The speed of light can’t 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’s son, has built a “superluminal” source based on Ardavan’s ideas. And any day now it could be switched on.

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

Ardavan’s 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. “More than 30 years after the discovery of pulsars, we still don’t know how the radio waves are produced,” says Janusz Gil of the J. Kepler Astronomical Center in Zielona G-ra, Poland. “Explaining pulsar radiation is one of the most difficult problems of astrophysics.” 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’s field – 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 “Tackling the pulse”, p 31).

Ardavan’s theory is radically different. He says that the pulses we see are “light booms”-shock 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’s 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’s 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-just 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’t work out what would happen if the source moved on a curved path-a 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 “cusp” of radiation that follows a spiral path (see Diagram, p 30).

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’s ideas aren’t popular, however. Among those who disagree violently is Tony Hewish of Cambridge University-one of the original discoverers of pulsars. The entire concept is wrong, he says. “Frankly, I think the error is in equation one.” He doesn’t 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-a 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’s 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. “Many of the unfamiliar superluminal effects are at first sight counter-intuitive,” 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: “The combination of an unconventional idea and an unconventional approach makes us all uncomfortable.”

Melrose is sympathetic to the theory, but doesn’t believe it actually applies to pulsars. The rotating charge patterns, he thinks, would be unstable. Other pulsar theorists object that Ardavan’s theory doesn’t 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’s son, Arzhang, became interested in the problem after completing a doctorate in experimental physics at Oxford. “My father’s theory was dinner table conversation for a long time,” he says. He joined forces with John Singleton of the Clarendon Laboratory, and together they now have £330,000 of research council funding to build a table-top pulsar. The money had been earmarked for developing “non-derivative” instruments-those based on original principles. “This is wildly non-derivative,” 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 “polarisation synchrotron” 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’s, 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’s 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-and 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’s profound scepticism extends to the Oxford project. “I think it’s a great pity they’re wasting their time on this.” Other astronomers take a slightly different view. “The physics is right: there’s nothing wrong with it,” says Melrose. “I just don’t believe that this mechanism can be set up and sustained in a pulsar.” In that case, the new transmitter could still work-even if its inspiration proved to be false.

“This is the most exciting experiment that I have worked on,” says Arzhang Ardavan. “Whatever happens, it will open up research in a whole area of physics that people just haven’t really considered before.” Singleton says: “I see no reason why it shouldn’t work. Let’s suck it and see.”

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.


Author: Nick Appleyard and Bridget Appleby are science writers based in Bristol

New Scientist issue: 28th April 2001

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