Electromagnetic radiation is of limited utility for interstellar communication, precisely because it is limited to the speed of light. It is not terribly difficult to imagine a scenario in which a civilization on a star was in EM communication with a colony of its own on another star one parsec away. Of necessity, this would be in the form of reciprocal one-way communications, perhaps the interstellar equivalent of three-year-old newsreels. But it seems unlikely that this sort of communication would be uninterrupted and continuous.
Over distances much larger than a parsec, any semblance of two-way communications
breaks down to the point of utter futility. The only place in our galaxy where EM has any conceivable utility for two-way communications over interstellar distances is near the galactic center, where “interstellar distances” are in fact quite short. However, with as much radiotelescope time as has been devoted to this region of the sky generally, we have not yet detected artificial signals there. For the rest of the galaxy, any species attempting two way communication across interstellar space using EM may, almost by definition, not be considered “intelligent”.
Unfortunately, we are still limited to searching in this spectrum, even with the
understanding that no species in their right mind would be broadcasting on it, at least in terms of two-way interstellar communication. So we must look for reasonable purposes for transmitting high-output, uniformly pulsed signals which are not intended to be responded to. As we have only earth-bound human culture as an analogue from which to anticipate alien technologies, we must look at human civilization and technologies which have, by design and intent, broadcast proportionally large-output transmissions which were deliberately and readily identifiable as artificial.
Perhaps the best candidates which come to mind are “aids to navigation” such as buoys and lighthouses. On earth, these tend to be self-powered, self-repairing, and capable of broadcasting light, sound and/or radio signals easily discernible from the background, in an easily recognizable and repeating pattern. An analogous structure in deep space might well remain “on station”, transmitting for eons after the civilization which constructed it had become extinct. Antiquity would be no great barrier to utility, as the age of the phased light or radio transmissions would be of little relevance for determining the object’s relative location. A “buoy” placed on-station ten thousand years ago by a species long extinct would still have utility for space-faring civilizations today, and would provide earthbound SETI researchers with proof of extra-terrestrial intelligence.
Buoys, by definition, are undeniably artifacts; they must be designed in such a way as to not be possibly mistaken for a naturally occurring phenomenon. They tend not, however, to be scintillating conversationalists. If in fact such artifacts do exist throughout the galaxy, and are in fact transmitting on frequencies which humans are capable of receiving, the first “intelligent” communication we receive from an alien civilization may be something like “dit dah dah dit”, repeated over, and over, and over, and over…tantalizing, as it would tell us absolutely nothing about the species which created it other than the fact that they had need of navigation aids at some point in their history. However, given their potential longevity, message redundancy, signal strength and likely ubiquity (if in fact other species in the galaxy are spacefaring, by whatever means), deep-space aids to navigation, if
such exist, seem very likely candidates for our first unmistakable and undeniable contact with an extraterrestrial intelligence.
The argument against navigational buoys being our first contact is that we haven’t heard any yet. There are four possible reasons for this. 1) they are too far away, or for whatever other reason the signal-to-noise-ratio is too low for us to detect with our existing telescopes, 2) they are not transmitting in the spectra we are looking, or even in a spectrum that we are aware of, 3) we are receiving transmissions from them already and have simply not recognized then as such, or 4) they don’t exist.
SETI today
Presently, SETI predominantly uses very large earth-based radiotelescopes such as
Arecibo Radio Observatory in Puerto Rico to “look” at the sky at that latitude as the earth rotates under it. Because this is a “fixed” antenna located on a rotating body, SETI first looks for signals which increase and then decrease at a rate consistent with planetary rotation and the passive field-lobe of the array; this signal-strength bell-curve is called a “Gaussian”. Any EM source which is not originating on earth will exhibit this, whether it is a star, an earth-orbiting satellite or an Aldebaranian disc-jockey. Strong narrowband EM pulses of smaller pulse-length than the duration of the Gaussian are also looked at, and “triplets” (evenly spaced short pulse-length EM pulses which conform in signal strength to a Gaussian) are especially interesting.
Once a signal of interest has been detected, verified by multiple computers and isolated from terrestrial (or near-extraterrestrial, such as satellite) radio frequency interference, it is then examined for persistency. A “persistent” signal is one which is observed on more than one occasion with the same frequency and same location, by one or more radiotelescopes. One current data set of 80,704 Gaussians contained 2,868 candidates which matched once (2 occurrences), 111 candidates which matched twice (3 occurrences), and 4 candidates which matched three times (4 occurrences). Within this particular data set there were no candidates which matched more than three times. As this particular search construed “persistency” as being within 2.5 arc minutes and 50 Hz frequency, the persistent signals in this sample must be presumed to be only randomly and incidentally so.
Much of the search to date has focused on the 1000 to 10,000 MHz “waterhole”, with the presumption that anyone willfully transmitting EM over interstellar distances would do so in frequencies least impeded by background noise. Again, the likelihood of any species technologically intelligent enough to do so actually attempting two-way communication across interstellar distances with EM is, one hopes, rather small; aids to navigation or similar beacons, however, would by necessity utilize the water hole if they were transmitting EM at all. All other things being equal, then, as EM is the only means we have of searching for extraterrestrial intelligence at this point in our history, if we are serious about locating proof of extraterrestrial intelligence we need to consider the virtue of searching for radio- or light-transmitting artifacts rather than actual coherent communication, and optimize our search toward finding those things which are most likely to be transmitting on the frequencies we’re searching.
The disadvantage of searching for buoys, obviously, is that once the initial excitement of discovery wears off, they really aren’t very interesting. The advantage is that what buoys lack in eloquence they make up for in tenacity, so there’s no worry about “missing the signal”. Any systematic search of the sky on the right frequency and sensitivity will find it eventually, and once it is located it is easy to verify independently with other antennas, and could be easily monitored continuously as the earth rotates by a series of antennae at different longitudes around the globe. For a search such as this, then, one large array is
infinitely preferable to a large number of smaller arrays.
However, for detecting more “interesting” and potentially more ephemeral signals, a
much larger number of smaller antennae would seem to be optimal. A 3-meter dish
antenna is the minimum needed to “see” in the water hole. Already, SETI-inclined
amateur radio enthusiasts have been building very small radio telescopes from old
satellite television antennas; the SETI League has established Project Argus to integrate the searches of these amateur radio-astronomers via e-mail and newsletters.
Concurrent with but independent from this “SETI at home” amateur radio astronomy is
“SETI@home”, which utilizes millions of personal computers around the world to
analyze SETI data obtained from Arecibo or other giant radio arrays. Essentially
SETI@home is using unused processing time on individual personal computers as a giant supercomputer. The individual user is given a screensaver-like program which processes units of parsed-out data from Arecibo. Because SETI@home has over 2,000,000 participants, extraordinary amounts of data are able to be processed in a very small amount of time.
“Combining the forces” of the SETI League and SETI@home, it would be possible to
create an array of perhaps millions of small antennae scattered around the world,
interfaced via personal computers. If an individual antenna picked up an interesting
candidate, the personal computer it was attached to could perform a preliminary data
analysis, and then automatically prompt all of the antennae in its hemisphere to train on the same Right Ascension and Declination. All data collected would then be transmitted to a central supercomputer for further analysis. In this way, a truly global antenna array (if only 1% of current SETI@home participants participated in this, that would commit 20,000 new antennae to the search) could be built for well under $500 per participant, and almost no hardware or software overhead whatsoever for the university organizing it.
The existing single massive antenna approach, used in tandem with the sort of
Shoestring Budget Global Array (SBGA) proposed here, could significantly accelerate
our search for extraterrestrial intelligence.
And, really, what could be cooler than turning your old satellite dish into a personal radiotelescope to hunt for aliens with?
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