Timeswitch

A time traveler from the 20th century is in a desperate race against time across 11th century England in an attempt to change the course of history. But which history? And which 20th century? With chilling overtones of the environmental crisis facing us today, but incorporating the latest scientific thinking about the Multiverse, John Gribbin offers a unique blend of real science and adventure fiction.

Old fashioned hard Sf at its best – all the science in the story is real, even the time travel. It just hasn't happened yet – or has it?