How to pull a fast one
Marcus Chown investigates the chinks in physics that make faster-than-light
ROBERT MATTHEWS says that serious doubts have been cast on Miguel
Alcubierre's ingenious scheme for a "warp drive" like that in Star Trek
to travel at speeds faster than light (This Week, 26 July, p 6). Alcubierre,
a physicist at the University of Wales in Cardiff, had suggested that the
Starship Enterprise could zoom around the Galaxy if the fabric of space-time
in front of it was scrunched up while that behind it was simultaneously
stretched-in effect bringing the stars closer. But two physicists at Tufts
University in Medford, Massachusetts, calculated that such a feat would require
far more energy than is contained in the Universe.
Probably, the new calculations are correct. After all, we know intuitively
that bringing the mountain to Mohammed is almost certainly less energy-efficient
than the reverse strategy. But it is worth remembering that in the past
calculations of this type have not always been watertight.
As Arthur C. Clarke pointed out in his book The Promise of Space,
calculations by a distinguished Canadian astronomer in 1941 showed that
a million tonnes of fuel would be required to take a one-pound payload on
a return trip to the Moon. The implication was that crewed lunar expeditions
were quite impracticable.
The astronomer had assumed that the entire rocket -every last nut and bolt
on the launchpad-would have to travel all the way to the Moon and back. A
more sensible strategy, as employed by NASA in 1969, is to throw away most
of the rocket as soon as it has done its job so that only a tiny part-the
cramped capsule containing three astronauts- completes the return journey
There was nothing wrong with the astronomer's calculation-only the assumptions
underpinning it, which could be the problem in the present case. Assuming
they are not, though, and that flitting between the stars with a warp-drive
is a nonstarter, are there any "scientific windows" that might permit
faster-than-light (FTL) travel?
Well, contrary to popular belief, Einstein's theory of relativity does not
forbid a material object from travelling faster than light, only at the speed
of light. Particles called tachyons may exist for which the speed of light
is a lower limit. In fact, some physicists have seriously suggested that
neutrinos are tachyons (This Week, 16 August, p 19).
Like photons, which are born travelling at the speed of light and need never
accelerate from zero, tachyons would be born moving faster than light and
never have to cross the light barrier. Unfortunately they would play havoc
with the order of cause and effect,creating all sorts of time-travel-like
paradoxes. Although such problems can be fixed, it invariably makes physics
less aesthetically appealing.
Tachyons would be useful for FTL communication but it is hard to see how
they could bring about FTL travel (unless of course there exists a way of
turning normal matter into tachyons and back again in the manner of the Star
Trek transporter). However, there are a number of other ways that FTL
travel might conceivably be achieved.
One is to use shortcuts through the topologies of space known as wormholes.
A voyager might enter one mouth of a wormhole near, say, the Earth, and after
travelling for a few hours, emerge from the other near Alpha Centauri or
the centre of the Galaxy.
Wormholes are bona fide solutions to Einstein's field equations of gravity.
However, wormholes have a nasty habit of snapping shut in a blink of an eye.
Keeping them open, while at the same time preventing tidal forces ripping
apart a spaceship, requires large amounts of "exotic" matter with bizarre
properties such as negative average energy density (such material
is also required by Alcubierre's warp drive). If the theory of inflation
is right, tremendous quantities of exotic matter existed in
the first split-second of Creation. But nobody
knows whether any survives today. Another possibility for
achieving FTL travel is provided by string theory,
which views fundamental particles of matter as submicroscopic pieces of "String"
vibrating in a space of at least
10 dimensions. The extra dimensions are rolled
up more than a billion billion times smaller than a proton, but conceivably
one might be unrolled to achieve FTL speeds, and then rolled up again. It
would be equivalent of a
shortcut through a higher dimension. Gary Bennett, Robert Forward and
Howard Frisbee considered many such ideas two years ago at a NASA/JPL workshop.
The three physicists were quite upbeat about the prospects for FTL travel
and communication: "There are enough cracks in the armour of physics that
FTL travel cannot be ruled out."
|Further reading:Possible applications of advanced
quantum/relativity theory propulsion to interstellar exploration: report
of a NASA/JPL Workshop (American Institute of Aeronautics and