It seems someone has come up with a theoretically workable time machine concept:
RONALD MALLETT thinks he has found a practical way to make a time machine. Mallett isn’t mad. None of the known laws of physics forbids time travel, and in theory, shunting matter back and forth through time shouldn’t be that difficult. […]
To twist time into a loop, Mallett worked out that he would have to add a second light beam, circulating in the opposite direction. Then if you increase the intensity of the light enough, space and time swap roles: inside the circulating light beam, time runs round and round, while what to an outsider looks like time becomes like an ordinary dimension of space. A person walking along in the right direction could actually be walking backwards in time–as measured outside the circle. So after walking for a while, you could leave the circle and meet yourself before you have entered it (see Diagram, opposite).
The energy needed to twist time into a loop is enormous, however. Perhaps this wouldn’t be a practical time machine after all? But when Mallett took another look at his solutions, he saw that the effect of circulating light depends on its velocity: the slower the light, the stronger the distortion in space-time. Though it seems counter-intuitive, light gains inertia as it is slowed down. “Increasing its inertia increases its energy, and this increases the effect,” Mallett says.
As luck would have it, slowing light down has just become a practical possibility. Lene Hau of Harvard University has slowed light from the usual 300,000 kilometres per second to just a few metres per second–and even to a standstill (New Scientist, 27 January, p 4). “Prior to this, I wouldn’t have thought time travel this way was a practical possibility,” Mallett says. “But the slow light opens up a domain we just haven’t had before.”
Update: Mallett has written a book called Time Traveler: A Scientist’s Personal Mission to Make Time Travel a Reality. His work has been heavily criticized. See the Ronald Mallett entry on Wikipedia.
Lene Hau‘s work is also worth looking into.