How would a biological radar work? [duplicate]

Question

In many games and sci-fi worlds, there's a kind of "biological radar" that somehow knows where nearby organisms are. How would such a “radar” be possible? I'm looking for a way to detect all large organisms, including plants and cold-blooded animals that don't show up on infra-red. Ideally, this would work underwater as well, detecting things like jellyfish. For the purposes of this question, microbes can be ignored- although a clever explanation for why this would be the case would be awesome.

This should be something possible on Earth with minimal hand-waving, so you don't have to worry about detecting aliens or beings that don't obey our current biological understanding of macroflora/macrofauna.

I’m hoping for something handheld, like a Star Trek tricorder, and which doesn’t require an extensive array or satellite/ship based mechanism.

EDIT: I am not looking for a way to use current radar to detect biology. I’m asking how sci-fi civilizations would be able to detect life forms using something like radar.

For example, from Star Trek TNG: "Captain, I am detecting life readings from the planet's surface". What would this instrument be "reading"?

Answer 1

TL;DR: I think it's not likely to work with radar, but sonar could be possible

The reason why radar works is reflection of electromagnetic waves on most metallic surfaces. If you don't have this strong reflection effect you can't measure where things are and your send waves just pass through the material or are absorbed by it.

Organic material is a lot different from metals. Biological bodies barely have any reflection in most parts of the electromagnetic spectrum. And there certainly is not that one thing you can detect any biological entity in air and in water with the same wavelength. Most likely you couldn't even use several different wavelengths, because the reflection is so weak you can not detect anything.

What might work is sonar, because the properties regarding acoustic waves in biological entities are vastly different from the surrounding medium (air or water). Reflection might be possible to use find most biological organism. Bats use a sonar system to find things in air and things like whales can be seen on sonar systems on boats. So i do think it is possible to detect most biological entities of sufficient size. (Jellyfish might be difficult, since they are thin and mostly consist of water, if i am correct)

EDIT: I just read that the maximum range of ultrasound in air is approximately 200 meters. If this is sufficient for you purposes sonar could work.

Answer 2

Larger predatory sharks, such as the great white or mako, do this with sensors in their nose that pick up the electrical impulses from nerve activity of potential prey. Needless to say, this works best in water, where electrosensing is a double bonus because prey can be impossible to see at any distance in murky water.

On land, heat would be a more likely approach. Pit vipers have heat sensors to locate prey.

Or sonar, as bats use to find bugs in flight, or dolphins or whales use to identify their prey. However, sonar doesn't work well with a cluttered background, which is probably why pit vipers use heat instead.

All of those systems are optimized to identify potential prey in the particular environment the creature operates, so much would depend on what your critter wanted to find, or avoid, and the environment in which it lives.

Answer 3

• Radar emits an electromagnetic pulse that is reflected by high-density material (like metal). The time between emission and reflection is measured to determine distance. Finding an electromagnetic emission that reflects off of bone or plant fiber (without cooking it) is ostensibly plausible. Likewise...

• Ladar (laser distance detection) works basically the same way, but with photonic reflection rather than electromagnetic reflection.

• Sonar and echolocation emits a compression wave (often known as "sound") and the wave bounces of of objects and returns. Distance is determined in pretty much the same way as radar. (From orbit, you could measure atmospheric disturbances much the same way a laser can be used to detect the vibration of glass and, thereby, decode the conversation going on within. Sneaky bounders those spies.)

• Electroreception (good catch, @MichaelKutz) is a passive detector (nothing is emitted) that detects minute amounts of electricity in a short range.

• Thermal detection works basically the same way, but with heat.

It's a glorious assumption that sci-fi shows like Star Trek aren't using some or all of these technologies to detect "life" on the planet. But there are other things that our modern astronomers use to detect "life."

• Spectrophotometry (did I spell that right? Probably not.) analyzes light to detect chemicals in the medium the light passed through. Very simplistically like a prism: how the light gets displayed tells you what the prism is made of. Specto-mo-photo-mooky-tree is therefore useful for detecting masses of oxygen emitters (plants), carbon-dioxide emitters (mammals), sulfer emitters (volcanoes), etc.

• The same technology can be used with other forms of electromagnetic radiation (gamma rays were used to detect the latest void in the Great Pyramid) to detect all kinds of things, both their presence and their absence.

• And that assumes you're not using something along the lines of photoluminenscence using electromagnetic radiation rather than visible light to cause materials to "glow" (emit something detectable), which can be identified uniquely by how they emit, what they emit, etc.

• And, of course, you can theoretically detect mass by the perturbations in the larger gravity well caused by smaller masses. Detecting "gravitons" is well beyond what we can do today, but certainly within the realm of Clarkian Magic.

And there must be more things that biology emits, reflects, or reacts to that can be used to detect its presence.