Hi All

One of the more coherent arguments with respect to ETIs is that exponential growth very rapidly causes them to come up against the speed of light limit, thus causing an inevitable finite range to their colonisation efforts before they crash ecologically - unless they change their natures in a fundamental way.

Paul Birch has argued that astrophysical engineering on a vast scale can allow indefinite growth - but a growth that approaches linear expansion asymptotically at late times. What isn't seen by SETI is any sign of such astrophysical manipulations to sustain such growth - no Dyson Clouds, no star-mining, no planetary manipulation.

Dyson Clouds/Spheres can be detected to surprisingly large distances, especially if they surround up a large fraction of a Galaxy - possible behaviour of Kardashev III civilisations. James Annis has argued, based on statistical analysis of a large sample of Galaxies, that Kardashev III civilsations don't yet exist in this Universe.

I think that pretty strongly indicates that we're early arrivals on the Cosmic stage contra Charles Lineweaver's suggestion that we've passed the epoch of most numerous Earth-like planets - something astrophysical has suppressed the evolution of life in the galaxies.

The worrying question is then - has it finished suppressing life???

Adam