TIME TRAVEL

INTRODUCTION

Time Travel, travel into the past or future. Long a staple theme of science fiction, it is now a subject of serious research. In the 1980s mathematical physicists made the surprising discovery that there is nothing to forbid time travel in the laws of physics as understood at present (in particular, the general theory of relativity). It would be extremely difficult to build a working time machine, but it is possible in principle, and there may even be naturally occurring objects in the universe today that function as time machines.

MASSIVE ROTATING CYLINDERS

Two theoretical types of time machine are known. Both depend on the fact that space and time are not distinct entities, but merge into one four-dimensional whole, the space-time of relativity. The first kind of time machine consists of an extremely dense object that rotates extremely rapidly. The strong gravitational grip of the massive object “drags space-time around” as it spins. To the physicist, this metaphor has a precise interpretation, and represents the interlinked distortions of the geometry of space and the passage of time near the rotating object.
A spaceship travelling on a precisely worked out path could approach a suitable massive spinning object and travel past it on a trajectory that seemed to all the occupants of the spaceship to be merely a journey through space, and affected every instrument on board accordingly, yet emerged on the other side in a different time, either in the past or in the future.
The kind of spinning object required to achieve this result would be equivalent to ten neutron stars (see Star: Pulsars and Neutron Stars), each containing as much mass as the Sun in a volume no bigger than Mount Everest, joined pole to pole to form a cylinder and spinning 2,000 times every second. No such object is known, and it is not clear that if one were formed it would be stable: apart from anything else, it seems that gravitation would soon crush such an object to a sphere, and then a black hole, a theoretical object from which no matter or radiation escapes. But millisecond pulsars, which are individual neutron stars rotating about 700 times per second, are intriguingly close to the conditions required for time travel.

WORMHOLES

The second approach to time travel involves black holes. The equations of relativity suggest that pairs of black holes may be connected by “tunnels” that make a short-cut through space-time. These tunnels are known as wormholes. The two black holes—the mouths of the tunnel—can be anywhere in space or time, and still be connected by a wormhole. Thus one mouth could be here now, while the other is in the same place a thousand years in the past. If so, an object could enter the present-day mouth and emerge a thousand years ago.
A problem here (apart from the difficulty of finding or manufacturing a black hole) is that gravity tends to snap the wormhole shut. It might, though, be possible to keep the wormhole open by threading it with exotic matter, material that physicists conjecture may exist but has not yet been detected (see Dark Matter). Black holes almost certainly exist, ranging from objects in our Milky Way galaxy with a few times the mass of the Sun, to objects with millions of times the mass of the Sun in the hearts of other galaxies and in quasars.
Although these speculative possibilities do not provide practical ways of building time machines, physicists continue to study this topic because of the possibility that the entire universe may be threaded by tiny wormholes, with mouths much smaller than a proton. Such wormholes could explain why the laws of physics are the same everywhere in the universe—why, for example, an electron on Earth has the same charge and mass as an electron in a distant galaxy. It has been seriously (though speculatively) suggested that it is information leaking across time and space through microscopic wormholes that keeps the laws of physics constant from one place to another and from one time to another.