I enjoyed this book on three levels, though initially I felt it made my brain expand.

**Firstly, understanding physics theories.**

Greene did a great job explaining the complex physics concepts using analogies and thought experiments. For example, I would have never imagined that quantum tunnel has anything to do with balancing bank accounts or people walking through walls. I’d prefer, however, that he emphasize the Fundamental Postulate and the logical reasoning underlying Special Relativity instead of describing many fictitious experiments that seem to obfuscate the logic.

String Theory unites General Relativity and Quantum Theory into one grand framework, which is one of its main appeals. String Theory solved/avoided the problem that plagued General Relativity, i.e., the singularity at zero radius, because of the dimension of the string particle, but it requires the introduction of additional dimensions. Moreover, the string particle possesses both vibration and winding energies, which balance each other as the radius of the circular dimension of the space increases or decreases, therefore the physical properties of a large circular dimension are symmetric (around the radius of Planck length) to those of an infinitely small circular dimension. Distance is relative indeed.

**Secondly, how to evaluate whether a theory is true?**

All theories are suspect until proved by experiments. String theory has not yet been proved, but, as Greene pointed out, Einstein proposed his theory of General Relativity long before it was corroborated by observations. On the other hand, theories that have been validated by experiments, such as Newtonian classical physics, may still be proved wrong (or at least incomplete) by more sophisticated experiments and measurements.

One of the pillars of the String Theory is the concept of symmetry, which is presumably mathematically and aesthetically elegant. I can’t help wonder if there may be higher dimensions or more complex levels of symmetry, which the String Theory overlooked because it focused on a lower level symmetry.

**Thirdly, the joy and excitement of discovery**

In describing his own research, Greene conveyed to the readers the joy and excitement of discovery. He may be biased, but his enthusiasm is certainly contagious. How many victory laps would he take if the string theory is proved to be true?

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