By Paul Rincon
BBC News science reporter
For budding time travellers, the future (or should that be the past?) is starting to look bleak.
Wormholes contort the fabric of the Universe
Hypothetical tunnels called wormholes once looked like the best bet for constructing a real time machine.
These cosmic shortcuts, which link one point in the Universe to another, are favoured by science fiction writers as a means both of explaining time travel and of circumventing the limitations imposed by the speed of light.
The concept of wormholes will be familiar to anyone who has watched the TV programmes Farscape, Stargate SG1 and Star Trek: Deep Space Nine.
The opening sequence of the BBC's new Doctor Who series shows the Tardis hurtling through a "vortex" that suspiciously resembles a wormhole - although the Doctor's preferred method of travel is not explained in detail.
But the idea of building these so-called traversable wormholes is looking increasingly shaky, according to two new scientific analyses.
A common analogy used to visualise these phenomena involves marking two holes at opposite ends of a sheet of paper, to represent distant points in the Universe. One can then bend the paper over so that the two remote points are positioned on top of each other.
If it were possible to contort space-time in this way, a person might step through a wormhole and emerge at a remote time or distant location.
The person would pass through a region of the wormhole called the throat, which flares out on either side.
According to one idea, a wormhole could be kept open by filling its throat, or the region around it, with an ingredient called exotic matter.
This is strange stuff indeed, and explaining it requires scientists to look beyond the laws of classical physics to the world of quantum mechanics.
Exotic matter is repelled, rather than attracted, by gravity and is said to have negative energy - meaning it has even less than empty space.
But according to a new study by Stephen Hsu and Roman Buniy, of the University of Oregon, US, this method of building a traversable wormhole may be fatally flawed. In a paper published on the arXiv pre-print server, the authors looked at a kind of wormhole in which the space-time "tube" shows only weak deviations from the laws of classical physics.
These "semi-classical" wormholes are the most desirable type for time travel because they potentially allow travellers to predict where and when they would emerge.
Wormholes entirely governed by the laws of quantum mechanics, on the other hand, would likely transport their payloads to an undesired time and place.
The concept is a favourite of science fiction writers
Calculations by the Oregon researchers show a wormhole that combines exotic matter with semi-classical space-time would be fundamentally unstable.
This result relies in part on a previous paper in which Hsu and Buniy argued that systems which violate a physical principle known as the null energy condition become unstable.
"We aren't saying you can't build a wormhole. But the ones you would like to build - the predictable ones where you can say Mr Spock will land in New York at 2pm on this day - those look like they will fall apart," Dr Hsu said.
A separate study by Chris Fewster, of the University of York, UK, and Thomas Roman, of Central Connecticut State University, US, takes a different approach to tackling the question of wormholes.
Amongst other things, their analysis deals with the proposal that wormhole throats could be kept open using arbitrarily small amounts of exotic matter.
Fewster and Roman calculated that, even if it were possible to build such 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, says Fewster.
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.
"Frankly no engineer is going to be able to do that," said the York researcher.
The authors are currently preparing a manuscript for publication.
However, there is still support for the idea of traversable wormholes in the scientific community. One physicist told BBC News there could be problems with Hsu's and Buniy's conclusions.
"Violations of the null energy condition are known to occur in a number of situations. And their argument would prohibit any violation of it," the scientist commented.
"If that's true, then don't worry about Hawking radiation from a black hole; the entire black hole vacuum becomes unstable."
The underlying physics of wormholes was not in doubt, the researcher argued. The real challenge was in explaining how to engineer wormholes big enough to be of practical use.
Cambridge astrophysicist Stephen Hawking is amongst those researchers who have pondered the question of wormholes.
In the 1980s, he argued that something fundamental in the laws of physics would prevent wormholes being used for time travel. This idea forms the basis of Hawking's Chronology Protection Conjecture.