ADVANCED extraterrestrial civilisations may be using neutrinos to broadcast information throughout our Galaxy, according to physicists in the US. If this is the case, the next generation of neutrino telescopes, now being built, may pick up the first intelligent messages from the stars.
Walter Simmons, Sandip Pakvasa, John Learned and Xerxes Tata at the University of Hawaii at Manoa came to this conclusion after considering how a civilisation which has spread through our Galaxy might send 鈥渢iming pulses鈥� over interstellar distances. The scientists believe that such pulses 鈥� the equivalent of the 鈥渟peaking clock鈥� 鈥� would be essential in order to synchronise clocks on different worlds.
鈥淎ccurate synchronised clocks are needed for a wide range of scientific measurements, particularly in astronomy,鈥� says Simmons. 鈥淏ut any advanced civilisation is bound to require time standards which are even more exacting.鈥�
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Without synchronisation, clocks at different places in the Galaxy would gradually drift out of step because of the effect of general relativity, which causes local time to flow at a rate dependent on the motion of nearby massive bodies. This effect is impossible to predict, because of 鈥渃haos鈥� in any many-body system such as the Solar System. 鈥淪tandard clocks in different parts of the Galaxy will need to exchange timing signals at the limits allowed by physics,鈥� says Simmons.
This is where neutrinos come in. Simmons and his colleagues have identified a fundamental physical process that might be used to provide phenomenally precise timing information: the decay of a Z0 boson into a neutrino and antineutrino. 鈥淭he decay takes just 10鈭�21 seconds,鈥� says Simmons. 鈥淚t鈥檚 the fastest process known in physics.鈥� (Quarterly Journal of the Royal Astronomical Society, vol 35, p 321).
However, being able to generate neutrino pulses of shorter duration than pulses of electromagnetic radiation does not in itself make neutrinos attractive for sending timing information across the Galaxy. It is also crucial that the pulses are penetrating, able to remain sharp as they travel, and that they can be made very bright. According to Simmons, neutrino pulses satisfy all these criteria. 鈥淭hey are not blocked by interstellar dust and they are not smeared out, or dispersed, by ionised gas in space,鈥� says Simmons. 鈥淭hey can also be produced with greater luminosity than electromagnetic radiation.鈥�
Simmons and his colleagues conclude that a very bright source of neutrinos, generated from the decay of Z0 bosons, could transmit timing pulses as narrow as 10鈭�21 seconds across many thousands of light years. Our Galaxy is about 150 000 light years across and the Sun is about 28 000 light years from the centre.
Simmons says that Z0 bosons could be created by colliding high-energy electrons and positrons head on. 鈥淭he transmitter would be a giant particle accelerator,鈥� he says. 鈥淚t would need to be about the size of the Earth and would probably be in space.鈥� The Z0 bosons could be made in bursts synchronised by a 鈥渕aster clock鈥�.
According to Simmons, an advanced civilisation would probably choose to broadcast neutrinos in all directions rather than just one. This is because a tight beam could be created only by taking advantage of an effect known as 鈥渞elativistic beaming鈥�, which would require accelerating the entire transmitter to close to the speed of light. Even for an advanced civilisation this might be a little too difficult.
Even illuminating the Galaxy with an 鈥渙mnidirectional鈥� beam would require a high neutrino luminosity. But this would be limited by particle processes to about the luminosity of the Sun. If the intensity of the neutrinos became too large, they would simply scatter off the converging electrons and positrons in the giant particle accelerator, destroying the timing information they carry.
Simmons and his colleagues have also identified a way of detecting the neutrino timing pulses. The decay of the Z0 boson produces equal numbers of all three types of neutrino, corresponding to the electron, muon and tau particles. One of these types, the electron antineutrino, could be detected by a 鈥渞eceiver鈥� consisting of high-energy electrons. If such a beam were aimed at the incoming electron antineutrinos, W鈭� bosons would be created, which would be detectable. This interaction is most likely to occur when the combined energies of the electron antineutrino and the electron equal the mass-energy of the W鈭� boson, about 80 gigaelectronvolts (GeV).
In fact, an interstellar neutrino signal could even be picked up by detectors currently under construction, says Simmons. One such experiment off Hawaii, the Deep Underwater Muon and Neutrino Detector, or DUMAND, will consist of strings of sensitive light detectors dangled vertically in the deep ocean capable of picking up tiny flashes of light produced when cosmic neutrinos interact with seawater. 鈥淭he detection of extraterrestrial neutrinos may be possible with as little as a cubic kilometre of seawater if there is a transmitter within about 3000 light years,鈥� says Simmons.
An intelligent signal should be quite distinctive. 鈥淣ot only will the neutrinos come from a single direction in the sky, but they will have a very well-defined energy as well,鈥� says Simmons.
Of course, any civilisation requiring such an extraordinary amount of power might be expected to reveal its presence indirectly. For instance, it might be 鈥渄amming鈥� all the energy flooding into space from its star by building a spherical shell around it, perhaps from the rubble of a dismantled planet. Although such a 鈥淒yson sphere鈥� would be invisible optically, it would be bright in the infrared.
Many such infrared sources were detected by NASA鈥檚 Infrared Astronomical Satellite in 1983. However, all can be explained as stars with normal shells of dust. 鈥淎stronomers like to exhaust the natural explanations before moving on to unnatural ones,鈥� says Simmons.
There is always the possibility that an civilisation will use a communication medium as yet undreamt of. What physicist of the mid-19th century could have imagined radio waves, which today carry voices around the planet? Simmons admits this is a possibility. 鈥淔or obvious reasons we have considered only technologies which use known science,鈥� he says. 鈥淎nything else is pure science fiction.鈥�