Quantum Field Theory (QFT) and Quantum Mechanics enable us to unite all the three fundamental forces of nature, except the gravity which could only be understood properly by General Theory of Relativity (GTR). Now, this is because General Relativity coins gravity as an effect of distortion of space time and has no subatomic particle in reality , on the other hand QFT deals with all real fundamental particles and hence they both clash with each other.

So we know have two theories which completely describes our universe:

1. General Theory of Relativity – Gives a separate but true understanding of the gravity force.

2. Quantum Theory – unites electromagnetism, strong and weak nuclear forces into one. (Which are all a part of the standard model of the universe)

Now something needed to be done in order to form a single unified theory which can describe the grand design of universe as a whole. So, some scientists introduced the topic of Quantum Gravity.

At the beginning when string theory was proposed, it was only for bosonic particles. It was later accounted for a presence of supersymmetry between fermions and bosons and thus led to formulation of Super-String theory. Five Consistent Versions of Super-String were developed thereafter and all are now considered to be limiting cases of a single theory in 11 Dimensions known as M-Theory.

Now I express what I feel as the most favourable candidate at the present. Disadvantages:-

1. String Theory – It assumes that the universe has 11 dimensions. All the 11 dimensions of universe are very difficult to detect and the strings also are next to impossible to detect as they are of very small order and we will need a very large particle collider which has yet not been developed. Moreover its vibrations are difficult to detect. It also talks of super-symmetries so many particles are still there in the universe whose counterparts are not present.

2. Loop Quantum Gravity Theory(LQGT) – The basic idea of Loop Quantum Gravity theory is very fascinating but it has deep flaws in it. It destroys Lorentz Invariance. Einstein’s Special Theory of Relativity states that the length of the object contracts in the moving direction of the observer. And the contraction is different for different observers moving with different speeds. This is valid for these energy packets. Hence , they disobey the principle of relativity which is the basis of special theory of relativity and thus talk of a special frame of reference. Now this is contrary to Einstein’s GTR also which cancels the idea of an absolute frame. So, this theory in this sense is self contradictory.

(a) But now some physicists also claim that the quantization of angular momentum of particles doesn’t mean there is any prefered direction of angular momentum, similarly quantization of space-time sheet doesn’t mean violation of Lorentz transformation, so no evidence of absolute frame .

(b) String Theory has no conflict in this matter as it very well Lorentz Invariant. They obey all the fundamental laws of nature , because they are inside the space time whereas loop quantum gravity does not talk of inside the space but its solutions reveal no. Of possibilities of space times , they rather constitute space time not inside the space time.

(c) String Theory has very correctly proved the Hawking Radiation for an extremal group of black holes . Whereas LQGT has also claimed to do so but only after introduction of a peculiar and artificial value for a parameter known as Barbero-Immirzi Parameter .

(d) String Theory works at the singularity of black hole where general relativity fails but LQGT also depicts the possibility that the black hole might be a gateway to another universe that might be a parallel universe!! (This is solely my idea) , because LQGT believes in Roger Penrose’s Spin Network or in layman’s terms universe might never ever die but continue in a repetitive cycle of birth, expansion and then again crunch down and again form a new universe and this cycle will continue till time immortal known as Big Bounce.

So, what I have personally realised studying all these that String Theory works well with the present conditions of universe as it has given correct evidences of many cosmic events but Loop Quantum Gravity Theory also is equally important. You cannot take one and leave another as it is (in others at present both win) we will need some other solid evidences to take one theory and reject other or take a new theory which is a combination of both of these and in which both will resonate with each other in a fine arrangement. One possible model for that is LQGT which is compatible with the theory of relativity and the string theory are found to be very much compatible according to a recent research (TWO SIDES OF SAME COIN).

So we know have two theories which completely describes our universe:

1. General Theory of Relativity – Gives a separate but true understanding of the gravity force.

2. Quantum Theory – unites electromagnetism, strong and weak nuclear forces into one. (Which are all a part of the standard model of the universe)

Now something needed to be done in order to form a single unified theory which can describe the grand design of universe as a whole. So, some scientists introduced the topic of Quantum Gravity.

**QUANTUM GRAVITY**– This theory gives a quantum picture to the gravity, says let gravity have a fundamental particle, Graviton Bosons. But there is a problem, the formulation of force of gravity using the laws of Quantum Field Theory via gravitons produce discrepancies, and cannot be used to make useful physical predictions. So, the physicists now take more radical approaches to this theory and we introduce two new concepts – STRING THEORY and LOOP QUANTUM GRAVITY THEORY.**String Theory**– String Theory, maintaining the originality of QFT , adds on gravity . Therefore, it hampers the originality of General Theory of Relativity and therefore is less favoured by general relativists. It replaces the zero dimensional points or particles of particle physics with 1 dimensional strings (either open or in closed loops) which have length of the order of 10^-35, planck’s length. So, when viewed from large dimensions, they are observed as zero dimensional particles. The vibrations in these strings are proposed responsible for the mass and charge of particles , one of such vibrations is believed to be giving rise to gravitons.At the beginning when string theory was proposed, it was only for bosonic particles. It was later accounted for a presence of supersymmetry between fermions and bosons and thus led to formulation of Super-String theory. Five Consistent Versions of Super-String were developed thereafter and all are now considered to be limiting cases of a single theory in 11 Dimensions known as M-Theory.

**Loop Quantum Gravity Theory**– This was another proposition made by the physicists. In this theory, they introduced quantum effects in general theory of relativity thus giving GTR the big stand here. Its goal is to unify the theory of gravity in the common theoretical framework as the other three fundamental forces . Just as there are energy packets (photons) in electromagnetic theory , in similar fashion the space time fabric is divided into discrete energy levels (loops ) or quantum packets , and these finite loops are woven with each other in network chain . Thus, the whole space and time is constructed of infinitely small quantum loops at Planck scale (10^-35), having a definite spin .Now I express what I feel as the most favourable candidate at the present. Disadvantages:-

1. String Theory – It assumes that the universe has 11 dimensions. All the 11 dimensions of universe are very difficult to detect and the strings also are next to impossible to detect as they are of very small order and we will need a very large particle collider which has yet not been developed. Moreover its vibrations are difficult to detect. It also talks of super-symmetries so many particles are still there in the universe whose counterparts are not present.

2. Loop Quantum Gravity Theory(LQGT) – The basic idea of Loop Quantum Gravity theory is very fascinating but it has deep flaws in it. It destroys Lorentz Invariance. Einstein’s Special Theory of Relativity states that the length of the object contracts in the moving direction of the observer. And the contraction is different for different observers moving with different speeds. This is valid for these energy packets. Hence , they disobey the principle of relativity which is the basis of special theory of relativity and thus talk of a special frame of reference. Now this is contrary to Einstein’s GTR also which cancels the idea of an absolute frame. So, this theory in this sense is self contradictory.

(a) But now some physicists also claim that the quantization of angular momentum of particles doesn’t mean there is any prefered direction of angular momentum, similarly quantization of space-time sheet doesn’t mean violation of Lorentz transformation, so no evidence of absolute frame .

(b) String Theory has no conflict in this matter as it very well Lorentz Invariant. They obey all the fundamental laws of nature , because they are inside the space time whereas loop quantum gravity does not talk of inside the space but its solutions reveal no. Of possibilities of space times , they rather constitute space time not inside the space time.

(c) String Theory has very correctly proved the Hawking Radiation for an extremal group of black holes . Whereas LQGT has also claimed to do so but only after introduction of a peculiar and artificial value for a parameter known as Barbero-Immirzi Parameter .

(d) String Theory works at the singularity of black hole where general relativity fails but LQGT also depicts the possibility that the black hole might be a gateway to another universe that might be a parallel universe!! (This is solely my idea) , because LQGT believes in Roger Penrose’s Spin Network or in layman’s terms universe might never ever die but continue in a repetitive cycle of birth, expansion and then again crunch down and again form a new universe and this cycle will continue till time immortal known as Big Bounce.

So, what I have personally realised studying all these that String Theory works well with the present conditions of universe as it has given correct evidences of many cosmic events but Loop Quantum Gravity Theory also is equally important. You cannot take one and leave another as it is (in others at present both win) we will need some other solid evidences to take one theory and reject other or take a new theory which is a combination of both of these and in which both will resonate with each other in a fine arrangement. One possible model for that is LQGT which is compatible with the theory of relativity and the string theory are found to be very much compatible according to a recent research (TWO SIDES OF SAME COIN).

This post was written by Pradipta Pathak, Delhi University.

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