At What Wavelengths Should We Search for Signals from Extraterrestrial Intelligence?

It has often been concluded that searches for extraterrestrial intelligence (SETI) should concentrate on attempts to receive signals in the microwave region, the argument being given that communication can occur there at minimum broadcasted power. Such a conclusion is shown to result only under a restricted set of assumptions. If generalized types of detection are considered--in particular, photon detection rather than linear detection alone--and if advantage is taken of the directivity of telescopes at short wavelengths, then somewhat less power is required for communication at infrared wavelengths than in the microwave region. Furthermore, a variety of parameters other than power alone may be chosen for optimization by an extraterrestrial civilization. Hence, while partially satisfying arguments may be given about optimal wavelengths for a search for signals from extraterrestrial intelligence, considerable uncertainty must remain. The above discussion indicates that the infrared is as good as, and may be a more favorable region for SETI than, the microwave region on the basis of reasonable assumptions. However, it does not indicate that we should search only in the infrared or even search at all in this wavelength region with present technology. There is considerable uncertainty as to what design parameters would be considered most critical for interstellar communication by an extraterrestrial civilization. Furthermore, the microwave region does have one unique property--that we are prepared now, during the coming decade, to search the microwave spectrum rather efficiently. Hence such searches are probably quite justified. But I believe the above discussion does show that we have no assurance the microwave region is the one of choice for a civilization trying to communicate with us. This may affect the scale and style with which SETI is carried out on Earth even in the immediate future.