Box 4. Optical SETI

There is an advantage in transmitting signals at short wavelengths. This explains the interest in optical SETI (OSETI) in which searches are done at optical wavelengths, which are very much shorter than radio wavelengths. In OSETI, receiver noise floor is very much lower, increasing receiver sensitivity. For transmission, it is simple to show that a 1-m telescope, operating in the optical range, can produce one hundred million times the ERP as a 100-m radio telescope operating at 50 cm. In addition, systematic effects such as interference should be less in the optical or infrared than the radio wavelength region and the rate of information transfer is faster because of the larger bandwidths. Filters in the optical range are less selective than in the radio range, but an advantage is that ETCs could send messages using nanosecond pulses designed to look artificial and thus distinguishable from natural sources of electromagnetic radiation.

OSETI proponents also make an argument based on properties of the interstellar medium. Ionized clouds in the interstellar medium²³ scatter and absorb light much less than radio signals. However, optical or infrared signals are absorbed by dust in interstellar clouds. The targets for OSETI are selected on the basis of visible light or near-infrared measurements, and so would not be affected by intervening material. Thus the overall effect of interstellar clouds on optical or near-infrared communications is smaller.

The following example illustrates the advantages of OSETI in regard to effective radiated power²⁷. An ETC orbiting a Sun-like star could use a laser to illuminate a 1-m optical telescope through narrowband optical filters. The ETC could then produce a short pulse lasting a microsecond or less. This would produce a flash 300,000 times as bright as their Sun. Even without optical filtering, the flash would still be 30 times as bright as their Sun, and this factor would rise to 3,000 if the diameter of the telescope were increased to 10 m, as with the Keck telescope. Because of the short pulse length, such OSETI signals would not be found in conventional optical surveys.