What is time? If you're a practising physicist, it's a quantity in your equations, t. This is the variable that you use for one of the four dimensions of the manifold of space-time, the term coined by mathematician Hermann Minkowski after Albert Einstein's theories of relativity began to show that time and space are fungible. And yet we can move freely back and forth in space but not in time. Why?

In Time Travel, science writer James Gleick reviews the science of time by focusing (mostly) on the science fiction of time travel. He starts from, and often returns to, H. G. Wells's The Time Machine, which predates Einstein's 1905 special theory of relativity by a decade. It's a pleasurable romp over Wells's fourth dimension and polished Victorian machinery; 'golden age' science-fiction authors such as Isaac Asimov, who provided the templates for modern treatments of time travel; and the Doctor Who franchise (A. Jaffe Nature 502, 620622; 2013). Gleick also explores more highbrow offerings from writers such as David Foster Wallace and Jorge Luis Borges (who envisaged time as a “Garden of Forking Paths”), and filmmaker Chris Marker, whose 1962 sci-fi short La Jetée inspired 1995 time-travel noir 12 Monkeys.

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