The first comprehensive search for radio signals produced by extraterrestrial civilisations has drawn a blank. As far as signs of intelligent life go, it’s as quiet as the grave from here out to a distance of 50 parsecs – 1550 trillion kilometres – in every direction.
That’s the slightly dispiriting conclusion reached by a team of researchers led by astronomer Emilio Enriquez from the University of California, Berkeley, in the US, which has analysed data gathered by the Robert C. Byrd Green Bank Telescope (GBT) in Virginia as part of a privately funded project called the Breakthrough Listen Initiative.
The initiative – funded by Russian oligarch Yuri Milner and his wife Julia – works on an idea first formulated by astronomers Philp Morison and Guiseppe Cocconi in 1959 and embraced by the Search for Extraterrestrial Life Institute (SETI) ever since.
Morison and Cocconi reasoned that technologically advanced civilisations positioned light years apart from each other could communicate using microwaves – electromagnetic radiation with frequencies between one and 10 gigahertz (GHz)
These frequencies, like all on the electromagnetic spectrum, travel at the speed of light, but are especially attractive for interstellar communications. Galaxies naturally create enormous amounts of radio waves at lower frequencies, making it impossible for communication signals within that range to pass through without getting lost.
Higher frequencies – at least on Earth – tend to get absorbed within the atmosphere (which also emits some all by itself). The same effect would happen on inhabited alien worlds, assuming a broadly similar atmosphere.
The one-to-10 GHz range, thus, is called the Microwave Window. Signals coming from space with frequencies within the window are automatically of interest to ET-hunters, and even more so if they display significant variation, indicating perhaps that they are encoding language or data streams rather than the uniform expression of microwave activity arising from some cosmic chemical interaction.
The Milners’ project aims to eventually monitor the microwaves emitted in the vicinity of one million stars, so the first tranche of results, covering just 692, is hardly conclusive. Neither, however, is it encouraging.
In addition to the GBT, an optical telescope, Enriquez and his colleagues also made use of the Parkes radio-telescope in Australia. The team made three five minute observations of each target, together with additional five minute observations at specific distances away.
The research focussed on the lower end of the Microwave Window, looking for signals at between 1.1 and 1.9 GHz. (The intention is cover the full window, but that is expected to take several years.)
At first, hopes of detecting an advanced alien civilisation may have been raised among the scientists. Eleven of the signals detected exceeded the thresholds established for identifying possible communications.
Sadly, however, detailed analysis showed all of them to be anthropogenic in origin. In effect, the telescopes were picking up our own species’ background babble.
In a paper posted on the preprint server arXiv, hosted by the Cornell University Library in the US, the researchers conclude that “none of the observed systems host high-duty-cycle radio transmitters emitting between 1.1 to 1.9 GHz”.
The chances of anywhere within 50 parsecs of Earth possessing such transmitters, they go on to estimate, is less than 0.1%.