The bookshelf in my office(well officle, I share the office with my colleague Sarah,) is my pride and joy. Considerable time has been spent cultivating my favorite Oxford books, raiding bookshelves of employees who are leaving, scouring in-house book sales and generally keeping my eyes open. One of my favorite “finds” is Magic Universe: The Oxford Guide To Modern Science by Nigel Calder. I am generally more of an “Oxford World Classics” kind of gal, but this book kept me reading about science until the wee hours of the morning. Calder makes science (gasp) fun. Anyways, I thought I would share one of my favorite entries. Keep reading to learn whether time travel is in our future.(Cue the Twilight Zone music )
A cartoon in Igor Novikov’s book The River of Time (1998) drawn by the author himself shows a scientist arriving in the Garden of Eden, just in time to stop Eve picking the forbidden apple. It illustrates the problem that seems to many people to rule out the feasibility of traveling back in time. You could not visit the past without altering it, which in turn would alter the present from which you started, so creating a logical impossibility.
More succinctly, you could murder your own grandmother. The conundrum for scientists is that, even if the laws of biology might seem to rule out time machines, the laws of physics do not. On the contrary, they positively incite the idea. Antiparticles, which certainly exist, are indistinguishable from ordinary particles moving backwards in time.
In relativity theory it is trivially easy (conceptually speaking) to travel into the future by making a return flight on a high-speed rocket ship, which brings you home when your relatives are long since dead. If you can spend a while in orbit around a black hole, where time runs very slowly, that will make the difference by your calendar even greater. These effects on time have been verified by observations, although involving particles and light, rather than time travelers.
To visit the past you must either travel faster than light, which is explicitly ruled out in relativity theory, or else use a wormhole in spacetime. This is very like the time machine of popular imagination. You go in at one point, proceed through a tunnel that is effectively outside our Universe, and reappear at a different place and/or time. Nobody knows how to construct a practical wormhole but physical laws don’t exclude it. Should a prohibition turn up some day, that would itself be an important addition to the principles of physics.
If time travel is feasible, then perhaps Mother Nature already makes use of it. If so, the future can affect the past and the implications for ideas of causality are horrendous. A lazy kind of billiards illustrates the possibilities. You just hit the balls in any direction you like. Wormholes linking the pockets enable the very same balls, reappearing from the past, to collide with themselves and send themselves into the desired pockets.
The nature of time has been debated since Plato’s day, yet with a strange lack of firm conclusions. For example, it has long been argued that the laws of heat provide an irreversible arrow of time, with all systems moving towards a more uniform temperature—described as an increase in entropy. Our personal experience, of past times remembered and the future unknown, is compatible with this idea of a thermodynamic arrow. Yet life itself defies entropy, and the theorists have never demonstrated that it rules out time travel.
The discovery of the expansion of the Universe was said by some experts to provide another arrow of time. If it should ever stop expanding and instead contract then, they said, time would reverse, the dead would climb out of their graves, and we should all experience our lives backwards, all the way to our mothers’ wombs. The eminent theorist Yakov Zel’dovich in the Space Research Institute in Moscow dismissed this notion as being as foolish as supposing that a clock in a rocket runs backwards after it has reached its maximum height and begins to fall back to the ground.
Zel’dovich and Novikov speculated about loops in spacetime, as did John Wheeler in the USA, but it was Kip Thorne at Caltech who began to try to pin down the possibility of practical wormholes. He offered it as a means of space travel in a letter to the astronomer Carl Sagan of Cornell, who needed a plausible way of dashing about the Universe for the characters in his sci-fi novel, Contact (1985).
Thorne recalled later, ‘It introduced to the world, to science fiction, and also reintroduced to serious scientists, the notion of a wormhole as something that is really worthy of thinking about.’ To convert a space machine into a time machine would not be a trivial task, but Thorne continued to puzzle over the problem and so did Novikov, when he moved from Moscow to Copenhagen in 1991.
‘Are time machines possible?’ Novikov asked. ‘That seems to me the most important question in contemporary physics. If they are feasible for Nature, we must look for their effects. And if wormholes ever become practicable for engineers, our descendants will no doubt want to use space machines to travel very widely in the Universe, and time machines to explore the past and the future.’
Antiparticles with anticlocks
Take a long, narrow strip of paper and paste the ends together, after putting a half-twist in one end. You then have a Mobius strip, an object popular with mathematicians ever since its description was found among the papers of August Mobius of Leipzig after his death in 1868. The innocuous-looking artifact remains a good starting point for further radical thoughts about the nature of space and time.
Suppose you live on a Mobius strip, like the ants in a well-known painting by Maurits Cornelis Escher. You find matters of orientation trouble-free as long as time machines you don’t travel very far. But take a walk all the way around the strip, and you’ll come back to your starting point on the underside of the strip, separated from your friends on the topside. Up and down have become ambiguous.
Worse inconveniences occur if you live inside a Mobius strip. Then, if you set off on your travels as a right-handed person, you’ll come back left handed. Or if you’re a subatomic particle spinning to the right, you’ll be spinning to the left on your return. The space in the strip is said by mathematicians to be nonorientable.
A theoretical physicist at Warwick, Mark Hadley, considered what would happen if time, too, were like a Mo¨bius strip, and non-orientable. Now a particle making the round trip comes back, not only spinning the other way but with time running backwards. The particle has changed into its own antiparticle. Hadley also imagined sending a clock around the loop and getting back an anticlock, which faithfully measures antitime.
This was no idle conjecture, or mathematician’s frolic, but part of a serious campaign to probe the link between relativistic and quantum views of the world. Hadley argued that the well-known mutual annihilation that occurs when a particle meets its antiparticle actually requires that time be non-orientable—in another word, ambiguous. ‘A failure of time orientability and particle– antiparticle annihilation are indistinguishable,’ he concluded. ‘They are alternative descriptions of the same phenomena.’
Perhaps this adds a couple of health warnings to ideas about time machines. The first is that you could go crazy thinking about them. From a more practical point of view, if you’re going to try travelling through space and/or time via wormholes, be sure you don’t get your personal dimensions of space and time scrambled en route, or you could finish up with arms and legs a microsecond long. Oh, and don’t waste money on an expensive watch, lest it finish up running backwards.