SETI at Different Wavelengths

Charles H. Townes, the inventor of the maser and laser, wrote a paper in 1983 to discuss the appropriate wavelength for the search for extraterrestrial intelligence (SETI). He argued that, while SETI developed, it was important to consider which wavelengths to conduct searches. Beginning with Drake’s Project Ozma, most SETI experiments have used the radio region of the electromagnetic spectrum with particular emphasis on the water hole. With the discovery of masers and lasers, it was possible to consider using them for interstellar communications at optical wavelengths. Schwartz and Townes in a 1961 letter first presented this idea of using masers to communicate over long distances, assuming we used a narrow-band receiver. In this article they proclaimed:

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, and appropriate techniques now available for detection will be discussed. Communication between planets within our own stellar system by beams from optical masers appears a fortiori quite practical.

They concluded “the frequency of the hydrogen line in the micro-wave region is not the only reasonable place at which to search for possible interstellar communications, and […] the optical region also seems a logical one”. This paper can largely be viewed as a continuation of his initial work.

Townes begins by motivating SETI at other wavelengths. An extensive search focusing on one regime in the electromagnetic spectrum would be a large endeavor and potentially a waste of resources. The communication capabilities of ETI were assumed to be analogous to our capabilities. The principals for SETI are described via various strategic questions and under the assumption ETI wishes to minimize costs of any technology they use. The first question addressed is the nature of the signal, primarily if ETI signals would be isotropic or directive. It is preferred that a civilization broadcast narrow band. Townes uses the excessive power an isotropic signal would require (9 ordered of magnitude more) to suggest ETI would favor sending a beam. Other assumptions regarding ETI and its capabilities:

  • regarding power sources, there is no necessary choice as a function of wavelength from the radio region down at least into the ultraviolet,
  • there are detectors of sensitivity close to the ultimate limit dictated by the quantum properties of radiation over the whole range of wavelengths, and
  • if needed, the use of space for the beacons is to be expected.

Townes consider numerical evaluations of the signal-to-noise ratio (SNR) for different wavelengths. One potential observation scheme involves using longer wavelengths with linear detection of all wavelengths and a constant antenna area but solid angles corresponding to the diffraction limit only for wavelengths >1 cm. The other observation scheme involves short wavelengths with a quantum counting detector and an antenna with a fixed diameter for long wavelengths down to 1 cm and then decreasing linearly in size to 10 m in the infrared.

These were but two examples discussed. Townes concludes that, depending on the assumptions, other regions, such as the infrared, should be considered. This was a marked departure from what was initially proposed by Cocconi and Morrison. While Townes initial suggest of using the infrared may not be used today, the discussion regarding where to look is still ongoing. Experiments in optical SETI have since been conducted (e.g. Reines & March, 2002), Laser SETI is a thing (see Movie 1), and it optical SETI is one of the projects of the SETI Institute. Recent papers have scrutinized both the wavelength of photons and even the nature of the particle observed by SETI. It may have taken over forty years since the first publication from Townes discussing masers, but at least proponents of SETI are no longer latching onto the microwave.

Movie 1. Laser SETI Wants Your Money
Laser SETI is an example of the types of searches Townes proposed – something not tied to the microwave region. The optimal wavelength to observe is an important discussion that is still ongoing.