Many years back I wrote an article for Glenn Hauser’s Review of International Broadcasting called “Where the Real DX Is.” DX is the shortwave radio term for seeking out distant signals, a sport in which the smaller and fainter the station, the more interesting the catch. I was laboring with an old FRG-7 receiver to attempt impossible receptions like the Falkland Islands and Tristan da Cunha (neither of which I ever heard), but in the back of my mind were the nearby stars. What about receiving a signal from one of them?
And while I wrote about the emerging SETI scene, my real thinking was that an extraterrestrial reception wouldn’t be from a beacon — I still doubt these exist — but from accidental leakage from a technological society. Now a new paper by Harvard’s Abraham Loeb and Matias Zaldarriaga suggests an interesting strategy for finding such leakage, via a a low-frequency radio telescope study that will look at highly redshifted 21 centimeter emissions from hydrogen. The Mileura Wide-Field Array (MWA) is designed to tell us about the state of neutral hydrogen in the early universe, before it became ionized by the first galaxies.
Which is a fascinating project in itself, because it tells us about the universe only a few hundreds of millions of years after the Big Bang, helping us learn about early star and black hole formation. But given the paucity of SETI funding (most of it today comes from non-profits), Loeb sees a wonderful synergy between this work and a SETI hunt in the frequencies the MWA will probe. Why not use MWA and other radio observations of the redshifted hydrogen to look for extraterrestrial DX?
This strategy is novel because by the time you’ve redshifted the 21 centimeter hydrogen (1420 MHz), you wind up in the frequency range between 80 and 300 MHz. That’s not where the typical SETI search is, but it’s in the middle of the band in which our own civilization would be the most luminous from another star. Indeed, where we’re putting out the most significant signals is in radio emissions from military radars, FM radio broadcasts, and TV, all of which can be found in a wide band from roughly 40 to 800 Mhz.
Thus the Mileura Wide-Field Array gives us the chance to piggyback SETI on the back of a well-funded area in cosmology, looking at a significantly different part of the spectrum than existing SETI searches. Again, we’re not looking for beacons here (conventional thinking says these might be best searched for in the 1420 to 1660 MHz water hole band) but incidental emissions from a technology at work. And yes, the frequency range Loeb and Zaldarriaga are talking about is tricky. From the paper:
Of course, our own radio broadcasting is far greater at lower frequencies, so at least for the purpose of “eavesdropping” on another civilization, lower frequencies might be more interesting. The fact that our civilization makes much use of the lower frequency spectrum presents severe technical diﬃculties for SETI programs trying to operate in this frequency range as they have to ﬁlter-out our own radio-frequency interference (RFI). Thus, 21cm cosmology is a case in which an unrelated science driver will open a new and potentially more suitable window for SETI programs. The interest in high-redshift 21 cm surveys means that there will be signiﬁcant efforts to control RFI by, for example, placing the observatories in remote locations with the lowest RFI record (such as China, Australia, Africa, or even the moon), as well as developing new ﬁltering techniques for RFI and ionospheric noise. Obviously SETI programs could beneﬁt signiﬁcantly from these technological developments.
Can the leakage of electromagnetic radiation be detected from planets around nearby stars like Centauri A and B? Loeb believes the Mileura Wide-Array may be up to the challenge. The Foundational Questions Institute agrees and has given him a grant to initiate such a search as the MWA work proceeds. You can read more about optimizing the software for detecting such signals in Loeb and Zaldarriaga’s paper “Eavesdropping on Radio Broadcasts from Galactic Civilizations with Upcoming Observatories for Redshifted 21cm Radiation,” available online at the arXiv site.