August 15, 1977: What may be the best evidence yet for extraterrestrial intelligence, the “Big Ear”, a radio telescope operated by Ohio State University as part of the SETI (Search for Extra Terrestrial Intelligence) project, received an extremely powerful radio signal from deep space; the event is named the “Wow! signal” from the notation made by a volunteer on the project. The signal bore the expected hallmarks of non-terrestrial and non-Solar System origin. It lasted for the full 72-second window that Big Ear was able to observe it, and has never been detected again. Amazed at how closely the signal matched the expected signature of an interstellar signal in the antenna used, researcher Jerry Ehman circled the signal on the computer printout and wrote the comment “Wow!” on its side. This comment became the name of the signal.
The frequency of the Wow! signal matches very closely with the hydrogen line, which is at 1420.40575177 MHz. The hydrogen line frequency is significant for SETI searchers because, it is reasoned, hydrogen is the most common element in the universe, and hydrogen resonates at about 1420.40575177 MHz, so extraterrestrials might use that frequency to transmit a strong signal.
Wow! has tantalized by evading almost every suggestion put forth to explain it. For one reason, that frequency range is protected; nobody on Earth is allowed to transmit on that frequency. We know the signal did not come from an aircraft or spacecraft passing overhead, because the signal was consistent with a point in the sky that was not moving. No known planets or asteroids were in a position that they could have reflected the signal toward Earth. Any space debris would have had to be absolutely still in space relative to the Big Ear, which is unlikely, and not tumbling, which is also unlikely. Even complicated astronomical effects like gravitational lensing and interstellar scintillation (basically twinkling like that which we observe stars doing visually) have technical reasons that make them very poor candidates to explain Wow!