A knotty topic unravelled for novices

Few other active research topics in physics can boast as many lay people willing to offer expert opinions as string theory. Bestselling books and TV shows about the elegance of string theory have brought their creators fame and fortune. A recent series of books claimed to exhibit the failures and lies string theory is based on.

Even among physicists, a discussion of string theory is often driven more by prejudices and emotions than by facts and calculations. String theorists tend to talk about the beauty and consistency of their theory in the abstract, holding back on the equations, which are deemed to be beyond anyone lacking a particle physics PhD. Barton Zwiebach's A First Course in String Theory does away with this myth. In this well-written text, he takes us through the hottest topics in string theory research, requiring only a solid background in mechanics and some basic quantum mechanics, nothing beyond the quantisation of the harmonic oscillator.

This is not just one more text in the ever-growing canon of popular books on string theory trying to convey ideas and the big picture without ever being concrete. On the contrary, Zwiebach takes a low-tech approach. A standard textbook on string theory would immediately simplify the analysis by introducing dynamical gravity as an auxiliary variable living on the string. This leads to more elegant equations at the price of requiring advanced mathematics and physics. Zwiebach forgoes such tricks. He spends a lot of time on the classical mechanics of the string. He lays out in detail how to parameterise the string motion and finally leads us to a simple method of writing the classical string, known as light cone gauge, from where quantisation simply amounts to replacing classical oscillators that represent the modes of the string by quantum oscillators.

By the end of the first half of the book, the reader will have rediscovered that strings describe quantised gravity and exist only in a critical space-dimension that is larger than the usual three. Next, instead of introducing string interactions - almost impossible without quantum field theory tools (though Zwiebach tries to do without these in the last two chapters of the book) - the second half of the book pushes the free string as far as it will go and manages to give the basic ideas behind forefront research topics: D-branes, black holes, duality and the gauge/gravity correspondence. Again, the calculations in the book are completely laid out and the practice problems are well designed. Only occasionally does Zwiebach have to appeal to material beyond the scope of the book.

Readers knowledgeable in mechanics and rudimentary quantum mechanics will be able to follow the formulas step by step; no miracles are invoked. The practice problems enable the reader to confirm quantitatively all the claims made in the text. True, this is no bedtime story; the text requires readers to work through the equations and is best used as a textbook for a class in undergraduate string theory. Following the pioneering lead of Zwiebach himself at the Massachusetts Institute of Technology, such classes exist now at various US universities. But one can always digest the text independently if one is willing to do the problems. The effort is well worth it, even if readers decide in the end that string theory is not for them: in the process of learning about strings, one has to reapply concepts scattered across one's undergraduate physics education and tie them all together in a new context.

Andreas Karch is assistant professor of physics, University of Washington, Seattle, US.