Exactly one hundred years ago, in 1895, H. G. Wells classic story The Time Machine was first published in book form. As befits the subject matter, that was the minus tenth anniversary of the first publication, in 1905, of Albert Einstein's special theory of relativity. It was Einstein, as every schoolchild knows, who first described time as "the fourth dimension" -- and every schoolchild is wrong. It was actually Wells who wrote, in The Time Machine, that "…showed first 75 words of 546 total…

…showed last 75 words of 546 total…clocks run slow. Yet all of these predictions of relativity theory have been born out many times in experiments, to an impressive number of decimal places. And when you look closely at the general theory of relativity, the best theory of time and space we have, it turns out that there is nothing in it to forbid time travel. The theory implies that time travel may be very difficult, to be sure; but not impossible.