Take a look at the frequency range of our SETI searches and you’ll see that we are probing into new territory. Project Phoenix, which ran from 1995 to 2004, used radio telescopes at Arecibo, Parkes (NSW, Australia) and Green Bank (WV, USA), working in a frequency range of 1.2 to 3 GHz. The BETA project used a 26-meter radio telescope to examine the so-called ‘waterhole’ frequencies between 1400 and 1720 MHz, which seemed a likely place to look for an extraterrestrial beacon because this range covers an unusually quiet band of the electromagnetic spectrum between the hydrogen spectral line and the strongest hydroxyl line.
With the Allen Telescope Array coming online, we can look forward to a search of 250,000 stars in the ‘waterhole’ region, but new facilities like LOFAR (Low-Frequency Array) are pushing into the megahertz area in pursuit not only of SETI but also astrophysical studies of the early universe. LOFAR makes me think back to my shortwave radio days, tuning around these frequencies looking for hard to catch transmitters in places like the Falkland Islands (extremely difficult) or Tristan da Cunha (impossible unless you were based in South Africa). I often wondered about SETI at these frequencies and dismissed it as absurd. But that was then.
From Interference to Silence
The problem with LOFAR’s frequency range is that it runs into massive interference from many of the things our civilization does to emit radio signals, from radar to television stations. New technologies have come along that allow us to filter out this interference with great efficiency. But it’s undeniably true that we are radiating strongly in these wavelengths, so it’s an interesting question whether we might be able to pick up a civilization doing the same things by using LOFAR or the upcoming Square Kilometer Array (SKA), which some studies tell us could detect signals like those we produce at a distance of up to 300 light years.
A new paper by Duncan Forgan (University of Edinburgh) and Robert Nichol (Institute of Cosmology and Gravitation, Portsmouth, UK) looks at these possibilities in the context not only of technological capability but the likelihood of running into such radio leakage in the first place. The duo worked with Monte Carlo realization techniques, generating a catalog of planets which can develop life that evolves toward intelligence, and creating statistical populations of ETIs that develop at various times and locations. The resulting realizations are run multiple times and the resulting distributions averaged to quantify the uncertainty in the modeling process.
The connectivity calculations are approached in two ways, the first setting no maximum distance or time limits on communication — imagine a civilization that puts out detectable radio signals for the duration of its existence. The second, however, is more like us and it is deeply constrained. It tends to go quiet at these wavelengths rather quickly, just as our own civilization is doing. Here the distance assumption is 100 parsecs and the time that a civilization leaks radiation into space is reduced to about 100 years. As the authors note:
…we should think carefully about what we might expect the SKA to see. While humans are still leaking radio emission into the Galaxy, the extent of this emission has diminished. Technological improvements have reduced the transmission power required to broadcast, and the dawn of the digital age has begun to supersede traditional radio entirely. These events have occurred in just over 100 years, putting us on the path to becoming a “radio quiet” civilisation. If the Biological Copernican Principle is true (i.e. humans are not atypical as intelligent species), then what happens if all civilisations rapidly become radio quiet?
Human-like Cultures Hard to Find
The first scenario is easy to analyze, with cultures around various stars exchanging messages at the speed of light for the duration of their technological maturity. But the second scenario is grim. Connectivity reduces to virtually zero: Human-like civilizations have a probability of communication in the area of 10-7 under these constraints. We can play with the parameters by extending the observation period of a SKA-like installation from the 30 days used in these calculations to 10 years, in which case the probability of detection becomes 10-4. Assume 105 civilizations and a small number of detections become possible, although the likelihood of using a facility like SKA for 10 years of constant observations on a SETI project is slim.
If other civilizations behave like we do in terms of radio communications, then, we are not likely to find them with SKA. These considerations are not new, but the authors have updated the discussion by using the most recent predictions for the possible distribution of habitable planets and the sensitivity of the latest radio detection technologies. As the paper notes:
While the SKA remains an important instrument for SETI researchers, its abilities are limited to detecting civilisations that are at a less advanced state than Mankind, i.e. they must develop radio technology that remains radio-loud for a significant period of time. This strengthens the argument for a multi-wavelength approach to SETI, as radio-quiet civilisations may be optically loud, or detectable at some other energy scale…
The last point is the most important conclusion of the paper. Because the detection of radiation leakage from other civilizations at our current level of technology is unlikely, the multi-wavelength approach cited above is the best way to proceed. If accidental communications are off the table, we are probably looking for beacons, or else for signals deliberately beamed at us. Such a signal would demand a civilization far more advanced than our own, and in the latter case, one that was aware of our existence because it had developed a capability for optical or radio detections that we have yet to achieve.
The paper is Forgan and Nichol, “A Failure of Serendipity: The Square Kilometre Array will struggle to eavesdrop on Human-like ETI,” accepted for publication in the International Journal of Astrobiology and available as a preprint.