What produced the Wow! signal?
Five answers, none aheadNone is ahead yet. See what would settle it.
Where it stands
Five answers have been proposed. None is ahead on the evidence yet. Five are still possible. Of the two narrower questions under it, none is answered yet.
Why it matters
It is the only candidate in fifty years of radio SETI that has every property a deliberate beacon was expected to have and no accepted natural or terrestrial explanation[1][2]. Whichever answer is right matters: a natural hydrogen flare would be a new class of astrophysical event and a new source of false positives[3]; a terrestrial origin would say something about the protected band; a transmitter would be the discovery the programme was built for.
Answers 5
History
- Ehman puts forward: a narrowband transmission from an extraterrestrial source, either a one-off or a beacon that repeats or…
- Ehman puts forward: terrestrial radio interference, direct or reflected from space debris, that happened to mimic a drifting…
- Ehman: the frequency lies in a band where transmission is prohibited worldwide.Against: Terrestrial radio interference, direct or reflected from space debris, that happened to mimic a drifting…
- Ehman: nothing in the data excludes a reflection from debris moving at the sidereal rate; Ehman regarded it as…For: Terrestrial radio interference, direct or reflected from space debris, that happened to mimic a drifting…
- Ehman: a comet would have appeared in both horns; the signal appeared in one.Against: Hydrogen emission from comets 266P/Christensen or P/2008 Y2 (Gibbs) passing through the region.
- Ehman: no modulation or content was or could have been recorded by a 10 kHz, 12-second receiver.Against: A narrowband transmission from an extraterrestrial source, either a one-off or a beacon that repeats or…
- Méndez et al.: no triggering transient was observed in 1977 and no second example of such a flare has been recorded.Against: A sudden maser-like or superradiant brightening of a small cold interstellar hydrogen cloud, triggered by a…
- Méndez et al.: arecibo drift scans found narrowband hydrogen-line signals from small cold clouds, about a hundred times…For: A sudden maser-like or superradiant brightening of a small cold interstellar hydrogen cloud, triggered by a…
- Méndez et al.: the 2025 reanalysis finds a frequency shift consistent with a Galactic source and argues interference is less…Against: Terrestrial radio interference, direct or reflected from space debris, that happened to mimic a drifting…
- Méndez et al.: the revised frequency of 1420.726 MHz implies a Galactic source with a larger radial velocity.For: A sudden maser-like or superradiant brightening of a small cold interstellar hydrogen cloud, triggered by a…
- Méndez et al.: single narrowband events were common at Big Ear, with more than 40,000 logged.For: Terrestrial radio interference, direct or reflected from space debris, that happened to mimic a drifting…
- Méndez et al.: about a hundred follow-ups, including VLA, META and 14 hours at Hobart, found nothing.Against: A narrowband transmission from an extraterrestrial source, either a one-off or a beacon that repeats or…
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