The Search for Extraterrestrial Intelligence, SETI, has been ongoing for almost half a century and in that time most of the attempts to make some kind of contact with any putative extraterrestrial intelligences have been by means of radio i.e. searching the skies in an attempt to catch any radio broadcasts by such civilizations that might have come our way. Given the vastness of our galaxy, not to say of the universe, this approach is a sensible and cost effective one. Even at the speed of light, which is the speed at which radio waves travel, it would take more than 4 years for a radio broadcast from a putative civilization in the neighbourhood of Alpha Centauri, our closest stellar neighbour, to get to us; when we consider that our galaxy stretches for over a hundred thousand light years, the problems involved in the search are placed in clear perspective. Given the present stage of terrestrial technology, radio offers the best hope; even our most powerful optical instruments can see only so far.
Yet, in spite of its inherent advantages, a search that is entirely dependent on radio suffer some inherent shortcomings, the most obvious of which is what of those civilizations who do not utilise radio, either because they have advanced far beyond this mode of communication or they are yet to discover radio (here on earth, radio is barely a century old), or, perhaps, they just dont care about radio. How do we determine the existence of such intelligences? It is in this area that optical SETI comes into play.
Some civilizations, it is suggested, might utilise powerful lasers as a means of communication rather than radio. Such communication would therefore be interceptible on the optical wavelengths rather than on radio wavelengths. The idea which was first postulated in the earliest days of SETI was endorsed in 1983 by Charles Townes, one of the inventors of laser in a detailed study. Although many SETI investigators, including Frank Drake, the pioneer of SETI and leader of the first SETI exploration Project Ozma, agreed with the premise, the 1971 NASA funded study, Project Cyclops, concluded that the idea was not feasible arguing that the construction of a laser which could outshine the sun of a distant system would be too difficult.
Two problems arise if lasers are to be used as a means of communication. The first is that lasers emit light in just one colour, which raises the difficulty of which frequency to search for a putative optical transmission. This problem is however answered by the fact that when light is emitted in narrow pulses, the emission is spread over a wide spectrum of frequencies, and the narrower the light pulses are, the wider the spectrum covered by the emission which should reduce the difficulty of detecting a putative optical transmission.
Secondly, lasers are essentially unidirectional, so that while radio waves can be scattered around in every direction, so to speak, a laser beam carrying some communication can easily get lost in the masses of interstellar clouds that abound in space.
However, there is hope. Paper studies indicate that if a high energy laser is focussed with a 10 metre mirror, the light from the laser would appear thousands of times brighter than the sun to an observer in the beams direct line of transmission. If such a system were programmed to send a message to a number of selected stars, an optical message could be sent to, say, all Sol like stars within a distance of 100 light years, or whatever distance is chosen. This would be somewhat similar to NASA's discontinued Microwave Observing Programme, MOP, which was continued by Project Phoenix, and which investigated some 800 Sol like stars within 200 light years, albeit by means of radio telescopy. Linked with a system of automatic detectors, such a system would also continuously scan the selected stars so as to intercept any message that might be sent from them.
Optical SETI is not just theoretical. In 1980, an optical SETI experiment was conducted in then Soviet Union, though like its radio sibling, nothing turned up. Also, between 1998 and 1999, a Harvard/Smithsonian Institute team carried out an optical survey of 2500 stars using a laser detector aligned with Harvard's 155cm optical telescope. Although nothing was detected in SETI terms, the effort is ongoing. The Harvard/Smithsonian Institute team is now working with a Princeton University team to carry out a joint effort using Princeton's 91cm optical telescope. Both telescopes will track the same stars thereby increasing efficiency and the possibility of detecting any intelligent transmission. Similar studies are also underway at the University of California, Berkeley.