<p>Time travel is a familiar theme of science fiction, but is it really possible?<br><br>Surprisingly, time travel is not forbidden by the laws of physics - and John Gribbin argues that if it is not impossible then it must be possible.<br><br>Gribbin brilliantly illustrates the possibilities of time travel by comparing familiar themes from science fiction with their real-world scientific counterparts, including Einstein's theories of relativ...