force not directly pulls the material in, but it causes the material to lose energy

so as the material loses it’s the energy it slows and spirals down into the BH

this causes the material of the disk to ultimately fall inside

By observing gravitational waves-

Short explanation- when 2 black holes merge, gravitational waves are produced, we can detect them to identify the location of black holes

Long explanation-

a question must be coming to your mind

“What are these gravitational waves?”

In simple terms these waves are like ripples through space

Here’s a simple explanation-

1- Space is not like something which we can’t touch

2- Space can also be touched

3- But, it is touched or affected in a different way, i.e. by using gravity

4- Think what happens when you throw a pebble into a pond?

5- It produces ripples in the water

6- Likewise is we throw a celestial body, say a planet into space, it will produce ripples, in the space

Now throwing a planet is not the only way to produce gravitational waves, they can be produced in several ways such as

Here’s a video showing gravitational waves (Walter Lewin's video)

Let’s see how these gravitational waves help us in detecting the blackholes

When 2 black holes merge, gravitational waves are sent throughout the fabric of space

But if the production source of these waves is very far away from the earth, their magnitude decreases, and it becomes very difficult to detect them

We need a highly precise setup to measure the gravitational waves coming from a source many light-years away

This precision instrument is known as LIGO (red more: LIGO project)

The basic idea behind the working of LIGO is

A gravitational wave can change the path of a light beam, and the effect of this deflected beam will be seen in its reflected wave from a mirror

Here’s a basic outline for the working of LIGO

1- 2 mirrors are placed on 2 far away locations on the earth

2- LASER light is shined on these mirrors

3- when a gravitational wave arrives, the path of these light waves are shaken

4- this small variation is observed from the reflected waves

5- a small change in the path of light can be due to a coincidence also

6- that’s why 2 mirrors are placed at 2 far away locations

7- results from one mirror are compared

8- if both mirrors provide the same result, this means the cause of deflection of light was a gravitational wave

9- calculations are done on this data and the location of the source of gravitational waves is found

By observing gravitational lensing-

Short explanation- Immense gravitational force of black holes results in strong gravitational lensing which suggests a possibility of black hole

Long explanation-

Let us first understand what is the meaning of gravitational lensing

This phenomenon is very similar to the working of lenses

We have 2 kinds of lenses-

1- Convex lense- it converges the light coming towards it

2- Concave lense- it diverges the light coming towards it

In both cases, it can be seen that the lens bends the light, only the direction of bending of light is different

This is exactly what a gravitational lens does

“a gravitational lens bends the light rays from a source”

We must keep in mind we can see an object when the light from it reaches our eyes

If we place an object in front of a light source, we are unable to see it because the object will block the light from the source

“Is there a way to see the light source behind the object?”

What about bending the light?

Yes, this will work and we will be able to see the source

“But, how can we bend light?”

There are so many ways to do it, but here I’ll discuss only 1, i.e. gravity

According to the general theory of relativity, an object with mass can bend space-time and this bending results in gravity

Lets put it like this

1- Suppose we have a massive galaxy

2- There is a star behind the galaxy

3- The massive galaxy bends the space around the galaxy

4- When light from the star behind comes near the galaxy, it is bent, because the space around the galaxy itself is curved, so the light has no other path to choose

5- Now this bent light can reach the observer in-front of the galaxy

6- Because of this, the observer in-front will be able to see the star behind

NOTE- the position of the star seen by the observer will be different from the actual position

We have an image of the black hole M87

One of the most obvious reasons for the existence of black holes is that we now have a pictorial proof of it

The first-ever image of the black hole, M87 was one of the most remarkable events in science

· A team of 200 scientists from different countries made this image possible

· 8 telescopes all around the world were used, they all were configured very precisely using atomic clocks, so they can work together as a single big telescope and data from them can be analyzed

· It took nearly 1 and a half years to combine all the data from these telescopes, analyze them and produce the image

· It took this long because the data files from the telescopes were so large that they could not be sent using the internet, so they were transported physically in hard drives

· Moreover, there was one telescope in Antarctica, from which the data could not be brought for months because of bad weather

Will blackhole disappear

Short answer- blackholes can disappear because of hawking radiation

Long answer-

At first, this might seem weird, because I mentioned earlier that nothing can escape from a black hole, then how it can emit a radiation

Blackhole can emit hawking radiation because this is a different kind of radiation

For understanding what is Hawking radiation, you first need to understand what are virtual particles

Here’s a simple explanation

1- Seemingly empty space is not actually empty

2- It is filled with a lot of quantum fields

3- These quantum fields are the fundamental unit of matter

4- Each fundamental particles has its own field, for example- an electron has an electron field

5- Whenever there is an imbalance in these fields, a particle is born, and the propagation of energy through the field is movement of that particle

6- Quantum mechanics states that is empty space, there must be certain pairs of particle and anti-particle popping into existence from these fields

7- These particles are called virtual particles

8- These particles are called virtual particles because they are not observed physically

INTERESTING FACT-

empty space is never perfectly empty, in fact, it requires a lot of energy to create a perfectly empty space

So now you know what is a virtual particle

Let's proceed further and explain hawking radiation

1- We now know that virtual particles are poping into existence in empty space

2- Select a region in space where a black hole is about to form

3- Before there was a BH in that area all the quantum fields were normal

4- As soon as BH arrives in that region the quantum fields there are disturbed and get scattered or deflected

5- So certain modes of vibrations of these fields are lost inside the event horizon of the BH

6- The fields outside the event horizon still have all the modes present

7- Now when the vacuum state is constructed using the distorted nodes, it appears as if the vacuum has particles

8- The type of modes which are distorted by the BH depend on the size of the BH

9- more massive the BH higher the wavelength of particles in its hawking radiation

10- This the way one can also predict the type of particles present in Hawking radiation of a BH, by knowing the size of that BH

This way we can say that a BH will not last forever, it will disappear because of Hawking radiation

You might be wondering the exact mechanism by which hawking radiation takes away the energy of a BH and ultimately vanishes it

By which mechanism hawking radiation reduces the mass of a black hole-

Hawking radiations coming from a BH have more energy that the radiations earlier at that point, this indicates that some energy is imparted to these radiations, so the black hole is losing mass

Here’s an elaborated explanation

1- We know that the vacuum has a lot of quantum fields

2- If we select an area in this region where BH is formed in future

3- If we compare the wavelength at a region outside the BH before and after it is formed

4- We find that the wavelength after the formation of BH has more energy that wavelength at the same point before BH formation

5- This suggests that the presence of BH have added energy to the radiations at that point

6- In other words, it can be said that BH has lost its energy

7- Alternatively, it can be said the BH is loosing mass (mass-energy equivalence)

This was a very broad picture of BH’s mass closing mechanism

But, how do we quantify this energy

Can we calculate this energy radiated?

Yes we can calculate it because

INTERESTING FACT-

One of the most interesting facts about Hawking radiation is that its spectrum is very similar to a body with temperature emitting radiations

Stephen hawking plotted the spectrum of hawking radiations, and surprisingly they were very much similar to thermal radiations emitted by a hot body

Thus he concluded that black holes should have a temperature

INTERESTING FACT-

black holes have a temperature that depends on their mass

Hawking made some calculations and found that-

The temperature of a black hole depends inversely on its mass

This means that

A smaller black hole with smaller mass will have a higher temperature as compared to a larger black hole with more mass

This also suggests

A less massive black hole will radiate at a faster rate than compared to a more massive black hole

This way energy from hawking radiations can be calculated by knowing the temperature

This ultimately leads to the death of a black hole

When a black hole dies what happens to it

When a black hole dies it becomes brighter and brighter and ultimately dies in a gamma-ray burst

BH loose energy (or mass) through hawking radiation which ultimately leads to their death

Here’s how a BH dies

1- As the BH radiates energy, its mass decreases

2- This decrease in mass is also accompanied by the increase of temperature

3- This increase in temperature leads to a faster evaporation

4- the BH evaporates at a faster rate as it loose it's mass

5- this increase in temperature causes BH to loose more and more energy, which makes it brighter and brighter

6- BH continues to shrink and becomes brighter

7- Ultimately they become so small that there temperature becomes infinite

8- At infinite temperature, BH radiates energy at an infinite rate, this is seen as a gamma-ray burst

9- This infinite temperature also corresponds to zero mass

10- Mass of BH becomes zero, it means that the BH is gone

These gamma-ray bursts upon the death of a BH are very powerful and can even be used as a source of energy (if possible)

INTERESTING FACT-

These tiny black holes are so powerful that they can have the energy of about 1000 nuclear bombs

But we have never observed any such tiny blackhole

This is because we currently don’t have any technology to see it

Although these super microscopic black holes could possibly exist because

Our satellites experience some time dilation, which is unexplained, if we consider the presence of these BHs, they could possibly be the reason for this

These small BHs are theorized to be formed within the first few minutes after the big bang, so the study of these tiny monsters could also reveal some information from the past of our universe

Blackhole information paradox-

When som information (matter) goes into a black hole, it is still unknown where this information goes this is called black hole information paradox

Although black holes emit radiation know as hawking radiation but this radiation seems to contain no information about the matter that fell in

Let us first begin by knowing what is meant by information

Information is a kind of description

It can be a description of mass, charge, position, etc

Thinking this way only 1 particle can contain a lot of information like

What mass the particle has?

What is its spin? Etc.

Answers to questions like this are 1 unit of information

Another interesting thing about information is that it is conserved

Meaning it can neither be created nor destroyed but it can certainly, be converted from one form to the other form

For example- if we burn a book what happens to information in the book?

Theoretically, the information is not lost it can still be brought back by combing all the ashes and energy

But what will happen if we throw a book inside a black hole?

This is where the information paradox comes in

One might think that the information is contained in the hawking radiations coming out of the BH

But hawking radiations seem to have no information about the matter that went in

Hawking radiations can give information about the black hole like what is the mass of the BH, what is its temperature, how far it is located

But it has no information about the matter that went in

And because of this hawking radiation, the BH will become smaller and smaller, and ultimately die in a gamma-ray burst

But what happened to the information that went in?

It appears that we have lost the information, but information is conserved so it cannot be destroyed, this is the paradox

It may be possible that we don’t understand how information works, it may be possible that information can be lost, if that’s the case, we will have to develop completely new physics to understand that

There are many solutions to the information paradox, which are an attempt to explain where the information that went in, goes?

one of them is

1- Hologram at the event horizon-

According to this solution to the information paradox, anything that goes into a black hole leaves its information at the event horizon as a hologram

To understand this let us first discuss what is a hologram?

A hologram is a mathematical concept of encoding information from a higher dimension to a lower dimension

Information is encoded to describe how the object in higher dimension would be

For example-

A photograph is a hologram, because a photograph simply encodes some information of a 3-dimensional object into a 2 dimensional are

In a similar way a video is a hologram as well because it is also encoding information 3^rd dimension to the second dimension

NOTE-

it must be noted here that in the process of making a hologram some information maybe lost, like a photograph cannot perfectly describe the depth, whereas a video can do that but still, some information such as what was behind the video camera is not present in the hologram (video)

now we know what is a hologram

let’s see how a hologram is formed at the event horizon

a massive object can cause time to run differently on or near it for an observer at a distance this concept is followed from the general theory of relativity, it is known as gravitational time dilation

in simple terms, it can be said the more the gravity of an object the slower time runs for it when seen far away from it

in the case of black holes, time dilation is so extreme that at the event horizon time moves infinitely slow for an outside observer

this means as if an observer sees an object going inside a BH

· he will see the object to slow down as it approaches closer and closer to the event horizon

· this happens because time becomes slower and slower for the observer

· the object will appear to be at rest at the event horizon

· because of an infinite slow down in time the object must take an infinite time to cross the event horizon

in a nutshell to an observer seeing something falling into a BH, the object appears to slow down and stop, and never cross the event horizon (never enter the BH)

but the time dilation effect is only observed from the outside observer’s frame of reference

in the object’s frame of reference, everything appears fine and the object does cross the event horizon

so which one of them is correct?

Both of them are correct because time is relative

Another way of saying this is that the object goes inside but its reflection (information) is stuck on the event horizon for an outside observer

This forms a kind of hologram at the event horizon

So if we break down matter into pure information we can also say that the black hole is consuming information

Spinning black holes and ring singularity-

Spinning black holes are very interesting because we don’t understand them, even our mathematics and physics fail to understand them

Let us first start by understanding what causes a black hole to spin

1- A black hole is formed from a star and stars can spin

2- When a spinning star dies, is angular momentum remains conserved

3- Because of this conservation of angular momentum the black holes need to spin

4- The black hole formed from a spinning star spins at a higher speed than the star from which it is formed

NOTE- a black hole spins because of the conservation of angular momentum of the spinning star which formed the black hole

When an object spins it is accompanied by a centrifugal force and BHs are no exception

So a spinning BH will also have a centrifugal force

Centrifugal force is one of the most interesting forces because it has an anti-gravity effect

NOTE-The centrifugal force according to the general theory of relativity can have anti gravity effect

This anti-gravity effect is not observed in daily life because it's very negligible

But at the level of BH it becomes very significant and causes an interesting thing, a ring singularity

Here’s how a ring singularity is formed-

1- Singularity is an infinitely dense point, at the center, where all the matter consumed by the black hole ultimately goes

2- antigravity effect of the centrifugal force matter cannot accumulate at the center in a spinning BH

3- now the matter cannot go to the center, so it gets accumulated in a ring, this ring is referred to as ring singularity

a very interesting conclusion that can be drawn from the presence of ring singularity is the presence of-

1- wormholes (read more: what are wormholes)

2- white hole

because the singularity is present in a ring shape, the space inside this ring could possibly be a wormhole

and this wormhole acts as a tunnel between BH and a white hole, this wormhole would open to a white hole at the other end

another interesting thing this ring singularity does is the formation of 2 event horizons

1- outer event horizon

2- inner event horizon

outer event horizon- this is the normal event horizon which every BH has

inner event horizon- this event horizon is formed between the ring singularity and the outer event horizon

here’s how the inner event horizon formed

1- as the matter crosses outer even horizon, it gets attracted because of the gravitational force

2- when the matter goes further it faces the antigravity effect of the centrifugal force

3- there will be a place where the gravitational and anti-gravitational forces become equal

4- this place is where the matter will stop falling in and gets accumulated, this place is called the inner event horizon

the internal event horizon is a place where all matter entering the BH gets accumulated and does not go any further

But

Doesn't this sound weird?

Ring singularity was supposed to be the place where all the matter will be accumulated, but now we have seen that the matter will be accumulated at the inner event horizon

This is a contradiction

Why does it arise?

Because we have no idea what we are doing, we are certainly wrong at some point

What would see if you enter a black hole

You will see the light from the outside universe to compress and form a band around your waist

There are a lot of misconceptions regarding what will you experience if you enter a black hole

It is believed that you experience an intense relativistic beaming, but this picture is incorrect

To understand why its incorrect lets first understand what is meant by relativistic beaming

Relativistic beaming is a phenomenon which occurs at the speed of light or close to the speed of light

Here are the things which you will experience close to the speed of light

1- space will form a kind of cone in front of your face, it's like your view will be compressed into a cone

2- all the wavelengths of light will get blue shifted, this means you will experience a burst of gamma rays and x rays onto your face

this experience is true only if you are hovering just above the event horizon

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