Throughout history, humans have
speculated about possibilities of other worlds and life elsewhere in the
universe. [1,2] Humans from different eras and regions
have set the momentum for the search for extraterrestrial intelligence (SETI)
for nearly twenty-five centuries. Hundreds of ancient documents from both
western and eastern civilizations exist about humanity's inquiry into other
life in the universe. While this is not the place to present all the ideas
and early speculations about extraterrestrial life, a passage by Teng Mu,
a chinese scholar in the Sung Dynasty (960-1127AD), eloquently places our
natural curiosity in historical perspective [1] :
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However, a concerted search in the optical spectrum did not soon occur. Technology in the 1960's was far from sufficiently advanced to consider constructing an optical transmitter of ample power, and the SETI community was reluctant to attempt detection of what were then considered to be unlikely optical signals. At the time of Schwartz's and Townes' paper, long-range radio and microwave communication was common practice on Earth and already a suggested phase space for interstellar communication. Not surprisingly then, experimental SETI began and flourished in the radio and microwave regime. Today, after four decades of Moore's Law growth of optical laser technology, we have the technological means to transmit effective interstellar optical signals. It is presently possible to construct lasers with continuous megawatt (10^6 W) output, as well as ones producing petawatt (10^15 W) peak power optical pulses down to picosecond pulse widths.[5] A group at Harvard-Smithsonian Observatory calculated that by using a modern pulse laser directed with a 10-meter telescope, it would be possible to outshine our sun by a factor of 5,000.[6] |
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ETI may well choose a transmitting technique which uses minimum energy per bit transmitted. Sending a nanosecond pulse satisfies this criterion. As implied above, a modern laser with nanosecond pulse widths boosts peak energy by a factor of a billion, over what might be achieved with a continuous wavelength laser. Nanosecond pulses are believed to be distinguishable from all astrophysical sources. The shortest known time-scale for astrophysical phenomena is in the microsecond range, corresponding to a light travel distance of 300 meters. In addition to astrophysical sources, there may be other sources of pulsed nanosecond signals, such as statistical fluctuations in the incoming data stream from a bright star, Cherenkov radiation from cosmic rays, various detector pathologies, and human induced signals.[8] Most of these other misleading pulsed signals may be eliminated with appropriate instrumentation, as will be discussed later. Our optical SETI search strategy is based on the assumption that a true optical SETI signal will consist of a strong, brief burst of radiation, whose signature will be the near simultaneous arrival of many photons at all instrument detectors. Systems to detect nanosecond optical pulses from extraterrestrial civilizations are now in use at Leuschner Observatory of UC Berkeley, Harvard-Smithsonian Observatory, Princeton, Columbus Ohio, and University of Western Sydney, Australia [9]. Instruments have used a beam splitter to divide the telescope's light between a pair of detectors, and then use coincidence logic to detect when both detectors see simultaneous pulses. However, at high flux levels from bright stars, two detector systems can still yield a high false alarm rate, due to coincidence of several photons at both detectors. Our three detector system, described in the instrument section, succeeds
in further reducing the rate of false positives.
References
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