The Wow! Signal Hasn't Been Settled in Nearly 50 Years: It Looks Like an Alien Call, but No Second One Ever Came
That 72-second narrowband hydrogen-line signal from 1977 has never repeated, and it falls short of the threshold for evidence of an alien civilization; its legacy isn't an answer but a reminder: in a noisy universe, the distance between detection and proof is an entire galaxy.
The argument · tap a timestamp to hear it
The 72-second length comes from Earth's rotation
Big Ear was a transit telescope: it didn't turn to track a target, it let Earth's rotation sweep the whole sky overhead. A point source fixed on the celestial sphere crossing the beam naturally takes about 72 seconds — exactly 1/1200 of a day. So the duration doesn't mean the sender deliberately spoke for only 72 seconds; what really matters is the gain curve: a real source enters the beam, strengthens, leaves the beam, weakens. The Wow! signal matched all of it, and any later terrestrial-interference explanation has to clear that bar first.
— Isaac Arthur6EQUJ5 is an intensity curve, not a code
6EQUJ5 is the signal-strength reading under Big Ear's early print format: each symbol represents an observation window of about 12 seconds, and the digits and letters indicate the multiple above background noise. In time order they are 6, 14, 26, 30, 19, 5 — strong in the middle, weak at both ends, like a brightness curve written sideways. Renormalized to 1–100, that's 17, 40, 74, 86, 54, 14. The host's quip is worth remembering: if the aliens' first sentence looks like a failed software activation code, that's a reasonable opening too.
— Isaac ArthurIt hugs the hydrogen line, yet sits 50 kHz off
1420 MHz is the radio spectral line of neutral hydrogen, the natural landmark of the universe's most abundant element, and the frequency SETI most often assumes a call would use. Because Big Ear had its master oscillator set 0.1 MHz high, the Wow! signal's frequency has two corrected figures: 1420.356 MHz or 1420.456 MHz; the true hydrogen line is at about 1420.406 MHz. Both are only about 50 kHz off, as if deliberately avoiding hydrogen noise while using the hydrogen line as a coordinate. But that spot is also full of natural hydrogen-cloud signal sources, and frequency alone can't distinguish cleverness from coincidence.
— Isaac ArthurOne horn received it, the other didn't
Big Ear had two feed horns side by side. A source fixed on the celestial sphere, swept by Earth's rotation, should enter the two horns in succession and be recorded twice. The Wow! signal appeared in only one horn, and the records don't say which, so the sky retains two candidate positions. Possible reasons include: the source itself lit up only briefly, a narrow beam happened to graze past, or the interference came from nearby. Every later search — including the 2022 Breakthrough Listen hunt for sun-like stars in the candidate region with Green Bank and the Allen array — heard no repeat. The missing second detection is the core reason it can't make the confirmed list.
— Isaac ArthurTerrestrial interference and satellites are hard to imitate
Earth is noisy, but a genuine candidate signal has to pass the 72-second profile test: an ordinary aircraft crossing the beam wouldn't last so conveniently long; a low-orbit satellite usually streaks through in just seconds. For a satellite to do it in high orbit, via a side lobe, or on a special trajectory would require a chain of coincidences. Terrestrial transmissions are banned in the internationally protected 1420 MHz band, but illegal equipment or faulty electronics could produce out-of-band signals that leak in through the antenna's side lobe. There's also an effect called interstellar scintillation that can briefly brighten a distant signal, but that requires an amplified source to exist first. None of these explanations is perfect, which is exactly why the puzzle survives.
— Isaac ArthurA magnetar could pump a hydrogen cloud into a maser
Research drawing on Arecibo archives has produced a natural explanation: some high-energy transient — a magnetar flare, say — pumps atoms in a cold neutral hydrogen cloud into population inversion, briefly turning the cloud into a natural microwave-band maser, much as a laser amplifies microwaves. This can explain several things at once: the signal came from space, sat near the hydrogen line, was strong enough, yet appeared only once; the hydrogen cloud was glowing as the first horn swept past, and had gone dark by the time the second horn arrived. The host says this is his personal favorite, because it solves multiple problems within a single mechanism, but it remains a speculative path with no conclusive evidence.
— Isaac ArthurBecoming a discovery means passing four gates
If a signal like this arrives again, calling it a true technosignature would require at least four conditions: one, it repeats at the same position, so we know it's still there; two, telescopes at two different locations receive it simultaneously, ruling out Earth interference; three, the signal carries modulation, encoding, or mathematical structure, rather than being a plain carrier; four, known natural processes are ruled out one by one. Historically, when pulsars were first discovered they were nicknamed "little green men," showing that nature can perfectly impersonate intelligence. The right posture isn't to pick a side, belief or mockery, but to allow the excitement and then do everything possible to prove you're fooling yourself.
— Isaac ArthurIts legacy is a window that was far too narrow
The Wow! signal appeared for 72 seconds in exactly the form early SETI expected to know — narrowband, near the hydrogen line, strong enough, shaped like a real source in the sky — and then nothing more. It showed what a great discovery might look like before the evidence is sufficient. The host's landing point: the galaxy is a hundred thousand light-years wide and billions of years old, and even the largest civilization is just one flickering spark within it. If civilizations don't expand, don't transmit often, and don't often point their beams at us, the Fermi paradox may not exist. With that 1/1200 of a day, the Wow! signal briefly put that possibility in front of humanity.
— Isaac ArthurIn their own words · checked verbatim
Science is happiest when things are either confirmed or discarded. The Wow signal has managed to neither.
Isaac Arthur1:05
Being near the hydrogen line also means nature has plenty ways to be involved.
Isaac Arthur8:11
A one-time signal can be interesting, but it cannot carry the weight we would want for a claim as huge as extraterrestrial intelligence.
Isaac Arthur11:13
A possibility that cannot be confirmed, repeated, or tested gets weaker with time. It may remain interesting, but does not become evidence merely by aging well.
Isaac Arthur14:16
even the mightiest civilization we have ever built is still only a flickering spark against the immense scale of a galaxy 100,000 light-years wide and billions of years old.
Isaac Arthur23:22
The most efficient way to talk loudly across interstellar space may look to the recipient like a random, non-repeating glitch, but it also pushes back into the idea as a deliberate contact strategy.
Isaac Arthur25:24
And again, the 6EQUJ5 signal is not text we got. It's just a measurement of brightness over six measured intervals.
Isaac Arthur28:27
That is not cynicism. That's how you avoid fooling yourself in a universe full of noise.
Isaac Arthur30:28
Figures
| Detection date | August 15, 1977 | 0:03 |
| Big Ear beam scan duration | about 72 seconds = 1/1200 of a day | 3:09 |
| Signal strength sequence (time order) | 6, 14, 26, 30, 19, 5 | 5:10 |
| Strength normalized to 1–100 | 17, 40, 74, 86, 54, 14 | 5:10 |
| Hydrogen line center frequency | about 1420 MHz | 6:10 |
| Master oscillator offset | 0.1 MHz | 7:11 |
| Two corrected values for the Wow! frequency | 1420.356 MHz or 1420.456 MHz | 7:11 |
| Hydroxyl line band | 1665–1667 MHz | 9:12 |
| Later re-search | In 2022 Breakthrough Listen used two telescopes and found no repeating signal | 13:14 |
| Galaxy width | 100,000 light-years | 23:22 |
Glossary
- transit telescope
- A radio telescope with a fixed orientation that doesn't track celestial objects, relying on Earth's rotation to sweep the sky through its beam.
- hydrogen line
- The spectral line emitted by neutral hydrogen near 1420 MHz, the most common radio landmark in the universe.
- water hole
- The quiet radio band between the hydrogen line and the hydroxyl lines, where SETI often assumes civilizations would call.
- maser
- A microwave-band stimulated-emission amplification mechanism that can produce narrowband strong signals like a laser.
- side lobe
- A secondary reception direction outside the main beam; strong interference can leak into a telescope through a side lobe.
- technosignature
- An observable signal or trace indicating a non-natural technological origin, the search target of SETI.
How to listen
Podcast listeners curious about SETI's evidentiary bar, science fiction writers, astronomy new-media editors; also suited to anyone who has been misled by clickbait alien news.
The channel and Nebula membership promo at the end can be fast-forwarded.