Time travel is an intriguing concept. Many of us have imagined journeying into the future to see the technological wonders that await us there. Still more of us have dreamed of travelling into the past: to walk with dinosaurs, listen to the Sermon on the Mount, or perhaps watch England's triumph in the 1966 World Cup final.
HG Wells tapped into our fascination with the past and the future with the publication in 1895 of his first novel, The Time Machine. Since then, generations of film-goers and science-fiction fans have thrilled to the time-travelling adventures of Dr Who, the Terminator, Bill and Ted, Marty McFly, and others.
Commonsense tells us that all of this is strictly fiction. In reality, time cannot be altered or manipulated. It is an "ever-rolling stream" flowing relentlessly from past to future, sweeping along everything and everyone at the same unvarying pace. We cannot return to the past, because the past no longer exists. We cannot visit the future, because the future has yet to unfold.
This was the view of time adopted by Sir Isaac Newton (1642-1727), whose scientific theories held sway among physicists for more than 200 years. In Newton's universe, time is absolute, irreversible and unvarying - a principle that is the same for everybody, everywhere.
But Newton was wrong. One-hundred years ago, in 1905, Albert Einstein (see page 10) published his special theory of relativity and astonished the scientific world. He demolished the Newtonian conception of time by showing that time can be stretched and shrunk. A decade later Einstein published his general theory of relativity, which further eroded our commonsense view of time by showing that it can be manipulated - sometimes quite dramatically - by the presence of mass andor energy.
The special theory of relativity (which includes the famous equation E=mc2) shows that time travel into the future is scientifically unproblematic, and the general theory of relativity opens up the tantalising possibility of time travel into the past. Thanks to Einstein, future generations may conquer both space and time.
Time travel to the future
In 1905 Einstein replaced the concept of absolute time with the notion of relative time. He had been studying the way light moves and could make sense of it only by subjecting our notion of time to a thorough overhaul.
His special theory of relativity showed that the rate at which time passes for different observers depends upon their relative velocities.
Einstein predicted that a clock travelling away and then returning to its starting position will lose time compared with a clock remaining stationary at the starting point. The faster the clock travels the more time it will lose. This slowing down of time by motion is known as time dilation.
For the effect to be noticeable, velocities approaching the speed of light are required. This is very fast indeed since light travels at 300,000km per second (spacecraft reach only a fraction of 1 per cent of this speed.) The closer to the speed of light a clock travels, the bigger the time dilation effect becomes. A clock travelling at light-speed would grind completely to a halt. However, the special theory of relativity shows that light-speed is a limit that ordinary material bodies can never quite reach.
Special relativity is a startling theory, but one that has been verified experimentally many times. Physicists Joe Hafele and Richard Keating tested Einstein's time dilation prediction in the early 1970s by sending highly accurate atomic clocks on a long-distance journey aboard a jet-plane. They compared their readings with identical clocks left on the ground. The moving clocks lost precisely the amount of time predicted by Einstein. The difference was only a very tiny amount (0.000000059 seconds) since aeroplanes travel very much slower than the speed of light. But the result was nonetheless a resounding confirmation of Einstein's theory.
It is not only clocks that are affected by motion. All physical and biological processes are slowed down in precisely the same way. In fact, it is correct to say that time itself slows down. This slowing-down of time by motion can have strange consequences. One of these, the so-called twin paradox, provides the key to visiting the future. Imagine a pair of identical twins. If one twin remains stationary on earth while the other makes a round trip at a very high velocity, the second twin will age more slowly than the first. If she travels close to the speed of light for one year on her return to Earth she will find that everyone else has aged 10 years while she will only have travelled forward a year but she won't be able to go back in time to catch up on her lost nine years. She will - quite literally - have travelled forward in time.
This brand of time travel is uncontroversial. It may take centuries of technological advances, but there's no reason why it shouldn't one day become a reality. A time machine capable of travelling into the future is nothing more than a spacecraft able to travel close to the speed of light.
Sadly, the special theory of relativity does not allow us to visit the past since this would require faster-than-light travel, which is forbidden by the same theory. Fortunately, all is not lost for those who dream of travelling backwards in time, thanks to Einstein's later work: the general theory of relativity.
Cosmic deja vu
We normally think of space and time as entirely separate entities. But in Einstein's universe the three dimensions of space and one dimension of time are combined together into a four-dimensional continuum known as spacetime.
All events - past, present and future - can be located by four co-ordinates that specify when (time) and where (along, up and down) they take place.
Rather than thinking of time as flowing from past to future, most physicists accept the idea that events simply exist in spacetime. This conception of time is known as the block universe. The terms past, present and future have no special significance in the block universe. All events, whatever their location in spacetime, are considered equally real. In a letter to a friend, Einstein wrote: "We physicists believe the separation between past, present, and future is only an illusion, although a convincing one."
By this reckoning, the past and the future exist just as surely as the present. They are both "out there". Special relativity already provides us with a method of travelling to the future. Events from the past can also be visited provided we can find a way to navigate through spacetime towards them. This is where general relativity comes in.
In his general theory of relativity, Einstein extended special relativity to take account of gravitational effects. Central to the theory is the idea that gravity is the result of mass and energy warping spacetime.
Very dense matter can warp space and time quite dramatically. Theoretically it is possible for spacetime to become sufficiently warped to lead to the existence of closed time-like curves (CTCs). These are pathways through spacetime that loop back on themselves. CTCs are potentially corridors to the past. If an astronaut could safely navigate one, he could travel into the past and participate in events there.
In 1949, the Austrian logician Kurt Godel offered solutions to Einstein's field equations of gravitation that allow such journeys into the past.
However, his solutions apply only to a rotating, non-expanding universe.
Our universe is expanding and does not appear to rotate. Nonetheless, Godel's calculations demonstrated that unrestricted time travel is possible at least in principle, and led to the search for other situations in which time travel to the past might be possible.
Some physicists have suggested that a rotating black hole could distort space-time sufficiently to produce CTCs (a black hole is a celestial object with a gravitational pull so strong that not even light can escape its clutches). An astronaut plunging into a rotating black hole could, in theory, emerge from it at a time earlier than that at which he entered, but only if it rotated sufficiently quickly to allow him to escape from it.
Unfortunately for would-be time travellers, it seems unlikely that any naturally occurring black holes rotate at the required rate.
If there are no naturally occurring CTCs, then perhaps it will be possible to create them artificially. In the 1970s, physicist Frank Tipler calculated that a rapidly rotating, super-dense cylinder could create a gravitational field capable of distorting spacetime enough to allow time travel. A long, thin cylinder with a mass of 10 suns, spinning at a few billion revolutions per minute would do the trick. The creation of a time machine of this type is, however, not physically realistic.
Kip Thorne of the California Institute of Technology has argued that by manipulating the two ends of a wormhole (a type of shortcut through spacetime) one could form a CTC. Creating a traversable wormhole suitable for travelling through time is fraught with scientific and technological difficulties. But that is not to say that it is impossible.
If such a thing is possible, it will require the resources of a civilisation far in advance of our own. One thing is for sure, if we ever do succeed in constructing a time machine it will be a very complex and curious affair. Not at all like the lever-operated contraption in H G Wells' The Time Machine, or the souped-up DeLorean sports car in the film Back to the Future.
Perhaps time travel into the past is just a pipe dream. Scientists may uncover a physical law that forbids it. Or the technical challenges may prove insurmountable. But for the moment it remains a tantalising possibility...