Space Travel with WormholesA wormhole is a hypothetical shortcut between two distant regions ofspace-time. Although a three dimensional wormhole is impossible tofully visualize, a two dimensional analogue can be constructed toaid visualization. Imagine an intrinsically flat, two dimensional,space as a folded piece of paper embedded in a higher threedimensional space, where a tube connects two distant points, A andB, on the paper. The length through the tube (the wormhole) can bemuch less than the distance from A to B along the paper, creating ashortcut. A full three dimensional wormhole would have entrances and exitsthat are three dimensional spheres rather than two dimensional ringslike the mouths of the paper tube. Such lower dimensional, humanfriendly,visualizations are termed embedding diagrams, and theiconic wormhole image is usually shown as the well knownSchwarzchild embedding diagram, which is the wormhole analogue for astatic, non-rotating, Schwarzchild black hole.An observer passing through such a wormhole could, in principle,traverse the wormhole in less time than it would take to travel frompoint A to point B through normal space-time outside the wormhole.
Moreover, if A and B are sufficiently distant in space and thewormhole length is sufficiently short, an observer could potentiallytraverse the wormhole in a time less than it would take to send alight signal from A to B through normal space. Wormholes could thusbe used as a cosmic “cheat” to effectively bypass the limitationthat no object can travel faster than the speed of light in specialand general relativity. Faster than light travel itself presentsmany paradoxes, since it could be used to send messages andinformation back in time. Certain other uses of wormholes couldalso potentially allow observers themselves to travel into the past.Time Travel with WormholesIn addition to facilitating effectively faster than light travel,wormholes could potentially be used as time machines, in thefollowing sense first developed by Caltech theoretical physicist KipThorne.
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 aspacecraft capable of traveling through space at some reasonablefraction of the speed of light. Imagine the wormhole connecting thelab and spaceship is created in some future year, say 2500. Now keepone end on Earth and send the spaceship off in any direction at someappreciable fraction of the speed of light for a finite durationafter which it will decelerate, turn around, come back to Earth, andstop, so the wormholes ends are brought back together.Relativity tells us that the clocks of observers left on Earth andthose in a relativistically moving spacecraft will begin to differby an amount that depends on the speed of the craft. Since movingclocks run slow in relativity, a spaceship observer might experiencea short subjective duration of say, a few weeks, but thousands ormillions of years could pass in the external universe depending onhow fast they were traveling. In this sense, time travel to thefuture is easy, and does not require wormholes, just a ship capableof moving at relativistic speeds.
A spaceship executing the abovemaneuver might find itself thousands or millions of years in thefuture after stopping. Yet an observer at the wormhole mouth in thelaboratory on Earth would still have its clock synchronized with theshipboard wormhole. If the ship finds itself, say in year 3500,after returning to Earth, any observers on the ship could return tothe year 2500, traveling 1000 years into the past, simply bystepping through their shipboard wormhole back into the laboratoryon Earth.In this way, wormholes could theoretically be used to travel intothe past. However, in this case, the shipboard time travelers couldnever travel to before the year 2500. This poses a striking answerto the question, “If time travel to the past is possible, how comewe aren’t being constantly visited by time travelers from thefuture?” For these types of wormhole time machines, the answer issimply, because they haven’t been invented yet! Time travel of thissort can never take an observer back to before the original datewhen the wormhole connection was set up.
This is a particularlyclever resolution to an interesting time travel paradox.Do Wormholes Actually Exist?No observational evidence for wormholes currently exists, butmathematical solutions describing wormholes have long been known tobe valid theoretical solutions to Einstein’s field equations ofGeneral Relativity. However, wormholes made completely of normalmatter with positive energy density would be inherently unstable,and would be likely to collapse in the presence of nearby matter ormatter that tried to traverse the wormhole.