Carrying on with the notion of structure and regularity in messages, I was wondering about SETI and just how they go about attempting to verify that certain signals are from intelligent sources.

The thing is, it's not just a matter of detecting a high signal-to-noise ratio, since we know that there are many natural phenomenon that exhibit a high degree of structure. For example, pulsars at one point were thought to be candidates for signals from extraterrestrial life, but it turns out they simply exhibit a high degree of regularity in the signals they generate.

So how would we know if aliens were trying to contact us? In the novel Contact, the signal is extremely strong, and is comprised of prime numbers in bursts at a temporal scale that we can comprehend. The signal ended up having a huge amount of underlying structure just waiting to be decoded as well. Presumably if they were trying to contact us, they'd try to make the signal as transparent and interpretable as possible. But what if it were more subtle than that, or their minds work on a different spatial or temporal scale?

It's been a long time since I read Contact, so I don't remember if Sagan goes into that much detail. I'm not very pleased with the on-line sources I've poked around at talking about how to assess the likelihood that a signal was generated by intelligent life. Anybody else know of any? Or of any other books or other references on the subject?