[Paleopsych] BBC: Wormhole 'no use' for time travel
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Sat May 28 19:36:37 UTC 2005
Wormhole 'no use' for time travel
By Paul Rincon
BBC News science reporter
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.
[The wormholes] 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
Stephen Hsu, University of Oregon
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
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
This is strange stuff indeed, and explaining it requires scientists
to look beyond the laws of classical physics to the world of quantum
Exotic matter is repelled, rather than attracted, by gravity and is
said to have negative energy - meaning it has even less than empty
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
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
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
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
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 they could
see 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," they 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 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.
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