The early SETI pioneers were on the right track.

1) "Intelligence" is DEFINED as the ability and desire to build microwave transmitters. Anyone else, no matter how intelligent, would be invisible to us and therefore irrelevant to us. I'm not saying only technical civilizations are worth talking to, just that they're the only ones we can talk to.

2) Why microwaves? Because, the Universe is very background- and quantum- noise-free at many microwave frequencies and signals can be generated at very low energy costs which which will carry a long way at useful Signal/Noise ratios. Besides, microwaves would be very useful to alien astronomers, just as they are to ours, and they would know that and they would know we were listening and knew how to exploit them too. If not, see 1). Lasers would be extremely useful to communicate with, but would be worthless as a hailing device (see my original post)

3) Robert is right, we may not be able to decipher their signals, but we would know they were artificially produced. For example, no one would transmit at 21-cm, but they just might at 10.5 or 42. Besides information theory, there would be band-width issues etc. which anyone who understood radio engineering would take advantage of. If they didn't know radio astronomy, see 1).

4) My question is, are there likely to be enough of them out there to make it worth our while to look at all? My post suggests that even if ETIs exist, and if they are technically compatible with us, they would still be so scarce (for evolutionary reasons) that it would be highly unlikely one would be close enough to us in space or time for us to recognize its activities as artifacts. And if they knew this too, they wouldn't bother calling us at all. To put it another way, even if other extraterrestial civilizations exist, we may still be wasting our time and money trying to locate them.

For strictly philosophical and emotional reasons, I would be delighted if anyone could convince me I was wrong.

Henry