There are great challenges in searching the universe for signs of intelligent life, including their identification and interpretation. As various SETI projects have progressed, their claims have been criticized by researchers as being too euphoric. Scientific investigation of the phenomenon began shortly after the advent of radio in the early 1900s. Focused international efforts to answer a variety of questions have been going on since the 1980s. In 2015, Stephen Hawking and Russian billionaire Yuri Milner announced a well-funded effort, There have been many earlier searches for extraterrestrial intelligence within the Solar System. In 1896, Nikola Tesla suggested that a version of his wireless electrical transmission system could be used to contact beings on Mars. On August 21–23,1924, Mars entered an opposition closer to Earth than at any time in the century before or the next 80 years. In the United States, a National Radio Silence Day was promoted during a 36-hour period from August 21–23, with all radios quiet for five minutes on the hour, every hour. The program was led by David Peck Todd with the assistance of Admiral Edward W. Eberle, with William F. Friedman. A1959 paper by Philip Morrison and Giuseppe Cocconi first pointed out the possibility of searching the microwave spectrum, and proposed frequencies and a set of initial targets. In 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named Project Ozma, a 400 kilohertz band around the marker frequency was scanned, using a single-channel receiver with a bandwidth of 100 hertz. The Soviet scientists took a strong interest in SETI during the 1960s, in the March 1955 issue of Scientific American, John D. Kraus described an idea to scan the cosmos for natural radio signals using a flat-plane radio telescope equipped with a parabolic reflector. Within two years, his concept was approved for construction by Ohio State University, with a total of US$71,000 in grants from the National Science Foundation, construction began on an 8-hectare plot in Delaware, Ohio. This Ohio State University Radio Observatory telescope was called Big Ear, later, it began the worlds first continuous SETI program, called the Ohio State University SETI program. In 1971, NASA funded a SETI study that involved Drake, Bernard M. Oliver of Hewlett-Packard Corporation, the resulting report proposed the construction of an Earth-based radio telescope array with 1,500 dishes known as Project Cyclops. The price tag for the Cyclops array was US$10 billion, Cyclops was not built, but the report formed the basis of much SETI work that followed. The Ohio State SETI program gained fame on August 15,1977, when Jerry Ehman and he quickly circled the indication on a printout and scribbled the exclamation Wow. in the margin. In 1980, Carl Sagan, Bruce Murray, and Louis Friedman founded the U. S, Planetary Society, partly as a vehicle for SETI studies
Screen shot of the screensaver for SETI@home, a distributed computing project in which volunteers donate idle computer power to analyze radio signals for signs of extraterrestrial intelligence.
The WOW! Signal Credit: The Ohio State University Radio Observatory and the North American AstroPhysical Observatory (NAAPO).
Microwave window as seen by a ground based system. From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence