The Search for Extraterrestrial Intelligence

star field


SETI Methods

Radio Search

The radio search makes use of radio telescopes like the 26-meter Green Bank antenna in West Virginia, which was pointed to the stars Tau Ceti and Epsilon Eridani in 1960. As is usual the incoming signal was stored for later analysis. The general procedure is to point the narrow beam

Greenbank antenna

One of the earliest searches for signals indicative of extraterrestrial intelligence was made in 1960 with this 26-meter antenna at Greenbank Virginia.( Photo courtesy National Radio Astronomy Observatory / Associated Universities, Inc. / NSF.

of the antenna in a selected direction and choose a range of radio frequencies for scanning, making sure that they are capable of passing through the Earth’s atmosphere and are not masked by background noise or interference. A human observer or a computer program then steps through the frequency range at narrow intervals to examine at each point whether there is any evidence of the presence of an artificial signal..

Initially SETI made use of available time on radio telescopes that were carrying out a multitude of radio astronomy tasks. More recently, specific instruments for the search have been developed. SERENDIP is an ongoing development started in 1979 at Berkeley and currently operating at Arecibo. Berkeley is also cooperating with the SETI Institute in the development of the Allen Telescope Array at Hat Creek Observatory in northern California, which will be used for both radio astronomy and SETI. Some 350 6.1-meter parabolic dishes produced by low cost commercial dish technology will be linked to provide the equivalent of a single 100-meter dish. An array of 42 

The Allen Telescope Array

The Allen Telescope array at the Hat Creek Radio Astronomy observatory of the University of California at Berkely. This is an initial array of 42 antennas that are combined electronically to perform like a much large array. Ultimately, 350 of these 6.1-meter dishes could perform like a single 100-meter dish.

antennas was funded privately by Paul Allen, co-founder of Microsoft. The ATA has a wider field of view than other large telescopes, which enables it to image whole galaxies. It is thus particularly useful for mapping the whole sky in search of new radio sources.


Optical Search

After the invention of optical lasers in the 1950’s it became apparent that an extraterrestrial civilization could use laser beams for communication as an alternative to radio waves. Various theoretical studies showed that particular types of laser signals, notably those in the infrared, could outshine their local star by many times and should be detectable at distances of up to 100 light years. A short optical search was conducted in Russia in the 1980’s but turned up nothing. More recently interest has picked up. A SETI laser detector is mounted on a Harvard optical telescope being used for a sky survey.

The UC Berkely Optical SETI effort, SEVENDIP, employs a 30-inch automated telescope located in Lafayette, California, to scan the sky for potential extraterrestrial signals. Since its inception in 1997, SEVENDIP has searched for nanosecond pulses that could be generated by powerful lasers of a distant civilizations. This search plans to observe 2500 nearby stars using UC Berkeley's 30 inch automated telescope at Leuschner observatory and an instrument originally built for optical SETI in 1997

A second search at Berkeley is observing 1000 stars for ultra narrow band laser signals in the visible that are on continuously. A search is being made through thousands of extremely high resolution spectra for very sharp lines. As well as data from the Berkeley experiment, data is coming in from Lick and Keck observatories, and from the Southern Hemisphere search for planets in Australia.


Infrared Excess

In 1960, Freeman Dyson suggested that an advanced civilization would encapsulate its sun and planets in a giant sphere in order to meet its energy requirements by capturing the maximum amount of energy from its star. A complete Dyson sphere would not be detectable from earth, but a partially completed sphere could be revealed by its star appearing to radiate excessively in the infrared.

DYSON, the UC Berkeley Infrared Excess Search, attempts to identify excess thermal infrared radiation emitted from highly advanced civilizations by searching the IRAS and 2MASS catalogs and performing follow-up observations on likely candidates with the SETI@home , Serendip V, and Optical SETI databases. The initial dataset consists of 1000 stars with ages greater than a billion years, to reduce the likelihood of confusion from infra-red radiation coming from a protoplanetary disk. Thirty three candidates for partial Dyson spheres were found and examined for indications of extraterrestrial radio or optical signals. None were found. Future tests will make use of survey data from the Spitzer Space Telescope and the Wide Field Infrared Survey Explorer.


Neutrino Communication

For SETI, neutrino communications are a beguiling region for investigation because neutrinos penetrate massive interstellar clouds and solid bodies with ease. And even if these near-zero mass particles cannot travel faster than the speed of light, they can travel at very close to that speed. So neutrinos are proposed as a means of interstellar communication or navigation that could be used by advanced extraterrestrial civilizations. Detecting neutrino transmissions carrying information would provide evidence of such civilizations. The challenge is the ease with which neutrinos travel through materials. This makes them extremely difficult to detect and means huge detectors are required. Large as they are, current neutrino detectors operate at very low efficiencies and would have difficulty in separating intelligent signals out of the neutrino background.

A recent demonstration of information transmission using neutrinos illustrates the scale of the challenge. The transmitter was the Fermilab particle accelerator, itself of considerable size. It accelerated protons around its 2.5 mile track to 120 GeV and sent them through the main injector into a carbon target. This generated a dense stream of neutrinos carrying a message by means of a pattern of pulses achieved by switching the proton stream on and off. The neutrinos then travelled 1,035 meters, including 240 meters of earth, to the MINERVA neutrino detector, where the message was decoded. A massive surplus of neutrinos was required because only about one in ten billion could be detected. An advanced civilization might find neutrinos useful for communication or for beacons signaling the civilization's presence. An effective approach for the latter application would be to transmit beams of neutrinos that are only produced by accelerators and do not exist in nature. (More ./..)