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Searching for extraterrestrial intelligence signals in astronomical spectra, including 
existing data

 

Source

 Ermanno F. Borra,

Centre d'Optique, Photonique et Laser,

Département de Physique, Université Laval, Québec, Qc, Canada G1K 7P4

 (email: [email protected])


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1. INTRODUCTION


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.


Read the whole SETI paper in PDF  HERE

6. CONCLUSION

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.

The outstanding advantage of the proposition is the simplicity of the data analysis. 
A strong signal could be found by visual inspection.  One could also incorporate signal finding algorithms into existing software and use it with existing databases and future 
spectroscopic data. As discussed in section 4, one could use Fourier transforms to detect 
the signal. Using a Fourier transform would be particularly useful for an automatized 14
analysis of a large quantity of spectra (e.g. from a survey). This can be done with a few 
lines of code in Matlab. It is a very small effort worth doing because finding ETI, would 
be of enormous interest. It is a small effort with a potentially huge pay-off. 
Finally the proposition meets the criterium (Tarter 2001) that new instrument that 
opens up pristine cell of observational phase space may surprise us with unexpected 
manifestations of ETI
Note also, that, although this article considers the optical-infrared spectral 
window, the signals could also be generated in other spectral regions (e.g. the radio 
region).

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