I love good sci-fi. Hell, I love bad sci-fi, but sometimes I make the mistake of thinking too much about it and then I start to get grouchy. Today I’m grouchy about detecting life.

In almost any space-faring sci-fi story, there is a moment when someone pushes a button, looks at a display, and proclaims that there are lifesigns aboard another ship. They do it with startling accuracy at orbital distances, with sensors able to determine not only the presence, but also the species, gender, and state of health of the occupants from tens of thousands of kilometers away.

How the hell does that work?

The short and pithy answer is that it doesn’t. It’s made up. But why stop there when we can overthink it enough to ruin all future books and movies. Let’s do that now.

1: Radiated Light

Life doesn’t produce any kind of mystical aura that can be read like an RFID tag, but living things do glow. Most don’t do it in the visible spectrum, but we have ways of detecting other wavelengths of light. Warm-blooded animals radiate across a decent chunk of infrared. This can pass through some solids making it possible to see life forms through walls with a sensitive infrared camera.

Unfortunately, the wavelengths that people emit would only pass through thin walls of certain materials. Metals and ceramics and even some paint would reflect or absorb all of the light, making it impossible to detect it from outside of a spaceship. Not just difficult, Impossible. A really sensitive camera wouldn’t help. None of the light would be escaping.

2: Reflected Light

We may not be able to detect the dim glow of a living thing through the hull of a ship, but we might be able to bounce some light off of them.

The easiest way to check for life is to use good, old fashioned reflected visible light through optical ports. ie. look in the windows. If the occupants aren’t polite enough to stand in front of a window, we could try another wavelength.

Different wavelengths on the electromagnetic spectrum, from radio waves up to gamma rays, will be reflected or absorbed by different materials, and will pass straight through others. This is how x-rays work. They pass through skin, but not so much bone.

Shining a wide spectrum of light at a ship and seeing which wavelengths bounce back in what ways could tell you something about the contents of the ship. That could theoretically include any watery sacks of meat moving around inside.

Unfortunately, radio waves long enough to pass through the wall of the ship, would similarly ignore something a small as a body. Shorter wavelengths light with enough energy to penetrate the hull would also pass straight through any people, not helping you detect anything and possibly killing everyone inside in the process.

Also, spaceships are necessarily designed to protect their occupants from many kinds of radiation. The light from stars includes a lot of very high energy rays which you would need to protect anything living inside from. The surface of ships would need to be highly reflective. Even without their ubiquitous invisible “shields”, your ship’s hull would probably block any wavelength that might be useful to throw at them. All you would pick up is the surface of the ship.

3: Magnetic Fields

You can do some pretty thorough imaging by spinning a magnet around. That’s how MRIs do their thing. Our current technology requires a gigantic magnet and a physical ring of sensors around whatever you’re trying to scan, but maybe sci-fi future tech can solve that problem (probably not though).

What it can’t solve is the fact that any decent ship would be shielded against that kind of scan. Surviving in the space between planets requires that your ship and its electronics are protected from the vicious magnetic fields generated by stars and some planets. Again, the hull of the ship will prevent your detector from detecting anything inside.

4: Dark Energy

Dark energy is a placeholder name for something we know that we don’t understand. Same with dark matter. There is no reason to think it would help determine if an adult female was suffering heart palpitations on the engineering deck of a ship on the other side of a solar system.

It’s a lazy excuse for an explanation and I’m annoyed at you for suggesting it.

5: Neutrinos

See section 4 above, but replace “we don’t understand” with “we are absolutely certain this won’t work.” Neutrinos don’t interact with matter, in general, and not in any special way with living things, either. They’re not magic. They’re just very small uncharged particles.

6: Sound Waves

Sure, there is no sound in space, but noises are just vibrations and there are other, more effective ways to detect vibrations than sticking your head out of an airlock and listening.

It’s possible to bounce lasers off of things and detect the vibrations of that object by watching how the laser wobbles. This can be absurdly precise. We have devices now that can measure movement in the sub-nanometer range by detecting the doppler shift in laser light bouncing off objects.

If you bounce many lasers off of a ship in different locations, you can build up an image of how vibrations are moving through it, and infer their source and location.

That’s great and all, but it’s not like living things make particularly strong vibrations. This method only works if the vibrations they make transmit to the surface of the ship and don’t get absorbed and diffused. Maybe on smaller ships you could hear someone walking, but you probably couldn’t pick up a heartbeat or a conversation. Those vibrations just wouldn’t penetrate the whole way through the ship. They’d be absorbed and converted into a very tiny amount of heat.

It might be possible to augment this system a bit; you could ring the ship like a bell. If you impose known vibrations on something and then detect how they move through and bounce around inside of it, you can build up a map of the contents. That is basically how ultrasound imaging works, and how we work out the structure of the inside of the earth.

How you impose those vibrations is a bit dodgy. Maybe you grab it with your tractor beam and rattle it around a bit. It seems an odd sort of handshake for ships to make, but who am I to judge.

Of all the methods for detecting life on other ships, this might be the most believable one. Even so, getting a precise count of passengers, including their state of health seems beyond the limits of physics on this one.