What else can be sensed passively?

July 17, 2007 4:13 PM

What else can be sensed passively?

Help me construct a list of things that can be sensed passively by machines. I came up with a list of the more obvious subjects of passive sensing, including:

smell
touch
taste
sound
location
motion
pressure (e.g. of the air and blood)
temperature
levels of certain gases (e.g. CO2 levels)
humidity
seismic activity
electromagnetic radiation (e.g. just about everything else I could imagine)

Help me fill in the obvious while also providing the more obscure, such as wireless networks (which, admittedly, may be categorized as electromagnetic radiation). What other things can be sensed passively by machines?

posted by viewofdelft to technology (17 comments total)

Orientation? As in the vestibular system?
posted by teleskiving at 4:16 PM on July 17, 2007

Light levels and color.
posted by Faint of Butt at 4:20 PM on July 17, 2007

Gravity.
posted by bru at 4:25 PM on July 17, 2007

By machines?

conductivity
pH
nuclear magnetic resonance

I'd argue that location would be subsequent to EM radiation (ie., GPS) and seismic activity as a function of motion.
posted by porpoise at 4:29 PM on July 17, 2007

Air composition
- smell, gas levels, humidity etc.

Material composition
- texture, "taste" etc.

Dynamics and Motion
- Velocity, temperature, pressure, seismic activity, rotational orientation, sound, spin

Electromagnetic Radiation
- light, x-rays, infrared, ultraviolet, radio

Forces
- electromagnetic charges, gravitational strength

Time
posted by vacapinta at 4:29 PM on July 17, 2007

Really, lots of these items can be generalized into just a few categories. For example, sensing taste, sensing smell, and sensing CO2 levels are really the same thing - recognizing the presence of certain types of molecules.
posted by chrisamiller at 4:41 PM on July 17, 2007

Chrisamiller: You're right. I was looking for more things like "wireless networks" and I wanted to provide a list of about ten items, so that people could get an idea of where I was going.

The suggestions are good so far. Thanks!
posted by viewofdelft at 4:49 PM on July 17, 2007

Everything we see. (Seconding Faint of Butt) Because of the speed of light, many people don't understand that sight is passive, just like hearing. When we observe the world, we see echoes of the light source (e.g., the sun) with some of the wavelengths removed. The grass is really red; it doesn't want the green, so reflects those wavelengths over to our eyes.
posted by weapons-grade pandemonium at 4:52 PM on July 17, 2007

I could sense the drop in temperature today when I told a girl I like that her hair looked nice up rather than down. Even though she had very obviously hastily put it up because of the heat, and generally wears it down.

WTF wfrgms?
posted by wfrgms at 5:10 PM on July 17, 2007

Right. Echoing Chrisamiller, light and color are included under "electromagnetic radiation."

Can you think of anything else I might be able to sense passively with a hypothetical machine were I to take it around the typical American city?
posted by viewofdelft at 5:14 PM on July 17, 2007

"Can you think of anything else I might be able to sense passively with a hypothetical machine were I to take it around the typical American city?"

The efficiency and speed-of-response of the local counter-terrorism squad. Make sure your hypothetical machine doesn't have much in the way of blinkenlights or numeric readouts... you can be singled out for taking pictures nowadays, who knows what would happen to you if someone saw you checking out your hypothetical device in a public space.
posted by foobario at 5:34 PM on July 17, 2007 [1 favorite has favorites]

Hm. I don't think that a portable device could detect seismic activity as distinct from other motion.

You could add a Geiger counter
posted by winston at 7:26 PM on July 17, 2007

Nuclear magnetic resonance is the phenomenon--doesn't NMR spectroscopy also simply listen to a slice of the electromagnetic spectrum (close to the one your FM radio is sensitive to, in fact)?
posted by pullayup at 7:33 PM on July 17, 2007

pullayap - you're absolutely correct.

viewofdelft - I wanted to provide a list of about ten items, so that people could get an idea of where I was going.

So what direction are you going?

- magnetism/electrons moving in a coherent manner (as vacapinta mentions)

I guess the question comes down to what kinds of detectors have been invented thus far.
posted by porpoise at 8:01 PM on July 17, 2007

Windows boxes can sense fear.
posted by flabdablet at 12:00 AM on July 18, 2007 [1 favorite has favorites]

Direction (the humble compass deserves a mention)
Cancel the Earth's magnetic field and you get a magnetometer, which detects the sun's interactions with the atmosphere, or local geology.
Nearby (but not necessarily visible) lightning (ie an electrometer)

Yeah, many geiger counters (and scintillators) can detect alpha and beta radiation, neither of which are electromagnetic radiation, and when you throw in the electromagnetic stuff (x-ray and gamma) you can extrapolate some info about the local geography.

(With the right accessories, you can add more non-electromagnetic particles to that list too, like muons, neutrons, etc.)


I'd argue that location would be subsequent to EM radiation (ie., GPS)

No, both lat. and long. can be deduced from passive things - clocks, sundials, stargazing, etc.


nitpick: is a compass needle considered passive? On the one hand, it has it's own magnetic field, but on the other, it's field is not the limit of it's range - the earth's field is.
No matter, it sounds like it meets the criteria of interest to the question.
posted by -harlequin- at 4:04 AM on July 18, 2007

Nuclear magnetic resonance is the phenomenon--doesn't NMR spectroscopy also simply listen to a slice of the electromagnetic spectrum

NMR spectroscopy requires both a fairly strong magnetic field and a radio source.
posted by DevilsAdvocate at 10:09 AM on July 18, 2007