Will living creatures arise on every planet where favorable physical conditions occur? No certain answer can be given, but those best qualified to judge the matter, the biologists, seem to think that life would in fact arise wherever conditions were able to support it. Accepting this, we can proceed with greater assurance. The extremely powerful process of natural selection would come into operation and would shape the evolution of life on each of these distant planets. Would creatures arise having some sort of similarity to those on the Earth? The distinguished biologist, C.D. Darlington, suggests that this is by no means unlikely. To quote Darlington's own words: "There are such great advantages in walking on two legs, in carrying one's brain in one's head, in having two eyes on the same eminence at a height of five or six feet, that we might as well take quite seriously the possibility of a pseudo man and a pseudo woman with some physical resemblance to ourselves....

A further question: Will travel between different planetary systems ever be possible? I am sorry to give an unpopular answer, but I believe this to be an uncompromising, no. Communication is a different matter. If living creatures at a high technological level exist on planets belonging to any of the nearest thousand stars it would be feasible to establish communication. A two-way interchange of information would take many centuries to develop. Even so, perhaps we should be starting now? (Hoyle, 1950)

Classification of Galactic Civilizations

Type I civilizations have a technological level close to the level presently attained on Earth. The energy consumption of Type I societies is approximately 4x10¹⁹ erg/s.

Type II civilizations have the capability of harnessing the energy radiated by their own star (4x10³³ erg/s). They may construct large Dyson Spheres to capture the energy from their primary star.

Type III civilizations are those who have an energy utilization equal to that of their galaxy (4x10⁴⁴ erg/s).

What Would a Sought-After Signal Look and Sound Like?

The narrower the band-width of a transmitted signal is of fixed total power, the greater the range of successful transmission. Thus, we can expect the signals to be of narrow band-width. This has the advantage of providing simple methods for distinguishing the signals from naturally occurring cosmic noise, which is extremely broad in band-width. For those using Microsoft's Internet Explorer you should hear what a narrow-band signal would sound like.

Detection Range of Cosmic Signals

Given an effective radiated power of the transmitter (in watts), the effective area of the receiving antenna (in square meters), the excess receiver noise temperature of the receiver used (in K), the averaging time of the receiver (in seconds), and the accepted band-width of the signal (in Hz), the range at which we can detect a signal transmitted by an intelligent civilization, is

where the constant is calculated from 1/[9.4608x10¹⁵(4*pi*k) ^1/2]. Here the constant is the number of meters per one light year, and k is the boltzmann constant.

A Good SETI Receiver Should

1. Operate at a frequency greater than 1420 MHz, with 
performance near 1420 MHz (21 cm) desirable.

2. Operate over a very large range of band-widths, including
10 MHz and less.

3. Have receiver frequency much less than the band-width
during the averaging time of the receiver, specifically
not more than about 1 MHz in several minutes.

4. Have the highest sensitivity available.



5. Eliminate receiver noise effects as much as possible.

6. Discriminate against broad-band cosmic noise, if possible.



7. Preferably reject terrestrial interference that mimics
the sought-after signal.

The "WOW" Signal

On August 15, 1977 the Ohio State Radio Observatory detected the most promising extraterrestrial candidate signal to date. The so-called "WOW!" signal received its name from the note written in the margin of the computer printout by Dr. Jerry Ehman. The signal rises about 15 dB above the mean background noise, in a single channel. The actual symbols (shown in the printout below) are a time series representation of the signal amplitude, as received by the Big Ear radiotelescope. The symbols (6EQUJ5) represent the number of standard deviations by which the received signal exceeds the average background noise, on a scale of 0 to 35. For example, a "0" means no stronger than background noise, a "1" is one sigma above the noise level, a "9" means nine sigma above the background level, an "A" would be ten sigma, and a "U" (the strongest peak of the Wow! signal) is 30 standard deviations above the mean background level.

An analysis of the Wow! signal indicates that its source was moving with the background stars. From its Doppler shift signature, terrestrial interference, aircraft, and spacecraft can be ruled out as possible origins of the signal. The antenna coordinates indicated that the signal was coming from no known nearby solar-type stars. The only condition required for a SETI signal not met was that of repeatability. However, since the Ohio State Big Ear radiotelescope has an extremely narrow bandwidth, viewing just one part in a million of the sky at any given time, one would not expect the signal to repeat. Assuming the Wow! signal is a typical SETI-like transmission, we can expect valid SETI hits to be very strong, high intermittent signals which appear once (as the transit beam sweeps past Earth), and never repeat again.

Signals from Interstellar Messengers

The waterfall plot below depicts one of the sidebands when Pioneer 10 transmits data. The lower portion of the plot shows a part (at 643 Hz) of the spectrum containing the carrier signal. The upper two thirds of the plot shows a different part of the spectrum containing the data signal. Pioneer 10 is currently more than 10 billion kilometers from Earth broadcasting with a power of only 6 watts.

Attached to the Pioneer 10 spacecraft is a plaque which acts like a "message in a bottle" to any interstellar traveler that might come across the spacecraft. The plaque was designed by SETI pioneers Frank Drake and Carl Sagan. The plaque is inscribed with symbols, binary numbers, and drawings depicting a universally understandable message. The message shows the location of the solar system and shows that Pioneer was launched from Earth and portrays a terrestrial man and woman.

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