Quantum Gravity :
In the field of theoretical physics Quantum gravity is a field that describes the gravity according to the principle of quantum mechanics(Quantum physics) in which even Quantum effects cannot be ignored, such as in the locality or visibility a black hole or similarly on the large compact astronomical objects where the effects of gravity are so strong, such as neutron stars and planets like Jupiter.
As we all know there are Four Fundamental forces:
•Weak Nuclear Force
•Strong Nuclear Force
Out of these four forces three can be described within the framework of quantum mechanics and Quantum Field theory.
But the fourth force that is gravity is not within the framework of quantum mechanics. The present understanding of the gravity is based on general theory of relativity which was given by Albert Einstein.
Since this theory is formulated within the entirely different framework of classical physics that is the reason why quantum mechanics and quantum field theory are not included in it. And also this is the reason why gravity is not described at quantum level.
However this also fails in describing the gravitational field of a black hole in the general theory of relativity. And also it fails to describe physical quantity such as the spacetime curvature diverge at the centre of the black hole.
So from above failure of general theory of relativity It clearly shows that we are in need for a theory that goes beyond general relativity into quantum. So to fulfill this need scientists are trying to Succeed Quantum gravity theory.
This theory will help in explanation of gravitational field of a black hole and gravity at distances very close to the Centre of the black hole.
Some scientists also believe that in this, quantum fluctuations of spacetime are expected to play a vital role. So we badly need this quantum gravity theory to describe these quantum effects.
Now we have learnt many things about quantum gravity. Now lets talk about how our theorists are actively exploring and developing a variety of approaches to the problem in the field of quantum gravity.
The most popular approaches so far are M-theory and loop quantum gravity. All of these approaches are done aiming to describe the Quantum behaviour of the gravitational field.
However, many approaches to quantum gravity such as most popular string theory, these types of theory try to develop a Framework that can describe all fundamental forces (gravity, electromagnetic, Weak and strong nuclear force). such theories are often referred to as a theory of everything.
While others, such that loop quantum gravity make no such attempt in explaining theory of everything. Instead, they make an effort to quantize the gravitational field and merge it with other three force group rather than separating from the other forces.
Now the question is if this theory is so important then why can’t it is done so far. So the answer is: one of the difficulties of formulating a quantum gravity theory is that the effects of quantum gravitation only appears at length scales near the planck scale, around 10^-35 meters.
which is so smaller and it can be only accessible with so higher energies than, those which are currently available in high energy particle accelerators. Because of this these type of thought experiment approaches are So difficult to do.
After observing all fundamental forces (except gravity) We found that all Fundamental forces (except gravity) have one or more than one known messenger particles. This Observation lead to the concept of Graviton.
According to theorists a graviton is hypothetical particle which is messenger particle of gravity. As we all know Photon of the electromagnetic interaction.
In the similar way this graviton work for gravity interaction. But again I would like to recall, it is Just a Concept of hypothetical particle And according weinberg -witten theorem graviton is a composite particle in their theories.
While many theorists believe that gravitons are an vital theoretical step in a quantum mechanical description of gravity. And also generally many believed that gravitons are in-detectable because they interact too weakly.