Research carried out in the late 1980's showed that genuine time travel is not forbidden by the known laws of physics. This means that it may be possible to build a time machine, but not that it may be easy. Help may be at hand though, it is possible there are naturally occurring objects in the universe that act as time machines.
There are at least two ways to build a time machine. Frank Tipler published a possibility in the highly respected journal Physical Review in 1974. This involves making a naked singularity, a singularity that is not concealed from view behind the event horizon of a black hole. To make a naked singularity involves rotating a singularity extremely rapidly, and if rotated sufficiently fast it would fling away the event horizon and exposes the singularity. We know that spacetime is extremely distorted by the singularity's strong gravitational field and the effect of this rotation would be to twist spacetime, and tip it over so that one of the dimensions of the space dimensions is replaced by the time dimension. A carefully piloted spaceship taken close to the singularity would enter the time dimension and journey through time instead of space, although to the astronauts all would appear as normal. When the spaceship moved away from the distorted area around the singularity, it would be in a different time from when they had entered the area.
According to Tipler's calculations, the same effect could be achieved with a cylinder about 100 km long and about 10 km across, made of material compressed to just over the density of a neutron star, and rotating twice every millisecond. It would be like ten neutron stars joined pole to pole and given a strong twist. Curiously, there are objects in the universe which nearly fulfill the other requirements - so-called millisecond pulsars are known which contain almost the right density of matter and spin once every 1.5 milliseconds, at one-third the speed needed to make a time machine. Such objects are so close to being time machines that they hold out the tantalising possibility that an advanced civilisation might be able to tweak them up in the right way to allow time travel.
That such things as naturally occurring time machines exist in the universe, with only a little tweaking needed, raises the prospect that an advanced civilisation may have already done the trick! This raises the interesting possibility that such a civilisation would have the capacity to travel between the galaxies; a journey of a few million light years would be as nothing. Something for the UFO brigade to mull over!
The other possibility for building a naturally occurring time machine involves worm holes - tunnels through spacetime which may, according to relativity, connect a black hole in one part of the universe to a black hole in another part of the universe. Before the mid-1980's physicists believed that such objects as wormholes could not 'really' exist, and that a better understanding of Einstein's equations would prove this. They were forced to change their minds as a result of careful investigation of wormholes carried out by Kip Thorne and his colleagues at Caltech in 1985. It is interesting to note that this research was triggered by Carl Sagan, a well known scientist, who was writing the science fiction novel 'Contact', a best seller that went on to become a highly successful film. Sagan wanted his wormhole to be as scientifically accurate as possible and approached Thorne to check out the idea as presented in the book. What neither Sagan nor Thorne first realised from the results of Thorne's study was that this short-cut through space would also work as a short-cut through time. In 1988 Morris, Thorne and Yurtsever (Morris and Yurtsever were students of Thorne) published their conclusions in the journal Physical Review Letters, that Einstein's equations really did allow for the existence of wormholes that link different times, and could be used as time machines.
We have seen that the laws of physics do not preclude the possibility of time travel, and further, that it may be possible to construct a time machine by tweaking naturally occurring objects in the universe. It would appear that we only need the technology to make time travel a reality.
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