This book is an attempt to make a popular account of the seemingly difficult concept called quantum gravity. Lee Smolin (the author) have been working in two of the approaches to quantum gravity, (string theory and LQG), and in the book also explains a third approach, black hole thermodinamics.(adsbygoogle = window.adsbygoogle || []).push({});

Here is a resumee of the chapters:

1)THERE IS NOTHING OUTSIDE THE UNIVERSE: The author emphasizes that a reasonable theory of quantum gravity should postulate that spacetime have no physical existence, but has to be interpreted as a relationship between objects

2)IN THE FUTURE WE SHALL KNOW MORE: The limit of the speed of light puts a limit to our knowledge of the universe

3)MANY OBSERVERS, NOT MANY WORLDS: Chapter dedicated to quantum theory, explain things like the superposition principle and decoherence

4)THE UNIVERSE IS MADE OF PROCESSES, NOT THINGS: There's not such a thing like an object. In fact, all what we observe pertains to a continuous flow, called process, and an event is defined like the smallest possible unit of a process

5)BLACK HOLES AND HIDDEN REGIONS: Explains what's an event horizon.

Explain how to see the atomic structure of the space taking profit that the light coming from the exterior of an event horizon is redshifted by the gravitational field of the BH

6)ACCELERATION AND HEAT: The Unruh effect: An observer standing still in a vacuum does not see nothing around him, but as it accelerates see himself surrounded by photons.

7)BLACK HOLES ARE HOT: Explains Hawking radiation

8)AREA AND INFORMATION: Introduce some concepts of thermodynamics

9)HOW TO COUNT SPACE: Tell the beginning of the story of LQG. At first, Smolin tried to develop a quantum gravity theory based on lattice gauge theory of Wilson. This attempt failed because it resulted that the theory was based on a fixed lattice and didn't incorporate that space is relational

10)KNOTS,LINKS AND KINKS: Amitaba Sen reformulated general relativity in an easier formalism, and this was profited by Smolin to found LQG, in which space wasn't absolute, and the only thing of importance was the relationship between loops. After a years, he replaced this formalism by a concept called spin networks. Spin networks are evolving graphs. They do not live in space, but they define space in its evolution. The surfaces and volumes of our world are quantized and only can have discrete values. The area of a surface depends on the values of the edges that pass trough it, and the volume of an object is proportional to the number of nodes inside it

11)THE SOUND OF SPACE IS A STRING: Chapter dedicated to string theory.

Smolin see that string theory has the shortcoming that is background dependent

12)THE HOLOGRAPHIC PRINCIPLE: It's a derivation of the concept of Bekenstein bound. Smolin thinks that the holographic principle can serve to unify all 3 approaches to quantum gravity

13)HOW TO WEAVE A STRING: He continues with the hypothesis that string theory and LQG are two faces of the same coin

14)WHAT CHOOSES THE LAWS OF NATURE: Speaks about his theory about cosmological natural selection, and about the anthropic principle

EPILOGUE: He posits some of his predictions, one of them is that we shall have the basic framework of the quantum theory of gravity in between 2010 and 2015

The book is 245 pages long, and also include a glossary.

Not a mathematical book, it include barely 5 equations. Will be interesting to begginners to quantum gravity, but a bit light to experts. If you like books by Hawking, this probably will be good

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# Three roads to quantum gravity

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