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SETI Searches Today
Searches today are not limited to microwave radio. As early as 1961 Charles H. Townes (co-inventor of the laser) and Robert N. Schwartz proposed that laser signaling would be an attractive alternative to interstellar radio. Stuart Kingsley of Columbus, Ohio, picked up on this idea and championed it for years. His small Columbus Optical SETI Observatory performed a pioneering targeted search of stars — both for narrowband laser signals and for extremely brief pulsed signals — at visible wavelengths using just a 10-inch amateur telescope and commercial equipment.
Optical SETI finally entered the scientific mainstream by the late 1990s. Analyses by Kingsley, Paul Horowitz, and many others demonstrated that brief, nanosecond laser pulses would indeed be a very attractive means of interstellar communication. A laser no more powerful than those already on engineers' drawing boards might direct a short burst of beacon signals to perhaps a million stars each day. Such signals could be detected across 1,000 light-years by today's best optical telescopes. If the aliens use a bigger laser, the signals could be detected by an amateur telescope equipped with a pair of low-cost high-speed photomultipliers.
Such a brief, pulsed signal would be so clear — and so plainly artificial — that we could see it by watching a single, wide-frequency channel spanning much of the visible or infrared spectrum! This sounds very attractive compared to radio SETI, where we are laboriously sifting through billions of narrow channels for a continuous signal. Nor are optical signals subject to the interstellar scintillation that degrades radio signals. Would ETs make the same judgments?
For more on this idea and possible amateur participation, see Kingsley's Optical SETI Network site. Kingsley and laser communications expert Monte Ross hoped to start an amateur optical SETI project, named PhotonStar, but it has not happened. However, high-end amateur CCD cameras (made by SBIG) should soon have nanosecond optical SETI capability built in, so such a project could yet get off the ground.
At Harvard, Paul Horowitz has looked for nanosecond pulses in the light of thousands of Sunlike stars. The Harvard Targeted Optical SETI Project was an example of piggybacking applied to optics. The stars were already having their radial velocities measured by a spectrograph on Harvard's 61-inch telescope. About a third of a star's light reflects off the jaws of the spectrograph slit and is normally lost. Horowitz and his group designed their SETI instrument to capture and examine this wasted light while the telescope went about its other business.
The project began observations in 1998 and was soon recording lots of signals — from electrical discharges in the equipment during humid weather and, perhaps, from cosmic-ray events in the atmosphere and radioactive decays in glass parts. Most of these problems were eventually solved. The program logged good observations of 6,176 stars for a total of 2,378 hours, or an average of 23 minutes per star.
False signals still came about once a night. To eliminate these, the Harvard group recruited volunteers at Princeton University, several hundred miles away, to set up a duplicate detector on Princeton's 36-inch telescope. "When this telescope is coordinated with the Oak Ridge 61-inch, we can be sure that short pulses of light seen simultaneously by both systems are truly from astronomically distant sources," wrote Horowitz. The Princeton Optical SETI program began coordinated observations with the Harvard telescope in November 2001. It watched 1,397 stars for a total of 494 hours thanks to amateur volunteers. (Only some of this time was in dual-observing mode with Harvard; the dual-observing log included 1,142 stars watched for 244 hours.)
After about five years the Harvard targeted project came to an end. A detailed paper on the project and its results appeared in the October 1, 2004, Astrophysical Journal. The Smithsonian Institution ceased funding any operations at the 61-inch telescope as of August 2005, and the telescope itself, the largest all-purpose astronomical telescope east of the Mississippi, was reportedly to be dismantled.
Since then, however, Horowitz and his students have built an all-sky, rather than just a targeted, optical survey instrument. The Harvard All-Sky Optical SETI Survey, funded by The Planetary Society, uses a short-focus, 1.8-meter (72-inch) "light bucket" telescope aimed at the meridian. A grid of nanosecond-speed pulse detectors in its focal plane covers a patch of sky measuring 1.6° by 0.2°. This patch scans a strip of declination continuously as the Earth's rotation moves the sky from east to west across the telescope's view. The telescope always points at the sky's north-south meridian; it can only move its aim up and down, not side to side.
The system gives not just a few stars but every point on more than half the celestial sphere (from declination +60° to –20°) at least 48 seconds of examination in the course of just 200 clear nights.
The stubby 72-inch telescope, with its state-of-the-art detector array, was dedicated on April 11, 2006 (see our article). It is housed in a roll-off-roof observatory only a few yards from the site of the old 61-inch. As of November 2009 it had performed 3,182 hours of observations and covered the full northern sky three times, including with upgraded detectors and circuitry that improved the system's original sensitivity to optical pulses by a factor of five.
"We observe every clear night," wrote Horowitz in the Nov/Dec 2008 Planetary Report, "and you can check out the operation at The Planetary Society's website."
False alarms happen regularly. In the first two years of observing, there were a few hundred such "events." "Some of these are easily spotted as artifacts," wrote Horowitz. "Others are more persuasive. . . . We re-observe these candidates," both by re-sweeping the site on another night and by enlisting other, more powerful, targeted instruments to track them for longer periods of time. So far, no "event" has repeated. Horowitz hopes someday to build an identical second copy of the 72-inch sky-scanner at another site so the two can observe in parallel. This should reduce the false-event rate to zero and would immediately confirm any actual faint signal from the stars.
On the other side of the continent, two optical SETI programs have been run from Berkeley. One, named SEVENDIP, is a search for nanosecond pulses (directed by SETI@home's Dan Werthimer) using Berkeley's 30-inch automated telescope at Leuschner Observatory in California. As of February 2009 this project had looked at about 10,000 stars and 100 galaxies, according to Werthimer. For the other program, Amy E. Reines and extrasolar-planet hunter Geoffrey Marcy checked high-resolution spectra of 577 Sunlike stars that are more than 2 billion years old for any narrow, continuous laser lines.
The SETI Institute too has built an optical SETI instrument, which has been working on a 1-meter telescope at Lick Observatory in California since 2001. As of January 2004, the Lick Optical SETI Program had observed 3,999 of its 5,039 target stars for 10 minutes each. The instrument divides starlight from the telescope into three beams directed into three high-speed photometers, not just two. This level of redundancy reduces the false-hit rate from about once per night to once per year. "This technique will be adopted by the Harvard [targeted] program as well during a future program upgrade," according to a September 2003 SETI Institute statement. "Iowa State is currently establishing an OSETI program that will also use the triple photomultiplier technique." So does the Berkeley program at Leuschner.
Another optical SETI project was run at Mount Wilson Observatory. Albert Betz (University of Colorado) and Charles Townes (University of California, Berkeley) examined 200 nearby solar-type stars at the 10-micron infrared wavelengths of carbon-dioxide lasers, using a 1.7-meter telescope during times when it was not being employed for its normal infrared observing projects.
Infrared is better than visible light for distant optical signaling because it better penetrates interstellar dust. Moreover, notes Horowitz, "more photons are received for the same pulse energy." It may well be that extraterrestrials have decided that infrared optical signaling is the only way to go, and that if creatures like us were smart enough to be interesting, we would realize this.
In 2000 an Australian optical SETI project named Oz OSETI was set up by Ragbir Bhathal of the University of Western Sydney, using two telescopes (0.4 and 0.3 meters aperture), each with two photometers, in separate domes about 20 meters apart to eliminate false hits from simultaneous noise events. In its first year the project looked at 100 Sunlike stars and 15 globular star clusters. The project was continuing as of 2010. (In May 2009 a probable false alarm in Bhathal's data was overhyped by the Australian press). Bhathal has also considered plans to build a 1-meter wide-sky OSETI survey telescope, rather like a smaller version of Horowitz's wide-sky OSETI telescope in the Northern Hemisphere.
Several other optical SETI projects have been carried out in the past, and more are planned. David Eichler (Ben Gurion University, Israel) and Gregory Beskin (Special Astrophysical Observatory, Russia) have published a paper pointing out that giant "light bucket" cosmic-ray detectors now in operation and under construction could serve as sensitive optical SETI detectors covering many square degrees of sky at once. Another paper exploring this possibility was published (June 2005) by J. Holder (University of Leeds, UK) and four colleagues; they are looking for optical SETI signals in archived data from the 10-meter Whipple cosmic-ray detector. The STACEE cosmic-ray detector in New Mexico has already been used to watch targeted stars for optical signals during its idle time.
Wider Frontiers
Our ideas of the kind of signals that aliens logically "ought" to broadcast, if they're trying to attract cosmic attention, have already changed a lot in the mere half-century of SETI work so far. They'll surely continue to change, perhaps beyond recognition.
For instance, in August 2009 Gregory, James, and Dominic Benford published a paper, based on their attempts at a universal cost analysis for radio-beacon design, concluding that any radio signals coming our way are far more likely to be brief and very infrequent than continuous; are probably at frequencies far above the magic hydrogen frequency or the "water hole" (that is, at least as high as 10 GHz); are probably not very narrowband; and will be aimed radially toward or away from the galactic center, more or less, for the greatest star-catching efficiency in the Galactic Habitable Zone in the midrange of the Milky Way’s disk. And, the transmitting civilization will be too far away to have any prior knowledge of life on Earth no matter how powerful its telescopes may be (due to interstellar absorption). Every recent and current SETI search ignores most or all of these characteristics.
And why assume that radio or light will be the signal medium at all? Another way that aliens might think to communicate, for instance, is by neutrino beams. This idea is not as farfetched as it sounds. Three physicists report (November 2008) that an appropriate beam generator "can be accomplished with presently foreseeable technology. Such signals from an advanced civilization, should they exist, will be eminently detectable in neutrino detectors now under construction."
Looking even farther afield, longtime SETI theorist Allen Tough proposes a wider range of SETI strategies. These include keeping a lookout for alien artifacts in Earth's fossil record or in today's solar system; recognizing signs of distant astro-engineering projects (for instance, planet-sized artificial shapes transiting the faces of stars or signs of other huge constructions); and, on the hypothesis that ETs might already be watching us right now, simply inviting them to come forward. Proposing such ideas means rubbing shoulders with the lunatic fringe of the UFO movement, but these ideas are not unreasonable in principle. If nothing else, negative results in such areas help to put upper limits on very advanced civilizations that might be around us.
After 50 years of no results, however, it's all too easy to overestimate the upper limits that we have already set. We still know extremely little; it's far too early to speak of any "great silence." Compared to the huge radio search space yet to be looked at, all of our SETI projects so far are nothing more than proof-of-concept trial runs. It is humbling to realize that dozens of exasperated civilizations could be blasting Earth right now with radio wakeup calls at dozens of "logical" hailing frequencies, and they would all be easily missed by every SETI search under way or planned.
While today's SETI programs show a widening range of coverage and strategies, they have not even begun to test all the possibilities.
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Alan M. MacRobert is a senior editor of Sky & Telescope.
Please send corrections or suggested updates to macrobert [at] SkyandTelescope [dot] com.
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