Why do we think that there is life
"out there"?
M74, Spiral Galaxy
Over the last half century, scientists have developed a theory of "cosmic evolution" which predicts that life is a natural phenomenon likely to develop on planets with suitable environmental conditions. Scientific evidence shows that life arose on the Earth relatively quickly, suggesting that life will occur on similar planets orbiting Sun-like stars. With the vast number of stars in the observable universe (up to 400 billion in our galaxy alone) and the probable number of Earth-like, habitable planets around other stars, it is likely that advanced technological civilizations are widely distributed in space. SETI tests this hypothesis by searching for specific technological manifestations of intelligent life.
Technology has many uses, among them are communication and active detection and ranging (radar, lidar, etc.). To accomplish these activities on Earth, our technology uses electromagnetic waves such as light, radio, and infrared. To be detectable over interstellar distances, such signals must not be absorbed by interstellar plasma. Radio waves travel through space with the least absorption or distortion. Most SETI searches concentrate on microwaves, radio waves in the frequency range from 1,000 MHz to 10,000 MHz. Radio waves emitted by natural astronomical objects are spread over bands of frequencies wider than a few hundred Hertz, are seldom polarized, and are not constant in phase. Artificial signals, produced by a transmitter and antenna, are often confined to a narrow range of frequencies, are highly polarized, and have the peaks of the waves in phase. Artificial signals may contain encoded information, while natural signals do not.
With our best rocket technology a flight to the Sun's nearest neighbor, Alpha Centauri, only 4 light years away, would take about 40,000 years. Even a far more advanced technology cannot avoid either paying a huge energy cost or going very slow. Relativity and the limit of the speed of light apply throughout the universe. About a thousand stars like the Sun are within 100 light years of us. To search around all of them with spacecraft would take more than a million years and vast amounts of money. Alternatively, we can search for radio waves now, with state-of-the-art technology, at a modest cost. The observational phase of the HRMS would have cost about a nickel per taxpayer per year.
University of California, Berkeley, astronomers are carrying out a search called SERENDIP III at the Arecibo Observatory in Puerto Rico. The Planetary Society, an independent, privately funded organization, operates Project META at Harvard University and in Argentina. Ohio State University conducts an ongoing full-time search with a large volunteer effort. Besides the HRMS, NASA has also funded a search at infrared wavelengths at UC Berkeley, part of the SERENDIP program, upgrades to the META system at Harvard and the Ohio State system. Smaller scale, limited searches have been and continue to be conducted by individual scientists in the United States and other countries. The SETI Institute is now raising private funds to continue the Targeted Search portion of NASA's HRMS as Project Phoenix.
In order to confirm that a signal is from another civilization, at least two observatories must be able to receive it. Once an artificial signal is confirmed as being of extraterrestrial intelligent origin, the discovery will be announced as quickly and as widely as possible. A Declaration of Principles Concerning Activities Following the Detection of Extraterrestrial Intelligence, endorsed by six international space organizations, addresses how to make such an announcement. The SETI Institute has a plan for action that resembles the Declaration of Principles. The intent of the plan is to ensure that news is distributed rapidly and widely. In fact, as part of the process of confirming a potential signal, SETI Institute scientists will contact other observatories to investigate candidate signals with their own equipment.
If the signal is intentional, it is likely to be easy to decode. In order to send or receive a signal over interstellar distances, a civilization must understand basic science and mathematics. Hence, a message from another civilization would probably use a language based on universal mathematical and physical principles. Signals that a civilization uses for its own purposes may be difficult to decipher. Such emissions may have no detectable message content.
Not right away. For the senders to know, we would have to send a message in reply. The SETI Institute has no plan for replying. Under an International SETI Post-detection Protocol now under consideration, the nations of the Earth would decide together whether and how to reply.
Project Phoenix is designed only to listen for signals, not to send them. However, since the early part of this century, the cultures of the planet Earth have been unintentionally transmitting signals into space -- radio, television, and other communications transmissions as well as military radar. Our earliest TV transmissions have traveled out into space more than fifty light years. A few mostly symbolic intentional messages have been sent. One message, broadcast in 1974 from the Arecibo Observatory, was a simple picture describing our Solar System, the elements important for life, the structure of the DNA molecule, and the form of a human being. The message was transmitted in the direction of the globular star cluster, M13, about 25,000 light years away.
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