[LINK] "Toward an Interstellar Archaeology"

[rfmcdonald]

The always interesting Centauri Dreams has a fascinating discussion on how we 21st century humans could possibly detect vastly superior civilizations.

Suppose a civilization somewhere in the cosmos is approaching Kardashev type III status. In other words, it is already capable of using all the power resources of its star (4*1026 W for a star like the Sun) and is on the way to exploiting the power of its galaxy (4*1037 W). Imagine it expanding out of its galactic niche, turning stars in its stellar neighborhood into a series of Dyson spheres. If we were to observe such activity in a distant galaxy, we would presumably detect a growing void in visible light from the area of the galaxy where this activity was happening, and an upturn in the infrared. Call it a ‘Fermi bubble.’

That’s the term used by Richard Carrigan (Fermi National Accelerator Laboratory) in his latest work on what he calls ‘interstellar archaeology,’ the search for cosmic-scale artifacts like Dyson spheres or Kardashev civilizations. A Fermi bubble would grow as the civilization creating it diffused through space. Carrigan notes that, as Carl Sagan and others observed, the time to colonize an individual system is small compared to the travel time between stars. An expanding front of colonization might then move forward at a rate roughly comparable to the space travel velocity. A civilization could engulf its galaxy on a time scale comparable to the rotation period of the galaxy, and perhaps a good bit shorter.

You might think a galaxy like the M51 Whirlpool galaxy would be ideal for such study, but Carrigan says a rough qualitative estimate shows there are no unexplained ‘bubbles’ at the level of 5 percent of the M51 galactic area. The quest is tricky because spiral galaxy structure includes natural voids — even if a void in visible light with infrared enhancement were traced, it would be hard to regard it as anything other than natural. In fact, James Annis has suggested that elliptical galaxies , which exhibit little structure, might be a better place to look for Fermi bubbles than spiral galaxies. Whatever the case, we’ve moved a long way from conventional SETI, listening for intentional transmissions from other civilizations.

Further down, the author suggests that the discovery of unusual contaminants in the atmospheres of planets or stars, or excessive infrared radiation around stars indicative of large-scale constructions, or unusual galaxies, could be useful markers. We've not found anything. So far.