How invisible is he really?
November 23, 2008 4:43 PM   Subscribe

Would a invisible man set off a motion detector?

Would it depend on the type of invisible he is? So if the invisible man was visibly invisible, but not infrared invisble? Or he was visibly and infrared invisible but not something else?
posted by pokeedog to Grab Bag (11 answers total) 4 users marked this as a favorite
Well, obviously, it depends on the motion detector.

If it's looking for infrared, if our invisible man has body heat, he'd be visible to it.

If it's sonar, presumably the sound waves'll bounce off of him, and he'd be 'visible.'

Now if it's radar-based, sending out pings - depends on what wavelengths he's invisible on. After all, radio waves are just another type of electromagnetic radiation; if it's just visible light, he'll show up, but if he's transparent to all forms of EM, he wouldn't.
posted by Tomorrowful at 4:47 PM on November 23, 2008

Theoretically, there's ways to defeat all three common types of motion detection sensors (passive IR, active ultrasound and active microwave), but they're all too expensive / cumbersome to make into a wearable suit for everyday breaking & entering.
posted by randomstriker at 5:09 PM on November 23, 2008

Here's a piece about how different sorts of "people sensors" work. You can probably determine for yourself just how invisible you'd have to be to defeat each type.
posted by Salvor Hardin at 5:46 PM on November 23, 2008 [1 favorite]

You have to tell us what, exactly, makes him invisible. Does light go around him, or through him? Does sound pass around or through him? Is it just magic? Etc. Since there are a number of ways that one can imagine a man becoming invisible, there are a number of possible answers to your question.
posted by bricoleur at 5:52 PM on November 23, 2008

It would depend on your definition of "invisible" and the particular type of motion detector in use.

If you were only 'invisible' or camouflaged within the visible spectrum, as would be the case for someone wearing some sort of hypothetical perfect-active-camo suit, you would still set off IR, radar, and sonar sensors. You might be able to make it past systems that use visible-light video cameras and then analyze the output (sometimes used in conjunction with networked video cameras), but that would be about it — and some cameras have a range that extends into the near-IR as well.

Active camo would also not protect you against break-the-beam systems (whether using visible light or some other part of the spectrum), unless the suit was capable not only of altering its reflectivity, but also of emitting light (so that the moment you broke the beam with your leg, the side opposite where the beam was hitting you would actually produce the beam, to fool the receiver). I've never heard of such a thing, even from the wildest DARPA fantasies, so I think it's pretty far out there. A true "invisible man" who was actually transparent to electromagnetic radiation would be safe, though.
posted by Kadin2048 at 8:18 PM on November 23, 2008

These Mythbusters episodes are somewhat related to your question:
Episode 54
Episode 59
posted by lukemeister at 8:38 PM on November 23, 2008 [1 favorite]

Response by poster: Tomorrowful, I like where you're going. randomstriker/kadin2048, I don't think it's a suit, more along the lines of a super I don't know what made him invisible bricoleur. I'll check out the links salbor hardin & lukemesiter.

I guess I'm seeking the theoretical without being to literal. No suits, he's au naturale.
posted by pokeedog at 8:59 PM on November 23, 2008

I guess I'm seeking the theoretical without being to literal.

And this is the problem. Because there's a bunch of different ways, all theoretical, that someone can be invisible, up to being in a different parallel dimension or using telepathic powers to erase yourself from people's perception. All have different answers to the question.

But to be a little more practical: The simplest form of invisibility would mean that anything external (light, sound, etc) would be passed through/around without change. (Never mind that he'd be blind and deaf.) So optical/laser motion sensors would not work.

However most fictional concepts of invisibility don't take into account internally generated things. So anything that detected internally generated body heat would detect them. As would pressure sensors. (And smell sensors, etc.) Probably wouldn't show up on an MRI either.
posted by Ookseer at 9:35 PM on November 23, 2008

I'll answer in the context of the short-lived Sci-Fi channel series "The Invisible Man" starring Vincent Ventresca.

In this case, his "power" bends light somehow but it only works with visible light. All three types of detector (IR, ultrasound, radar) would detect him just fine.

Your invisible man's performance may vary.
posted by mmoncur at 9:37 PM on November 23, 2008

The motion detectors in Alien(s) would detect him, since they don't detect him. They detect changes in the air he moves through.
posted by ROU_Xenophobe at 10:30 PM on November 23, 2008

If it's a camera that looks at pixels to see if there's motion, then yeah as long as he's invisible to the camera, he's invisible to the little engine that sees motion.

Infrared, the same - as long as he's invisible on the infrared spectrum then he's invisible to the camera & therefore whatever detections motion.

Whether or not he's invisible to ultrasonic or microwave transmission, depends on whether or not he's invisible to ultrasonic or microwave transmission (duh). One could theoretically make a suit of microfiber that would absorb sound on specific frequencies, and the sensor wouldn't know you're there... but it also wouldn't know if the wall behind you was there either. E.g. a "black hole" of nothingness would be the same as a "white spot" of somethingness if the sensor is just looking for changes.

Or to put it another way, it may not detect you moving, but it would detect the wall behind you "moving" as it reflected sound waves back at the sensor that it wasn't the moment you were standing in front of it.
posted by Muffy at 1:33 PM on November 24, 2008

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