A wormhole is a geometry of four-dimensional spacetime (for an explanation of spacetime see "spacetime" and "spacetime diagrams") in which two regions of the universe are connected by a short narrow throat. A classical large scale wormhole is a solution of the Einstein's field equations, which governs the curvature of spacetime. The most interesting thing with wormholes is that they could provide relatively easy means of travelling to distant regions of space or even of travelling backwards in time.
The real problem with transversing a wormhole is that none of the geodesics connecting these two external regions are time-like or light like. For information to cross from one external region to the other, it would have to follow a space-like path during at least part of the journey. In other words, in order for information to cross from one side of a wormhole to the other without winding up hitting the physical singularities it would have to travel faster than light in a way not allowed even by general relativity. If information could do this, then the first problem of stability wouldn't be a problem at all as the information could travel through the wormhole arbitrarily fast and make it through before the wormhole connection was broken. This is why the second problem is the real problem with transversing this type of wormhole.
Wormholes present time travel opportunities
Kip Thome, a relative theorist, devised a theory where wormholes could act as time machines. He believed that, according to the principles of quantum mechanics, if a space ship was strong enough to travel at something near light speed through a wormhole, time would travel backwards. By following the principles of the special theory of relativity that say time moves slower for objects traveling at the speed of light, it would be possible to travel in time.
The idea is that the geometry of space and time is sufficiently contorted to form a bridge through to another universe. Early ideas along these lines are due to Albert Einstein and Nathan Rosen , with further developments by John Archibald Wheeler. The most recent flurry of interest in traversable wormholes was prompted by Kip Thome and Michael Morris.
If the wormhole connects our universe to itself, it is an intra-universe wormhole. With wormholes that connect our universe to itself there is (classically) a risk of time travel paradoxes.
SAKHAROV CLAIMS WE CAN INSTANTANEOUSLY CROSS SPACE
As reported in the July 1999 issue of "Physics Today," two and a half months before his death, the world-renowned Russian physicist Andrei Sakharov gave a speech in Lyons, France. Included in that speech was the following: "We are looking into the fantastic possibility that regions of space separated from each other by billions of light years are, at the same time, connected to each other with the help of additional parallel entrances, often called 'wormholes.' In other words, we do not exclude the possibility of a miracle: the instantaneous crossing from one region of space to another. The elapsed time would be so short that we would appear in the new place quite unexpectedly, or, vice versa, someone would suddenly appear next to us. I talk of such things in order to show what kinds of questions are being raised and discussed at the cutting edge of science."
Thorne believes there could be another type of tunnel-like structure existing in the universe that could be used for a time travel portal. Wormholes, also called Einstein-Rosen Bridges, are considered to have the most potential for time travel if they do exist. Not only could they allow us to travel through time, they could allow us to travel many light-years from Earth in only a fraction of the amount of time that it would take us with conventional space travel methods.
Wormholes are considered possible based on Einstein's theory of relativity, which states that any mass curves spacetime. To understand this curvature, think about two people holding a bed sheet up and stretching that sheet tight. If one person were to place a baseball on the bed sheet, the weight of the baseball would roll to the middle of the sheet and cause the sheet to curve at that point. Now, if a marble were placed on the edge of the same bed sheet it would travel toward the baseball because of the curve.
In space, masses that place pressure on different parts of the universe could eventually come together to form a tunnel -- this is a wormhole. We could then travel from Earth to another galaxy and back relatively quickly (within a lifetime). For instance, let's picture a scenario in which we would want to travel to Sirius, a star that's seen in the Canis Major constellation just below Orion. Sirius is about 9 light-years from Earth, which is about 54 trillion miles (90 trillion km). Obviously, this distance would be far too great for space travelers to traverse and return in time to tell us about what they saw there. So far, the farthest people have traveled into space is to the moon, which is only about 248,548 miles (about 400,000 km) away from Earth. If we could find a wormhole that connected us to the space around Sirius, then we could cut the time considerably by avoiding the trillions of miles that we would have to cross with traditional space travel.
Many theorists believe black holes and white holes are connected by wormholes. In the 1960ís, a mathematician named Roy Kerr came up with a theory that it would be possible to dive your time machine into a black hole, go through a wormhole, and come out through a white hole, ending up at a new time and space. This existing system would constitute the means for time travel. Many scientists disagree with this theory because they believe that no object can enter a black hole and reach singularity without first getting crushed by the weight of the gravitational pull.
Masses in space place pressure on these two sheets which causes them to curve and bend toward each other. At one point, these two space times meet and create a tunnel. This tunnel is also called a wormhole. Through this hole, we would be able to traverse through space more quickly, taking our time machine from one end of the universe to the other. For example, if we wanted to go from Earth to the star Sirius, it would take at least take 9 light years, but with a tunnel connecting Earth to Sirius, the trip there and back could be made within a lifetime.