Why would we be looking for non-biological aliens? Well, argues Shostak, since we're already drawing on the experiences of life on our own planet to look for life elsewhere, we might as well take the full lesson from planet earth:
We are now building digital devices that can process information at blistering speeds. Our computers double in capability on timescales of only a few years. It's hardly outrageous to believe that we will successfully develop thinking machines within a handful of decades, or at most a century or two.
In other words, our self-improving machines might become the new space travelers. That leads to the following extrapolation:
If any species reaches the point of inventing radio, it is only a handful of centuries from inventing its intellectual successors. Biological intelligence is merely a short stepping stone on the path to the prodigious talents of machines. Consequently, the majority of the intelligence in the universe could well be artificial intelligence.
The conclusion? We should be looking for machines.