Optical SETI Rationale and Philosophy
With apologies to Philosopher Clive Goodall! Here is a comparison summary of the essential differences between the implied Conventional and Kingsley SETI Rationales:
---------------------------------------------------------------------------- MICROWAVE (CONVENTIONAL) SETI OPTICAL (UNCONVENTIONAL) SETI ---------------------------------------------------------------------------- 1. ETI technology mediocre ETI technology like "magic" 2. Signals weak Signals strong 3. Modulation bandwidths very small Modulation bandwidths large 4. Minimum bin bandwidth = 1 Hz Minimum bin bandwidth = 100 kHz 5. Number of frequencies = 9 X 10^9 Number of frequencies = 9 X 10^9 6. Chirp rate/bin bandwidth = 0.2 s^-1 Chirp rate/bin bandwidth = 0.2 s^-1 ----------------------------------------------------------------------------
While the sophisticated late 20th century man would recognize ETI technology as not really being "magic", it might take a long time to discover the physical laws behind much of ETI technology. Only human arrogance prevents us from realizing that the sum total of our knowledge today does not represent 99% of all knowledge that is to be obtained about our universe. We must humbly remind ourselves, that one hundred years ago, few people on this planet made any use of electricity. We have come a long way since then - a blink of the eye on the cosmic time-scale. We can expect that the next few hundred years will witness most startling discoveries and inventions. In all humility, the technology of ETIs, tens of thousands to millions of years more advance than us, will indeed have the appearance of "magic". Just as sophisticated man who is close to (peacefully) harnessing the very forces that power the Sun, can still rub two sticks together to make fire, so ETIs, who perhaps long ago ceased to use electromagnetic waves for communications, can still use the latter to contact emerging technical civilizations, with powers that could be in the terrawatt (10^12 W)+ level.
The assumption of strong, wideband signals, changes our signal processing approach and drastically reduces the search time. Strong signals mean that since ETIs can shout loudly, they don't need to choose frequencies at the quietest region of the spectrum. ETIs wouldn't be seen dead at the "water- hole" for fear that it would be interpreted as a sign of mediocrity, nor do they have concerns about their electricity bills or interstellar photon ballistics! Strong signals mean that larger bandwidths can be used. Larger bandwidths mean that laser linewidth problems are not significant, chirp becomes less of a problem, and the number of frequencies to search is kept reasonable. Indeed, the number of 1 Hz bins between 1 GHz and 10 GHz is 9 billion. The number of 100 kHz bins between the 10,600 nm CO2 infrared wavelength and the ultra-violet at 350 nm, is also about 9 billion.
With my rationale, the ratio of Doppler chirp rate (for a ground-based receiver) to minimum bin bandwidth, is approximately the same for both Microwave and Optical SETI. There is also a more subtle advantage that accrues from larger detection bandwidths and stronger signals. Since the minimum sampling time for a given bandwidth is approximately the inverse of the bandwidth, larger bandwidths may be sampled more rapidly. A 100 kHz filter can be sampled in approximately 10 microseconds, 100,000 faster than for a 1 Hz filter, and thus substantially decreasing the search time. By sampling faster, the Doppler drift during the sampling time is reduced. Hence, there is a strong-signal (beacon) scenario where it may not be necessary to de-chirp the laser transmitter or receiver prior to acquiring and locking onto a signal!
We can suggest that the optical signals are strong since so far, a systematic optical search has yet to be conducted. A detailed retrospective computerized analysis of a huge number of stellar spectrographic plates should tell us if such powerful signals are present. Thus, there might be strong ETI signals present in the optical spectrum, for it could be argued that we should have discovered their microwave signals by now if ETIs were using "magic" (powerful) microwave technology.
One last thought. Optical communications are likely to become very mature and the main means of communications on this planet within the next few decades, e.g., fiber-optics to the home, optical computers, and GEO to GEO/LEO orbital links. They are probably the ideal "compact" communications technology for space-faring civilizations who have yet to master non- electromagnetic communications technology.
In the coming decades, despite concerns of the SETI community about radio frequency interference problems, the radio brightness of the Earth may drop drastically as the more efficient beamed optical technology becomes dominant, and high-power terrestrial transmitters and military radars are turned off (there will be more microwave satellite communication links between the surface of the Earth and GEO/LEO orbits). Because the high- power sources of radio frequencies will eventually be decommissioned, I wouldn't be too concerned or depressed at this time that we haven't been able to eavesdrop on other technical civilizations leaking radio waves into space. The period of time during which an emerging technical civilization wastefully radiates large amounts of radio frequency energy into space may be less than 100 years. Thus, the probability of us detecting another technical civilization via their accidental radio frequency emissions may be very low, even if our part of the Galaxy is crawling with life. Of course, our present technical ability would limit us to only detecting carrier- waves, not the modulation envelops.
Optical communications will become our main communications technology across the solar-system and deep-space. This will be the technology used in interstellar space, as later next century, we send out non-relativistic probes to investigate and relay back pictures and data about our nearest star systems. Early next century, men will walk on Mars. That second "small step for a man . . ." will be seen by mankind via HDTV signals beamed to Earth. These signals will probably traverse most of the distance between Mars and Earth on an optical carrier, and may be relayed around the globe via direct laser links between geosynchronous communications satellites. Most viewers will receive these pictures via fiber-optic Cable TV.
The world of the future will be dominated by photonics (optoelectronics). It is very unlikely that microwave technology will remain the dominant form of beamed communications. It is more likely that for the greater span of the lifetime of a technical civilization based on electromagnetic technology, that photonics will have the greatest impact on society. For this reason, if our civilization is typical of emerging technical societies, it would appear most natural for ETIs to communicate via optical photons. In their 1961 Nature paper, Schwartz and Townes virtually hinted at this. It is a pity that they were not listened to by the rest of the then embryonic SETI community. The time has come to shake off our proclivity for microwave technology.
May 3, 1991 RADOBS.33 BBOARD No. 499
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