star trek - How do Starfleet Sensors Actually Work?

I recently asked how Starfleet sensors detect life signs and previously someone asked how Starfleet sensors receive information faster than light.

In the answers to my question (in the first link), there are references to points like watching for respiration and echoes or sounds. In the second link, there are comments about how subspace is used.

But this still leaves something open: How do the sensors actually work? Today we can "scan" things by radar or sonar or even lidar. But all of these involve sending out a beam (or a sound) and studying what comes back. In short, we have to work with reflected information.

But in The Enterprise Incident Checkov is able to find Spock on a Romulan vessel, where they would certainly be blocking any incoming electromagnetic beam that would be used for active scanning. Also, the Commander's quarters (or any quarters) are not likely to be directly up against the hull, where vibrations could be easily detected.

And when they're scanning a planet, that would mean the sensor beams (light, radio, or anything else in the EM spectrum) would have to be reflected back to provide information. That would mean that to scan a city, the sensors and computer would have to send out beams that detect and are reflected back by the outside of the building, but are also reflected back by the life forms in the building, as well as everything else in the building.

This would be true of scanning a spaceship, too. The sensor beams would either be reflected back by the hull or they'd penetrate. If they're powerful enough to penetrate the hull, then they'd go through a lifeform inside and not be reflected back.

So what do Starfleet ships use to read information about what is inside other ships or behind objects (like in buildings) or what's on the parts of a planet that aren't directly in their line of sight?

Answer

I highly recommend picking up a copy of the Star Trek: The Next Generation Technical Manual, which answers these types of technical questions. From the same section that answered the life signs question:

Planetary surface analysis. A broad range of short-range sensors provide extensive mapping and survey capabilities from planetary orbit. Besides high-resolution optical and EM scanning, virtual neutrino spectrometers and short-range quark resonance scanners provide geologic structure analysis.

The same section goes on to describe the long-range sensors of the Enterprise-D, which consists of a package of both active (i.e., like sonar) and passive (i.e., like human eyesight) EM, subspace flux, gravimetric distortion, thermal and neutrino imaging scanners.

In a nutshell, sensors aboard starships work just like sensors work in real life, just with some extra technobabble. When a starship wants to scan something passively, it uses its neutrino sensors. Otherwise, it uses its array of various active scanners that can see well beyond visible light (and even well beyond normal spacetime).

Why neutrino sensors? Neutrinos have the distinction of being some of the most weakly interacting matter in the Universe: they can pass through nearly anything without any problems. For example, a cloud of neutrinos can pass through the Earth without losing much of its mass.

While a neutrino sensor wouldn't be especially powerful, it would provide at least some information even through the thickest of barriers (like planets, starship hulls, and walls).