Quantum gravity is a theory that attempts to explain gravitational physics in terms of quantum mechanics. It is a theory which has both the principles of general relativity and quantum theory. Quantum gravity does not yet exist as a working physical theory.

Quantum gravity can be considered to be an effective field theory. This is primarily because the energy levels required to observe the conjectures cannot be generated in testing experiments. The quantum gravitational effects only become apparent near the Planck scale. The Planck length is a scale at which the ideas of gravity and space cease to be valid and quantum effects dominate.

Did you know: Spacetime is a term used to describe any mathematical model that brings together dimensions of space and time into a single four-dimensional continuum.

**The two main approaches to quantum gravity are:**

**Loop quantum gravity**

**Loop Quantum Gravity** | Image source: wikipedia.org

It is a theory that tries to explain the quantum properties of the universe and gravity. It is also the theory of quantum spacetime as according to general relativity gravity is the manifestation of the geometry of spacetime. It is an attempt to merge quantum mechanics and general relativity. Gravity cannot be treated as a quantum force as it is a property of spacetime itself. It is an attempt to develop a quantum theory of gravity based on Einstein’s geometrical formulation. The output of the theory is a physical picture of space where space is granular. This granularity is a direct result of the quantization.

In this theory, space itself is discrete and there is minimum distance possible to travel through it. Space can be viewed as an extremely fine fabric or network woven of finite loops. This network of loops is referred to as spin networks. The evolution of spin networks over a period of time is called spin foam. The theory predicts that space has an atomic structure.

All the research in loop quantum theory use the basic physical assumption and mathematical description of quantum space. The theory is being developed in two directions. One is traditional canonical loop quantum gravity and the new covariant loop quantum gravity which is also called the spin foam theory.

One of the most developed application of this theory in cosmology is called loop quantum cosmology. One use of this theory is to study the early universe and the physics of the big bang theory.

**String theory**

**String Theory** | Image source: pinterest.com

This theory uses a model of one-dimensional strings instead of the particles of quantum physics. The springs vibrate at a specific resonant frequencies.

The formulae of this theory predict more than four dimensions and the extra dimensions are curled up within the Planck length. Along with string this theory contains one more fundamental object called ‘brane’. It has many more dimensions.

Research in this theory may lead to a number of theories that could be constructed and may actually develop into ‘theory of everything’. The string theory has mathematical features which give it a great appeal.

The loop quantum gravity studies space and time whereas the string theory studied the behavior of objects within space. Loop quantum gravity does not work in higher dimensions where as string theory requires 10 dimensions. String theory implies the existence of supersymmetry whereas loop quantum gravity does not have supersymmetry.

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