Time travel paradoxes
But there are some logical objections, since travel to the past throws up a number of paradoxes. The best-known are the grandfather paradox and the knowledge paradox.
The grandfather paradox
If backwards time travel is possible, then what is to stop someone from going back to a time before the birth of his father, in order to kill his grandfather? On the face of it, nothing. However, if he succeeds he will prevent the birth of his father, and thus guarantee his own non-existence - which seems absurd.
The knowledge paradox
Backwards time travel seems to allow scenarios such as the following. A young man is given a book by an old man. The book contains information about how to build a time machine. The young man builds the time machine.
Years later, as an old man, he travels back in time and gives the very same book to his younger self.
This is a very weird state of affairs. The book is neither created nor destroyed. Furthermore, the knowledge contained in the book comes from nowhere, without anybody expending any effort to acquire it.
Can the paradoxes be resolved?
Some scientists, anxious to avoid such paradoxes, have opted to rule out the possibility of time travel by invoking some kind of physical law that forbids it. For example, Professor Stephen Hawking, author of A Brief History of Time, has proposed a chronology protection conjecture, which says that the laws of physics will always conspire to prevent backwards time travel.
Other scientists believe that the paradoxes can be resolved. One school of thought maintains that time travellers' actions will be subject to unusual constraints. Put simply: you can visit the past, but it will be impossible to change it.
Another school of thought maintains that the many-universes interpretation of quantum theory holds the key to resolving time travel paradoxes.
According to this theory, physical reality consists of a collection of parallel universes. A traveller journeying backwards through time emerges in a different universe from the one where his journey began. Imagine that Fred travels back in time and murders his grandfather, Bill. His journey begins in a universe where Bill is not murdered - and where Fred himself is eventually born. But when he travels back through time, he emerges in an alternate universe where Bill meets an untimely death - and where Fred is never born.
* The Time Machine by HG Wells (above), Penguin pound;6.99. Classic sci-fi novel: the first to feature a time machine.
* ABC of Relativity by Bertrand Russell, Routledge pound;12.99.
Old-fashioned but elegant introduction to Einstein's theories.
* Time Travel in Einstein's Universe by J Richard Gott, Phoenix pound;7.99. Accessible introduction to time travel.
* How to Build a Time Machine by Paul Davies, Penguin pound;7.99.
Entertaining romp through the physics of time travel.