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] :

Empty space is like a kingdom, and earth and sky are no more than a single individual person in that kingdom. Upon one tree are many fruits, and in one kingdom there are many people. How unreasonable it would be to suppose that, besides the earth and the sky we can see, there are no other skies and no other earths.

Today the search continues in our scientific world of SETI. In 1959 Cocconi and Morrison published the first modern scientific paper[2] on possible methods of the search for ETI. They suggested that the 21 cm hydrogen line was a promising wavelength at which to search for interstellar communications. In 1960, at the National Radio Astronomy Observatory in Green Bank, West Virginia, Frank Drake conducted the first such search, called Project Ozma. One year later Drake and Peter Pearman hosted the first SETI conference in the world. It was at this conference that Drake first presented the now famous equation -- presently known as the Drake equation -- containing seven factors that would allow one to predict how many civilizations might be conducting interstellar communication.

That same year (1961), Schwartz and Townes suggested searching for extraterrestrial signals in the optical spectrum.[3] Motivation for the optical search originated with invention of the maser and laser in 1959 by Townes[4], then with Bell Laboratories.

Frank Drake at NRAO, Green Bank, West Virginia, 1960.

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]

©Laurie Hatch 2003

Advantages of optical pulse beacons include high gain of optical transmitters, negligible dispersion at optical wavelengths, and the computational analysis is much simpler than for radio and microwave searches. Advantages for targeted optical searches are that they are more cost effective than radio searches, and the data requires comparatively rudimentary analysis. Optical detectors are considerably less expensive then radio and microwave instruments today. The computational power and Fourier transforms required for data analysis for radio and microwave searches are much more expensive and sophisticated than that required for optical searches.

In order to transmit a detectable interstellar signal, ETI would need to use a unique beacon, one that can be distinguished from background noise and from astrophysical sources. For optical lasers it has been shown that strong pulsed signals at nanosecond (or faster) intervals are distinguishable from astrophysical sources, and give a sufficiently high intensity over optical background noise. Scenarios for, and feasibility of transmitting signals with lasers has been previously well documented.[3,6,7]

The importance of signal to noise in SETI, whether in radio or optical searches, is crucial. Extraterrestrial civilizations transmitting a beacon would need to send a signal with a greater intensity than the background noise at that specific frequency.

Lick's 3-meter telescope emitting a laser beam into earth's atmosphere to create an artifical star for adaptive optics. This illustrates just one of the many practical, innovative uses of lasers today. Photo courtesy of Laurie Hatch Photography.

 

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

1. W. Sullivan, We Are Not Alone, Penguin Group, New York, NY, 1964, 1993, P.4.
2. G. Cocconi and P. Morrison, "Searching for interstellar communications," Nature 184, pp. 884-846, 1959.
3. R. Schwartz and C. Townes, "Interplanetary and interstellar communication by optical masers," Nature 190, pp. 205-208, 1961.
4. C. Townes, How the Laser Happened, Oxford University Press, Oxford, NY, 1999.
5. R. Slusher, "Laser Technology," Rev. Modern Physics. 71, pp. 471-479, 1999.
6. A. Howard et al., "Optical SETI at Harvard Smithsonian," in Bioastronomy '99 - A New Era in Bioastronomy, G. Lemarchand and K. Meech, eds., ASP Conference Series 213, pp. 545-552, 2000.
7. C. Townes, "At what wavelengths should we search for signals from extraterrestrial intelligence?," Proc Natl. Acad. Sci. 80, pp.1147-1151, 1983.
8. A. Howard and P. Horowitz, "Is there "RFI" in pulsed optical SETI?," Proceeding of SETI in the Optical Spectrum III, S. Kinglsey eds., SPIE 4913, 2001, in press.
9. R. Bhathal, "Australian optical SETI Project," in Bioastronomy '99 - A New Era in Bioastronomy, G. Lemarchand and K. Meech, eds., ASP Conference Series 213, pp. 553-557, 2000.