Quantum loop gravity
Loop quantum gravity (LQG) is a theory of quantum gravity, merging quantum mechanics and general relativity. It is an effort to unify gravity in a common theoretical framework with the other three fundamental forces of nature. It is the leading competitor to string theory. LQG begins with relativity and tries to add quantum features, while string theory, conversely, begins with quantum field theory and tries to add gravity.
From the point of view of Albert Einstein's theory, it comes as no surprise that all attempts to treat gravity simply like one more quantum force (on par with electromagnetism and the nuclear forces) have failed. According to Einstein, gravity is not a force – it is a property of space-time itself. Loop quantum gravity is an attempt to develop a quantum theory of gravity based directly on Einstein's geometrical formulation.
In order to do this, in LQG theory space and time are quantized, divided into discrete units. The theory gives a physical picture of spacetime where space and time are "granular". The granularity is a direct consequence of the quantization. It has the same nature as the granularity of the photons in the quantum theory of electromagnetism and the discrete energy levels of atoms. Here, it is space itself that is discrete. In other words, there is a minimum distance possible to travel through it.
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 10−35 meters. 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 space itself, has an atomic structure.
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