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.
By Paul Rincon