Difference Between Loop Quantum Gravity and String Theory

The key difference between loop quantum gravity and string theory is that loop quantum gravity does not attempt to unify fundamental interactions, whereas string theory is a theoretical attempt to unify all four fundamental interactions.

Loop quantum gravity is a theory that comes under quantum gravity, and it aims to merge quantum mechanics and general relativity. String theory is a theoretical framework where the point-like particles (of particle physics) are replaced with D objects names strings. The four fundamental interactions that are discussed above in the key difference section are gravitational interactions, electromagnetic interactions, strong interactions, and weak interactions.

1. Overview and Key Difference
2. What is Loop Quantum Gravity
3. What is String Theory
4. Loop Quantum Gravity vs String Theory in Tabular Form
5. Summary – Loop Quantum Gravity vs String Theory

What is Loop Quantum Gravity?

Loop quantum gravity is a theory that comes under quantum gravity, and it aims to merge quantum mechanics and general relativity. This is done through the incorporation of a standard model into the framework of the pure quantum gravity case. We can abbreviate this theory as LQG, and it is a candidate for quantum gravity, where it competes with string theory.

We can understand this theory as an attempt to develop a quantum theory of gravity. We can do this development depending on Einstein’s geometric formulation where we do not treat gravity as a force. There, we need to assume that the theory of loop quantum gravity has quantized space and time analogously to the quantization of energy and momentum in quantum mechanics. Therefore, this theory gives us an indication of spacetime where space and time appear granular and discrete directly due to the quantization, which is similar to the photons in the quantum theory regarding electromagnetism and the discrete energy levels of atoms.

Figure 01: CMS Particle Detector

Moreover, this theory postulates that space’s structure consists of finite loops woven into a fine network similar to fabric. We call these networks spin networks. However, space itself prefers an atomic structure. There are two research approaches for this theory, which include the more traditional canonical loop quantum gravity and the new covariant loop quantum gravity.

What is String Theory?

String theory is a theoretical framework where point-like particles of particle physics are replaced with D objects names strings. This theory can describe the propagation of strings through space and their interactions with each other. When it comes to larger scales, a string tends to appear as an ordinary particle having its mass, charge, etc., and we can determine these through the vibrational state of that string.

We can observe that the string theory considers one or more vibrational states of a particle string as a corresponding property to gravitation, which is a quantum mechanical particle carrying gravitational force. Therefore, we can say that string theory is a theory of quantum gravity.

Moreover, string theory contributes to the advances of mathematical physics that are applied in a variety of problems regarding black hole physics as well as in early universe cosmology, nuclear physics, etc.

When considering the history of this theory, it first came to stage in the 1960s as a theory of the strong nuclear force. However, it was abandoned in favor of quantum chromodynamics. Later, scientists understood that the major properties of string theory make it unsuitable for nuclear physics and were assigned to the quantum theory of gravity. We can identify the earliest model of the string theory as bosonic string theory. This theory included only boson particles, which was later developed into superstring theory which indicated the relationship or the “supersymmetry” of bosons and fermions.

What is the Difference Between Loop Quantum Gravity and String Theory?

Loop quantum gravity is a theory that comes under quantum gravity, and it aims to merge quantum mechanics and general relativity. String theory is a theoretical framework where the point-like particles (of particle physics) are replaced with D objects names strings. The key difference between loop quantum gravity and string theory is that loop quantum gravity does not attempt to unify fundamental interactions, whereas string theory is a theoretical attempt in unifying all four fundamental interactions.

The below infographic summarizes the differences between loop quantum gravity and string theory in tabular form.

Summary – Loop Quantum Gravity vs String Theory

The key difference between loop quantum gravity and string theory is that loop quantum gravity does not attempt to unify fundamental interactions, whereas string theory is a theoretical attempt in unifying all four fundamental interactions. The four fundamental interactions that are discussed above in the key difference section are the gravitational and electromagnetic interactions, strong and weak interactions.

Reference:

1. “String Theory Explained: A Basic Guide to String Theory – 2021.” MasterClass, MasterClass, 5 May 2021.

Image Courtesy:

1. “CMS Higgs-event” By Lucas Taylor/CERN – (CC BY-SA 3.0) via Commons Wikimedia
2. “Calabi yau formatted” By derivative work: Polytope24 / Calabi yau.jpg: Jbourjai (using Mathematica output) – This file was derived from: Calabi yau.jpg (Public Domain) via Commons Wikimedia