## Summary

In 1988, Stephen Hawking, Lucasian Professor of Mathematics at the University of Cambridge, published *A Brief History of Time*. It was among the most successful popular books about science ever written. *The Universe in a Nutshell* is its sequel. Unlike the first book, the sequel comes with stunning graphics and photographs to illustrate the concepts in the text, and each has an extended caption to clarify its relevance. The text itself, however, is much more cursory in presenting the intellectual and scientific background of current cosmology, and so readers who recall the first book are best prepared to appreciate the second.

Always charming, frequently funny, and at times bewildering, *The Universe in a Nutshell* describes a universe that sounds like a carnival fun house: Nothing is really as it seems; mysteries and perils lie hidden because of humans’ confined, specialized point of view; and at the end of the book, readers, thoroughly entertained, may yet wonder what exactly has gone on. Hawking wrote the seven chapters so that each could be read more or less independently of the rest, with the exception of the first two, which explain the foundation of modern cosmological theories. Accordingly, there is some repetition among the chapters, but it is never tedious—often, indeed, it is helpful, given the complexity of the concepts involved. The narrative is intended for a general readership, and so Hawking faces the dilemma confronting all popularizers of physics. He must explain theories that are mathematical in nature to readers who do not have a background in the abstruse mathematics. Although he includes equations now and then, he relies for the most part on descriptions and analogy. He is brilliant at it, too. The graphics help, but even those readers who can imagine, for instance, eleven-dimension space-time or recognize it in the two dimensions of an illustration must accept a fair amount simply on faith—faith in such baffling entities as supersymmetry, string theory, superstrings, and M-theory, which have emerged from the theorists’ equations like Proteus rising from the sea.

The first chapter, “A Brief History of Relativity,” is familiar territory for the armchair scientist. It recounts how Albert Einstein’s special (1905) and general (1915) theories of relativity permanently changed the way scientists conceive of time and space. The special theory demonstrated that matter and energy are forms of one another and that the speed of light is the fastest velocity possible. These findings led to conceptual revolutions. Specifically, Einstein demolished the idea of a fixed medium in the universe (“ether”) and with it the possibility of a single, absolute perspective on motion; also, the interchangeability of mass and energy led to development of atomic weapons and energy. The general theory was even more radical. It showed that gravity and acceleration are the same thing and that mass curves space near it. From the general theory grew modern cosmology and the exotica of astrophysics: black holes, gravitational lensing, the “big bang” origin of space-time, and the expansion of the universe.

The second chapter, “The Shape of Time,” extends the discussion of Einstein’s general theory of relativity. Hawking considers how the theory prescribes the character of time. In the extreme curvature of space near a black hole, time actually stands still, and here Hawking’s own pioneering contributions to theoretical physics come into play. He explains his theorem, derived with Roger Penrose, establishing that general relativity requires the universe to have originated from a single event and that when large stars eventually collapse upon themselves and form bodies so dense that not even light can escape, time comes to an end for that mass.

The greatest problem in modern cosmology, Hawking continues, is that general relativity does not accord with the other great physical theory of the twentieth century, quantum theory, which Einstein also helped develop. General relativity appears to break down at the infinitesimal scales at which quantum mechanical effects apply. It is therefore the goal of theoreticians in the twenty-first century to create a theory that weds quantum mechanics and relativity. There have been many attempts. The leading contenders, Hawking believes, are supersymmetric supergravity theories and string theories, which were published mostly during the 1980’s and 1990’s. Both, he further contends, are probably...

(The entire section is 1836 words.)