The Search for Extraterrestrial Intelligence (SETI) is an ongoing scientific effort to detect signs of technological civilizations that may exist elsewhere in the universe. SETI researchers use radio telescopes to listen for narrow-bandwidth radio signals that could indicate the presence of artificial transmitters, as these would stand out against the broad-spectrum radio noise seen from natural cosmic sources.
The modern era of SETI began in 1959 when physicists Philip Morrison and Giuseppe Cocconi proposed that extraterrestrials could use radio to communicate across interstellar distances. The first actual SETI observations were conducted in 1960 by astronomer Frank Drake, who used an 85-foot radio telescope in Green Bank, West Virginia to monitor two nearby sun-like stars at a frequency of 1420 MHz. This region of the radio spectrum is known as the “water hole” since it sits between the hydrogen and hydroxyl spectral lines, which mark the most abundant molecules in the universe.
While no signals were detected in Drake’s pioneering project, it set the stage for expanded searches in the coming decades. NASA funded a SETI program from the 1970s to early 1990s that systematically observed thousands of target stars across a wide range of frequencies using large radio telescopes like Arecibo in Puerto Rico. Funding was cut in 1993 but SETI continued through private institutions like the SETI Institute, UC Berkeley, and Harvard.
Modern SETI experiments take two main approaches – targeted searches of nearby stars and habitable exoplanet systems, and wide-area sky surveys over large regions. Targeted searches examine stars within a few hundred light years that have conditions suitable for life. Sky surveys cover millions of stars across the entire sky but with less sensitivity.
The workhorse instruments of SETI are large radio telescope arrays, which provide high sensitivity as well as the ability to precisely pinpoint the origin of any detected signals. Major facilities used include the Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia, Parkes Observatory in Australia, and the Allen Telescope Array in California.
The Allen Telescope Array, operated by the SETI Institute, consists of 42 dishes each 6 meters wide. It is the first radio telescope designed specifically for continuous SETI searches across a wide frequency range from 1 to 10 GHz. Its wide field of view allows observations of multiple star systems at once.
At these facilities, astronomers collect raw data across the microwave spectrum from target stars or sky regions. The data is then digitized and sent to high-performance computers to search for signals of interest. Powerful digital signal processing algorithms parse the data into chunks in time and frequency to be analyzed for variations that stand out from the background noise.
SETI searches apply filters to identify narrowband signals a few Hz wide that would be expected from artificial transmitters. Radio frequency interference from human-generated sources is also filtered out. Thresholding and pattern recognition techniques identify candidate signals with the properties of a potential alien transmission.
Promising signals undergo further scrutiny to rule out false positives. Multi-antenna observations help distinguish true point sources from radio interference. Follow-up observations are critical to confirm any candidates. Consistent detection from the same region of sky would provide compelling evidence of an artificial signal.
Over 60 years of SETI searches have turned up no confirmed extraterrestrial transmissions. However, only a miniscule fraction of the search space of frequencies, sky locations, and star systems has been explored. The number of potentially habitable worlds in our galaxy alone may number in the billions.
Advances in radio astronomy instrumentation, digital processing, and data analytics aim to greatly expand SETI’s search capabilities in coming decades. Machine learning algorithms trained on signal databases can help identify promising signal candidates. Optical SETI experiments also search for pulsed laser signals from other civilizations.
While the odds seem low, a definitive detection of intelligent life beyond Earth would be a transformative discovery. Confirming we are not alone in the cosmos and being able to study such a signal would revolutionize science and likely shake long-held assumptions about our place in the universe. SETI researchers remain optimistic that perseverance in methodically searching the sky will one day uncover evidence that we have cosmic company.
