Universe Today
Conventional SETI (Search For Extraterrestrial Intelligence) strategies have long been built on the idea that intelligent extraterrestrials (ETI) would aim to communicate with other intelligent civilizations along a very narrow band of the electromagnetic spectrum, preferably in the radio spectrum.
The hypothesis is that any putative technological civilizations out there would use electromagnetic signaling for virtual meetings at a so-called galactic watering hole. That is, a band of radio-quiet frequencies extending from 1420 MHz to 1662 MHz, encompassing the spectral lines of hydrogen (H) and hydroxyl (OH), both of which can combine to form water.
The idea of a cosmic electromagnetic watering hole makes for a powerful metaphor for potential galactic communication between advanced technological intelligences. If indeed such intelligence is out there.
But in a new paper just accepted for publication by *The Astrophysical Journal*, Ben Zuckerman, the paper’s sole author and a long-time UCLA professor of physics and astronomy, argues instead that we should be searching via what he terms broadband SETI. That is, from the radio on up through the infrared and optical portions of the electromagnetic spectrum.
Our principal assumption is that a purposely communicative technological civilization will do its technological best to establish communication with other ETI, writes Zuckerman. This opens the possibility for the serendipitous detection of an alien transmitter in electromagnetic sky surveys undertaken for reasons that have nothing to do with SETI, he writes.
Thus, if a nearby extraterrestrial intelligence wants to communicate with other nearby terrestrial intelligence, then it can and will transmit signals that can be detected even by a civilization like ours, Zuckerman notes. That is, engaged in astronomical research that uses modest size telescopes, he writes.
The Bad News?
The totality of published astronomical surveys at radio and optical wavelengths are already sufficiently extensive to suggest that cosmically speaking there are very few, perhaps zero, communicative technological civilizations near us, Zuckerman told me via email.
But that doesn’t mean we shouldn’t continue searching. We just need to be smarter about it.
To that end, Zuckerman proposes an improved search technique that more closely corresponds to astronomical surveys that have been undertaken for reasons that have nothing to do with SETI. Published non-SETI radio and optical surveys are sufficiently extensive that they already supply meaningful constraints on the prevalence of nearby purposely communicative alien civilizations, he writes.
Most of the stars covered in such programs are located well beyond the 650-light-year distance we have considered here, Zuckerman writes. In total, radio SETI surveys have observed only a modest percentage of the few 100,000 relevant nearby stars that should be observed, he writes.
In contrast, non-SETI radio surveys have arguably covered more of the position and wavelength space in a hypothetical 650-light-year sphere around Earth than dedicated radio SETI search programs, Zuckerman notes.
What Does Zuckerman Propose?
Use much wider channels and try to cover the entire radio/microwave band from 1 Ghz to 100 Ghz, Zuckerman told me. Cosmically nearby extraterrestrials who possess large space telescopes will know that Earth is special and will beam strong signals toward Earth, he says.
*The NRAO's Very Large Array in New Mexico. Credit: NRAO via Wikipedia*
According to a recent all-sky survey in a volume of radius of about 650 light years, there are about 500,000 single, solar-type stars, Zuckerman writes. So, we will assume that 200,000 of these are sufficiently old (greater than 4.5 billion years) will be of interest for hosting a technological civilization, he writes. Yet a targeted SETI search program may have to point its telescopes toward some 300,000 nearby stars to be sure that all old ones are included, he notes.
That’s in part because determining the age of an isolated star is not easy.
For a complete survey of "old" stars one should look also at some with poorly determined ages that might be younger than 4.5 billion years, says Zuckerman.
NASA’s Kepler satellite data suggests that about 30% of 200,000 planetary systems will include a potentially habitable rocky planet, or about 60,000 potentially habitable planets, Zuckerman writes. And Kepler projects that some 30% of sunlike stars will be orbited by an approximately earth-size rocky planet in the habitable zone, he notes in his paper.
But observations in the infrared spectrum will likely be key to finding any nearby ETI.
Nearby ETI are probably not transmitting at optical wavelengths because if they were, then their signal would have been detected by accident in one (or more) of the many published optical surveys of the sky, says Zuckerman.
Even so, SETI searchers would be wise to conduct a careful quantitative study of the ensemble of optical observations dating back over a hundred years. That would be painstaking but would be prudent for a comprehensive search.
ETI could well be transmitting at infrared wavelengths, but very little of relevance has been observed/published at IR wavelengths, says Zuckerman. For SETI, the infrared is still a big unknown, he says.
When should humanity have a handle on whether ETIs exist within 650 light years of Earth?
For a ‘good handle,’ infrared wavelengths will have to be well surveyed with a space antenna -- because much of the IR region of the electromagnetic spectrum can't be accessed from below the atmosphere, says Zuckerman. And I don't know when an appropriate satellite will be in orbit, he says.
