BRITAIN's leading cosmic physicist, Professor Stephen Hawking, has

accepted the possibility of time travel -- an idea he ridiculed for

years.

''If you combine Einstein's general theory of relativity with quantum

theory, it does begin to seem a possibility,'' he told The Sunday Times.

However, the Cambridge mathematics professor added that he thought

time travel would probably never be practical.

Professor Hawkings had previously scoffed at the idea, using advanced

science and ordinary logic to argue that something which could let you

kill your ancestors or alter your past was unthinkable.

''The best evidence that time travel will never be possible is that we

have not been invaded by hordes of tourists from the future,'' he said

two years ago.

His change of heart is spelled out in the foreword to a new book, The

Physics of Star Trek, by US astronomer Lawrence Krauss, which is due to

be published next month.

''One of the consequences of rapid interstellar travel would be that

one could also travel back in time,'' writes Professor Hawking.

Speculation that time travel may be possible was prompted by Einstein,

who showed that time and space are closely related and that both are

affected by gravity.

His ground-breaking theories led to the idea that enormous

gravitational fields, such as those around collapsed stars, known as

black holes, could reverse the flow of time.

However, harnessing enough energy for a machine to send travellers

back through time would pose a major problem.

Professor Hawking adds: ''We may not yet be able to boldly go where no

man or woman has gone before but at least we can do it in the mind.''