Albert Einstein, arguably the greatest physicist of the twentieth century, showed several decades ago that there are nexuses between gravity, motion, space and time. In his celebrated general theory of relativity, he demonstrated that gravity and motion are very closely related, not only to each other but also to the geometry of space and time. This theory was so dramatic in terms of its impact on physics because it departed orthogonally from previous thinking, which regarded space and time as absolute and fixed. Einstein’s theory of general relativity and his subsequent theory for matter and radiation (the quantum theory) required researchers to break definitively with Newtonian physics. Despite the dramatic progress made by Einstein, author Lee Smolin points out that a lot more remained to be done. This is because Einstein’s theories had defects and were also incomplete. Given this state of affairs, physicists all over the world have been hard at work attempting to come up with a “third theory” that will unify all of physics. This book is about the attempts that have been made by various physicists in their quest to shed light on some of the key foundational issues that have confronted and continue to confront the discipline of physics.

Smolin begins the proceedings by enumerating what he believes are the five “great” problems in theoretical physics. The first problem, the problem of quantum gravity, involves combining general relativity and quantum theory into a single coherent theory. The second problem involves resolving the current difficulties in the foundations of quantum mechanics. The third problem requires the researcher to ascertain whether the various particles and forces can be unified in a theory that explains them all as expressions of a single basic entity. The fourth problem requires the researcher to show how one chooses the values of the free constants in the so-called standard model of particle physics. The fifth and final problem involves explaining both dark matter and dark energy. As Smolin helpfully points out, these five problems are “great” because they denote the boundaries of current knowledge in physics and because these are the very problems that must be solved by any theory claiming to be a fundamental theory of nature.

Is string theory such a fundamental theory of nature? A basic idea in string theory is that elementary particles ought to be viewed not as point-like particles but instead as vibrations of strings. This central idea and the body of work that has developed from and around it have revolutionized contemporary theoretical physics. The initial promise of string theory was tremendous and, as a result, a number of very talented physicists began to conduct research in this field. From obscure beginnings, string theory gradually came to dominate theoretical physics to such an extent that individuals working in other areas or pursuing alternate approaches to the questions being addressed by string theorists were progressively either marginalized or were simply unable to obtain post-doctoral and faculty positions in the leading research universities in the United States.

This overarching dominance notwithstanding, Smolin tells us in painstaking detail that string theory thus far has shed little useful light on the five great problems of theoretical physics. In his words, string theory “has not been successful enough on any level to justify putting nearly all our eggs in one basket.” Therefore, claims Smolin, it is now time to look beyond string theory and, more generally, to completely alter the way in which research in theoretical physics is conducted. This will involve recognizing that it is in general bad for science when a small community of researchers comes to dominate a field before its theory has met the usual evidentiary tests. Further, it needs to be understood that even though theoretical physics is now in a “revolutionary period,” most theorists are attempting to work on the key outstanding research questions using the ill-suited techniques of normal science. Finally, to remedy the unsavory present state of affairs, university administrators and others must not only recognize the key differences between researchers who are “craftspersons” and those who are “seers”, but they must also take steps to ensure that young and potentially high-risk “seers” get a fair shake in the faculty recruitment process.

Occasionally, this book rambles when making specific points. In other instances, Smolin’s prose is less than exemplary. One could also credibly claim that Smolin pays excessive attention to his favored approaches and less attention to other interesting approaches, such as the non-commutative geometry-based approach to quantum gravity pioneered by Alain Connes. These caveats notwithstanding, it is important to note that this is a thought-provoking book that provides an informative and, for the most part, well-reasoned perspective on the problems confronting modern theoretical physics. Laypersons who are interested in learning more about the key unsolved problems in theoretical physics, and the directions that future research on these problems might take, will profit by perusing this book.