Radio telescopes designed to study the primordial universe could also eavesdrop on extraterrestrial civilisations similar to our own. "By a happy accident," says abraham Loeb of Harvard University, "the telescopes will be sensitive to justthe kind of radio emission that our civilisation is leaking into space."
The next generation of radio telescopes are designed to pick up radio waves emitted by neutral hydrogen molecules in the early universe. These signals originally had a wavelength of 21 centimetres, but the universe has expanded since they were emitted, stretching the waves in the process. Today, these signals have a wavelength of several metres, corresponding to a frequency of tens or hundreds of megahertz. "This overlaps with our civilisation's radio emissions, which are in the range 50 to 400 megahertz," says Loeb.
Loeb and his Harvard colleague Matias Zaldarriaga say that the most powerful emissions from our own planet come from military radars, and TV and FM radio transmitters. They span a small range of frequencies, and if ET is producing similar signals, these "spikes" in the radio spectrum will be discernible by telescopes such as the Low- Frequency array (LoFaR) which is now being built in the Netherlands (http://www.arxiv.org/abs/astro-ph/0610377). "This is radio SETi, but at wavelengths that SETi experiments haven't probed in the past," says Seth Shostak of the SETi institute in Mountain View, California. "That by itself is reason enough to recommend it."
The ability of telescopes such as LoFaR to detect spectral spikes means they will also be able to detect the doppler shift in wavelength as an ET planet orbits its parent star. according to Loeb and Zaldarriaga, this will make it possible to deduce the shape of the orbit, the tilt of the planet as it spins on its axis, and the planet's distance from the star. "This in turn will allow an estimate of the planet's surface temperature, indicating whether liquid water is a possibility," he says.
Loeb and Zaldarriaga say that the technique would pick up radio leakage from alien civilisations within about 1000 light years of Earth. By some estimates, there could be as many as 100 million stars with planets within this volume of space. Of course, the success of Loeb and Zaldarriaga's proposal depends crucially on how many of these planets have civilisations that are roughly at the same stage of development as ours. "This is very difficult to quantify," says Loeb.
There are practical difficulties, too. "By looking in the bands that we humans fill with signals – radar, TV and so on – the SETi researchers are guaranteed to encounter enormous terrestrial interference," Shostak says. Sorting out ET from the BBC will be a substantial challenge."
Written by Marcus Chown
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