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NS 28 apr 2001

Little white lies
At the heart of all long-term relationships lies a fundamental deception NS 28 apr 2001

WOMEN only stay with men for security, and men only stay with women for sex. It's a cynical view of human relationships, but researchers now say it is the driving force behind the evolution of monogamy-and women started it. By offering sex afl the time, females in monogamous species disguise whether they are fertile and trick males into sticking around. In most species, females only have sex when they are fertile. This is because sex takes energy, and carries the risk of disease. But it also means males can easily tell which females are fertile, so they don't waste time on mates that won't get pregnant. Indeed, males usually give females no help in raising their offspring. "The male strategy is to stay with the female for as long as she is fertile, and then to leave," says zoologist Magnus Enquist of Stockholm University. But in some species, including birds, porcupines and humans, the girls have wised up. By cutting down on visual and chemical cues, and by having sex all the time, they stop males ftom telling whether they are fertile. 'The male has no cue,' says Enquist. 'All he can see is the behaviour of the female."

Once males are blind to a female's condition, he says, it's no longer worth their while chasing lots of partners, because the one they're with is as likely to be fertile as any other. "There is a search cost. It takes some time to find a female.' Although this idea makes intuitive sense, until now it was a mystery whether the trick works. Normally, a male choosing a stable relationship over a philandering lifestyle would have fewer offspring, putting him at an evolutionary disadvantage. Would women hiding their fertility by offering sex continuously be enough to tip the balance?

Enquist and his colleague Miguel Girones from the Netherlands Institute of Ecology in Nieuwersluis decided to investigate. They created a mathematical model to test the theory, and found that under certain conditions, monogamy is the preferred option. Even in a population where males were used to havi@ng many partners, if females started to conceal their fertility, the males settled down into long-term partnerships.

"Classical explanations of sexual behaviour always focus on the male,' says Enquist 'But this gives stronger focus on the woman.'

Evolutionary biologist Anders Moller from the CNRS, France's centre for scientific research in Paris, agrees. 'This is driven by females," he says. 'When ovulation becomes concealed, the males stay with the females longer." But animal behaviour expert Mike SivaJothy of Sheffield University argues that tricking males into being monogamous isn't the only reason for females' high sex drive. Having lots of sex with lots of different males might ensure that at least some of their offspring were fathered by good-quality mates. Although this idea doesn't fit with the traditional view of monogamous societies, Siva-jothy points out that even in species where pairs bond for life, the females cheat. 'When avian biologists went out and looked at the DNA profiles of the offspring, they found that everyone was having a romping time,' he says. But so long as females can fool males into thinking they are being faithful, their strategy of hidden fertility will still work. 'They have to be cryptic because they don't want their partner to find out,' says Siva-jothy. Joanna MarchoM More at: Animal Behaviour (vol 61, p 6951

Matchmakers All you need to find a photon's distant twin is the right mirror

QUANTUM cryptography and quantum teleportation could become easier thanks to a new way of spotting special photons, say scientists in Austria. The technique allows researchers to separate ordinary photons from those that are mysteriously 'entangled' with a distant twin. "This separation is crucial for quantum communications," says Richard Hughes, a physicist at the Los Alamos National Laboratory in New Mexico.

Entangled photons are the Siamese twins of the quantum world. According to the laws of quantum mechanics, they are linked in such a fundamental way that carrying out a measurement on one determines the state of the other, no matter how far apart they might be. This ability to influence a distant photon by measuring one nearby is important for quantum communication.

But distinguishing entangled photons from ordinary ones is not easy. Until now, the most promising proposal has been to use the quantum equivalent of a logic gate, known as a CNOT. The gate flips the state of one photon-for example from horizontally to vertically polarised-depending on the state of another photon. Because the states of entangled photons are linked, it is possible to use a CNOT gate to distinguish them from pairs that are not entangled.

But CNOT gates are difficult to build. 'Right now, CNOTs don't even exist that work for photons,' says Anton Zeilinger, a physicist at the University of Vienna in Austria. And others doubt that quantum logic gates will ever be good enough for highquality quantum communication.

With a quantum sleight of hand, however, Zeilinger and his colleagues have proposed using mirrors known as polarising beam splitters to do the same job as CNOTS. The mirrors work by reflecting photons that are vertically polarised while transmitting horizontal ones, for example. If two photons enter the mirror from two directions and emerge from two directions, they are well entangled. But if they emerge in one direction, they are usually not entangled and so can be ignored (see Diagram).

The technique has the added bonus of improving the entanglement of pairs that pass through it. 'The amazing thing about this is that you can transfer entanglement from one pair to another," says Hughes.

He is not expecting the technique to be perfect, however. Half of the entangled pairs are lost. But Zeilinger says: "When it works, it works perfectly'.

Nicola Jones More at: Nature (vot 410, p 1067)



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 G6ra, 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 barriel 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. R,adiation 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 slowdecay 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 E330,000 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 datahungry 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 ftom 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, lowpower 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 barriel 0

Nick Appieyard and Bridget Appleby are science writers based in Bristot

Takling the pulse

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. lt is this motion that makes the electrons emit radio waves. But the pulses we see are staggeringly powerful. To give off so much energy, @ Nng 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 construct@, massively increasing the brightness of the pulse. One idea is called the 'streaming' 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 vall give off waves at once. In an aftemative klea developed by MaMn Ruderman of Columbia University In New York and Poter S nd of McMaster University in Hamifton, Ontario, small clouds of electrons are c by S"le, highly energetic particles. This is know as the 'spark gap' model. But most astrophysicists find these models contrived cobbled together to fit the data, rather then following naturally from the known properties of pulsars.