Optical SETI is the search for pulsed and continuous wave laser beacon signals from intelligent extraterrestrials in the visible and infrared part of the spectrum. The idea was first suggested in 1961 only a year after the Nobel-winning invention of the maser by Townes and Schalow but received little attention at the time because radio technology was in a much more mature state. Indeed, over the past few decades there have been many, large-scale SETI programs aimed at detecting artificial microwave signals but only a handful designed to operate in the visible/infrared region. That situation is now changing as a result both of the rapid development of laser technology and the realization by astronomers that optical communications over interstellar distances makes good, practical sense.

 

As an alternative to radio waves for sending messages to the stars, lasers offer a number of advantages. They are not produced as "noise" by natural processes and are highly distinctive. Quasars, pulsars, and black holes were all found or confirmed because of their unique radiation characteristics, not because of their resemblance to ordinary stars. So, it may be that a civilization wanting to send a message would choose an "unnatural" wavelength or technology to achieve maximum contrast to the natural environment (an argument directly counter to that used in support of 21-centimeter line SETI). Lasers also have the advantage that they can be aimed accurately at a target, thereby enabling the best use of the transmitted energy. A well-designed laser would be very bright with a high signal-to-noise ratio. Optical communication, enabling five times as much information to be carried, would be the logical choice for an advanced civilization, argued Russian astronomer Viktory Shvartsman. A search for such signals, begun in 1989, was carried out using the BTA-6 optical telescope (then the world's largest) of the Soviet Union's Special Astrophysical Observatory in the Caucasus, linked to a computer and another instrument, known as MANIA. The wide bandwidth available at optical frequencies could be exploited for sending complex, information-rich signals. This could even include, suggested John Rather of the Kaman Aerospace Corporation, the genetic code for a human being (see genetic code, interstellar transmission).

 

Many of the most recent developments in optical SETI stemmed from the discussions of the 1998 "SETI Science and Technology Working Group," sponsored by the SETI Institute. In the wake of this, the Planetary Society and the SETI Institute funded projects at the University of California, Berkeley (see optical SETI at Berkeley) and Harvard University (see optical SETI at Harvard/Smithsonian) to search for laser signals coming from nearby Sun-like stars, as well as from globular clusters and external galaxies.

 

References

1. Ross, M. "The Likelihood of Finding Extraterrestrial Laser Signals," Journal of the British Interplanetary Society, 32, 203–208 (1979).

 

Abstract: Communications at optical frequencies from extraterrestrial sources many light years away are shown to be quite feasible and can be received using present receiver technology. It is explained that the exact optical frequency need not be known and that short pulses in the nanosecond regime at high energy per pulse at very low duty cycle are most likely to be sent. Large optical collectors (10 to 20 metres diameter) of non-image quality can be built at low cost to collect the signal, it is explained why present optical telescopes and instrumentation would not normally detect such signals due to the planet's starlight and how this can be overcome by optical communication modulation formats. These formats enable a number of bits of information per received pulse. With many bits/pulse transmitted, each pulse may represent a line-word allowing for a small number of pulses to transmit a crude picture. it is further explained how high resolution pictures may be obtained. A suggestion is made relating to possible optical frequencies but it is made clear that it is not essential to detection of extraterrestrial laser signals. It is suggested that laser signals should be given consideration in addition to radio frequency signals for extraterrestrial communications.

 

2. Ross, M. "Design of an Optical Receiver for Space Signals," Journal of the British Interplanetary Society, 33, 89–94 (1980).

 

Introductory paragraphs: In a recent paper [1] it was explained how present technology is sufficient to detect likely laser signals from extraterrestrial sources. It was further explained that the likely nature of modulation of those signals and the laser output energy requirements were the same order as those of which we are presently capable. It was also discussed why the exact wavelength of the source need not be known and the reasons why laser transmission might be used. In this paper, we review briefly the above and provide a more detailed discussion of the optical receiver to detect extraterrestrial signals. The major obstacles to finding a laser signal from outer space have been (1) no knowledge of the optical frequency and (2) masking of the signal by the signal star's light. Both of these obstacles can be overcome by present technology and understanding of laser communications.