ALMOST as soon as radios were invented, people speculated about using them to listen to—and maybe even talk to—extraterrestrial civilisations. Since the 1960s attempts have been made to do so by sifting through signals from outer space in search of alien chit-chat. More recently, the use of lasers in telecommunications has suggested to some that they might be a better way to communicate across vast distances, so searching for telltale flashes from the sky is now in vogue.

But techniques that work well on Earth are not necessarily ideal for talking across the vast chasms that separate stars. And for several years John Learned of the University of Hawaii and Anthony Zee of the University of California, Santa Barbara, have been promulgating what they believe is a better idea. They suggest that any alien civilisation worth its salt would alight not on the photons of the electromagnetic spectrum—whether optical or radio-frequency—to send messages to other solar systems. Rather, it would focus its attention on a different fundamental particle, one that is rather neglected by human technologists. That particle is the neutrino.

Neutrinos, it must be confessed, are neglected for a reason. Though abundant (the universe probably contains more of them than any other sort of particle except photons), they are fiendishly difficult to detect. That is because they interact only occasionally with other forms of matter. But that is precisely why Dr Learned and Dr Zee like the look of them. Light and radio waves are absorbed and scattered by interstellar gas and dust. Neutrinos would pass straight through such obstacles, and could easily be detected by neutrino telescopes on Earth (which typically consist of giant vats of water or, more recently, huge chunks of Antarctic ice).

The two researchers go further. They argue that powerful beams of neutrinos could be used to turn entire stars into flashing beacons, broadcasting information across the galaxy. Outlandish as this sounds, it is an idea that can easily be checked, for astronomers are already sitting on the data that might contain these extraterrestrial messages. They just need to analyse those data from a new perspective. Dr Learned and Dr Zee are therefore trying to persuade someone who studies the data in question to take their idea seriously and spend a little time having a look.

Motes and beams

To detect artificial neutrinos using existing telescopes means screening out the natural neutrino background. Fortunately, much of that is produced by nuclear reactions in stars, and such stellar neutrinos have relatively low energies. If the aliens made their beams out of neutrinos that were a billion times more energetic than the ones emanating from stars (something the researchers argue is not completely beyond the bounds of current technological imagination), the background noise would disappear. At high enough energies the rest of the galaxy is so quiet that if someone detected even a couple of energetic neutrinos arriving from the same direction, it would almost certainly mean they were artificial.

Moreover, Dr Learned proposes a specific energy that aliens might favour. The magic number is 6.3 quadrillion electron-volts. (An electron-volt is the energy with which a one-volt battery can accelerate an electron.) A neutrino with this energy has a good chance of producing a particle known as a W- when it passes through a detector. The W- particle will then decay in a characteristic way, leaving an unambiguous record of the neutrino's passage.

This is not the first time that beams of neutrinos have been proposed as a means of interstellar communication. But past suggestions required enormous energies (of the order of the entire output of the sun) to create a sufficiently intense beam. Dr Learned and Dr Zee have come up with a design for a particle accelerator that would do the job a good deal more modestly, using another type of subatomic particle, the pion, as an intermediary. According to their estimates, this should require no more energy than a typical Earth-bound power station can provide. Aliens might therefore be willing to give it a go—as, indeed, might humanity, if that were thought wise.

Once a civilisation has mastered the trick of generating high-energy neutrinos, though, Dr Learned's imagination suggests it might signal its existence to the waiting universe another way—and this is where the existing astronomical archives come in. For 100 years astronomers have paid particular attention to a class of variable star known as the Cepheids. These are important because they produce regular pulses of light and the period of those pulses is precisely related to the luminosity of the star. A Cepheid's distance can thus be calculated with great accuracy. And, since individual Cepheids are bright enough to be seen in other galaxies, they were the first tool used to estimate the distances to such galaxies.

Dr Learned, however, reckons the pulse-period of a Cepheid could be modulated by a suitable beam of neutrinos. Such a beam would easily penetrate the star's outer layer, but the stellar core would be sufficiently dense to absorb part of it. That would be enough to heat the core slightly and this heating would, in turn, accelerate the Cepheid's pulse rate. Time the beam right and the star could be turned, in effect, into an FM transmitter—broadcasting to the universe on the underlying carrier-wave of the Cepheid's pulsation.

Tickling a star

Admittedly, the energy needed to produce a Cepheid-modulating neutrino beam really would be a sizeable fraction of the output of the sun. But putting aside such minor engineering challenges, the point Dr Learned is keen to make is that because Cepheids have been so thoroughly examined, looking for transmissions from the Betelgeuse Broadcasting Corporation might just be a matter of re-examining the archives. If an intergalactic version of “Yesterday in Parliament” showed up in such a trawl it might not demonstrate the existence of truly intelligent aliens. By contrast, coverage of the cricket on “Test Match Special” would surely be proof positive.