In addition to facilitating effectively faster than light travel, wormholes could potentially be used as time machines, in the following sense first developed by Caltech theoretical physicist Kip Thorne. Imagine an advanced technology capable of creating, manipulating, and containing both ends of a stable, traversible, wormhole. Place one end in a laboratory on Earth and the other on a spacecraft capable of traveling through space at some reasonable fraction of the speed of light. Imagine the wormhole connecting the lab and spaceship is created in some future year, say 2500. Now keep one end on Earth and send the spaceship off in any direction at some appreciable fraction of the speed of light for a finite duration after which it will decelerate, turn around, come back to Earth, and stop, so the wormholes ends are brought back together.
Relativity tells us that the clocks of observers left on Earth and those in a relativistically moving spacecraft will begin to differ by an amount that depends on the speed of the craft. Since moving clocks run slow in relativity, a spaceship observer might experience a short subjective duration of say, a few weeks, but thousands or millions of years could pass in the external universe depending on how fast they were traveling. In this sense, time travel to the future is easy, and does not require wormholes, just a ship capable of moving at relativistic speeds. A spaceship executing the above maneuver might find itself thousands or millions of years in the future after stopping. Yet an observer at the wormhole mouth in the laboratory on Earth would still have its clock synchronized with the shipboard wormhole. If the ship finds itself, say in year 3500, after returning to Earth, any observers on the ship could return to the year 2500, traveling 1000 years into the past, simply by stepping through their shipboard wormhole back into the laboratory on Earth.
In this way, wormholes could theoretically be used to travel into the past. However, in this case, the shipboard time travelers could never travel to before the year 2500. This poses a striking answer to the question, ``If time travel to the past is possible, how come we aren't being constantly visited by time travelers from the future?" For these types of wormhole time machines, the answer is simply, because they haven't been invented yet! Time travel of this sort can never take an observer back to before the original date when the wormhole connection was set up. This is a particularly clever resolution to an interesting time travel paradox.
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