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Geoscientist Online

Five Photons: Remarkable Journeys of Light Across Space and Time

Geach five photonsThe Hitchhiker’s Guide to the Galaxy says that “the Universe is an unsettlingly big place”. James Geach confirms the truth of that in this book. In doing so he draws a clear and helpful distinction between the observable universe—constrained by the distance light could travel in the time since the universe formed—and its potential actual size, which has no such constraints. The universe is about 14 billion years old, but, when expansion is taken into account, the radius of the observable patch is about 45 billion light years. (It’s big and the “excess” accounted for, apparently, by its expansion since the Epoch of Recombination, when light first escaped from the early universe some 380,000 years after the Big Bang, so that the observable distance is significantly greater than that which light could have travelled at its maximum measurable speed.)

The book comprises seven chapters, of which each of the central five is about one of the photons in the title. The opening chapter deals with light in terms of both electromagnetic radiation and particles (photons)—a conceptual duality that is hard to explain and harder to understand. Subsequent chapters cover old light, starlight, dark energy, black holes and long-wave (radio) emissions from the universal dawn. It’s all compelling, because the explanations are good—making sense while being relatively simple—and it’s where we live. The illustrations (31 of them) are all monochrome and support the text well. Perhaps strangely, for a book about light, this is one of the rare situations where colour would probably be of little, if any help.

Many abstruse issues are involved in understanding the nature and behaviour of light—taken to include the full electromagnetic (or photon) range, from the shortest (most energetic) gamma rays to the longest radio waves—in astrophysics. Geach provides well thought out explanations of many facets and issues. It is hard to see, for instance, how the conventional concept of massless photons can be applied to a system in which the path of light is bent by a gravitational field. The explanations offered extend from baryons and black holes, via Cepheid variables and the cosmic microwave background (CMB), to general relativity and wave-particle duality. Some explanations are more successful than others, but all work, and the book is interesting and relatively easy to read. It is recommended to anyone with an interest in light and/or space/time.

Reviewed by Jeremy Joseph

FIVE PHOTONS: REMARKABLE JOURNEYS OF LIGHT ACROSS SPACE AND TIME, byJames Geach, 2018. Published by: Reaktion Books, London, UK. ISBN: 978-1-78023-991-0. 184 pp. (hbk.) List Price: £14.95 W: