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20 Jun 2018
Taken from the June 2018 issue of Physics World

Benjamin Skuse reviews On Gravity: a Brief Tour of a Weighty Subject by Anthony Zee

Physicist and writer Anthony Zee has written a number of specialized books on various physics topics, including a trio of (ironically named) weighty tomes for graduate physicists: Quantum Field Theory in a Nutshell (608pp), Einstein Gravity in a Nutshell (888pp) and Group Theory in a Nutshell for Physicists (632pp). But he has also penned more accessible popular-science books – his Fearful Symmetry: the Search for Beauty in Modern Physics, for instance, garnered rave reviews for its exposition of the physicist’s search for an understanding of the universe.

His latest offering, On Gravity: a Brief Tour of a Weighty Subject, is ostensibly neither of these, being written “to help people bridge the gap between popular books and textbooks on Einstein gravity”. Inside, Zee’s deep enthusiasm and wit shine through, as the reader pauses to visit the four fundamental forces of nature, zoning in on Newtonian gravity, before reaching the book’s final destination of Einstein gravity.

Here Zee invites the reader to get off the tour bus and explore concepts familiar to lay physics enthusiasts, such as curved space–time, but also more up-to-date topics such as the great effort required to confirm Einstein gravity through the Laser Interferometer Gravitational-Wave Observatory (LIGO) detection of gravitational waves announced in 2016. The tour ends on a high, revealing continuing efforts to bring gravity and quantum physics together in the form of quantum gravity, and what the LIGO detection may herald in terms of opening a new window to the cosmos, which might just expose the nature of dark matter and dark energy.

Though pop-sci in spirit, there is a sense the author is yearning to break the shackles of simple exposition in the book, frequently halting flow with asides or sending the reader on endless diversions to footnotes and a whopping 177 endnotes. Often coming across like a “Choose Your Own Adventure” children’s gamebook from the 1980s, the reader is constantly impelled to flick back and forth between the paragraph they are reading and the footnotes/endnotes, occasionally more than once in a single sentence. Sometimes this effort bears fruit, providing unusual titbits like the fact that French mathematician Pierre de Fermat’s year of birth is unknown because his father named two sons from two different wives both Pierre. At other times it is highly frustrating, interrupting the story only for the reader to discover the endnote is simply referencing the author’s other more detailed textbooks.

Leaving aside the awkward reading experience and the feeling that the book is simply an abridged version of his weightier back catalogue, Zee is more often than not accomplished in combining analogies and humour to make challenging topics understandable. Like many writers before, he uses the passing train analogy to clarify subtleties in special relativity, and uses Newtonian gravity’s instant influence on distant bodies to explain away the mystery of quantum entanglement that Einstein famously called “spooky action at a distance”. But he also has his own analogies, quirkily likening the least time principle for light to a chiselled Richard Feynman saving a drowning girl by taking not the shortest but the best possible path to the stricken swimmer.

Zee does not rely solely on these tricks of the trade to draw in the reader. In a chapter dedicated to the LIGO experiments, he tones down his idiosyncratic style and provides a clear and concise introduction to gravitational-wave astronomy. This serves as a stalwart explanation of the experiments involved, as well as the scale of the achievement, in terms of both physics and politics.

Zee offers a new and refreshing base from which to delve deeper than most popular-science books

Where Zee shines brightest, though, is in explaining often-ignored concepts and basic mathematics that help the reader gain a more fundamental understanding of the subject. By clearly describing the action principle, for instance, On Gravity offers a new and refreshing base from which to delve deeper than most popular-science books into the most pressing problems in fundamental physics.

The action principle provides a means of looking at a physics problem in a different way, allowing complicated equations of motion in classical and quantum physics to be written concisely. As an example, Zee tells us: “Maxwell’s eight electromagnetic equations are replaced by a single action, specifying a single number for each possible history describing how the electromagnetic field changes.” This neat action formulation is then put to use to help peel back the curtains on the quest for a “Grand Unified Theory” of physics, and why dark energy is the leading candidate to explain the accelerating expansion of the universe.

By refusing to patronize the reader (as some pop-sci authors do when they only include fully established theories or omit mathematics), Zee takes us on a whirlwind tour of gravity that opens a window to advanced topics including Hawking radiation, the cosmological constant problem and quantum gravity. As a result, On Gravity provides a fresh way to understand the concepts behind relativity and a good introduction to the latest challenges in fundamental physics.

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