My latest book, on the history of the cosmic multiverse, will be published in September 2019 by Harvard University Press. It's called The Number of the Heavens: A History of the Multiverse and the Quest to Understand the Cosmos. The Amazon page is here.   

This book tells the story of the millennia-old debate about whether the universe humans occupy is the only one there is. That debate began with the ancient Greeks, was rekindled in the Middle Ages and was reignited in the late 20th century. Time and again some scholars pondering the cosmos have suggested that the universe as then conceived was not alone, but rather was accompanied in a grander existence by many (possibly an infinite number) of others: a multiverse.

Today, as in ancient and medieval times, this issue is hotly contested. Multiverse advocates cite various lines of evidence indicating that multiple universes might very well exist. Multiverse deniers argue that the multiverse is meaningless metaphysics. My book tells the story of this recurring debate from its beginnings. That story shows that the question of multiple universes has been reformulated multiple times as the concept of "universe" is redefined in light of new knowledge. Whether the current debate will someday be resolved in favor of a multiverse, I can't say. But each time the issue has arisen in the past, the advocates of multiple universes have turned out to be correct. This story has implications not only for the nature of the cosmos, but for what counts as science and the rules for practicing it it properly. 

A nice summary of my book has appeared in Publishers Weekly, which gave it a starred review.

Recently the journal Nature included a brief review in its list of top new science books.


The Number of the Heavens is a thrilling history of our quest to grasp the whole of reality and determine our place within it. Whether there is one universe or many, Siegfried's masterful prose allows us all to delight in our species' passionate urge to look up and wonder.”―Brian Greene, author of The Elegant Universe

“Packed with surprising historical tidbits and witty asides, Siegfried tells the riveting tale of millennia-long efforts to define not merely the extent of existence, but also the nature of science itself.”―K. C. Cole, author of The Universe and the Teacup: The Mathematics of Truth and Beauty

“Prepare to enter the mysterious realm of the multiverse! The Number of the Heavens displays unusual depth across several fields of research, allowing scientists, historians, and the general public to experience firsthand a debate of great cosmological import.”―Steven J. Dick, former NASA Chief Historian

“Examining the positions of medieval thinkers and today’s physicists alike, this book is a very thorough and timely study of the concept of the multiverse through the ages.”―Marcelo Gleiser, author of The Simple Beauty of the Unexpected