How do you detect life signs in space?

[Leviathan]

The plot needs a way to say "everybody's dead" or "someone is alive over there". Rather than muck about with all that nasty docking and looking inside nonsense we have sensors that read "life signs" though the hull.

How does it work?

Maybe you can pick up the resonance of a heartbeat on the hull? Thermal through a ships hull? Number of people looking out a window?

 

[DSG2k]

It's often a bit silly, especially on some occasions like the computer failing to recognize Borg drone lifesigns on the suggested grounds that they read as part of the ship or somesuch.

That said, your ideas of thermal and electrical signals imply a passive sensor receiving them, and indeed they shouldn't meaningfully convey through the hull. However, typically, the scan that can help the writers inform the crew is an active scan, utilizing some funky whatzits that then bounce an indication of some kind back to the scanning ship or are otherwise read.

Similarly, eyeballs, ears, and even radio antennae doing their passive sensor thing are unlikely to pick up an Me-109 crossing the channel, but once you ping that puppy with radar you get a blip.

 

[Christopher]

Maybe you can pick up the resonance of a heartbeat on the hull?

Possibly, if you use something like a laser microphone to detect vibrations in the hull, not just of hearbeats but of breathing, motion, etc.

Thermal through a ships hull?

Unlikely; even in real life here on Earth, thermal scans can't actually see through walls the way they're shown to in fiction. After all, keeping heat from passing through is one of the basic functions of a wall. https://www.flir.com/discover/home-outdoor/can-thermal-imaging-see-through-walls/

On the other hand, even though you couldn't see individual heat signatures, if there are warm bodies inside a ship, that heat would eventually warm up the hull itself, which would differentiate it from a ship whose crew had been dead for a while. But that's only the case if the engines and other heat-generating systems have been inactive for a while as well. Contrary to what fiction shows (because fiction gets a ton of physics wrong), vacuum is an insulator, so things in space don't instantly freeze; if anything, the greater risk in a spaceship or spacesuit is overheating. One of the most pervasive failings of science fiction spaceship design is the lack of the external heat radiator fins that would realistically be necessary to keep the crew from cooking inside the hull. (I consider this a missed aesthetic opportunity as well -- wouldn't spaceships be even cooler with sails?) Anyway, the point is that the waste heat from a ship's engine would presumably swamp the thermal signature of the crew, so if the engines or power generators were active, you probably couldn't tell if there was a living crew inside or not.

Presumably, life signs would be detected by some kind of active scan in a frequency that penetrates the hull, but you'd presumably want a spaceship hull to block most forms of radiation, at least the high-energy kind that's harmful to life. We already ruled out infrared, and visible light is out except for the looking-in-a-window suggestion, so that leaves lower-energy wavelengths like microwaves... but the whole way radar works is by reflecting off of vehicle hulls, so that probably wouldn't be good for looking inside. So we're probably talking about some kind of subspace frequency, since subspace radiation is imaginary and can do whatever the story says it can.

Of course, we shouldn't dismiss looking through the windows. Even aside from directly imaging the crew, you could take spectral and thermal readings of the atmosphere inside and potentially determine whether there was respiration occurring, whether there were subtle air currents resulting from crew movement, etc.

 

[hbquikcomjamesl]

Lifeforms! You tiny little lifeforms! You precious little lifeforms! Where are you?

 

[Christopher]

Here's a StackExchange thread about the science of scanning for life signs, although it focuses on ways to scan planets rather than spaceships:

How might you detect a "life sign"?

It's a common trope in science-fiction: "scanning for life signs". A few taps on the screen, and you get an instant population count, often with the ability to distinguish species, and even personal

worldbuilding.stackexchange.com

One possibility it suggests that I didn't consider is terahertz or submillimeter radiation, basically the transitional range between microwave and infrared, which is very good at seeing through things, apparently -- except it can't see through metal. So the question is, what would a spaceship hull be made of? It's generally assumed to be metal -- Trek tends to favor imaginary alloys like duranium and tritanium -- but in a more realistic universe, it might be made of some form of lightweight carbon composite. On the other hand, realistic models for lightweight spaceship or habitat hulls' meteoroid-impact defenses tend to include a thin layer of Mylar, so that alone might be enough to block scans.

There's also electroreception, scanning for the bioelectric fields of nerves and muscles, but that generally only works at close range except in water (a number of fish species use it).

 

[DSG2k]

One possibility it suggests that I didn't consider is terahertz or submillimeter radiation, basically the transitional range between microwave and infrared, which is very good at seeing through things, apparently -- except it can't see through metal.

Yeah, I didn't bring up UWB and such, mostly because one would hope the hull blocks most of that. Then again, we saw Voyager's hull fail to block AM radio (ugh).

As for looking through windows, there is some wacky tech that can reconstruct imagery from around corners and such by processing refractions, so even those little under-door gaps might allow a fancy version (mixed with a sensor-provided map) to see through a window and down the corridors based only on visible light . . . enough to see people walking.

(That's another reason for my excuse about model windows being plain white-out most of the time . . . I argued it was anti-snooping tech in use. After all, why let your cloaked foes read your screens and padds?)

However, I think we have to acknowledge that it is some sort of subspace tech, simply because IIRC they've detected lifesigns from light-years away (e.g. Chakotay and Tuvok presumably that far in "Cathexis"). While there could obviously be many different things that qualify as lifesigns, an assumption of commonality would suggest the shorter-range examples are the same thing.