[espadrine]

Let's imagine a remote island on Earth that was colonized five thousand years ago (and completely forgot about how they got there). They may spend a very long time sending and listening to sonar signals. However, we (the aliens from their point of view) use sonar for ourselves, not to detect intelligence. It would be very hard to detect intelligence from our use of sonars (which is similar to that of a few animals). Even if they did guess that we were intelligent, their understanding of who we are and how we live would be minuscule.

On the other hand, if they spent their energy sending ships, they would eventually find us, and they would learn phenomenally about technology, science and their place in the world, even before they reach us.

[pavel_lishin]

> It would be very hard to detect intelligence from our use of sonars.

Why would it be hard to determine that we're intelligent? We've clearly created some sort of apparatus for generating sonar. I would imagine that someone listening in our oceans would be able to tell fairly quickly that we're a technical civilization.

[espadrine]

> Why would it be hard to determine that we're intelligent?

Underwater sounds can come from a wide range of sources, including tectonics, natural events and animals. Those did not create some sort of apparatus. Yet, they can have a similar signature.

It is not even a hypothetical situation. A SETI lab spent a lot of time trying to figure out what a particular signature that their telescope picked up was, only to discover it was caused by their microwave oven: http://www.independent.co.uk/news/science/seti-the-hunt-for-....