On the evening of 15 August 1977, a radio telescope in Delaware, Ohio, recorded something it never recorded again. The telescope was called Big Ear, run by Ohio State University, and for years it had been sweeping the sky as part of the first serious, sustained effort to listen for a deliberate signal from somewhere other than Earth.
The record of what it heard survives as a single line of numbers, one of which somebody circled, and next to which somebody wrote a word. The word was "Wow!"
The handwriting belonged to Jerry Ehman, who was reviewing the telescope's computer printouts, a task he did largely as a volunteer. The signal he was looking at lasted seventy-two seconds, which is not a coincidence: it is exactly how long the telescope's beam dwells on a point in the sky as the Earth turns. The signal came in near 1420 megahertz, the frequency at which neutral hydrogen radiates. That choice of frequency is not arbitrary. Anyone searching for an interstellar transmission would choose that band precisely because it is a natural landmark on the dial, a frequency that any civilisation with radio would recognise as meaningful.
The printout scored the signal "6EQUJ5", a code the system used to represent intensity, where the letters carried the strongest values. A "U" was the highest the scale could go. Nothing like it had appeared in the data before, and nothing like it appeared afterward, despite many attempts to look again at the same patch of sky. The direction pointed toward the constellation Sagittarius, near the galactic centre. It was detected in one of the telescope's two feed horns and not the other, which is itself a clue about where it came from.
That is the whole of the evidence: seventy-two seconds, one frequency, one direction, no repeat. Everything else about the Wow! signal is interpretation, and the interpretations have never settled. Natural hydrogen clouds, reflections off space debris, interference from the ground, an aircraft or satellite crossing the beam, even a passing comet: each has been proposed, and none has been conclusively shown to account for every feature of the detection.
In 2017, one researcher suggested two comets had been in the right part of the sky on the right dates, and that a cloud of hydrogen around them could have produced the effect. The idea was appealing because it was ordinary, but it was also contested, because the comets were not where the original notes said the beam had been pointed, and the timing was not a clean match. Later reanalysis of the data argued that the signal's shape was more consistent with a natural source than with a transmitter, but that analysis, too, has not closed the question. The sober summary is that we have a strong, clean, unexplained detection and no explanation that fully fits it.
What is interesting is how the signal is remembered. It is famous as a possible message from another civilisation, which it may not be. Read as a fact rather than a story, it is a small, mundane thing: two minutes of radio energy at a natural frequency, from a patch of sky, once. Its fame rests entirely on the question it raises, not on what it actually was.
That is a common feature of the things we choose to preserve. A blank page is kept because of what someone hoped to write on it, and very often the keeping says more about us than any answer would have. We are, it turns out, unusually willing to archive our own hope.
It is worth remembering how thin the thread of evidence actually is. A single printout, a single ruled line of numbers that a volunteer happened to notice, one word written in the margin in pen. If Ehman had not been reviewing that particular page, on that particular day, the detection might have been filed and forgotten, a small anomaly in a mountain of paper. The signal would still have arrived; nobody would have known to care. What we have, in the end, is not just a measurement. It is a measurement that survived because one person decided a strange number was worth a word of punctuation. That is a fragile thing to rest a legend on, and also the most human part of the story.
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