Origins of Optical SETI

In their famous paper 40 years ago Cocconi and Morrison (Nature,
184, 844, 1959) suggested the possibility of interstellar
communication via microwaves -- that is, radio waves of very high
frequency, in the region of gigahertz (billions of cycles per
second).  At that time there were no lasers.

But within a year Schalow and Townes made the Nobel-winning
invention of the laser, and the very next year there was
published the first suggestion of laser SETI: "Interstellar and
Interplanetary Communication by Optical Masers" (Nature [again!],
190,205, 1961).

At that time lasers were new, tricky, low-power devices; by
contrast, radio technology had been developing for decades and was
relatively mature.  Perhaps that is why most searches for signals
from advanced civilizations were carried out in the microwave
region of the spectrum, most commonly at the special emission
frequency (1.4 gigahertz) of neutral atomic hydrogen -- the most
abundant atom in the universe -- the so-called "21-centimeter
line."

The rapid development of laser technology since that time -- a
Moore's Law doubling of capability roughly every two years --
along with the discovery of many other microwave lines of
astronomical interest, have lessened somewhat the allure of
hydrogen-line SETI.  Indeed, on earth the exploitation of
photonics has revolutionized communications technology, with
high-capacity fibers replacing both the historical copper cables
and the long-haul repeater chains of microwave towers. 
Furthermore, calculations and observations show that the galaxy
is not particularly kind to gigahertz radio waves -- even
operating under the prevailing criterion of minimum energy per
bit transmitted, one is driven upward to millimetric wavelengths.

Although most of the 50-some searches have been at radio
frequencies, there have been a handful of searches for optical
signals.  Russian (Shvartsman et al.) and American groups
(Werthimer; Kingsley) have looked for short laser pulses;
American groups have looked also for the spectral lines of lasers
(Betz et al.), as well as non-laser optical artifacts caused by
astroengineering or waste management.

As laser technology has matured, the SETI community has begun to
listen to these pioneers.  There have been a series of technical
papers (e.g., by Townes in 1983, 1993, and 1996; by Rather in
1991; and by Kingsley in 1993).  Furthermore, in the last year
the SETI Institute has been conducting a series of workshops to
take a fresh look at SETI strategies, both radiowave and optical.
From these workshops originated the optical SETI projects being
built both at Harvard/Smithsonian and at Berkeley.