Another article by Paul Davies in the current issue of Scientific American.
Traveling into the past is rather trickier. Relativity theory allows it in certain spacetime configurations: a rotating universe, a rotating cylinder and, most famously, a wormhole -- a tunnel through space and time.
Time dillation occurs when two observers move relative to each other. Hence the twins paradox: the space-traveler twin returns to find a much older twin back on earth. This happens on airplanes, too, but a few nanoseconds here and there are easily overlooked.
Gravity also slows time. A clock in the attic runs faster than one on the ground. The amount is trivial close to Earth but must be factored in by the GPS system.
Davies proposes a time machine constructed of a couple of wormholes. Place one next to a neutron star -- that will slow down time a bit. Go in one place, come out somewhere else in space and in time. How are we going to do this? Quantum mechanics pops up. Sounds like voodoo to me.
