"Life Signs" -- what might they realistically be?

[Zeropoint]

"Sensors show no life signs aboard the vessel, captain."

We've seen it in a dozen different sci-fi works; the skipper wants to know if a spaceship has anyone aboard, so the sensor operator fiddles with their console, maybe looks at some wiggly lines, and confidently reports that there don't appear to be any living things aboard. (whether they're correct is totally dependent on the needs of drama, naturally)

I'm bringing this up in the SLAM sub-forum instead of the sci-fi forum because I'm curious as to what reality-based sensors (including things more advanced than what we currently have, as long as they're solidly based in known science) might be able to detect that would indicate whether anything was alive aboard a space ship. For the sake of this discussion, let's assume that "life" means "Earth mammals".

So far, the most obvious thing I've thought of is temperature. If the ship in question is as cold as interstellar space, there's probably nothing alive there. On the other hand, if the ship IS warm and maintaining a comfortable temperature inside (check the life support radiators) that doesn't necessarily imply that there's anyone there.

Another possibility is the use of a laser microphone to listen for "people noises" remotely, maybe. Radar, including millimeter wave radar and terahertz radar could see through composite hulls.

Anyway, what are your thoughts?

[Khaat]

Well, bouncing a low-powered laser off the hull might give you vibrations (sounds) that could be filtered for probably-biologically-produced sounds (heartbeat, voices, possibly even respiration, etc.) A pattern-recognition system could separate engine/reactor noise (Red October, anyone?), mechanical pumps, etc., from all the rest, and then check the remaining sounds against a library of "probably life". Somewhen I seem to recall Spock mentioning respiration detected, but not how he got that information.

Otherwise, depending on if the tech (and biology) is comparable with your own, you might be able to measure energy expended for active life support (rather than idling.) I have no idea how you would reach said conclusions.

Gravitic detectors? Moving objects (mass) within the other ship? You'd have to have incredibly sensitive sensors and great computer filters for all the noise.

It would have to be something that a) reflects off water/carbon/whatever, b) yet somehow managed to penetrate shields and/or hulls c) and doesn't register as a weapon. I seem to recall in Trek, "sensor scans" were detectable by the scanned ship, which pushes firmly into "active emissions" territory ("pinging"), as opposed to sensors, which would be passive ("listening").

EDIT - tags

[Lord Revan]

yeah I suspect it's case of looking for specific patterns in the sensor scans (at least most of the time) so that you're not so much scanning for "life" as "patterns that generally indicate a lifeform", that would explain why some exotic creatures don't show up in lifeform scans (they simply don't match the pre-existing patterns) or how they can not only say if there's life but sometimes even the species (the pattern detected matches species x).

[Shroom Man 777]

If the marooned vessel had emergency protocols activated before whatever accident happened, or during the accident, it's distress beacon might also send information on the states of its passengers.

[Sea Skimmer]

Windows would help, then you could use laser spectrography to measure the contents of the air inside and given sufficient resolution, deduce all kinds of things concerning life or decay of life inside the ship.

Terrahertz wave sensors are notionally incredibly useful within short range LOS, but they will not penetrate solid material of any real structural significance. Ultrawideband radar can see through some metal, but only within narrow constraints, so using it through a hatch to check the far would certainly work, while a 'simple' double hull on a spacecraft would already present an enormous problem for a wide area scan. Ramping up the power could create dangerous side effects too, anything above IIRC 30-50 watts transmitted power is a safety issue for unshielded people and flammables.

I can think of one or two methods using particle beams that might work too, but would also kill people. Find life, and end it!

Gravity gradiometry methods are a real possibility, subject to serious limitations of the ship contains a lot of other motion, such as flow in drain pipes. Also you would probably need to engulf the ship in an array of sensors to ever have the hope of detecting a human by breathing, mainly to cancel out exterior interference (think of all the rocks flying around a star system). But that's not a deal killer. Some of this technology is being used on UAVs around Mosul to try to find ISIL tunnels and bunkers, it does not sound like it's been overwhelmingly effective but it beats nothing.


The best best would be hyperspectral imaging of the hull, followed by just knock on the damn door. It's what MARINES are actually for.

[The_Saint]

I can think of one or two methods using particle beams that might work too, but would also kill people. Find life, and end it!

"Sensors, report?"
"Captain, no life signs detected!"
"Strange, every damn ship so far, everyone dead, so weird"





Something's knocking around in the back of my skull about making a super size magnetic resonance imager utilising drones/remotes that can be on the opposite side of the ship from the detecting ship. I think it was probably part of a plot in a film/book but someone might know whether it's complete fiction or nearish-future, vaguely plausible.

[Sea Skimmer]

Something's knocking around in the back of my skull about making a super size magnetic resonance imager utilising drones/remotes that can be on the opposite side of the ship from the detecting ship. I think it was probably part of a plot in a film/book but someone might know whether it's complete fiction or nearish-future, vaguely plausible.

MRIs work off radio waves, those resonance off specific kinds of material in the human body, and a magnetic field is used to boost that signal strength. It has the same problem radar systems do of any amount of solid metal being a obstruction.

That's exactly where the neutron particle beam death option comes into play. With that you can make ones that can shoot right through a fair bit of metal, and use sensors to measure the ionizing radioactive backscatter this induces that also penetrates non trivial thicknesses of metal. But you know, it's going to make stuff radioactive while detecting it because its kind of like the neutron effect of a nuclear bomb your using for this. This might make sense in some situations, if the threat was serious enough or for an evil regime to find people smugglers. Detectors exist that work like this for finding nuclear material in shipping containers. Gamma and X-ray beams could also be useful for kill-sensors.

If we had superconducting magnets then a hand portable MRI machine might be possible and do a lot of what the Trek tricorder could do.

[LaCroix]

I was always going with the assumption that they were measuring neutrinos that went through the ship against the background radiation, and then computing stuff from it.

[Sea Skimmer]

Neutrinos directly induce radioactivity though, they are after all radioactive decay products in the first place and since they weakly interact with normal matter if at all, you'd need a very powerful neutrino beam to have any reliability of detection for something as small and low density as a person. It's kind of not going to work out great for anything but finding nuclear material.

Actually a proposal been around for a long time for the IAEA to build a giant passive neutrino detector into an oil tanker and sail it around looking for covert nuclear reactors.

If your talking Trek the answer seems to be pretty simply that they are using subspace particles of unknown properties. One assumes that they can pass through normal matter without harm, or that any harm they do cause is funneled back into subspace rather then affecting realspace tissues.

[Zeropoint]

Oh yes, Star Trek cheats like hell. However, at least for the purpose of this question, I was curious as to what people could come up with that would be physically plausible. I couldn't think of anything that would give good (i.e. reliable and easy to interpret) results, and it seems that no one else can, either. This is certainly not proof of anything, but I'm thinking that there's just no physically realistic way to check for life aboard a ship with a conductive hull.

[Lord Revan]

A group of various sensors looking for certain patterns seems like the most physically plausible to me, not mention it avoids the problem with using just a single sensor such an action aka it reduces the probability of accidental false positives due to the fact that while every sensor could be getting false readings the chance that they all were getting exactly the right kind of false readings is low.

[Swindle1984]

I would assume that in addition to heat and bioelectrical fields, they may look for other signs of life such as respiration and other chemical reactions. We know from several episodes that severely injured and/or dying people had 'weak life signs', and I believe in more than one episode they were difficult to detect as a result.

In the TNG episode with the super soldier who didn't want to be trapped on Planet Retirement Home and wanted to escape and return to society (can't remember the title off the top of my head, I'm sure six people will show up to tell me what it was), part of the chemical and surgical enhancements that gave him faster reflexes and a killing instinct also made him undetectable on the sensors. That, to me, seems to indicate that the sensors were looking for some kind of energy field or chemical process that his enhancement had muted or altered entirely.

[Shroom Man 777]

Not-so-hard-sci-fi suggestion: What if the remote-sensor involves nanomachines? Something that can - as a cloud or spurt of nanites - make contact with the hull and use conductive sensors to detect more than just heat, but also vibrations and even any chemical seepage. Or the nanomachines can even seep into the ship or interface with its externally-accessible systems* and gather data? The soft sci-fi element would have to be in the projector-tech. And this is already breaching the OP's terms because this is pretty much sending an unmanned system, not just some remote non-physical, non-contact sensor system...

A variation of the idea would be if civilian ships and other non-military or non-stealthy craft create their own nanomachine ecosystem-cloud that transmits onboard information to the external cloud. In the void it would be useless... but some kind of nanomachine CHEMTRAIL for nearby crafts and entities to access data and such without resorting to active sensors... that might be a cool thing.

Something like animal-pheromones, but synthetically-generated by vehicles.

*Externally-accessible systems might include standardized "Safe Medical/Emergency Remote Sensory Nanomachine Ports" built into civilian craft, even military ones, with safeties to prevent malicious exploitation?

[Zeropoint]

After thinking about this a bit, I have to agree that remote spectroscopy to look for metabolic byproducts is certainly a feasible approach. If a ship had windows, you might (or might NOT; I don't know enough about how such things work) be able to get a spectroscope reading on the atmosphere inside that could detect fresh metabolites in the air. This would absolutely be possible for detecting large concentrations of life on a planetary scale.

The nanomachine trail idea is . . . well, plausible technically but seems overcomplicated (and is odd enough that I need some time to roll it around in my mind). You could certainly have a legal mandate that all civilian ships return information on the status of crew and passengers when their transponders are pinged.

[Shroom Man 777]

I guess the nanomachine trails don't have to be as dense as some real-life coral reef mating-spawning season thing. And the nanomachines don't have to be so nano-ey. They could be visible buoys, microsatellites that can be recycled... and existing space-lanes that are regularly used can be dotted with long-lasting buoys that can ping and be pinged by passing ships...

But that's already digressing from the OP...

[Sea Skimmer]

Not-so-hard-sci-fi suggestion: What if the remote-sensor involves nanomachines? Something that can - as a cloud or spurt of nanites - make contact with the hull and use conductive sensors to detect more than just heat, but also vibrations and even any chemical seepage.

So some pretty advanced nanomachines are probably possible, if you made the cloud out of multiple different kinds. Say one senses gas, one heat ect... but what's the point compared to just building one small robot that does all of those functions together and has it's own propulsion source so it can go where it wants? Some use might exist for locally releasing nanomachines though as bio agent sensors.

Also very small but not cloud like robot avoids possible problems like the nanomachins causing cancer or shorting out electronics. You are talking about metal dust in the air.

Nanomachines can make sense for a lot of purposes, but active search isn't one of them. Notionally for one thing nanotech explosive are possible, in which basically the entire explosive is now a bunch of tiny capacitor circuits made of dense metal, all of it detonation combusting from it's own coordinated powr surge. So instead of an explosive wave travel through the explosive detonating all of it as it gose, the entire explosive explodes at once limited by the speed of light only in how fast it could be. You'd still have slight delay from the circuit paths. The result is intensity of explosion goes up far above any conventional explosion, though not to the nuclear order of magntitude.

I dunno how that would help find life, but it would help small search robots do exotic things like destroy enemy weapons directly, but not kill people. Use tiny shaped charge nanotech cannon to destroy the crazy people on the ship's rifle kind of scenario.

Or the nanomachines can even seep into the ship or interface with its externally-accessible systems* and gather data? The soft sci-fi element would have to be in the projector-tech. And this is already breaching the OP's terms because this is pretty much sending an unmanned system, not just some remote non-physical, non-contact sensor system...

If you have a lot of time to blow infiltrating enemy electronics in space and on the ground is plausible. If this threat existed though it would also be very straightforward to make countermeasures against it, since for one thing you'd just plain be able to see the nanomachines with thermal imaging if they did anything useful. They can't have zero power emission and hack electrical cables that carry 5 volt currents.

A variation of the idea would be if civilian ships and other non-military or non-stealthy craft create their own nanomachine ecosystem-cloud that transmits onboard information to the external cloud. In the void it would be useless... but some kind of nanomachine CHEMTRAIL for nearby crafts and entities to access data and such without resorting to active sensors... that might be a cool thing.

You'd need a lot of CHEMTRAIL... sounds like the chemtrail industrial complex at work to me!

Perhaps a better use would be if you wanted to survey an entire new world you could rain a very thin coating of chemtrail nanomachine over it and use that to start a massive world wide ground and climate survey. Get billions of datapoints so you can learn the weather of a planet quickly before you decide where to colonize it for example, and mineral surveying. This is demanding a lot from any plausible technology though, just because of the need for a radio or something to actually transmit information back to some kind of pickup.

*Externally-accessible systems might include standardized "Safe Medical/Emergency Remote Sensory Nanomachine Ports" built into civilian craft, even military ones, with safeties to prevent malicious exploitation?

You'd also want some ports so you can cycle the air inside the ship and vent what was onboard into space. Realistically you have to worry about buildup of explosive gas from rotting human corpses and fresh food that has spoiled. Spoiled vegetables in confined spaces have killed many people onboard ships, you must have ventilation. A sealed can in space is very dangerous if anything goes wrong, searching for life is only one priority in a realistic 'approaching derelict' situation.

Also bombs, chemicals, explosive residues (not tiny amounts, like meth lab in space situations), bioagents left behind, all of which nanomachines might be good for ultra decontaminating with. Many nanoscale reactant products already exist for specific decomposition tasks. Someone made a powder that can burn TNT without exploding it that way. It was on one of the episodes of Future Weapons, though they never said why it worked at the time. The same concept can apply to other explosives with more nanotech. Magic robots aren't its only uses! People pounded nanotech into swords with hammers and grass in ~1400 AD and it actually worked well.

[Shroom Man 777]

If you have a lot of time to blow infiltrating enemy electronics in space and on the ground is plausible. If this threat existed though it would also be very straightforward to make countermeasures against it, since for one thing you'd just plain be able to see the nanomachines with thermal imaging if they did anything useful. They can't have zero power emission and hack electrical cables that carry 5 volt currents.

Ah the point would be in this strange setting there'd be standard commonly-accepted normal clouds/miasmas of benign nanomachines in the regularly used transportation lanes for civilian utility purposes. The nanomachines would have IFFs. The nanomachines don't have to be capable of defeating countermeasures - vessels, civilian or military, should totally have countermeasures to defeat malicious nanites.

I wonder if these same nanomachines can be used to monitor hull integrity, using ultrasound emitters to find microfractures...

I guess the nanomachines can be spewed out of and return to buoys and drones that are deployed in regular intervals in the space lanes.

[Sea Skimmer]

Embedded sensors are starting to appear in composite materials to check them for damage, often by using ultrasound in fact, but also by acting as tiny strain gauges. This is actually leading to fiberglass-ceramic armors with video game like color displays for the commander showing where the armor is damaged and degraded, and where it should still be okay.

Pipes and wiring runs might also be kept under constant monitoring this way, and could can create sensors for corrosion too. Star Trek showed us a future with no rust, which I for one do not believe in.

The thing with using actual nanobots spread around is they are...spread around. They wont stay in place and that will make it hard for them to act as force sensors to ensure 100% monitoring. A nanosensor paint that comes with a free ap for your space phone is what you want.

Any kind of emergency government backdoor is a bad idea. Its super totalitarian. Also if the government can inject your ship with nanobots they might as well just open the door, the life sign scan thing seems more relevant to totally unknown ships.

[Khaat]

I think we're also overlooking the "shifted perspective point" - i.e., when a point of view other than "here" + "telescopic enhancement" is used (like in STIV, when they're going after George & Gracie, to save them from the whalers.) Is that just a computer-rendered simulation of what the view would be, or do they have actual warped-space telemetry capabilities? Can they warp space/time with fine enough controls to actually be capturing images from over there? Suddenly distance doesn't mean as much.

And when it comes down to sensor scans (picking Spock out in a ship full of Romulans - Enterprise Incident), how do they manage that? There was mention of "Spock's lifesigns being very similar to Romulans'", but again, not how they were getting those. OTOH, he was testifying, so maybe they were able to triangulate his position from that....

[LaCroix]

Also, nanites are very easily destroyed. Engine exhausts, active EM radiation (e.g. Radio), or even static electricity from pushing through them will zap them by huge numbers. A very expensive sensor network.

[Ultonius]

Atomic Rockets has a section on this, covering some of the means of detection that people have already talked about.