Hypothetical tunnels called wormholes were once the best bet for constructing a real time machine.
These cosmic shortcuts, which link one point in the Universe to another, are favoured by scifi writers as a means of explaining time travel.
Wormholes are familiar to anyone who has watched the TV shows Farscape, Stargate SG1 and Star Trek: Deep Space Nine.
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.
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 studies this method of building a traversable wormhole may be fatally flawed. 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.
"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," one researcher said.
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Wormhole 'no use' for time travel