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The Search for Extraterrestrial Intelligence (SETI) is a recent scientific endeavor.
The first published suggestion was made by Cocconi & Morrison (1959) and interest has
gradually increased within the scientific community. There presently are several
dedicated SETI projects now underway. An article in Annual Reviews of Astronomy &
Astrophysics by Tarter (2001) gives a summary of the history and the relevant physical
and sociological issues in SETI. The review concludes that there presently is no evidence
whatsoever for or against the existence of ETI. However, as Tarter (2001) mentions in the
introduction, SETI is a long shot endeavor with an immense payoff: Arguably finding
evidence of an ETI would be one of the most important event in the history of humanity.
The first astronomical searches for ETI were carried out with radio telescopes.
Following the suggestion by Schwartz & Townes (1961) and Townes (1983) that infrared
and optical lasers could be used for interstellar communications, SETI has begun in the
optical region. For example, the 1.5-m diameter Wyeth Telescope at the
Harvard/Smithsonian Oak Ridge Observatory carries out searches for nanosecond pulses
from ETI (Howard et al. 2004). Howard et al. (2004) discuss in details the issues relevant
to a search for nanosecond pulses. A recent article by Korpela et al. (2011) summarizes
the status of the UC-Berkeley SETI effort which includes radio and optical telescopes.
The SEVENDIP instrument at UC-Berkeley uses an automated 0.8-meter telescope to
search for nanosecond pulses in the 300-700 nm wavelength region (Korpela et al 2011).
There also have been suggestions of searches for ETI in astronomical spectra (e.g.
by Whitmire & Wright (1980) and by Paprotny (1977). They suggested searching for
anomalous spectral lines originating from radioactive fissile waste material. Reines &
Marcy (2002) searched, in 577 nearby stars, for emission lines too narrow to natural from
the host star, like lines originating from lasers.
Present techniques used in optical SETI to measure intensity time variations have
several limitations. They can only observe one object at a time. They are limited to bright
objects. Their major inconvenience is that they need dedicated instruments or require
precious telescope time on standard telescopes.
Borra (2010) shows that periodic time variations of the intensity signal originating
from a pulsating source modulate its frequency spectrum with periodic structures.
Periodic time variations of the intensity signal originating from a pulsating source with
periods between 10-10 and 10-15 seconds
would modulate its spectrum with periodic
structures detectable in standard astronomical spectra. Periods shorter than 10-10 seconds
could be detected in high-resolution spectra. Note that the modulation is rigorously
periodic in the frequency units spectrum but not in the wavelength units spectrum.
In this article I suggest that searches for extraterrestrial intelligence should be
carried out by analyzing astronomical spectra, including spectra already taken. The
outstanding advantage of the technique is that it does not require any specialized
equipment: To the contrary, one can use existing spectroscopic data acquired for other
purposes. The data analysis technique is also extremely simple. The data can be analyzed
by direct eye inspection or with simple Fourier transform software.
ExtraTerrestrial Intelligence (ETI) could signal its existence to others by sending
light pulses with time separations of the order of 10-9
to 10-15 seconds that could be detected in spectra.
Signals with time separations considerably larger than nanoseconds
would however be difficult to detect because the resolution of the spectroscopic
equipment would be insufficient to resolve the spectroscopic signature. One also could
detect spectroscopic signals from ETIs that send bursts with periodic time signals (e.g.
pairs of pulses) separated by longer time scales (e.g milliseconds). The other advantage
of this procedure is that the signals could also be detected in SETIs that look for intensity
pulses within short time scales. For example searches for nanosecond pulses in the optical
region (Howard et al (2004).
As shown in section 4, the physical requirements (e.g. energy) needed to
communicate within a 1000 ly radius are reasonable. They could be met with lasers and
telescopes presently available on Earth.