“In reality, there is no such thing as a life detector. Vitalism long since having joined the scientific junk-heap, it is a regrettable fact of the universe that there is no quick, convenient, and universal ‘vital field’ that we can tap into to determine the presence of living beings.
“But there is a life detection routine in the computers of your scout ship, you ask? How does that work?
“The answer is: approximation. We know a variety of things that suggest the presence of life. The most obvious example are the signifiers of technological civilization: patterned electromagnetic emissions, the characteristic neutrino products of controlled fusion reactions, and so forth. Where there is technology, there was someone to build it – at least at some point or another, and so the probable detection of technology is also the probable detection of life.
“But there are those few common characteristics that all life does have in common. Self-replication is one, not – by and large – terribly useful for quick detection. Existing within a solvent – for a broad definition of solvent encompassing everything from nebulae to degenerate matter – is another, which can at least tell us where not to look. But of most use is the last: life is an entropy pump. It depends upon energy differentials and pumps against the natural flow, maintaining and causing inequilibria.
“That gives us something to look for.
“A life detection routine hunts through the data collected by primary sensors looking for such inequilibria. Reactive gases – such as oxygen – remaining a significant component of a planetary atmosphere, implying their continuous production. Sustained low-level thermal sources, suggesting managed combustion or other energy transaction – bearing in mind that what is to be considered low-level is very different for the outer-system múrast and the star-dwelling seb!nt!at! While almost impossible to detect at any but the closest range, the electromagnetic emissions of high-order informational complexity associated with cognition are the most reliable sign – for life that is both intelligent and which makes use of electronic or electrochemical signals in its ‘nervous system’. These, and tens of thousands of other experience-learnt rules, continuously updated, are programmed into the expert system that underlies the life detection routines used by the Exploratory Service.
“It’s still no more than 80% accurate, yielding commonly both false positives and – worse yet, if missed – false negatives, and so the wise scout never trusts such a system without a close, personal investigation. But it can tell you where to place your bets.”
– A Junior Explorer’s Handbook, Vevery Publishing