“Can you, citizen of the 21st century, recall when you first heard of time travel?” asks James Gleick. He doubts that you can, because, from “morning to night, children’s cartoons and adult fantasies invent and reinvent time machines, gates, doorways and windows, not to mention time ships and special closets, DeLoreans and police boxes”.
I do remember first coming across the idea of time travel; it was in 1968, in an episode of the TV series The Time Tunnel. “Two American scientists are lost in the swirling maze of past and future ages, during the first experiments on America’s greatest and most secret project: the Time Tunnel,” said the voice-over during the opening credits. This short-lived ABC series is a rare omission by Gleick – the acclaimed author of six books including a classic account of chaos theory – as he skilfully weaves together science, technology and culture in a dazzling history of time travel.
The concept of time travel, Gleick argues, is a fantasy of the modern era. He traces its origins to the publication of H G Wells’s novel The Time Machine: an Invention. The year was 1895. Why then? Even though mathematicians such as Bernhard Riemann and Nikolai Lobachevsky had begun to explore non-Euclidean geometry and the fourth dimension, Gleick explains that it was through new technology that time became vivid, concrete and spatial: people could witness the railways crossing great distances on “a co-ordinated schedule, co-ordinated by the electric telegraph, which was pinning time to the mat”.
Wells first learned of a fourth dimension around 1884 and thought it was just “a witticism”. Yet his treatment of time as the fourth dimension preceded by a decade Einstein’s use of the concept in his 1905 theory of relativity. The German physicist discovered that spatial distances and time intervals between events depended on the relative motion of observers. He found that space and time were knitted to form the fabric of the universe: space-time.
Yet there is one crucial difference between space and time. Although it is possible to move in any direction in space, the ticks of the clock send time marching forward for ever. This inexorable flight of time’s arrow from past to present to future is bound up with the second law of thermodynamics. Put simply, the degree of disorder – what physicists call entropy – increases with the passage of time.
Breaking eggs to make an omelette exhibits the irreversibility that is the hallmark of time’s arrow. It is the increase in entropy, in the disorderliness of the world, which makes such everyday events irreversible and separates the past from the future. Eggs cannot spontaneously unscramble, because that would lead to a decrease in entropy. Wells knew all about entropy – it is the fate of the nameless Time Traveller as his machine hurtles onward through millions of years and into a final twilight.
“No idea from science fiction has captured the human imagination as much as time travel,” argues the American astrophysicist J Richard Gott. The possibilities it offers are unlimited. Time travel leapt from the pages of sci-fi novels to those of peer-reviewed journals after it was discovered that Einstein’s theory of general relativity permitted the construction of a time machine – if space-time is twisted to create a loop, it allows a traveller heading into the future to circle back to an event in his or her own past. This is possible in curved space-time because it is like a roller coaster with a loop-the-loop: the cars always go forward but the track circles back to a previous point. These “closed timelike curves” are circular highways that defy ordinary notions of cause and effect: events are their own cause.
All paradoxes are time loops, and Gleick devotes an entertaining chapter to such classics as the “grandfather paradox”, in which you travel back in time to kill your grandfather before he meets your grandmother, thus erasing yourself from existence.
It was to handle such conundrums that the cosmologist Stephen Hawking suggested in 1991 “the chronology protection conjecture”, whereby the laws of physics conspire to prevent time travel to the past by outlawing closed, timelike curves. According to Hawking, strong experimental evidence in favour of the conjecture lies in the lack of tourists from the future. He tells the story of a party he once threw for time travellers: “I sat there a long time, but no one came.”
Gleick is fascinated by the nature of time itself, though he laments that “people keep asking what time is, as if the right combination of words could slip the lock and let in the light”. “Time is nature’s way of keeping everything from happening at once,” said the physicist John Wheeler, and this is one of the “fortune-cookie” definitions that Gleick offers up.
Why do we need time travel? “For history. For mystery. For nostalgia. For hope,” Gleick writes. “To examine our potential and explore our memories.” Will we achieve it? Only time will tell.
Manjit Kumar is the author of “Quantum: Einstein, Bohr and the Great Debate About the Nature of Reality” (Icon Books)
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This article appears in the 22 Feb 2017 issue of the New Statesman, The world after Brexit