How Do Starfleet Sensors Detect Life Signs?

Question

Often, in many versions of Star Trek, whoever is working with sensors reports life signs. They can detect life signs on other spaceships and on planets. They're even able to detect what type of life sign it is. (In The Enterprise Incident Checkov is able, with trouble, to distinguish between Romulans and a Vulcan.)

I remember at some point hearing a comment about using passive sensors only, which implies that there might be some type of energy beam sent so the sensor array can read the returning beam and the computer and use that to make conclusions about what is being detected. But that would mean that the beam is being reflected by whatever is inside a spaceship if it were detecting life signs in a ship.

I can buy that if the sensor picks up certain shapes, that the computer can decode them and determine if they're alive or not, but how does it get this information about distant objects in the first place?

How do sensors actually detect life signs on planets and on other spaceships?

Answer 1

The Star Trek: The Next Generation Technical Manual describes the capability as:

Remote lifeform analysis. A sophisticated array of charged cluster quark resonance scanners¹ provide detailed biological data across orbital distances. When used in conjunction with optical and chemical analysis sensors¹, the lifeform analysis software is typically able to extrapolate a bioform's gross structure and deduce the basic chemical composition.

Translation: complete techno-babble/magic.

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^{Note 1}: this is diagrammed as being part of the main deflector array.

Answer 2

It appears to be a collection of signals:

• Heart rate or other acoustic vibrations produced by fluid pumping organs
• Heat signature
• Breathing or other chemical interactions with the atmosphere
• Locomotion
• Detection of organic compounds and known organic molecules in the target object
• Electromagnetic field (the nervous system generates one)
• Chemical reactions -- oxidation etc.
• Various other bio-plasmic and psionic fields that are generated by intelligent beings which are visible to Federation sensors (in VOY: Bliss, the sensors are able to detect a bio-plasmic field).

The computer may filter these signals out of ambient noise and match them against a database of known species' signals. A large array of these sensors will provide the resolution required to detect minute signals or multiple signals. I doubt any visual inspection is performed, since it requires direct line of sight.

Example: Both Romulan and Vulcan life signs will indicate localized circulation of copper (in their bloodstream) but only one (the Vulcan) life sign will indicate a psionic field.

Answer 3

I guess x-ray and IR scans could be counted as visual type scans? Not that they are directly visible to our eyes, but are within the light spectrum seen by CCD's