Five Photons - James Geach ****

It is generally acknowledged that Stephen Hawking's A Brief History of Time is one of the most common books to be bought but not read beyond the first few pages. If you are the kind of popular science reader who found Hawking hard going, you can stop now - Five Photons is not for you. If, on the other hand, you found A Brief History of Time a piece of cake and wished you could get into more depth without resorting to heavy mathematics or a tedious textbook style, Five Photons could be just up your street.

Astrophysicist James Geach starts of fairly gently with a chapter on the nature of light that mostly sets aside quantum physics, leading up to the observation that light is our vehicle for for stripping back the history of the universe to its earliest times (or, at least, the point where the universe became transparent). From here on, the five photons of the title take us on different journeys, from the oldest surviving light of the cosmic microwave background radiation to that from around black holes.

All that makes this book sound like a fairly straightforward cosmological title, even if it does have the rather nice 'five photons' hook. What makes it different (and definitely not for everyone) is the depth that Geach goes to - not in a mathematical sense, but in describing subtleties of the work of astrophysicists and cosmologists that popular science titles usually gloss over. Each chapter opens fairly gently, but soon we're plunged into the detail.

A good example is the opening of the fourth chapter, Dark Energy's Imprint. Geach starts by telling us a cosmology joke from a seminar: 'I once sat in a cosmology seminar that opened with the line "What is the integrated Sachs-Wolfe effect?" This prompted a few moments of awkward silence from the audience until the speaker continued, "It's like the Sachs-Wolfe effect... only integrated!"' As Geach says, maybe you had to have been there. But Geach goes onto describe what the Sachs-Wolfe effect is - a variation in gravitational redshift of photons emerging from the early universe as the cosmic background, depending on the density of the region they emerge from - and, indeed also explains the integrated effect, which brings in similar variations on the photon's journey due to dark energy.

I need to stress again that Five Photons is only for the advanced cosmology buff. Think of it as a sequel to A Brief History that explains some of the real detail of what has been discovered and brings in factors cosmologists have to consider that you won't see anywhere else in a popular science title and you won't go far wrong. And if you are in the audience for that, it's great.

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Review by Brian Clegg