Searches for Extraterrestrial Intelligence (SETI)

Twinkling Transmitters

My work in SETI was motivated by the fact that pulsar observations are affected by scintillation. Scintillations are intensity fluctuations. There can be two possibilities: Scintillations cause a source to be more intense than it otherwise should be or scintillations cause a source to be less intense than otherwise. Clearly, if scintillations have made a source stronger, it will be easier to find. Conversely, if the source is weaker than it otherwise would be, it will be more difficult to find.

If scintillations were equally likely to cause a source to get brighter or dimmer, they wouldn't be any big deal. On average, they would average out. However, the intensity fluctuations caused by scintillation are skewed. A scintillating source is more likely to be dimmed by scintillation than be made brighter. However, if made brighter, the source can be much brighter, e.g., 10x brighter.

The curious thing about SETI is that many radio surveys have detected candidate signals. These candidate signals have all the characteristics one would expect from an artificially generated signal (typically they're narrow-band, sometimes seen in a "special" reference frame), except one. When the locations (on the sky) are surveyed again, the candidates are not detected. Perhaps the most famous example is the Wow! signal, detected at the Ohio State University.

It's quite possible that these candidates are nothing more than some kind of exotic radio signal from a terrestrial source or they are a momentary glitch in the radio telescope being used. On the other hand, these candidates also behave exactly how one would expect a scintillating source to behave---a quick brightening followed by a huge dimming.

Additional details as well as some intriquing conclusions are discussed in our paper Now You See Them, Now You Don't. There's also a professional paper "Scintillation-induced Intermittency in SETI" by Cordes, Lazio, & Sagan.