Space-time wormholes are hypothetical objects that are possible within the general theory of relativity put forward by German-born American theoretical physicist Albert Einstein (1879–1955). These wormholes are theorized to be intense gravitational fields that warp space and time (the four dimensions of the universe) to provide shortcuts from one part of the universe to another (or, perhaps, a route from one universe to some other universe). Physicists have not found solid experimental evidence that wormholes exist. However, there are reasonably convincing theoretical arguments that strongly suggest wormholes should be part of the theory of quantum gravity.
As theoretical objects, wormholes were named in the late 1950s by American theoretical physicist John Archibald Wheeler (1911–2008), an early pioneer in the quest for a theory of quantum gravity. Since then they have become a standard tool in science fiction (such as in the television series Star Trek and Farscape and the novel Einstein's Bridge), but they have also attracted a lot of serious scientific attention. Although physicists cannot conduct any experiments yet, wormholes can be used in "thought experiments" to see how solid and reliable certain theories are to the physical characteristics of the universe.
Science fiction stories make wormhole travel look relatively straightforward, if not exactly easy. The physicists' conception is more conservative and less encouraging: Naturally occurring wormholes, if they exist at all, are likely to be extremely small, about 10 septillion (1025) times smaller than a typical atom. They are assumed part of a quantum-mechanical "space-time foam" that is expected to arise at extremely short distances. Wormholes of this size are not useful for human travel, or even for sending signals.
Creating a large wormhole, or artificially enlarging a small naturally occurring wormhole (if one existed), would require the manipulation of tremendous quantities of matter—planet loads of mass. A wormhole that is one meter (about one yard) across would require the manipulation of objects with the mass of the planet Jupiter and the squeezing of them into a region about a meter wide. Worse yet, because the gravitational field of a wormhole is in some sense repulsive (one would not want a black holes to form), one would need to manipulate large quantities of what is called "exotic matter," which is basically negative energy matter (with less energy than the energy of an equal volume of vacuum) and negative mass (that is, a massive body whose gravity repels objects rather than attracts them).
Needless to say, humankind does not currently possess anywhere near the technology needed for such manipulation nor is there any realistic hope of acquiring such technologies in the foreseeable future. Scientists and engineers cannot even manipulate positive masses the size of Jupiter, nor have humans ever found large quantities of negative mass lying around anywhere. So realistically, the future prospects for space travel using wormholes are close to zero (if not actually zero). However, this will not stop theoretical physicists from investigating the subject, but one should try to not be overly enthusiastic about the chances of actually during such theory into reality.
Because wormholes connect distant places, and the laws of space-time physics seem to treat space and time on an almost equal footing, it has also been suggested that wormholes should be able connect distant times. If one finds a wormhole, it would seem at first glance (at least in science fiction) to be relatively easy to turn it into a time machine. Now, this does seem a bit scary, especially to scientists. Thus, allowing time travel into physics is, to say the least, awkward.
However, there is an idea called "chronology protection," formulated by English theoretical physicist Stephen Hawking (1942–), to the effect that quantum physics may "keep the universe safe for historians" by automatically destroying any wormhole that gets too close to forming a time machine. As Hawking put it: "there is considerable evidence in favour of [chronology protection] based on the fact that we have not been invaded by hordes of tourists from the future." Dr. Hawking conjectures that the laws of physics make time travel impossible for everything except sub-microscopic matter.