For budding time travellers, the future (or should that be the past?) is starting to look bleak.
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.
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.