How might you detect a "life sign"?

Question

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 details about the subjects (e.g. "40 life signs, 12 Klingon, 7 female, one of whom is pregnant").

But what data source, or cross-referenced data sources, might be used to gather such data? What would you scan for, and what would you compare it to?

Ideally, it should be able to meet the following criteria:

• The subjects should be at a significant distance, and/or behind obstacles (i.e. you can't just look and count).
• The system should be able to discern between living things and life-like things (e.g. robots, fires, or holograms).
• The system can disregard microscopic or other widely divergent lifeforms like plants (usually we're scanning for humanoid life, or similar). Although, bonus points if you can scan for anything.

Given these constraints, how might a system detect "life signs" within a given planet or structure?

Answer 1

If you know some basic information about the target this is fairly simple.

Heat:

Search for heat emissions from the planet, matching the expected heat emission of the creature. Remove any readings from things such as volcanoes and factor in the average surrounding temperature.

This method allows you to check inside buildings and underground (to a certain extent - depending on the surrounding temperature and the life form's heat output). As Frostfyre has pointed out, note that this method is completely useless if the environmental temperature is the same as the target's temperature.

Note that due to possible blocking from things in the atmosphere (such as clouds), it's likely that you'll get better results by dropping a drone or a beacon of some sort to get the scans from the immediate area.

Movement:

Trace movement on the surface of the planet, matching to the average speed of the creature +/- value X based on maximum running speed and walking speed. Eliminate values from any movement of lifeforms smaller/larger than the creature min/max recorded size.

This method helps to reduce the accidental counts of similar life forms - eg: a human may produce the same amount of heat as a dog, but dogs generally move faster than humans. By tracing movement we can subtract the amount of dogs and robots and other anomalies from our total life form count.

"Sonar":

Blast the area of the planet with sound waves, record them as they bounce back, and use them to generate a 3D visualization of the area. Use the computer to count.

This helps to remove accidental stacking of heat. For example: if you have people living in buildings and you happen to have 3 people on top of each other, you'll only see the heat signature for the first one. But sonar may allow you to visualize all 3 of them, and as such, apply a correction factor to your count.

Wave/Pattern detection:

Listen for anything like radio waves being broadcast. If you can pick up/detect/intercept any sort of radio communication, you'll at least know "hey, this planet has creatures that have radio/radar already". If there happens to be some sort of (hand)wave in the future that we haven't invented yet that your population knows about, perhaps you can try scanning for those too.

Pattern wise, look for lights on the dark side of the planet. Even from space, we can see our man-made lights from earth. You don't have to look just for lights either, if you can detect noise from certain areas that sound like machinery or if you can detect smoke columns from factories, you've likely found a planet containing sentient life. If you find them, you can perhaps send a drone, or something down to take a look and see if you actually found sentient life.

This is only useful for detecting life above a certain technology level, and it also doesn't give you any information as to population density, species, gender, etc. All it does is tell you if this planet has sentient life capable of creating the type of wave/pattern you're scanning for.

Combine the above methods and you'll have a highly sophisticated, fairly-semi accurate life-sign scanner.

BONUS METHOD: Thank you Twelfth for giving me this idea.

If you don't need a quick result, you can drop nanomachines over the surface of the planet to literally count, obtain images/video of areas, and broadcast said information back to the ship. This would likely give the most accurate result, but would likely take days or weeks for you to obtain any result in the first place. Of course, this assumes that the technology level has already advanced to the level where not you have access to so many nanomachines that you can dump them freely, but your computing systems are also able to process all the information that comes back quickly.

Answer 2

As I can't yet comment to ask for clarification (and this question is really old!) I'll answer to the letter of your question, rather than its spirit.

The problem is, of course, the medium in which you're doing detection. The other answers here are strongly biased towards detecting things in atmospheres, which is fair enough because there's a strong statistical prior on the answerers (and the questioner) being air-breathers. Looks like there might be a lot of ocean planets out in universe, though, and you'll want different techniques for places where light, heat and radio don't reach far.

1. Electroception. Organisms with muscles and nerves generate weak bioelectric fields which are susceptible to detection at close range in airor through the ground, and somewhat longer range in water (or wet ground), either passively or by their distortion of an actively generated electric field. This is popular in quite a wide range of species on earth.

   The further you are away from the organisms in question the harder they will be to discriminate or detect, but as with most other suggestions here large groupings will be easier to detect than individuals.

   This mechanism ticks the boxes for being able to find things you can't see (it works in extremely murky water and even mud) and for excluding "wildly divergent things", because they don't have nerves and muscles. Robots and holograms may generate signals (the latter by virtue of the generating equipment) but it will be quite different to that of the real thing.

2. Sound. And I don't even mean sonar, but literally just listening. Earth's ocean is full of low frequency sounds, and if you listen in the right place you can pick up whales chatting from thousands of miles away (and I bet it would work at longer ranges on a big ocean world). Sound is the best way to transmit signals over long distances underwater, where light and radio simply won't work, so many aquatic organisms make use of it. This won't work on everything; it isn't nearly as widely applicable as electroception, but it will give you the opportunity to listen for things that sound like intelligent communication (though it might take you a while to work out whether birdsong or whalesong or its equivalent is a language, of course).

Answer 3

Scanning for life signs seems to focus on self-aware life forms, no matter how primitive. You don't usually hear "scanning for life signs...only deer and whales."

In this answer I'm assuming we're reverse engineering already existing technology, not demonstrating feasibility.

So from that we can assume that life signs use some form of brainwave sensing. Brain activity does emit some low level signals. Now, the feasibility of detecting such a weak signal from a distance would probably require some preliminary focusing, like heat and/or pulse to locate living things.

The way I would design this system is to first do a low-resolution scan for heat, then narrow it by pulse. Finally run the brain waves from those pinpointed locations against known patterns to determine age, health, species, and gender. You could probably determine mental state with such systems. At short range the technology could replace betazoid empath skills.

Answer 4

I imagine a multi-step process beginning with.

Machine Vision

NOTE: This step is for orbital detection, not from an infantry's perspective.

Given that computers are getting faster and smaller, its most likely safe to assume that an on-board computer could handle a high definition image of the planet (think super HD google earth). The first step would be to isolate possible areas that life would have a higher probability to be present in. For example, focusing processing power closer to the equator rather than the frozen poles on an earth-like planet. The systems would "scan" said image for pixel structures that tend not to show up in nature (like perfect shapes or kilometer scale infrastructure). Then, now knowing which areas to search in detail, we begin the next step.

Terahertz Scanning

T-rays are a quite disputed range in the electromagnetic spectrum. Representing one side, T-rays have been shown to provide highly detailed information while also immensely minimizing the health risks associated with radiation. Conversely, photons in this range are easily absorbed by the abundant water molecules in the planet's atmosphere. With a small amount of hand-waving or "convenient scientific development," this method would allow for a somewhat deeper scan of the planet possibly extending even to temperature detection, although this would require amazing precision. After we have focused on each specific location, we can blast a quantity of T-rays to obtain an X-ray like model of the possible life-form.

3D machine vision

Finally we have to determine whether or not the newly created 3D model is alive. There is no 100% surefire way of determining if a rock-like structure is alive or not, however, due to the computer's large database of all known life-forms, the scanners can provide a usually accurate result about what we consider life.

Limitations

Scanning for such rigid ideas of life may not fare so well. The idea of searching for a quantity of life implies that the sentient beings are explicit, an idea which may offend the galactic empire of slime molds.

Answer 5

Spetroscopy

Every substance has a spectrographical signature and you can check for that with a spectrometer. These things are so cheap, you can buy them at Amazon for a handful hundred bucks. Though, to be fair, those only work at very close range, and take a long time to assess what they are looking at.

There are some really powerful spectrometers that are installed in satellites and can measure the amount of chlorophyll in lakes, but those are too bulky and expensive.

However, in a futuristic setting, mass production and miniaturization mean everyone can carry a pocket version of this. If you want to look for animals in general, some substances you will be looking for are CO₂, H₂S and CH₄. Every creature has a smell and if you know what substances make up for each one you can use those to identify what you are looking at.

Notice that by doing this, rather than knowing what is in there now, you will know what has been in there. Spectrography is kinda like smelling with cameras. But you can combine it with other signals such as sounds and heat to make sure there is something there.