BLACK HOLES IN DEPTH

BLACK HOLES IN DEPTH..

Black holes !! what an amazing and mysterious thing that universe has created. In my previous blog I discussed the basic things about black holes like how they are formed, types of black holes, what is inside black holes and I briefly touched upon the activity that takes place outside the black hole. Now, let us understand black holes in depth, its relationship with thermodynamics and quantum mechanics, some of the theories which enables us to unfold the mystery of this wonderful creation of the universe.

Black holes as we know is a region in space-time where gravitational attraction is so much that even light cannot escape from it. the fact that black holes and thermodynamics are inter-related came to light in 1970 when there was a mathematical discovery which showed that surface area of the event horizon of a black hole has a unique property that it increases as the additional matter or radiation falls into it. It was also said that when two black holes collide and merge to form a single black hole, surface area of the resulting event horizon is greater than sum of event horizons of original black hole. If we look closely at these properties, we will find a striking similarity between surface area of event horizon of black holes and concept of entropy in thermodynamics.

 Entropy as we know is a measure of disorder or randomness of a system, since its bit difficult to digest the concept of entropy let me give some example for it. disorder or randomness of a system refers to the spreading out of energy, for example: melting of ice cubes, a cup of hot coffee turning cold, spreading of cream in coffee. These all explain entropy where energy is being distributed to surroundings. The famous second law of thermodynamics states that entropy of a system always increases with time.

This similarity between black holes and thermodynamics was further continued by Dr Stephen hawking and his team. The first law of thermodynamics states that for a small change in the entropy of a system is accompanied by proportional change in the energy of a system. Here the proportionality constant is temperature of the system. While studying about black holes Stephen hawking and his team found similar law related to this first law. According to that law change in the mass of a black hole leads to change in the surface area of event horizon. Here the proportionality constant is surface gravity, which is the measure of strength of gravity at the event horizon. If we assume event horizon is similar to entropy then surface gravity is also similar to temperature. More over while researching they also found out that surface gravity is same at all points in the event horizon just as temperature is same at every point of a body in a thermal equilibrium.

In 1972 Jacob Bekenstein suggested a theorem about black holes where he said that, when a black hole collapses under its own gravity, it rapidly settles down to a stationary state which involves only three characteristics i.e. its mass, angular momentum and electric charge. Apart from these three properties no data is preserved by black holes. This is called ‘a black hole has no hair’ theorem. This theorem was completely proved by Stephen hawking and his team.

‘no hair’ theorem mainly implies that black hole does not contain any data except its mass, angular momentum and its electric charge i.e. its independent of its matter or antimatter, regular in shape or irregular. In other words, black holes can be formed by the large number of different configurations of matter which leaves us infinite configurations. This problem arises when we see it through classical mechanics. However, in quantum mechanics there is a different perspective. Let us take a look at this in quantum mechanics point of view.

In quantum mechanics uncertainty principle says that, a body of mass m behaves like wave of wavelength h/mc where, h is a Planck’s constant and c is velocity of light. If a cloud of particles is about to form a black hole then the wavelength of that wave should be less than the black holes that is about to form. This shows that a black hole of given mass, angular momentum and electric charge has large configurations but its finite.

This indicated that black holes have finite entropy which is proportional to surface of event horizon, it also has finite temperature which is proportional to surface gravity. This gives us a black hole can be in equilibrium with thermal radiation at some temperature other than zero. But according to classical theories there is no such equilibrium exists because black hole absorbs all the radiations and emits nothing. But in 1974 Stephen hawking proved that black holes actually emit some radiations. Consider black hole as thermodynamic system of certain temperature which is proportional to surface gravity and inversely proportional to mass. When we do this the result we get is black hole also emits particles thermally like a usual hot body of some temperature. This also proved that black holes a finite entropy at a equilibrium at some temperature other than zero.

This gave us a proof that black holes are not completely black at all instead they emit thermal radiation at a steady rate. This emission of thermal radiation is what we call as Hawking radiation. If we want to understand this emission still in deep we have to take a look at some of the theories of quantum mechanics. These theories and some of other interesting facts about black holes I will discuss in my upcoming blog.

SCIENCE OF BLACK HOLES

BLACK HOLES

Several questions strike our mind after hearing the word black hole.

What does black hole mean? How does it work? How does it look like? Who named it? How it is formed? Seems INTERESTING, in this blog I will be answering some of these questions in a very simple and understandable manner.

Let’s start our journey to the black holes by answering the simple question. What is black hole? And let’s see who coined it?

In simple terms a black hole is region in space which is massive and dense and has a strong gravitational force that even light cannot escape from it. Any light emitted from the surface of the region is dragged back by the gravitational field. These regions are what we call as black holes. The term black hole was coined by an American scientist John Wheeler in 1969.

The next question which pop up’s in our mind is How are these black holes formed?

To understand how a black hole is formed we have to first understand the life cycle of a star. A star is formed when a large amount of gas (hydrogen) starts to collapse under its own gravitational attraction. As the hydrogen gas contracts, the atoms of the gas collide each other. Initially when these atoms collide they tend to repel each other. But after sometime the collision takes place frequently and at such great speeds that the gas heats up and these hydrogen atoms combine to form Helium.

During this nuclear fusion reaction there is large amount of heat released which makes the star shine. This heat also increases the pressure of the gas until it is sufficient to balance the gravitational attraction, and the gas stops contracting. For ex: A balloon where pressure of air inside is trying to expand it and the tension in rubber is restricting it. This is how star is formed and remain stable.

Now you might think How does black hole forms because of this phenomenon?  

Answer is after a period of time all the hydrogen is combined to form helium and the star runs out of fuel i.e. all hydrogen is converted to helium. As the fuel gets over there is imbalance between the pressure and gravity and the gravitational force takes over the pressure, due to this core of the star starts to collapse under its own gravity and forms a baby black hole.

This baby black hole starts to feed on the star i.e. it starts to consume the mass of star slowly and after sometime the star explodes to supernova explosion.

But there is condition here. If the mass of the star is 1/4^th of the mass of the sun then it would form a neutron star and if the mass of star is more than that of the sun then it would form a black hole. This is how a black hole forms.

Interesting isn’t it?

Now let us see how many types of black holes are present and how they are differentiated?

There are mainly three types of black holes

Ø Stellar black holes

Ø Supermassive black holes

Ø Miniature black holes

Now let us understand a little about all the different types of black holes and how they are formed?

Ø Stellar black holes:

Stellar black holes are formed when a massive star explodes when it runs out of fuel, i.e. normal black holes.

Ø Supermassive black holes:

We don’t know how these black holes are formed but supermassive black holes are found in heart of every galaxy. Their mass is equivalent to billions of suns. Many believe that these are by products of galaxy formation. The largest supermassive black hole that we know is S50014+81, which is 40 billion times our mass of the sun.

Ø Miniature black holes:

No one has ever seen a miniature black hole. These black holes have mass less than that of sun and these are said to be formed at the time of big bang.

So, now to summarize what we learnt- What are black holes? How they are formed? and What are the different types?

Now let’s take a look at black holes and understand its basic properties.

The above image which you are seeing is a stellar black hole. If you observe it closely, there is shiny ring like part at the edge of black hole. This is called event horizon. An event horizon is a boundary in space time mainly associated with black holes where escape velocity becomes more than speed of light and even light cannot escape from it. The interesting fact about it is the closer you move, slower the time becomes. Time freezes when you enter it but remember one thing once you enter there is no way you can come out.

We now understood what happens outside the black hole does anyone know what happens inside of it? Let us study what happens inside the black hole.

When you enter the black hole through the event horizon time freezes but you will be observing everything moving in fast forward. That is like moving into the future. From the above image you can clearly see Once you enter the black hole there is only one direction you can move. The core of the black hole is called singularity. A singularity may be infinitely dense i.e. all its mass is concentrated in a single point in space with no surface or volume. We cannot describe it clearly, its like dividing anything by zero. There is no way you can go until singularity because, the moment you enter black hole you will be dead. The gravitational pull is so strong that your cells starts to stretch until they are torn apart and all you will find is hot plasma of your body. So, this is what happens inside a black hole. Therefore, you cannot enter a black hole practically.

Now a question maybe arising in your mind. Does a black hole have an end? Or when does it die?

Well, to answer this question we need to understand some phenomenon. As I said earlier at event horizon nothing can escape until its speed is more than speed of light but there is also a phenomenon taking place at the edge of the event horizon. Black holes radiate their mass in space like a hot pot of water losing its water in the form of steam. This phenomenon is known as HAWKING RADIATION. This process takes place at unbelievably slow speed. It would take a black hole with a mass of our sun 10,000 billion billion billion billion billion billion years to lose 0.0000001% of its mass. So black hole does die but it takes billions and billions of years to do so.

After all this information there are still some interesting facts about these black holes, let us take a look at some of them.

Ø If u fall into a black hole, you will see the entire future of the universe unfold in front of you in a matter of moments, and you will emerge into another spacetime created by the singularity of black hole.

Ø When two black holes are about to collide, objects around them can travel backwards in time

Ø There is a belief that if you can enter a black then you can time travel.

Ø If you were to orbit a black hole in its sphere and look to one direction you would see the back of your own head.

So, here we are at the end of our journey to the black holes. It may be end of our journey but there are still many things to learn about this strangest and mysterious thing in the universe.