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Carlo Rovelli and Smolin defined a nonperturbative and background-independent quantum theory of gravity in terms of these loop solutions. Jorge Pullin and Jerzy Lewandowski understood that the intersections of the loops are essential for the consistency of the theory, and the theory should be formulated in terms of intersecting loops, or graphs .
In 1988, Rovelli, Lee Smolin and Abhay Ashtekar introduced a theory of quantum gravity called loop quantum gravity.In 1995, Rovelli and Smolin obtained a basis of states of quantum gravity, labelled by Penrose's spin networks, and using this basis they were able to show that the theory predicts that area and volume are quantized.
Loop quantum gravity (LQG) thus became related to topological quantum field theory and group representation theory. In 1994, Rovelli and Smolin showed that the quantum operators of the theory associated to area and volume have a discrete spectrum. [11]
The Order of Time is divided into four sections, covering the theory of relativity, space-time, loop quantum gravity, and thermodynamics.The first section, The Crumbling of Time, opens with Rovelli explaining time, which is considered as a fourth dimension in space-time.
In loop quantum gravity, the present spin foam theory has been inspired by the work of Ponzano–Regge model. The idea was introduced by Reisenberger and Rovelli in 1997, [ 2 ] and later developed into the Barrett–Crane model .
The physical content of the theory has not to do with objects themselves, but the relations between them. As Rovelli puts it: "Quantum mechanics is a theory about the physical description of physical systems relative to other systems, and this is a complete description of the world". [3]
In relativistic physics, Lorentz invariance states that the laws of physics should remain unchanged under Lorentz transformation.In quantum gravity, Lorentz invariance measures the universal features in the hypothetical loop quantum gravity universes; which is a hypothetical theory that explains the quantum theory of gravity based on a geometrical interpretation of the theory of relativity.
In loop quantum gravity (LQG), a spin network represents a "quantum state" of the gravitational field on a 3-dimensional hypersurface. The set of all possible spin networks (or, more accurately, "s-knots" – that is, equivalence classes of spin networks under diffeomorphisms) is countable; it constitutes a basis of LQG Hilbert space.