杏吧原创

THE SPEED of light can鈥檛 be exceeded. Everyone knows that. Yet Houshang
Ardavan of Cambridge University claims that there are sources of radio waves out
in space that move faster than light. A team of physicists at Oxford, including
Ardavan鈥檚 son, has built a 鈥渟uperluminal鈥� source based on Ardavan鈥檚 ideas. And
any day now it could be switched on.

Many physicists think this idea is a complete waste of time. But if they鈥檙e
wrong, and the Oxford experiment succeeds, Ardavan鈥檚 patented superluminal
transmitters could soon turn up in your pocket. Their weird radiation could
transform technologies from medical scanners to mobile phones.

Ardavan鈥檚 work was inspired by the mysterious celestial objects known as
pulsars. Pulsars send out pulses of radio waves several times a second, with
timing so regular that they were at first thought to be alien transmissions.
Astronomers now believe that pulsars are the remnants of massive stars that
ended their lives in enormous supernova explosions. Each supernova is thought to
have left behind a ball of neutrons 1.4 times the mass of our Sun but only 20
kilometres across, its material so dense that a teaspoonful would weigh three
billion tonnes.

Although most astronomers agree on the basic nature of pulsars, the radio
pulses remain a bugbear. 鈥淢ore than 30 years after the discovery of pulsars, we
still don鈥檛 know how the radio waves are produced,鈥� says Janusz Gil of the J.
Kepler Astronomical Center in Zielona G贸ra, Poland. 鈥淓xplaining pulsar
radiation is one of the most difficult problems of astrophysics.鈥� The regularity
is thought to come from the fact that pulsars spin, typically 10 to 20 times a
second. Somehow they must send out a beam of radio waves that sweeps past the
Earth on each rotation.

But how? Most theories depend on the intense electric and magnetic fields
that are thought to surround a pulsar. The magnetic field can be a hundred
billion times the Earth鈥檚 field 鈥� strong enough to wrench a spanner out of
your hand from across the Atlantic. The general idea is that the fields
accelerate electrons near the pulsar, causing them to emit radio waves. But to
produce the required radio intensity, these theories have to include some
awkward assumptions (see 鈥淭ackling the pulse鈥�).

Ardavan鈥檚 theory is radically different. He says that the pulses we see are
鈥渓ight booms鈥濃�攕hock waves made by a source moving faster than light,
rather like the sonic boom created by a supersonic plane when it breaks the
sound barrier. The idea of a light boom is not new. Because light slows down
inside materials such as water or glass, particles moving in such a medium can
travel at speeds faster than sluggish light. These particles emit a flash of
blue light called Cerenkov radiation.

Ardavan proposes that light booms could occur even in a vacuum, where light
travels at top speed. How can this be? According to Einstein鈥檚 theory of
relativity, no material particle can move faster than light in a vacuum.

But a pattern can. For example, if a long line of drummers each hits a drum
in turn, the pattern of drumming can easily exceed light speed. Of course, real
drummers would have to be pretty skilled to hit their drum each at the precise
prearranged time.

Ardavan points out that faster-than-light patterns could be circling around
pulsars. A pulsar鈥檚 magnetic field rotates along with the star. As it does so,
it induces a rotating pattern of electrical charges and currents in the
surrounding gas. This pattern rotates as if rigid, so the farther out you go
from the pulsar, the faster it sweeps by鈥攋ust as the spot illuminated by a
lighthouse moves faster further out at sea. Beyond about 5000 kilometres from
the pulsar, the pattern will be circling faster than the speed of light.

Vitaly Ginzburg and his co-workers at the Lebedev Physical Institute in
Moscow were the first to examine radiation from faster-than-light patterns of
charge. They predicted the existence of light booms, but they didn鈥檛 work out
what would happen if the source moved on a curved path鈥攁 question that
requires fiendishly complicated maths. Ardavan realised that much of the
mathematics he needed had already been worked out for supersonic systems, so he
started by working on the theory of supersonic helicopter blades, which mimic
the whirling magnetic patterns around pulsars.

Adapting this work to his pulsar model, he calculated in 1994 that such a
rotating pattern would produce a shock wave of light. As each patch of charge
loops around, the waves it emits pile up in a complicated fashion, crowding
together especially strongly in a 鈥渃usp鈥� of radiation that follows a spiral path
(see Diagram).

To explain the whirling beam that flashes radio waves at Earth, Ardavan
presumes that the overall charge pattern around a pulsar must be lopsided.
Perhaps a few closely clustered regions in the pattern act as especially intense
sources, and their cusps of radiation combine into the radio beam.

This radiation has one strange unforeseen property. Radiation from any
ordinary source, be it lightbulbs or laser beams, spreads out and fades rapidly
as it travels. The intensity falls in proportion to the square of the distance
from the source. But in 1998, Ardavan published a paper showing that radiation
from a superluminal source should fade only in proportion to the distance (
Physical Review E, vol 58, p 6659). The reason for this slower decay is
that further from the source, the set of waves forming the cusp overlap and get
squashed together into a tighter beam, so the intensity will no longer drop off
as quickly.

Ardavan鈥檚 ideas aren鈥檛 popular, however. Among those who disagree violently
is Tony Hewish of Cambridge University鈥攐ne of the original discoverers of
pulsars. The entire concept is wrong, he says. 鈥淔rankly, I think the error is in
equation one.鈥� He doesn鈥檛 believe that a collection of particles all moving
slower than light can produce superluminal shock waves, even if the charge
pattern moves faster than light. Hewish also points out that many common types
of antenna and waveguide already carry superluminal charge patterns, yet this
slower decay in brightness has never been seen.

Ardavan accepts this, but says an antenna would have to be curved to emit his
slow-decay waves. The charge patterns not only have to be moving faster than
light, they must be accelerating. Speeding up, slowing down or moving around in
a circle would do, but steady motion will not produce the vital cusp.

Ardavan claims that there is already evidence to support his theory. In 1999,
Shauna Sallmen and co-workers from the University of California at Berkeley used
a high-resolution technique to examine the pulsar at the heart of the Crab
Nebula. Instead of a single source, they saw three separate points鈥攁
characteristic of superluminal sources. As Jean-Luc Picard of the USS Enterprise
has demonstrated, a starship that exceeds the speed of light can overtake its
own image and appear to be in more than one place at a time. So the Crab pulsar
seems to be performing the Picard manoeuvre, appearing in three places at once.
Ardavan was overjoyed.

Despite this, pulsar researchers have not embraced Ardavan鈥檚 model. He
believes that this lack of acceptance might partly be due to a poor
understanding of the superluminal regime of electrodynamics, which he says has
taken him 20 years to comprehend. 鈥淢any of the unfamiliar superluminal effects
are at first sight counter-intuitive,鈥� he says. Another problem is the
formidable mathematics. To do his integrals Ardavan uses an obscure technique
that almost nobody else understands. As pulsar theorist Don Melrose of the
University of Sydney says: 鈥淭he combination of an unconventional idea and an
unconventional approach makes us all uncomfortable.鈥�

Melrose is sympathetic to the theory, but doesn鈥檛 believe it actually applies
to pulsars. The rotating charge patterns, he thinks, would be unstable. Other
pulsar theorists object that Ardavan鈥檚 theory doesn鈥檛 explain every detail of
the observations.

This debate might have dragged on for decades, but two years ago a way to
test the theory appeared. Ardavan鈥檚 son, Arzhang, became interested in the
problem after completing a doctorate in experimental physics at Oxford. 鈥淢y
father鈥檚 theory was dinner table conversation for a long time,鈥� he says. He
joined forces with John Singleton of the Clarendon Laboratory, and together they
now have 拢330,000 of research council funding to build a table-top pulsar.
The money had been earmarked for developing 鈥渘on-derivative鈥�
instruments鈥攖hose based on original principles. 鈥淭his is wildly
non-derivative,鈥� says Arzhang Ardavan with a smile.

The core of their device is a curved rod of aluminium oxide covered with
electrodes. The voltage on each electrode will be oscillated at radio frequency,
with a slight time delay from one electrode to the next. The idea is that this
will mimic the line of drummers, to create a pattern of waves that moves along
the rod faster than light.

The hardware of their 鈥減olarisation synchrotron鈥� is all in place, so now
Singleton and Arzhang Ardavan just have to get the delicate timing of the
electrodes right.

If it works, the weird radiation produced will be tremendously useful. The
device could be tuned to emit radiation over a very wide range of the
electromagnetic spectrum, including previously inaccessible terahertz
radiation.

Terahertz waves penetrate the skin, but cause less tissue damage than X-rays,
so they could be used to diagnose skin and breast cancers, as well as rotten
teeth, with little health risk. Mobile phones using terahertz waves would have
access to a bandwidth thousands of times greater than today鈥檚, making
data-hungry applications like video streaming a breeze. And a terahertz source
could also be used as a computer clock, allowing elements to switch far faster
than in today鈥檚 PCs.

Houshang Ardavan predicts that the radiation from this table-top pulsar will
fade more slowly than that from any other source. This would be very useful for
long-distance communications: space probes could send information back to Earth
using small, low-power superluminal transmitters. Mobile phones could beam
directly to a satellite without needing a ground-based relay station, so you
could use them anywhere on Earth. Strangest of all would be a new means of
transmitting secure communications. With the right antenna, you could send out a
pulse that only assembles itself in one place, where the waves interfere
constructively鈥攁nd in theory you could modulate this pulse to transmit a
signal. No one outside the target area would be able to intercept it.

Not surprisingly, Hewish鈥檚 profound scepticism extends to the Oxford project.
鈥淚 think it鈥檚 a great pity they鈥檙e wasting their time on this.鈥� Other
astronomers take a slightly different view. 鈥淭he physics is right: there鈥檚
nothing wrong with it,鈥� says Melrose. 鈥淚 just don鈥檛 believe that this mechanism
can be set up and sustained in a pulsar.鈥� In that case, the new transmitter
could still work鈥攅ven if its inspiration proved to be false.

鈥淭his is the most exciting experiment that I have worked on,鈥� says Arzhang
Ardavan. 鈥淲hatever happens, it will open up research in a whole area of physics
that people just haven鈥檛 really considered before.鈥� Singleton says: 鈥淚 see no
reason why it shouldn鈥檛 work. Let鈥檚 suck it and see.鈥�

If the equipment does do something interesting when the switch is thrown,
tighten your seat belts. Twenty-first century technology is about to lurch
through the light barrier.

How do pulsars generate their regular blasts of radio waves? Astrophysicists
believe that electrons and other charged particles travelling at close to the
speed of light surround these compact stars. The particles are confined to
curved paths by a powerful magnetic field. It is this motion that makes the
electrons emit radio waves.

But the pulses we see are staggeringly powerful. To give off so much energy,
these flying electrons would have to have an absurd amount of energy, equivalent
to a temperature a billion billion billion times hotter than the Sun. Instead,
most physicists agree that the electrons must be collaborating. Somehow, bunches
of electrons are emitting electromagnetic waves all together, so that the waves
interfere constructively, massively increasing the brightness of the pulse.

One idea is called the 鈥渟treaming鈥� model: electrons streaming through the
ionised gas around the neutron star hit turbulence and clump together. As the
clump spirals around in the magnetic field, every electron will give off waves
at once. In an alternative idea developed by Malvin Ruderman of Columbia
University in New York and Peter Sutherland of McMaster University in Hamilton,
Ontario, small clouds of electrons are created by single, highly energetic
particles. This is know as the 鈥渟park gap鈥� model. But most astrophysicists find
these models rather contrived鈥攃obbled together to fit the data, rather
then following naturally from the known properties of pulsars.