The 1,000 foot Arecibo dish./A smaller pivoting antenna.
The Search for Extra-terrestrial Intelligence (SETI) has been carried on by scientific researchers since 1960. Four years earlier two Cornell University physicists, Giuseppi Cocconi and Philip Morrison, published a paper suggesting it might be possible to use microwave radio to communicate between the stars. By pointing a radio telescope at a near-by, Sun-like star, that might have a planets, astronomers might be able to detect radio waves generated by intelligent life there.
Radio waves are the best means available, given current technology, for trying to detect extra-terrestrial intelligence. Radio waves travel at the speed of light (which is the fastest theoretical speed possible) which is about 300,000 kilometers a second. At this speed a signal sent from our nearest neighbor star, Proxima Centauri, takes over four years to reach Earth. This may seem like a long time, but the fastest space probe currently built would take 300,000 years to make the same trip.
Radio waves are well suited to communications because they are relatively free from absorption in space and easily penetrate Earth's atmosphere. Researchers hope to detect either a message sent to by other intelligent life, or the accidental "leakage" of radio emissions caused by a civilizations TV or radio broadcasts. It might be possible for Earth researchers to pick up Proxima Centauri's equivalent of "Charlie's Angels"
Frank Drake, a radio astronomer, was the first to attempt a SETI search by using an 85 foot antenna to listen in the direction of two nearby stars. For two months he monitored the stars for signals on the 1,420 MHz frequency (that some scientists thought might be a logical choice for interstellar communications) without a positive result.
Additional SETI programs were conducted in the Soviet Union through the 1960's but the next serious attempt in the United States wasn't made until the early 70's when NASA's Ames Research Center put together a team of experts to consider how an effective search could be done. The result was known as Project Cyclops.
Radio astronomers, using the work in the Cyclops report, started conducting searches throughout the 70's using existing antennas and receivers. Later in the decade more organized programs were established at the Jet Propulsion Laboratory (JPL) in Pasadena, California, and the NASA Ames Research Center. Actual observations didn't start at Ames and JPL until 1992, and were terminated, in less than a year, a victim of congressional budget cutting.
Privately funded efforts for SETI continue through programs like Project Phoenix which will attempt to carefully examine the regions around some 1,000 nearby Sun-like stars. SERENDIP, another SETI program, is being conducted by the University of California, piggybacks upon other radio astronomy programs by constantly watching for interesting signals as the telescope is used. So far nobody has detected a signal so amazing that it virtually proves extra-terrestrial intelligence.
SETI observations are made by using large antenna designed for radio astronomy. Typically these are large dish-like structures designed to reflect radio waves coming in from a particular direction to a central focal point. The waves are concentrated at that point so even a weak signal from thousand of light years away might be detected. Most dishes pivot to allow any point in space to be examined, however a few, like the enormous 1,000 foot diameter antenna in Arecibo, Puerto Rico, remained fixed and move only the central detector around.
One of the challenges of SETI is the question of which of the millions of possible radio frequencies should be monitored. Initially 1470Mhz, a frequency associated with hydrogen, was chosen as a logical channel to listen to because of it's astronomical significance (Hydrogen is the most abundant element in the Universe). However, this channel is probably too noisy from natural causes to provide a good interstellar link. SETI programs now use receivers capable of scanning millions of channels a second linked to data processing equipment designed to sort natural signals from those that might possibly be artificial.
In addition to SETI programs some researchers propose a CETI program that would actively send out a signal and allow two way communications between Earth and others star systems. Some scientists, like prestigious Nobel prize winner Sir Martin Ryle, are concerned that Earth's culture would not survive a clash with a superior alien civilization and think we should not attract their attention.
Concentrate Light with a Dish
University of CA. SETI Project (SERENDIP)
A radio telescope dish concentrates a weak, distant signal.