Optical SETI experiments
While most SETI sky searches have studied the radio spectrum, some SETI researchers have considered the possibility that alien civilizations might be using powerful lasers for interstellar communications at optical wavelengths.
The idea was first suggested in a paper published in the British journal Nature in 1961, and in 1983 Charles Townes, one of the inventors of the laser, published a detailed study of the idea in the US journal Proceedings of the National Academy of Sciences.
Most SETI researchers were cool to the idea.
The 1971 Cyclops study discounted the possibility of optical SETI, reasoning that construction of a laser system that could outshine the bright central sun of a remote star system would be too difficult. Now some SETI advocates, such as Frank Drake, have suggested that such a judgement was too conservative.
There are two problems with optical SETI, one of which is easy to deal with, the second of which is troublesome.
The first problem is that lasers are highly "monochromatic", that is, they only emit light on one frequency, making it troublesome to figure out what frequency to look for.
However, according to Fourier analysis, emitting light in narrow pulses results in a broad spectrum of emission, with the frequencies becoming higher as the pulse width becomes narrower, and an interstellar communications system could use pulsed lasers.
The other problem is that while radio transmissions can be broadcast in all directions, lasers are highly directional. This means that a laser beam could be easily blocked by clouds of interstellar dust, and more to the point, we could only pick it up if we happened to cross its line of fire.
As it is unlikely an alien civilization would focus an interstellar laser communications beam on Earth deliberately, we would have to cross such a beam by accident.
However, as discussed earlier, the power requirements for omnidirectional interstellar radio broadcasts are tremendous, and narrow-beam radio communications are technically more plausible.
As SETI researchers have adjusted to the idea that interstellar radio communications may be over narrow beams, the idea of hunting for interstellar laser beams has become no more troublesome.
In the 1980s, two Soviet researchers conducted a short optical SETI search, but turned up nothing.
During much of the 1990s, the optical SETI cause was kept alive through searches by Stuart Kingsley, a British dedicated amateur living in the US state of Ohio.
Now the SETI old-timers have warmed to the concept of optical SETI.
Paul Horowitz of Harvard and researchers with the SETI institute have conducted simple optical SETI searches using a telescope and a photon pulse detection system, and are considering further searches.
Horowitz says: "Everyone's been mesmerized by radio, but we've done that experiment a lot and we're a little tired of it."
Optical SETI enthusiasts have conducted paper studies of the effectiveness of using contemporary high-energy lasers and a ten-meter focus mirror as an interstellar beacon.
The analysis shows that an infrared pulse from a laser, whose light output is not bound by the inverse-square law of light emitted from a hot body like the Sun, would appear thousands of times brighter than the Sun to a distant civilization in the beam's line of fire.
The Cyclops study proved incorrect in suggesting a laser beam would be inherently hard to see.
Such a system could be made to automatically steer itself through a target list, sending a pulse to each target at a rate, say, of once a second.
This would allow targeting of all Sun-like stars within a distance of 100 light-years. The studies have also described an automatic laser pulse detector system with a low-cost, two-meter mirror made of carbon composite materials, focusing on an array of light detectors.
This automatic detector system could perform sky surveys to detect laser flashes from civilizations attempting to contact us.
Several optical SETI experiments are now in progress.
A Harvard-Smithsonian group that includes Paul Horowitz designed a laser detector and mounted it on Harvard's 155 centimeter (61 inch) optical telescope.
This telescope is currently being used for a more conventional star survey, and the optical SETI survey is "piggybacking" on that effort.
Between October 1998 and November 1999, the survey inspected about 2,500 stars. Nothing that resembled an intentional laser signal was detected, but efforts continue.
The Harvard-Smithsonian group is now working with Princeton to mount a similar detector system on Princeton's 91-centimeter (36-inch) telescope.
The Harvard and Princeton telescopes will be "ganged" to track the same targets at the same time, with the intent being to detect the same signal in both locations as a means of reducing errors from detector noise.
The Harvard-Smithsonian group is now building a dedicated all-sky optical survey system along the lines of that described above, featuring a 1.8-meter (72-inch) telescope.
The new optical SETI survey telescope is being set up at the Oak Ridge Observatory in Harvard, Massachusetts.
The University of California, Berkeley, home of SERENDIP and SETI@home, is also conducting optical SETI searches.
One is being directed by Geoffrey Marcy, the well-known extrasolar planet hunter, and involves examination of records of spectra taken during extrasolar planet hunts for a continuous, rather than pulsed, laser signal.
The other Berkeley optical SETI effort is more like that being pursued by the Harvard-Smithsonian group and is being directed by Dan Wertheimer of Berkeley, who built the laser detector for the Harvard-Smithsonian group.
The Berkeley survey uses a 76-centimeter (30-inch) automated telescope and an older laser detector built by Wertheimer.
Where are they? / The interstellar Internet
SETI experiments performed so far have not found anything that resembles an interstellar communications signal.
Says Frank Drake of the SETI Institute: "All we know for sure is that the sky is not littered with powerful microwave transmitters."
The great Italian physicist Enrico Fermi suggested in the 1950s that if there was an interstellar civilization, its presence would be obvious once we bothered to look. This is known as the Fermi paradox.
While faster than light, or "superluminal", flight is ruled out by contemporary physics, no law of physics absolutely rules out interstellar flight at "subluminal" speeds, though the physical requirements are formidable.
Assuming that stars are on the average about ten light-years apart; that an interstellar mission can be conducted at a speed of 10% of the speed of light; and that it takes four centuries for an interstellar colony to grow to the point where it can launch a pair of interstellar missions, then the "doubling time" of the interstellar colonies created by this advanced civilization would be 500 years.
This would allow colonization of the entire galaxy in five million years.
Even limiting an interstellar mission to 1% of the speed of light and assuming it takes a millennium for a society to get to the point where it can mount two
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