One of the liveliest scientific debates is whether our universe is unique. Unlike some controversies, the multiverse debate is highly interdisciplinary. Philosophers, theologians, and of course physicists all cling to entrenched positions about the multiverse’s reality. Perhaps no other topic than the multiverse has so clearly and passionately raised the question of what constitutes science.

Into this debate comes The Number of the Heavens: A History of the Multiverse and the Quest to Understand the Cosmos by Tom Siegfried, the former editor-in-chief of Science News and author of three books including Strange Matters: Undiscovered Ideas at the Frontiers of Space and Time (2002). Despite his journalistic credentials, Siegfried is not a dispassionate observer or tour guide. He boldly asserts his opinions as he enumerates the many ways humanity has understood the multiverse from antiquity to today.

Combining interviews of modern physicists and philosophers with a detailed historical narrative of ancient, medieval, and Renaissance interpretations of the word “world,” Siegfried’s text fills an important gap in the expanding body of multiverse literature. His approach to the multiverse is liberal: Possible multiverses have, at times, included other planets in our solar system, other stars in our galaxy, planets around other stars in our galaxy, other galaxies in our universe, and parallel universes in both space and time. Siegfried thus gives the interpretation of the multiverse a wide berth. However, only the parallel-universes interpretation is relevant to modern physicists.

The Number of the Heavens is intended for a general audience. The book’s first two-thirds is replete with ancient conceptions of the multiverse—though clearly the ancient thinkers were usually considering other possibly habitable planets rather than multiple universes. Nowadays, no scientist or philosopher doubts the existence of such worlds, even Earth-like ones, whereas the evidence for multiple universes is as yet nonexistent.

Siegfried sees the modern interpretation of the multiverse as an inevitable logical evolution of Copernican thought. Why would our universe be singular? Seven other planets aside from Earth populate our solar system alone, 100 billion or more suns are in the Milky Way, and a similar number of galaxies exist in the observable universe.

Siegfried writes about many historical scientists, Copernicus included, who entertained the notion that some version of a multiverse may exist. The historical prologue, though interesting, takes up most of the book. That leaves only a few chapters for Siegfried’s juiciest prose on the modern meaning of the multiverse: brane worlds, Everett’s many-worlds theory, and the inflationary multiverse.

The imbalance between ancient and modern multiverse conceptions isn’t the book’s only shortcoming. Most notably, the perspectives of multiverse opponents are only minimally covered. As an experimental cosmologist working to constrain models of inflation, I was disappointed not to find a description of the most promising approach to discovering evidence for inflation and potentially the multiverse as well: B-mode polarization of the cosmic microwave background.

Siegfried rejects science philosopher Karl Popper’s idea that the demarcation between scientific and unscientific theories lies in the ability to be proven false. But Siegfried proposes no alternative. Instead, he cites the opinions of Steven Weinberg, Leonard Susskind, Sean Carroll, Lisa Randall, and other multiverse proponents, or asserts his own opinions. His referring to opponents of the multiverse with the pejorative “deniers” links them with deniers of evolution, climate change, or worse. A less partisan presentation would have improved the book’s balance. Finding multiverse opponents is not exactly difficult; an informative, if heated, debate recently took place between multiverse proponents and opponents in reaction to the article by Anna Ijjas, Paul Steinhardt, and Abraham Loeb in the February 2017 issue of Scientific American. Healthy debate is a welcome feature of the multiverse landscape.

Other books on the multiverse include Brian Greene’s The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos (2011) and Max Tegmark’s Our Mathematical Universe: My Quest for the Ultimate Nature of Reality (2014). Those texts largely eschew the history of the multiverse and cover the pertinent, if esoteric, physics more thoroughly.

The Number of the Heavens shines when Siegfried adopts a journalistic neutrality about the arguments for and against the multiverse. His wry wit is evident throughout, but nowhere more so than when relating past episodes of confusing and even contradictory interpretations of metaphysical ideas.

At the outset, Siegfried stresses that “there is no greater story in science than the human quest to comprehend the cosmos.” Our understanding of the universe is rapidly expanding. But physicists still debate the definition of the words we use to describe the cosmos’s capaciousness. Even the term multiverse constantly evolves. At least with The Number of the Heavens, we finally know where it began.

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Brian Keating is Chancellor’s Distinguished Professor of Physics at the University of California, San Diego, and author of the book Losing the Nobel Prize: A Story of Cosmology, Ambition, and the Perils of Science’s Highest Honor (2018; see the review in Physics Today, May 2018, page 57). His research focuses on the origin and evolution of the universe through cosmic microwave background measurements in Chile and Antarctica.