Wow! Signal: Mysterious Radio Pulse from Deep Space

Jerry Ehman was reviewing data printouts from Ohio State University's Big Ear radio telescope on August 15, 1977, when he spotted a signal so unusual that he grabbed a red pen and circled it, writing "Wow!" in the margin. The notation gave the signal its enduring name. For 72 seconds, a narrowband radio burst at the 1420 MHz hydrogen line had arrived from the direction of the constellation Sagittarius with an intensity 30 times above background noise. Nearly five decades later, no one has definitively explained what caused it. The Big Ear was part of the SETI project, the Search for Extraterrestrial Intelligence, scanning the sky for radio signals that might indicate a technological civilization. The 1420 MHz frequency was considered the most likely channel for interstellar communication because it corresponds to the emission line of neutral hydrogen, the most abundant element in the universe. Any civilization attempting to broadcast across interstellar distances would logically choose a frequency that other astronomers would already be monitoring. The signal matched the expected profile of an artificial extraterrestrial transmission with eerie precision. It rose and fell in intensity exactly as a point source in the sky would appear to a fixed radio telescope as the Earth rotated, confirming that the signal originated from beyond the solar system. But it was never detected again. Ehman and other astronomers pointed the Big Ear and other telescopes at the same patch of sky more than 100 times in subsequent years, finding nothing. Proposed explanations have ranged from a classified military satellite to a comet releasing hydrogen gas, though each hypothesis has significant weaknesses. The comet theory, advanced in 2017, was challenged by astronomers who noted that the signal's narrowband characteristics were inconsistent with a diffuse hydrogen cloud. The Wow! signal remains the strongest candidate for an extraterrestrial radio transmission ever recorded, a single tantalizing data point that raises the most consequential question in science without answering it.