IncidentThe Wow! Signal (15 August 1977)

The Big Ear’s chart printer recorded an intensity sequence ‘6EQUJ5’ (each character a ~10-second sample; ‘U’ a peak ~30 standard deviations above background) over the full 72-second window the beam takes to drift past a fixed point - matching a celestial point source, not a continuous local interferer. It was narrowband (<10 kHz) at ~1420.4556 MHz (just above the rest hydrogen line), and Ehman found it days later and wrote ‘Wow!’ (‘6EQUJ5’ is the intensity code, NOT an encoded message). It has never re-appeared despite decades of follow-ups (Ehman 1977; Gray’s META/VLA/Tasmania campaigns; Breakthrough Listen 2022 - all null). Proposed explanations all fail or remain unproven: terrestrial RFI/satellite (implausible at the protected hydrogen-line frequency), Antonio Paris’s 2017 comet-hydrogen hypothesis (widely rejected - comets aren’t 1420 MHz emitters and a cloud should appear in both feed horns), interstellar scintillation, and a 2024-25 archival ‘Arecibo Wow!’ magnetar-maser proposal (a non-peer-reviewed preprint, criticized as contrived). The most famous unexplained SETI radio anomaly - adding the radio-SETI dimension (the scientific search for ET intelligence, distinct from sightings), a genuine well-documented one-off that remains unexplained but unconfirmed.