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Optical SETI 101


Recently, as this update was being completed, the media was a buzz with news about the discovery of life on Mars. The hype was so intense that it provoked statements to the media by President Clinton and NASA Administrator, Dan Goldin. For those of us in the SETI community, the discovery of extraterrestrial life comes as no surprise, for we have long believed that life was ubiquitous throughout the galaxy and the universe. Next century, historians will look back at this century and find it difficult to understand how mankind thought that life was somehow unique to this insignificant speck in the cosmos. It will be as much a revolution in thinking as the Copernican revolution was five hundred years ago.

It is only a matter of time before we also have proof that intelligent life is also fairly common throughout this galaxy. Then we will know that we are not alone. Perhaps the thoughts expressed here within this Web site will help speed that day!




There are several reasons given by the SETI establishment for why the optical approach to SETI is vastly inferior to the microwave approach. You may have been informed by the Microwave SETI community that the optical approach is no good for the following main reason:

That the transmitter energy cost per optical photon landed on its target is much higher than for microwave photons.




Starfire Optical Range at Kirkland AFB (17792 bytes)

Starfire Optical Range at Kirkland AFB


This assumes one accepts the economic rationale for the relative energy costs of signalling, and this itself is debatable when dealing with very advanced civilizations with access to energy sources beyond our wildest dreams.

However, accepting the idea that the relative energy costs are very important, in actual fact, if ETIs are allowed to use uplinks with gains that would be typical of a 10-meter diameter diffraction limited telescope, i.e., gains of about 150 dB (1015), then it can be shown that the energy costs are about the same. It could be quite possible for a pulsed ETI beacon signal to briefly outshine the ETI's star by seven orders of magnitude.

Note that although a single 10-meter diameter transmitting telescope is often used for modelling purposes, it is not meant to be suggested that ETIs would use a single transmitting telescope with a 10-meter aperture. It is only suggested they could use a much larger phased-array with a defocused beam as narrow and a gain equivalent to that produced by a single 10-meter diffraction-limited telescope. This eliminates the energy density problem with "small" apertures.

Although it would appear that incredibly high peak optical transmission powers are being assumed, it should be realized that what is regarded as a strong signal captured by a moderate size of receiving telescope with sensitive and fast receivers, is very weak with respect to what is received by the naked eye. These so-called strong signals would not be detectable with the naked eye even if centered on the visual response and not obscured by the light of the star. There is no validity to the objection that if such signals existed in the visible spectrum, then we would have seen the stars flashing with the unaided eye. The number of photons detected by the naked eye is too small, and the pulse too short to be noticed. The mean intensity of the conjectured signal amounts to only about 1% of the star's intensity.

In the early 70's, the Microwave SETI community was persuaded that Optical SETI was useless, largely by the conclusions of the comparative analysis given in the Project Cyclops Study. In particular, Table 5-3 from page 50 of the Cyclops Report, put the nails in the Optical SETI coffin by proposing a ridiculously small aperture for an ETI transmitters uplink at a wavelength of 1.06 microns. This is illustrated in the extract from the table shown below:


                     Project Cyclops comparison scenarios
                       OPTICAL            INFRARED            MICROWAVE
PARAMETER            A         B         A         B         A         B
Wavelength           1.06 um   1.06 um   10.6 um   10.6 um   3 cm      3 cm
Antenna Diameter     22.5 cm   22.5 cm   2.25 m    2.25 m    100 m     3 km*
No. Of Elements      1         1         1         1         1         900
Element Diameter     22.5 cm   22.5 cm   2.25 m    2.25 m    100 m     100 m
                     ^         ^
                     |         |
                     |         |

If the reader observes the antenna diameter given in columns "A" and "B" on the left of the table under the heading "OPTICAL" (near-infrared), if will be seen that the transmitting telescope aperture (uplink) is constrained to be only 22.5 cm in diameter! This is smaller than the telescope aperture being used by the COSETI Observatory to detect such signals!

Click here for the complete Table 5-3 from the Project Cyclops Report


Project Cyclops Extract




Why was this assumption made?

Because is was assumed that ETIs were inept!

The author cannot understand how this assumption was accepted by the SETI community.

Even if one assumed that ETIs would have difficulty with the point-ahead targeting of nearby stars, since the beams would be smaller than the zones of life around these stars, why would one cripple the long range performance of the transmitter. One could simply build the largest diffraction limited transmitter affordable for ones civilization and just defocus it when targeting nearby stars. In this way, the inverse-square law would not apply, and the energy density of the beam would be essentially independent of distance out to thousands of light years!

The other strong justification for the optical approach to SETI, be it in the infrared or visible spectrums, is that only laser light has the capability of allowing wideband communications over interstellar distances. This is not just an issue of received signal strength and signal-to-noise ratio, rather it is a reflection of the fact that interstellar dispersion and scintillation (see Cordes) effects at radio frequencies make it very difficult, if not impossible to transmit wideband information over such distances.

If the reader finds these arguments for Optical SETI persuasive, then please browse this Web site for more fascinating information on the optical approach to SETI. This site covers both the professional and amateur approaches to Optical SETI, where the main difference between the approaches is the size and cost of the receiving telescope and the sophistication and cost of the signal processing electronics.

In conclusion, it may well be that Modern SETI is built on a house of cards. By removing the "Assumption of Ineptitude" on the part of ETIs in their knowledge base and targeting skills, we find that lasers are clearly superior for direct, point-to-point interstellar communications out to ranges of several thousand light years. If ETIs are fairly common within the galaxy, so that the mean distance between such communicating civilizations is less than several thousand light years, then the only form of electromagnetic communication that can support the "Galactic Internet" has to be laser based. Technically advanced and sophisticated civilizations will not waste large amount of power by beaming electromagnetic energy semi-isotropically into empty space!

We ourselves, are probably no more than fifty to one hundred years from being able to produce and accurately direct the powerful laser beams conjectured by this author. In the next decade, the momentum produced by NASA's Origins Program will increase the precision in our knowledge base of the peculiar and proper motions of other stars with respect to the Sun. Then we will be well on the way to being able to realize the huge antenna gains that this author has projected for ETIs but which were rejected by the Cyclops Study some twenty five years ago.


For more information, see:

First uploaded: August, 1997

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