Hawking Radiation

Hawking's Radiation

Black holes: What is Blackhole?, What we know about black holes? How they work?, Is there is any black hole in our Universe. All these questions annoying physicist for the last 5 decades.

We all know that Event horizon is Point of no return in black holes. In Actual, Event horizon is a boundary of black hole outside the event horizon an object can pass unless it faces the gravitational pull from the black hole, but if an object crosses the boundary of event horizon then it becomes impossible for an object to escapes from its strong gravitational pull, Even light cannot be able to escape from its strong gravitational pull.

Normally, when we are talking about strong gravitational pull actually we're talking about the boundary of the Event horizon.

Stephan Hawkings first discovered that light rays which enter event horizon and light rays which are outside the event horizon doesn't approach each other.

It means that the area of event horizon increases or remains the same but never decreases. In other words, as long as the black hole does not swallow any object, Its area remains the same, if it swallows something then its area increases. 

If two black holes collide with each other then their area increases and increase in the area always greater than the area of each black hole before the collision, Increase in the area means an increase in area of the event horizon.

The Properties of Blackhole resembles with the 2nd law of Thermodynamics which means the entropy of the system always remains constant or increases with time, but never decreases.

If we combine the two systems, then the entropy of the resultant system always greater than the entropy of the individual system, but never decreases.

Increase in entropy means if we left any system which is initially in perfect order state then it moves towards disorder state,

The biggest problem which faced the physicists is that Black holes violate the 2nd law of thermodynamics. 

What happens when an object which has high entropy enters a black hole for eg  like gas particles,

what happens next is that the entropy of the matter which do not fall into black hole decreases which violates the 2nd law of Thermodynamics because there is always increasing in entropy but in this case entropy decreases.

According to some Physicists, The entropy of matter which is outside the black hole never decreases. They believe that the entropy of the matter outside the black hole is balanced by the entropy of the matter which fall into black hole. It looks like entropy remains same.

But it is proved wrong because we cannot see what's inside a black hole and how it looks and we cannot calculate the entropy of the matter which fall inside black hole.

One night Stephan Hawkings was enjoying the beautiful view of moon and he thought if there was any way that could help us to calculate the entropy of the matter which fall inside the black hole then we could be able to tell eveyone how the entropy of the matter which falls increases.

And at the time Jacob Beckenstein came forward and suggested Hawkings that the measurement of the area of the event horizon is the measurement of entropy because there is always increase in area of event horizon whenever a new object fall inside black hole.

If we sum up the entropy of the matter which presents outside the black hole and increases in the black hole after falling off object, then it was found that it never decreases, it means it follow 2nd law of thermodynamics.

It seems that the problems about Blackhole is solved but there is something which bothered him and he was constantly thinking about that problem. And the problem is that if black holes have an entropy which means it also have temperature and any object which has some temperature always emits some kind of radiations.

And it is necessary for an object to emit radiations which has some temperature in order to follow 2nd law of thermodynamics.

But we know that black holes have strong gravitational field which cannot even allows light to escape through it then how it is possible for an black hole to emit radiations. 

To solve this problem, Stephan Hawking's meet too many physicists everyone has same results. If Black holes have temperature then it must emit radiations which depend upon the mass of the black hole.

According to Calculations by many Physicists- It was found that more the mass of the black hole, the low the temperature.

But question still remains the same, HOW?

And this problem is solved by Quantum Theory, According to this theory, The radiations are not emitted by black hole but the space present near the event horizon.

In Quantum Mechanics, Empty space doesn't mean Emptiness in space, It means the net result of the field is zero. Change in these values is just like change in position and velocity of an object.

According to the Heisenberg Uncertainty Principle, It is not possible to determine both the position and momentum of the particle at the same time.

If we know the exact position of an object in space then we are uncertain about the Momentum of that object and vice verse.

That's why in empty space the value of the different field is not zero, It means there is change in values in both cases which actually means there are some quantum fluctuations.

In empty space, the matter and anti-matter both occur in pairs and for a shorter period of time and then combines with each other and destroy itself. This type of particle are called Virtual Particles.

Virtual particles are not easily detected but we know that it exists in our universe because we see only its effects like Energy changes occur in Electron orbit which resembles approximately to the theoretical proof of it.

According to the Uncertainty principle, The real particles like Electron, protons, quarks also have their virtual particles that is one is matter and the other is antimatter.

We know that the virtual particles come into existence in empty space and destroy itself.

What Happens if these particles come into existence near the event horizon?

In this case, virtual particles come into existence but before destruction, the negative particle get absorbs by the black hole and the positive particle get emitted out which were seen as radiations emitted from the black hole and such kind of radiations emitted from the black hole is known as Hawking Radiations.

The reason behind why negative particle get attracted towards the black hole is that the energy of the negative particle is much lesser as compared to the energy of the positive energy particles. Positive energy particles have sufficient amount of energy which prevent it to fall on black hole.