SETI (Search for Extraterrestrial Intelligence) is an attempt to detect technological civilizations elsewhere in the universe, particularly in our galaxy. Billions of places outside our solar system may host life, that we could discover, if it has developed to a technological level at least as advanced as our own.
In a very real sense this search for extraterrestrial intelligence is a search for a cosmic context for mankind, a search for who we are, where we have come from, and what possibilities there are for our future - in a universe vaster both in extent and duration than our forefathers ever dreamed of.
-- Communication with Extraterrestrial Intelligence (CETI), Carl Sagan, Editor, 1973, MIT Press, "Introduction", pp. ix-x.
You can see a few thousand stars with the naked eye on a clear night. This is only a tiny fraction of the actual number of stars in our Milky Way galaxy - some 400 billion - which itself is only one of some 50 - 100 billion galaxies observable by the Hubble Space Telescope.. It seems only reasonable to wonder, if there might not be creatures on other worlds, wondering if there might not be creatures on other worlds, wondering...
People have wondered "Are we alone?" since first we realized that the points of light in the night sky are other suns. Only within the late twentieth century, however, has our technology advanced to the point where we can seek scientific evidence to help answer this age-old question.
Scientists have concluded that life is a natural phenomenon likely to develop on planets with suitable environmental conditions. Life started on Earth relatively soon after it was formed, suggesting that life will occur on similar planets orbiting sun-like stars.
Astronomers have recently discovered large planets orbiting other stars in our galaxy - almost certainly too large to support life, but suggestive that there could also be smaller, more Earth-like planets too. With the recent discoveries of extrasolar planetary systems, and the suggestive evidence that life may once have existed on Mars, this scenario appears even more likely. Considerations such as these have led many scientists to believe that there is a likelihood of life elsewhere in our galaxy, that we stand a chance of detecting.
Cornell University physicists Giuseppe Cocconi and Philip Morrison published an article "Searching for Interstellar Communications," in the September 19, 1959 issue of the British science journal Nature. In that article, a strategy was discussed for scanning nearby sun-like stars for microwave radiation which could not be explained by natural causes.
They pointed out the potential for using microwave radio to communicate between star systems. They suggested that radio waves might be the most effective means of communication across interstellar distances, and therefore the best way to detect the existence of an extra-terrestrial civilisation.
Light and radio waves are both forms of electromagnetic radiation - i.e. the same kind of energy - but the differ only in wavelength / frequency. The electromagnetic spectrum also comprises infrared, ultraviolet, X-rays, gamma-rays, etc. These all travel at the 'speed of light' (186,000 miles or 300,000 kilometres, per second). This is the highest speed attainable, according to Einstein's theory of relativity, but even so, light (or radio) from the stars and galaxies takes many years to reach us; about 4 years for the closest stars, and up to 15 billion years for the farthest galaxies. So any two-way conversation would probably take lifetimes!
As Morrison and Coccini were writing their article, radio astronomer Frank Drake was preparing to perform the very experiment which they outlined. He pointed a 25-metre (85-foot) radio telescope at two nearby stars called Epsilon Eridani and Tau Ceti on 8 April, 1960, near Green Bank, West Virginia, USA. The telescope surveyed for just a few weeks, tuned to the 21 cm (1,420 Mhz) line of neutral hydrogen, and detected no extra-terrestrial intelligent signals.
This was the first attempt to detect interstellar radio signals. Drake's Project Ozma - named after a princess in Baum's Wizard of Oz books - initiated the modern scientific search for extra-terrestrial intelligence.
The world's first SETI meeting was convened at Green Bank by Drake in 1961. Drake drafted an equation for estimating the number of possible communicative technologies in the cosmos, as the agenda for that conference. Today Drake's Equation is the primary mathematical tool with which SETI scientists assess their prospects of success. Its seven factors encompass cosmology, planetology, atmospheric science, evolutionary biology, psychology, technology, and sociology. Thus SETI is possibly the most interdisciplinary of the sciences.
N = R* . Fp . Ne . Fl . Fi . Fc . L
N = number of civilisations,
R* = rate of star formation in our galaxy,
Fp = fraction of stars with planets,
Ne = number of habitable planets per planetary system,
Fl = fraction of those in which life starts,
Fi = fraction of life that evolves to intelligence,
Fc = intelligent species that develops communication,
L = technological lifespan.
Making reasonable guesses, it turns out that the first few factors are either approximately 1, or effectively cancel each other, so that the final term dominates the equation - i.e. N is approximately equal to L. This last factor, L, is the hardest to estimate. It depends on how likely it is that civilisations destroy themselves, or are destroyed, or become reclusive, etc. We can only make the roughest of guesses, at somewhere between 1,000 years, and 1,000,000 years. This implies that we could expect our galaxy to be home to between 1,000 to 1,000,000 communicative civilisations!
These civilisations could be detectable with present day technology, either through attempts on their part to communicate, or through by-products of their activities, such as TV and radio broadcasts. Such signals would likely be extremely weak, but modern electronics is capable of extremely high amplification. But, it's not simply a matter of pointing an antenna at the sky and hoping - SETI researchers face several challenges. There are several issues to address, such as knowing where to look, for how long, what to look for, how to differentiate between ET and local interference, the most likely frequency where ET may be broadcasting, either as a deliberate message or as interstellar communication.
In 1971/2, a team of radio astronomers and engineers headed by Bernard Oliver, a vice president of engineering for Hewlett-Packard (and later the chief of the NASA SETI program), wrote "Project Cyclops," a detailed plan for a SETI search. The ambitious plan was widely distributed by NASA and sparked several search efforts, but it also spurred a congressional backlash that eventually led to a ban on SETI funding in 1993.
During the Cyclops study, Oliver hypothesized that the Hydrogen (H) and Hydroxyl (HO) lines - at about 1420 MHz (corresponding to a wavelength around 21 cm) and 1660 MHz (a wavelength of 18 cm) respectively, constituted obvious signposts to a natural interstellar communications band.
Since H and HO are the disassociation products of water, the spectrum between them is known as the "Water-Hole". Cosmic background noise (left over from the Big Bang) and absorption in the atmosphere are both low in this range. Since the H and OH lines are visible throughout the universe, in the quietest part of the radio spectrum, it would be a logical band for other civilizations to choose to send out a signal.
Project Phoenix is the successor to the ambitious NASA SETI program that was cancelled by Congress. It's the world's most sensitive and comprehensive search for extraterrestrial intelligence. Phoenix began observations in February, 1995 using the Parkes 210 foot radio telescope in New South Wales, Australia - the largest radio telescope in the Southern Hemisphere.
When SETI began, the technology was barely available to make the search. Drake had one channel to scan two nearby star systems in 1960. Today he is president of the SETI Institute and its Project Phoenix is studying 1,000 nearby stars sifting through 28 million channels simultaneously every second.
Upon Congressional cancellation of the NASA SETI program in 1993, many former NASA staff found themselves continuing their work through The SETI Institute's Project Phoenix. This is a highly professional organization doing more than just a radio search for signals; it is also the home for over three dozen first rate scientific and educational projects involved in many aspects of the Drake Equation.
The SETI Institute and the University of California, Berkeley, recently signed a memorandum of understanding that could result in the construction of a world-class telescope array that would be an order of magnitude less expensive than comparably sized, contemporary instruments. The new telescope, called the Allen Telescope Array would be used for both nonstop SETI and cutting-edge radio astronomy research.
SERENDIP: The UC Berkeley SETI Program, SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) is an ongoing scientific research effort aimed at detecting radio signals from extraterrestrial civilizations. The project is the world's only "piggyback" SETI system, operating alongside simultaneously conducted conventional radio astronomy observations. SERENDIP is currently piggybacking on the 1,000-foot dish at Arecibo Observatory in Puerto Rico, scanning 168 million radio channels. This data will be fed to thousands of SETI@Home participants.
SETI@Home is a scientific experiment that uses Internet-connected computers in the Search for Extraterrestrial Intelligence (SETI). You can participate by running a free program that downloads and analyzes radio telescope data.