The Search for Extraterrestrial Intelligence

star field

Arecibo Antenna Picture

The 305-meter radio antenna at Arecibo, Puerto Rico, carries a UC Berkeley experiment to detect signals from extraterrestrial civilizations. Connected to the Arecibo L-Band feed, the SERENDIP V instrument provided a stream of data analyzed by over seven million home computers for evidence of intelligent communications. The program is currently in hibernation.( Photo courtesy of the NAIC-Arecibo Observatory)

1 Introduction

The Search for Extra-Terrestrial Intelligence (SETI) takes a number of different forms. The most active is searching for signs of intelligence in electromagnetic waves coming from outer space. The search is with a narrow-beam antenna and a receiver sensitive to a narrow frequency band somewhere in the spectrum from radio waves to ultraviolet light. As the receiving systems become more sensitive to signals from greater distances, the number of combinations of frequency, bandwidth and pointing angle goes up exponentially. Only a minute portion of the total search volume has been explored during the 60-year life of the search.

Another method involves a search for evidence of visits from other civilizations to the neighborhood of the earth. Thus far there is no evidence of artifacts left on the Earth, Moon, or Mars or in stable orbits in the Earth-Moon system, or of impacts from manufactured objects (other than those launched from Earth.)


Signal Content

As we continue to invest in equipment, operations and human effort in the search for extraterrestrial intelligence (SETI), we recognize the need to consider the content of the signals we might receive, because we will have to know  how to interpret them. A signal seeking to establish first contact with an unknown civilization may well use mathematics to draw attention to the presence of intelligence in the signal. This was suggested in the initial scientific paper on interstellar communication by Giuseppe Cocconi and  Philip Morrison (Searching for Interstellar Communication, Nature, 184, 844, 1959). They expected the signal to be pulse modulated with a speed not very fast or slow compared to a second, on the grounds of band-width and of rotations. Adding, “For indisputable identification as an artificial signal, one signal might contain, for example, a sequence of small prime numbers of pulses, or simple arithmetic sums.” Morrison later developed this idea further to suggest using a repeating frame of numbers to convey a simple representation of a circle.

In 1974 the staff of the National Astronomy and Ionosphere Center, used the 1,000-foot Arecibo Antenna, operating at an effective average radiated power of 3 x 1012  W, to transmit a 2380 MHz signal to the Great Cluster (Messier 13) in Hercules. An astronomer in that cluster of some 3,000 stars would see the sun become the brightest star in the sky as the transmission outshone the sun ten million times at the chosen frequency band.  The transmission switched between two frequencies, intended to represent the characters 0 and 1, for a total of 73 groups of 23 characters each. Coded into the signal were a summary of terrestrial biochemistry, a simplified sketch of a human, an indication of a four-billion population, and identification of Earth as the third planet from the sun

However, this approach that we find practical may not be followed by an older, more advanced civilization. And it is such civilizations that we hope to communicate with. If there are other civilizations near us, they will probably have lasted much longer than us, and we may have much to learn from them. However, they may well be uninterested in communicating with what they consider inferior civilizations of doubtful stability. And we may never encounter their signals because they may be using techniques of instantaneous communication that we currently find impossible

Extra-Solar Attitudes

So, in general terms, what we might expect by way of information in a successful SETI interception depends very much on the level of civilization and the attitude of the sender. We have been warned by eminent scientists, for example, that it would be unwise to communicate with an extraterrestrial civilization because it is likely to see us and our planet as a useful resource to be taken over and exploited. After all, that is we do things on Earth. On the other hand, there have been times when one civilization on Earth has helped another on compassionate grounds, particularly in the event of natural disasters.  So we can recognize the possibility of three types of extraterrestrial attitudes: exploitation, indifference, compassion. We should perhaps also add morbid curiosity, as there may be some interest in how intelligence can be so self-defeating as to destroy its own environment.

The weakness that we can observe in aggressive predatory civilizations is that they do not last very long. On Earth, their average lifetime appears to be getting shorter as time goes by. The problem seems to be that aggressive organizations are run by aggressive, self-centered individuals and these lack the temperament to win the hearts and minds of their people in a way that permits an effective succession to power. On the timescale where a global civilization controls a whole planet this is likely to be fatal, because constant challenges from the environment put a premium on flexibility and continuity for achieving survival.

Even in the unlikely event that aggressive, predatory civilizations could achieve a lifetime of 10,000 years, they probably will be separated by such enormous distances that they are a very low level threat compared with other events in the Galaxy. 5/25/2020

Back  Contents    Next

. . .that prolonged hesitation between sound and . . .