In Star Trek, a micro-wormhole is a wormhole of very small size. A micro- wormhole could have originally been of a much greater size, but is in an advanced state of collapse, and over time degraded and shrunk to a smaller size. It is a wormhole whose singularity has mostly dissipated, collapsing and leaving an extremely narrow passageway through subspace. In 2371, the USS Voyager discovered a naturally-occurring micro-wormhole thirty centimeters in diameter which led from the Delta Quadrant to Romulan space (20 years in the past). (VOY: "Eye of the Needle"). In 2376, the Pathfinder Project at Starfleet Communications was able to create a temporary, artificial micro-wormhole using the MIDAS array, to briefly communicate with Voyager over 30,000 light years away in the Delta Quadrant. (VOY: "Pathfinder").

In physics, a wormhole is a hypothetical topological feature of spacetime that is fundamentally a 'shortcut' through space and time. Spacetime can be viewed as a 2D surface, and when 'folded' over, a wormhole bridge can be formed. A wormhole has at least two mouths which are connected to a single throat or tube. If the wormhole is traversable, matter can 'travel' from one mouth to the other by passing through the throat. While there is no observational evidence for wormholes, spacetimes-containing wormholes are known to be valid solutions in general relativity.

However, a study in 2005 by Chris Fewster, of the University of York, UK, and Thomas Roman, of Central Connecticut State University, US, demonstrated that even if it were possible for humans to build such wormholes, they would probably be too small to fit spacecraft or even humans and could probably only be possible at the scale of Star Trek's micro-wormholes. The study analysed the proposal that wormhole throats could be kept open using arbitrarily small amounts of exotic matter. Exotic matter is a hypothetical concept of particle physics and refers to any material which violates one or more classical conditions or is not made of known baryonic particles. Such materials would possess qualities like negative mass or being repelled rather than attracted by gravity.

Fewster and Roman calculated that, even if it were possible to build a wormhole, its throat would probably be too small for time travel. It might - in theory - be possible to carefully fine-tune the geometry of the wormhole so that the wormhole throat became big enough for a person to fit through. But building a wormhole with a throat radius big enough to just fit a proton would require fine-tuning to within one part in 10 to the power of 30. A human-sized wormhole would require fine-tuning to within one part in 10 to the power of 60.

It is currently speculated that tiny quantum wormholes - at the level of quarks and electrons - may exist for short periods of time. But most experts suspect that some fundamental law of physics prevents the formation of large wormholes. Wormholes that were large enough for humans to travel through would need to be crammed with 'exotic matter' to keep them open.

However, a Russian expert in relativity, Sergei Krasnikov of the Pulkovo Observatory in St Petersburgh has developed a new type of wormhole model that is compatible with the known laws of physics, yet can be as big and stable as you like. His wormholes create their own exotic matter out of nothing when space and time are curved in the right way, in sufficient quantities to make it big enough and keep it open long enough for people to use. This wormhole actually generates enough to make it arbitrarily large. However, Krasnikov accepts that testing his claims by building a wormhole is far beyond present technology.