1. Background
In 2010 the Search for Extraterrestrial Intelligence
(SETI) was 50 years old, and the anniversary provided
an appropriate opportunity to take stock. In particular,
there have been calls to widen the search by considering
not just purposefully-directed radio messages, but the
most general signatures of intelligence [1]. Alien intelligence,
if it exists, or has existed, might have endured for a
very long duration, measured in millions, tens or hundreds
of millions or even (in principle) billions of years,
and will almost certainly be dominated by post-biological
intelligent systems [1,2]. Thus it may lie far beyond
human capacity to meaningfully extrapolate regarding
its specific characteristics. If we therefore have no fixed
idea what to look for, it makes sense to search all
available and emerging databases for ‘‘artificiality,’’
whether deliberate (as in a message) or inadvertent (as
in environmental impact)