Probably the most popular representation of a wormhole in science fiction is the stargate from the franchise of the same name. Within the franchise the stargates are described as utilizing wormholes to allow travel between various habitable worlds. However the behavior of the stargate system is much more like a teleportation device, where matter is dematerialized at one stargate portal and re-materialized at the other end. Similar gateways have been used in a range of science fiction stories, such as Carl Sagan’s Contact, the Star Trek episode “City on the Edge of Forever”, and Heinlein’s Tunnel in the Sky. Similar concepts appear in fantasy stories as well, such as the magic door in Howl’s Moving Castle.
A better example of a wormhole can be seen in the video game Portal. In this game a portal gun allows you to place two ends of a wormhole on different walls, thus creating a portal between separate locations in space. By stepping through the portal you can step between widely separated locations while (to you) it is but a single step. In the video game wormholes are only formed between different parts of a building, but in principle it would be the same between locations light years apart.
The first appearance of a wormhole in physics comes from Albert Einstein himself. Together with Nathan Rosen he developed a solution to general relativity that could be interpreted as a shortcut between two widely separated locations. They presented this solution in a paper titled “The Particle Problem in the General Theory of Relativity”. Einstein and Rosen weren’t trying to create cosmic shortcuts, but rather were trying to introduce particle physics into general relativity. Still, such a cosmic shortcut was soon known as an Einstein-Rosen bridge.
The Einstein-Rosen solution looks like a black hole on one side and a kind of inverted black hole on the other. The latter was sometimes referred to as a white hole, and it was proposed as a possible explanation of the tremendously bright quasars. (We now know that quasars are powered by galactic black holes.) One way where the E-R bridge differs from the common wormhole concept is that they only work one way. Basically you would enter what looks like a black hole on one end, and leave through a white hole on the other. You couldn’t turn around and go back in the opposite direction. Still, it beats all that tedious mucking about in hyperspace.
Then in 1962 John Wheeler and Robert Fuller demonstrated that an E-R bridge is unstable. Even if you could warp space to create one, its own curvature and energy would cause it to pinch off into separate black holes before anything could travel through it. Not even light could get through fast enough. In other words, even if wormholes existed, they wouldn’t be traversable.
In the 1980s there was a renewed interest in wormholes, and Kip Thorne proposed making a traversable wormhole by lining its interior with “exotic matter”. This is the same type of hypothetical negative mass stuff we came across yesterday when discussing warp drive. Thorne’s idea was to fill the throat of the wormhole with negative mass-energy before it could collapse, thus creating a traversable wormhole. The advantage of Thorne’s wormhole is that not only is it traversable, but you could travel through it from either direction.
Thorne’s model was another “what-if” scenario intended to test the limits of general relativity. He demonstrated that keeping a wormhole open required negative mass-energy stuff, so he simply introduced it into the model to see what would happen. It turns out this caused all sorts of problems. For one, traversable wormholes can be turned into time machines very easily. Because of special relativity, the motion of one end of a wormhole relative to the other would mean that clocks near each opening would tick at slightly different times. So if you took one end of a wormhole and spun it in a circle for a while, its clock would be increasingly slower than a clock near the stationary end. Do this long enough, and the rotating end would be so far behind that you could enter the stationary mouth of the wormhole and find yourself in the past. You would then have plenty of time to travel back through regular space and prevent yourself from entering the wormhole.
It is the grandfather paradox all over again. All the causality problems with time travel also exist for wormholes. All sorts of ideas have been proposed solve those problems, and as with time travel there aren’t really any clear solutions. But since traversable wormholes require the same kind of exotic matter needed for warp drive, it is all pretty hypothetical.
However the wormhole idea in physics isn’t completely dead. There are extensions to general relativity, such as Einstein-Cartan relativity, that allow for traversable E-R bridges without requiring exotic matter. Then there are some approaches to quantum gravity, such as string theory, where space and time become foamy on very tiny scales. Within this quantum foam micro-wormholes would constantly be forming and collapsing, similar to the quantum fluctuations we observe in the Casimir effect.
It should be emphasized that there is currently no evidence for string theory or extensions of general relativity. We know that general relativity and quantum mechanics don’t play well together, but we don’t have any evidence to guide us toward a working theory of quantum gravity. Until we are able to find that evidence we can only play with ideas, though wormholes do appear in several of them.
So it seems that Einstein’s idea of a cosmic bridge is in fact still alive.
Tomorrow: An ansible is a device capable of communicating across light years instantly. Light signals are limited by the speed of light, but can quantum mechanics and “spooky” action at a distance allow signals to break the light barrier? Find out next time.