Cocconi and Morrison initially proposed interstellar communication using radio waves, particularly near the hyperfine transition of hydrogen. The first SETI observations, conducted by Frank Drake, followed this suggestion of where to look. The focus in the microwave was a result of technological limitations, as observations at other frequencies were unimaginable when Cocconi and Morrison initially presented their work. With the discover of the maser in the 1950s, a new vector for communication became available. The authors postulated that “maser oscillators and other appropriate apparatus in or near the optical region [will] allow detectable light signals to be beamed between planets of two stars separated by a number of light years”. The creation of the laser in 1960, a more practical device than a maser, served as further justification for this claim.
Townes and Schwartz note the physics behind the maser was first described by Einstein. There was no theoretical deficit precluding its discovery or delaying development. The authors use this to argue that there may exist an extraterrestrial society comparable to Earth that discovered and developed masers before radio waves. They boldly state:
We propose to examine the possibility of broadcasting an optical beam from a planet associated with a star some few or some tens of light-years away at sufficient power-levels to establish communications with the Earth. There is some chance that such broadcasts from another society approximately as advanced as we are could be adequately detected by present telescopes and spectrographs […]
A maser, much like its optical counterpart, could theoretically operate continuously at high power and would be almost monochromatic. Townes and Schwartz note the limit to producing an ideal maser would be the technological problems in mirror accuracy and control of any optical distortions. They considered two masers (1) one energetic maser and (2) twenty-five masers pointing in the same direction and specified two criteria for the detectability of either maser:
- it must produce enough photons per unit area at the receiver to be detectable with a lens of practical size and in a reasonable time and
- it must be distinguishable from the background stellar light.
They argued that the intensity of radiation from the group of masers would produce a beam of high intensity capable of being observed by the naked eye or binoculars out to 0.1 or 0.4 light years, respectively. A larger telescope of long integration would be required for masers further than 10 light years. The authors note that, from the work of Cocconi and Morrison, there were ten Sun-like stars within a distance of 10 light years, making masers very applicable to SETI. Spectra would be another useful diagnostic for a maser. A grating spectrograph in the 1960s could have resolved the energetic maser as a signal equal to the stellar background. Noting this, Townes and Schwartz propose that “[a] spectral line sought can be expected to be exceptionally narrow, at an abnormal frequency for the type of star in question, and varying in intensity [o]bservation of any of these characteristics should lead to closer examination” of an object.
In less than a decade after discovery, masers were already being considered for SETI. The advantages include the coherence of radiation over a very large aperture and the theoretical possibility of obtaining coherence among several maser sources. Given that nay plausible atmosphere would prevent emission of masers, the authors propose utilizing a “very high-altitude balloon, a space platform, or natural Moon”. Only two years passed after Cocconi and Morrison published and people began considering where to look. This is an on-going discussion, but the authors correctly argue that charged particles would be deflected while UV and IR emission would be absorbed by an atmosphere. This blogger considers this to be an important discussion. When this was originally published, SETI was still in its infancy and the authors emphasized the need to consider other wavelengths before all of SETI focused on microwaves. While the development of masers may have stymied after the discovery of the more practical laser, optical SETI now exists. Recent progress in masers (see Movie 1) suggest their applicability will soon increase. Perhaps the masers of the future will become useful for SETI as Schwartz and Townes initially proposed.