Beam smashes light barrier
Wednesday, 19 July, 2000, 18:09
GMT 19:09 UK
By BBC News Online science editor Dr David Whitehouse
Scientists
have seen a pulse of light emerge from a cloud of gas
before it even entered. This astonishing and baffling observation was made by
researchers from the NEC Research Institute in
Princeton, US.
They conducted an experiment that involved lasers, a
chamber containing cold caesium atoms and a super-fast
stopwatch.
The end result was a beam of light that moved at 300
times the theoretical limit for the speed of light.
It was Einstein who said nothing physical could break
this barrier because, among other things, to do so would
also mean traveling back in time.
Dramatic demonstration
But the NEC scientists believe their work does not
violate Einstein's theory.
Writing in the journal Nature, Dr Lijun Wang and
colleagues say their light beam raced through the atom
trap so quickly that the leading edge of the pulse's
peak actually exited before it had entered.
If this sounds confusing, then do not worry. Many
physicists are uncomfortable with it too despite their
explanations that it is a natural consequence of the
wave nature of light.
Although the work of Dr Wang's team is remarkable, it
is not the first time that this sort of
"trick" has been performed - but it is
certainly the most dramatic demonstration.
Earlier this year, a team of physicists made a
microwave beam travel 7% faster than light speed. Last
year, they announced that they had even slowed light
down to almost a crawl.
Anomalous refractive index
To achieve their peculiar effect, Dr Wang's group
fired laser beams through a trap of caesium atoms.
By adjusting the frequency of the laser beams to
match those of the energy levels in the atoms, the
researchers were able to achieve an effect called
"anomalous refractive index." This boosts the
pulses' so-called "group velocity" to a speed
faster than what we understand to be the speed of light
- just short of 300 million meters per second.
The group velocity of a light pulse depends upon the
mixture of frequencies within the pulse and the medium
through which it travels. It need not be the speed of
the pulse itself.
The important thing, however, is that whilst the
group velocity can be manipulated to be faster than the
speed of light, it is not possible to use this effect to
send information faster than the speed of light.
Because of the fast group velocity, the leading edge
of the pulse appears to leave the caesium-filled chamber
62 billionths of a second before it arrives.
Causality principle
And according to Dr Wang, this strange result does
not threaten Einstein's theories - in particular, the
causality principle, which states that a cause must
precede its effect.
Or so almost all physicists think - for now.
Privately, some admit that experiments such as Dr Wang's
may force a reassessment of some cherished ideas.
According to Dr Guenter Nimtz, of the University of
Cologne, who has carried out similar experiments, the
NEC work is very exciting.
He told BBC News Online: "The effect cannot be
used to go back in time, only to reduce the time between
cause and effect a little bit.
"The reason for this," he said, "is
because the light pulse has a finite length of time,
much longer than any gain obtained by a
faster-than-light speed."
