An Introduction to Loop Quantum Gravity (Three Roads to Quantum Gravity by Lee Smolin)

Since the postulation of theory of relativity (theory of cosmos, which describes the structure of space and time), and quantum mechanics (laws of microcosm, which describes atomic structure, nuclear forces, and nature of basic component of matter); physicists until now have struggled to explain gravity (which is a manifestation of spacetime fabric in presence of matter) in terms of quantum mechanics (quantum gravity). In this book the author attempts to explain three different approaches to quantum gravity; Loop Quantum Gravity (LQG), Superstring - M theory (S. -M), and Blackhole Thermodynamics (BT). While each takes a different starting point, they all agree when viewed on Planck scale, and they also view space and time are not continuous, and space is composed of discrete units. LQG gives us a detailed picture of these units in terms of spin networks, where as S.-M theory proposes continuous space in terms of a continuous string (with compactified extra dimensions) made of string bits, which is governed by uncertainty principle. BT theory states that amount information in any given space is finite and is proportional to the area of the boundary of the region in Planck units. The author is a pioneer in the field of LQG and provides the reader with a good introduction of the theory in a non-mathematical form and then compares with S.-M and BT theories. The book is described in three parts; the first part is a general introduction, which describes historical development of three theories, the second part introduces LQG and then compares with S.-M and BT theories, and the final part attempts to unify the three approaches into a single theory using Holographic Principle.

The author gives us several interesting accounts of physicists working in these fields are in a climate of mutual ignorance and complacency with the belief that their theory is correct and others are wrong. There are instances when one group can't solve certain problems, and they seek the help from the other camp. The author also briefly explains other theories such as Twister theory, and Non-Commutative Geometry. This is one of the few books I have read which describes LQG in some detail, although there are several books in literature, which describes S.-M theory. The author is very honest in comparing the three approaches to offer the best explanation for quantum gravity. Anyone who wants to understand LQG must have this book.

Reference: Three Roads to Quantum Gravity by Lee Smolin