Introduce to Loop quantum gravity

Loop quantum gravity (LQG) is a theory that attempts to describe the quantum properties of gravity. It is also a theory of quantum space and quantum time, because, according to general relativity, the geometry of spacetime is a manifestation of gravity. LQG is an attempt to merge and adapt standard quantum mechanics and standard general relativity. The main output of the theory is a physical picture of space where space is granular. The granularity is a direct consequence of the quantization. It has the same nature of the granularity of the photons in the quantum theory ofelectromagnetism or the discrete levels of the energy of the atoms. But here, it is space itself which is discrete.

More precisely, space can be viewed as an extremely fine fabric or network "woven" of finite loops. These networks of loops are called spin networks. The evolution of a spin network over time is called a spin foam. The predicted size of this structure is the Planck length, which is approximately 10−35meters. According to the theory, there is no meaning to distance at scales smaller than the Planck scale. Therefore, LQG predicts that not just matter, but also space itself has an atomic structure.

Today LQG is a vast area of research, developing in several directions, which involves about 50 research groups worldwide.[1] They all share the basic physical assumptions and the mathematical description of quantum space. The full development of the theory is being pursued in two directions: the more traditional canonical loop quantum gravity, and the newer covariant loop quantum gravity, more commonly called spin foam theory.

Research into the physical consequences of the theory is proceeding in several directions. Among these, the most well-developed is the application of LQG to cosmology, called Loop quantum cosmology (LQC). LQC applies LQG ideas to the study of the early universe and the physics of the Big Bang. Its most spectacular consequence is that the evolution of the universe can be continued beyond the Big Bang. The Big Bang appears thus to be replaced by a sort of cosmic Big Bounce.

 

Loop quantum gravity is string theory’s biggest competitor. It gets less press than string theory, in part because it has a fundamentally more limited goal: a quantum theory of gravity. Loop quantum gravity performs this feat by trying to quantize space itself — in other words, treat space like it comes in small chunks.

In contrast, string theory starts with methods of particle physics and frequently hopes to not only provide a method of creating a quantum theory of gravity, but also explain all of particle physics, unifying gravity with the other forces at the same time. Oh, and it predicts extra dimensions, which is very cool!

source : http://en.wikipedia.org/wiki/Loop_quantum_gravity ; www.dummies.com/how-to/content/string-theory-and-loop-quantum-gravity.html