Why String Theory?,

String theory is among the most ambitious and elaborate theoretical frameworks ever conceived. Even the theory’s most skeptical critics would not deny its mathematical elegance. But skeptics do question its relevance to our physical world. In his delightful little book *Why String Theory?*, theoretical physicist Joseph Conlon takes up that question. In laying out his arguments, he also ventures onto a road less taken by touching on the sociology of string theory, and not just its scientific merits.

Conlon’s book begins with a whirlwind tour of the great advances in physics prior to string theory. The theoretical jigsaw puzzle mapped out by the giants in physics remained, near the end of the 20th century, an unfinished one with several large and crucial pieces missing. Those gaps in our understanding of nature called for a critical rethinking of the principles that govern the universe.

String theory is a part of that critical assessment. After his introductory section, Conlon goes on to describe what string theory was and the twists and turns that shaped the theory into what it is today. Originally proposed in the 1960s as a theory of the strong interaction, string theory has evolved to become “a magnificent theoretical framework that interrelates a very wide range of topics in physics and mathematics,” according to Michael Green, one of the theory’s founding fathers. String theory describes not just the strong force and quantum gravity, but a whole lot more.

The next few chapters, devoted to the theory’s successes, are fast-paced and not overloaded with detail. Conlon does an excellent job of captivating his readers and sparking them to appreciate the marvels of modern string theory. He first discusses how the advent of the anti–de Sitter/conformal field theory (AdS/CFT) correspondence offers a new way to attack vexing problems in strongly coupled quantum field theories, such as those describing strongly correlated condensed-matter systems or the quark–gluon plasma, a novel state of matter that exists at extreme temperature or density.

The author then turns to physics and mathematics, which, as he stresses, have had a long and healthy marriage. String theory spices up that marriage by converting physical intuition into new groundbreaking areas of research in mathematics. One example is mirror symmetry, in which two different geometric spaces known as Calabi–Yau manifolds can be used to describe the same phenomenon. Through string theoretical insight, difficult computations for a Calabi–Yau manifold can be transformed into easy problems for its mirror dual. Conlon also discusses how string theory not only relates to known ideas and scenarios in particle physics and cosmology but also suggests novel ways to go beyond them.

As an active practitioner making those connections, Conlon gives a firsthand account of the research that elucidates the potential experimental con sequences of moduli and axion-like particles—features that arise generically in string theory when relating the 10- or 11-dimensional theory to our 4-dimensional world. String theory is, moreover, the prime candidate for a quantum theory of gravity, a status that was sealed by its microscopic account of black hole entropy. All those successes taken together have led many to believe that string theory is not only a mathematical curiosity but also highly relevant to our physical universe.

Despite its remarkable successes, string theory has a tendency to inspire some strong emotion. It is not uncommon to hear “If you cannot test string theory directly, how do you know it is right? And if you do not know it is right, and you will not know anytime soon, why do so many of you work on it?” Such questions are raised not only by critics in the scientific community but also by curious members of the general public who are excited about the fundamental laws of physics. The author carefully dissects those questions and offers his honest, thoughtful responses. Instead of forcing his conclusions on the reader, he presents a set of compelling arguments for the value of string theory while acknowledging its weaknesses and open challenges. Like courtroom juries, readers are encouraged to draw their own logical conclusions.

A physicist colleague once remarked to me, “There is rarely any doubt about the existence of dinosaurs, although almost certainly no humans were alive 65 million years ago to observe the extinction.” Like paleontologists finding evidence for ancient life, string theorists work to tease out subtle signals from nature even though direct experimental verifications are currently out of reach. Conlon writes with flair and wit to give a lively account of the struggles and joys of theoretical physicists seeking to uncover the inner workings of a beautiful, wide-ranging theory.

**Gary Shiu** is a theoretical physicist at the University of Wisconsin–Madison. His research explores the deep connections of string theory to particle physics and cosmology, particularly the early universe.