Three Roads to Quantum Gravity

I'm almost done reading this book called "Three Roads to Quantum Gravity" by Lee Smolin. It's a fascinating subject matter. Quantum gravity (QG) is the theory that unifies the general theory of relativity and quantum mechanics.

So how are these 2 seemingly disparate areas unified? After all, general relativity describes the world of large objects like galaxies, stars, and planets, while quantum mechanics describes the world of small objects like atoms, electrons, and quarks. Smolin argues that it requires a radically different view of space and time.

The three roads to QG are respectively derived from quantum mechanics (string theory), relativity (loop quantum gravity), and independent mathematical approaches that have unexpectedly appeared in the other approaches (e.g. spin networks described below).

I've written about string and superstring theory before, but this is the first time I've read about loop quantum gravity (LQG). LQG derives from relativity theory. Some physicists have reformulated the general theory of relativity including Einstein's gravitational equations. Applying quantum theory to these equations led to already known equations describing quantum gravity (the Wheeler-Dewitt equations), but the simpler form of relativity enabled exact solutions to be found (Smolin was among the group to discover them). These solutions describe the quantum states of the geometry of space and time. It was found that these states were loops. As long as the loops didn't intersect, link or knot with each other, they solved the QG equations. Smolin explains that other conditions need to be satisfied in order to handle the intersections, links and knots. Also, the loops can be grouped together to form spin networks invented by Roger Penrose. These spin networks describe the quantum geometry of space!

There are several ideas advocated by the LQG approach. The first is that space and time are discrete, not continuous. This isn't apparent in everyday reality, but is at the miniscule Plank scale. The second is that they are relational, and not absolute stationary entities. LQG thus advances a background-independent view of space & time. All the World is not a stage, but rather a network of interrelated processes.

In later chapters, Smolin shows how the three pathways could be converging into one theory. He is an advocate of M-Theory which unifies (supercedes?) all the previous string theories. I don't know how his thinking has changed since the book was published in 2002; but I've enjoyed reading this book as it is!