Quantum Gravity

Standard model of elementary particle is a model which tells us in good way that how matter is formed, it also explains how three forces out of four fundamental forces works at quantum level. We can say that this model is answerable to all the questions. Still this model is incomplete because this model doesn’t tell how gravitational force works at quantum level. We know all the carriers bosons of three fundamental forces, but no boson particle is observed which is responsible for gravitational force. Many theoretical physicists believe that these particles should be present in this universe. This hypothetical particle is named as Gravitons.

Fundamental force
Carrier Boson Particle
Electromagnetic force
Weak nuclear force
W/Z Bosons
Strong nuclear force
Gravitational force
Gravitons  (Still not found)
Our universe is well explained by the general theory of relativity
Our Universe

We can easily understand the behavior of our universe if we successfully find gravitons. Beside this we will get a unified theory which will be called Theory of Everything. 
It is assumed that reader has elementary knowledge of standard model of elementary particles. Elementary particles are classified into two categories namely fermions and bosons, fermions are building block of matter while boson particles works as carrier of fundamental forces. In other way we can say that fundamental forces exist only due to bosons. The name bosons was given to the particle on the name of Satendra Nath Bose who was a Indian Origin theoretical physicist. He is well known for contribution in field of quantum mechanics. Especially he kept the foundation of Bose-Einstein statistics and Theory of the Bose-Einstein Condensate. Paul Dirac was responsible to give the name boson to these elementary particles who was famous for Paul Dirac Equation. He named the particles as bosons because these particles obeys Bose-Einstein statistics. 

Gravitons and Quantum Gravity

In early twentieth century two theories changed the world first theory is quantum theory and the second one is the general theory of relativity. Quantum theory helps to understand behavior of elementary particles at quantum level like how these particle interact with each other, how they formed, how they behave and show properties etc. At quantum level matter behaves strangely, at this stage it is possible that at one time particle may present at two different positions. i.e. they possess dual character of particle and wave. This leads to the conclusion that simultaneously we cannot predict their momentum and position with 100% accuracy. Quantum theory may be very strange but the predictions made by this theory are true, that’s why theoretical physicist believes that the theory is correct. 
On the other hand the general theory of relativity tells us what is gravitational force and how does it works. According to this theory the massive objects warps the space-time due to which distortion occurs in fabric of space-time which results in gravitational force. The predictions made by the general theory of relativity are true like gravitational lensing and gravitational waves are main. It can be said that general theory of relativity is correct in its domain. The Quantum theory and the general theory of relativity work very well in their respective domain. In today modern world the subject of quantum gravity is a subject of focus among theoretical physicist because it may join general theory of relativity and the quantum theory. 

What is quantum gravity?

As the name indicates quantum gravity is that theory which explains gravity on basis of principles of quantum mechanics and not on the basis of classical mechanics which can explain the gravitational force on quantum level like other three forces.
In simple words
General theory of relativity + Quantum theory = Quantum Gravity.

Why do we need it?

There are many reasons whose answer can be quantum gravity. All the objects in this universe can be categorized into three categories: 

1. The object that we counter in our daily life and the laws of classical mechanics explains the behavior of these objects very well.

2.The elementary particle like electrons, protons, neutrons etc. quantum theory explains behavior of these particles with accuracy but the quantum theory is applicable only to those particles which are so small and light that the gravitational force between them can be neglected. 

3.The massive object like stars, planets, black-holes etc. general theory of relativity explains the behavior of existing gravitational force between these massive objects but this theory is applicable only to those objects which are so large and heavy that their quantum properties can be neglected. 

What if an object is tiny and heavy at the same time like center of black-hole or singularity? This is very small in size but dense in weight. At present time we have not a theory which can tells us what exactly is present at center of black-hole. The big bang was also due to singularity. We don’t know why big bang was happened or what was before big bang? Quantum gravity can answer this question. Second reason to give importance to quantum gravity is that in standard model we have explained the behavior of matter and three forces and gravitational force cannot be the exceptional one means in future we can explain gravitational force at quantum level. At the time of Large Hadrons Collider when the conditions like early universe was recreated it was found that at a stage electromagnetic force and weak nuclear force was unified to a single force called electroweak force. Similarly all the four fundamental forces are the manifestation of a single unified force. This theory is called Grand Unified Theory. 
The predicted properties of gravitons:

1. The gravitons should be massless because the range of gravitational force is infinite like electromagnetic force which is possible only if its carrier particle will be massless like photon in case of electromagnetic wave. 

2. The spin value should be 2 because gravity is a second rank tensor field, the spin value is different from rest of the particles because spin value for photon is 1 and it is ½ for electron. 

3.  It should be electrically neutral.
It is impossible to find gravitons in LHC because it is a very weak force that we know at present stage. Why this force is so weak is also a mystery. At present time we have only one theory which can explain the existence of graviton if it will proven. The theory is string theory. 
I shall take this theory in other article.

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