The Wow! signal

https://en.wikipedia.org/wiki/Wow!_signal

was an unexplained burst of radio emission from the constellation Sagittarius detected by the Ohio State radio telescope in 1977. The signal received its name from a scribble on the computer printout monitoring the telescope by an enthusiastic researcher. Although this observation is considered the best possible evidence of a signal by an alien civilization, it was never confirmed as such because it was never observed again. Several other incidents like this have been observed, but they have been traced to non-extraterrestrial transmissions, or clandestine radio emission from earth satellites (the frequency, the 21 cm line, is protected by International Law because of its importance to radio astronomy). The Wow!’s duration, location, frequency characteristics and other clues are such that an alien source cannot be ruled out, although neither do they guarantee its non-natural origin. We simply don’t know, one way or the other. Radio astronomers in Australia are still trying to live down one suspected alien transmission that was eventually traced to the microwave oven in the observatory’s break room.

Suspected non-natural extraterrestrial signals are subjected to a strict protocol to confirm their authenticity. The most important of these checks is the insistence the signal be observed again, preferably by an independent observer. One isolated incident tells us nothing. On the other hand, there is no reason to suspect that a radio transmission by an extraterrestrial civilization can’t be a transient event. A signal is not necessarily an attempt to communicate, which implies multiple calls to increase the probability of detection. It is possible to conceive of many signals that are accidentally received which are industrial, scientific, or even military artifacts. The earth might be accidentally intersecting a tight beam from a planetary defense radar, a navigational beacon, or perhaps even a message intended for someone else. For example, during the Cold War, America’s Dew Line defense system swept the Arctic region seeking early warning of a Soviet attack. As the earth rotated, and revolved around the sun, these radar beams were flashed around the Galaxy like reflections off a mirrored disco ball. Any detection by an alien civilization would be limited to a brief chirp as the beam swept by—just like the Wow!

SETI researchers know this, and a number of these transients have been detected and catalogued. So far, none has been shown to be the real thing.