In this concept, a person is confined by a precisely-defined invisible barrier that causes extreme pain when any part of them crosses it. It affects only them, thus the source of the pain must be within them, but triggered externally. I'm thinking something like nano-sized GPS receivers (either organic/chemical or mechanical) that have been put into their body and have dispersed to all of their nerve receptors. When any part of their body crosses the threshhold of the GPS coordinates, the receivers do something to excite the relevant nerve receptors to an excruciating degree. I'd be curious to hear plausible ideas about nano/biological/biochemical systems that could produce spatially-determined effects within the human body.

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    $\begingroup$ If you really want this to be precisely defined so that a single body part crossing the threshold will trigger pain, you'll need something more local than GPS. Even the most accurate GPS available doesn't come near to providing accuracy down to a few inches. $\endgroup$
    – barbecue
    Dec 7, 2023 at 23:25
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    $\begingroup$ If you cannot rely on GPS signal, perhaps you can use proprioception. Transport the prisoner to the center of the designated location, then prime and implant the nanobots/anthrobots, which spread all over and can tell how much the prisoner moved. Move more than the specified parameters and prisoner gets punished. $\endgroup$
    – KC Wong
    Dec 8, 2023 at 4:43
  • $\begingroup$ Steven Gould describes a mechanism for this in Reflex. Wikipedia provides details (that completely spoil the book). $\endgroup$ Dec 8, 2023 at 11:32
  • $\begingroup$ As a suggestion, why not simply two laser 'walls' after each other? If the laser is interrupted, the receiver will not see the laser and instruct the pain. With two laser 'walls' after each other you can also pretty accurately see if they returned or exited, to either keep the pain or turn it off again. $\endgroup$
    – Trioxidane
    Dec 8, 2023 at 15:50
  • $\begingroup$ "precisely-defined invisible barrier" - how precise does it need to be? Does it surround them, to keep them contained? Or is it meant to keep them out of an area? How large is that area? $\endgroup$
    – Bergi
    Dec 9, 2023 at 14:11

7 Answers 7


Ferrite nanocrystals dispersed in one's body, and a microwave source can do what you are looking for.

When the subject gets outside of the shadow area for the microwave source, the ferrite will start absorbing the microwave and convert them to heat, with related discomfort/pain.

  • $\begingroup$ Could you hold them in an area via magnetic field? $\endgroup$
    – DKNguyen
    Dec 7, 2023 at 15:26
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    $\begingroup$ Ambiguous syntax, @DKNguyen - do you mean the crystals, or the person so afflicted? The former, maybe. The latter, unlikely, though you could probably make them suffer and die. $\endgroup$
    – jdunlop
    Dec 7, 2023 at 19:33
  • $\begingroup$ Could you hold the ferrite nanocrystals airborne in an defined area via a magnetic field? $\endgroup$
    – DKNguyen
    Dec 7, 2023 at 19:34
  • $\begingroup$ A low tech version of the same idea is a collar made of a conductive material shaped in a way to maximize microwave reception. More than pain it will cause burns but the effect might be equally deterrent. $\endgroup$
    – Pere
    Dec 7, 2023 at 20:19
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    $\begingroup$ This might work without ferrite, just use microwaves of frequencies that heat water, but control the barrier so it won't hit the person enclosed with rebounds off its surroundings. $\endgroup$
    – Vesper
    Dec 8, 2023 at 7:11

You can buy these things from Amazon. They are usually meant for pets. I find this to be a kind of animal cruelty, but if you apply it to a human, it might be something more humane than prison.

With pets, you use a set of transmitters to make an invisible fence. When the critter crosses them, their collar will give them a little jolt. With humans, all you need is to adapt the collar.

If you want something really fancy and you like DYI, you can instead take an electronic tag - the kind worn by people under house arrest - and combine it with a tazer. If you want to be cruel, you can make an implant, kinda like the internal part of a pacemaker.

Heck, I'm even gonna prototype one now and make a Kickstarter to sell it.

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    $\begingroup$ Many hospitals have something similar for babies. Except the baby only has the tag anklet, not the shock collar, of course. If a baby is kidnapped from the hospital, the hospital security and police are immediately warned. Of course the "invisible wall" is only defined by the range of the transmitters. I think the important point in your answer is that there are two distinct parts to the technology: the electronic tag to detect that the individual is on the wrong side of the "wall", and the punishment device. The OP is free to adapt both parts separately to best fit with their story. $\endgroup$
    – Stef
    Dec 8, 2023 at 19:47

Camera and Laser

The invisible wall is defined via software. There is a camera with depth perception. Any part of the prisoner's body that touches the wall (as seen by the camera) is shot by the laser, which will hurt. The laser can even be outside the visible light spectrum. The software will not aim the laser at non-prisoners.


I don't think nanotech is necessarily required when you could just put a chip in the pain center of the brain. From there, you could remotely activate it to cause the sensation of pain, modulate exactly how painful it is, and prevent it being solved by anaesthetics or narcotics. And since you already have a chip inside the brain, you could also add the failsafe option of inducing a seizure or unconsciousness remotely.

As far as the granularity of the zone is concerned: if you just want it to be "when crossing X point" then you could have the equivalent of an RFID chip or some sort of GPS signal and give it a bounding zone. I prefer the latter because then you could also set up a failsafe where if the GPS signal isn't detected for X period of time it starts inflicting pain, which would prevent blocking the signal with a tinfoil hat or what not.
If you want more granularity, a few well-placed sensors will allow an excellent amount of coverage. Placing relay sensors in the T1/L4 vertebrae, elbows, hands, knees, and feet would be very effective in preventing any part of the body being extended past the zone without the need for more comprehensive solutions. At that point, what could possibly get out of the zone? At most, some fingers or toes, and at that point just increase the size of the bounding box by six inches (and honestly another two feet just for good measure) and you're set.

This system could actually be relatively cost-effective, all things considered. The brain chip is obviously the main expense, but that's just a GPS/RFID chip and a simple way to stimulate the brain. Have a surgeon or robot install it and you're away. Then 11 relay chips that are just RFID/connect to the brain chip somehow, and RFID is cheap enough even with today's technology. Have a nurse or another robot install those (which are even easier to install), and that's that. Plus, this allows for easy removal of the system if the prisoner is released or if the plot demands it.


If you can put implants in the target's body, and those implants either

  • have long-lived batteries (weeks-months with current tech, dependent on the size of the battery pack)
  • draw energy parasitically from the target's system (moderate-future tech, maybe 20-50 years?) and the implants are large enough to have a GPS transceiver (think cell phone, though phones are dominated by the size & weight of the screen + battery),

then you can do pretty much anything you want to the target. <10 mA across the appropriate nerves could probably cause excruciating pain and/or let you manipulate the target like a puppet... depending on how good your surgical procedures are. Pain & spasms are going to be easier to do than fine motor control. No hardware required other than the implants and the existing GPS network.

If you can't put an implant in your target, you could ward specific areas with directed energy weapons and put facial recognition or other sensors in the area whose access you want to control. Turn on the system only when the target approaches. Of course, those systems may be vulnerable to both false-positives and false-negatives, and creative targets might try to cross at the same time as someone you don't want to fry.

More exotic, target-specific wards / pain-walls might become available further in the future by having swarms of nanites that look for genetic markers and restrict themselves to particular areas. They'd effectively be infecting your target if small enough to enter the body unnoticed, or infesting / attacking the target if a bit larger. But they could presumably induce all the pain & control options for implants above.

  • $\begingroup$ Great ideas. Thanks. $\endgroup$
    – Gene Terry
    Dec 8, 2023 at 13:30

We can kind of do this today with millimeter waves. This operates on a principle similar to a microwave oven, set to use a wavelength that completely and fully activates the pain receptors of any skin they hit, forcing a complete flight response. You do NOT want to be there.

For the first millisecond, it just felt like the skin was warming up. Then it got warmer and warmer and you felt like it was on fire. ... As soon as you're away from that beam your skin returns to normal and there is no pain.

This has four major drawbacks based on the question, but I list it here because I feel like it's the most "sci-fi" solution, and depending on the exact details of the story it might still work:

  1. They don't discriminate against the prisoner. You may be able to work around this if you can allow visitors and jailers to wear special clothes, or have other protections, especially if its something like fibers woven into normal-looking items so the prisoner might not perceive they're doing anything special. Combine this with things like disabling parts of an emitter array as people approach.
  2. You can't put them everywhere. The barrier is only going to be so thick, and once through it the prisoner is completely free. You'd need to fill a fairly thick area to be effective.
  3. They won't protect against something like tunneling. Even a piece of cardboard is enough to totally block the effect based on current capability. For a science fiction story, you might get by saying the captors use a different band or type of radiation that can penetrate solid materials to a much greater depth.
  4. It can actually cause burning if the exposure lasts too long.
  • $\begingroup$ This was my first thought, but it's indiscriminate, so it doesn't fill the requirement of the field affecting only him. $\endgroup$
    – Aos Sidhe
    Dec 8, 2023 at 15:03
  • $\begingroup$ @AosSidhe Correct, though I do address that weakness. $\endgroup$ Dec 8, 2023 at 15:06

Against my better judgment, I'm going to give you a tech that isn't here yet.

Nanobots, not even smart ones.

They spread to predetermined places where nerve traverses, sit dormant absorbing your body's energy and filling some kind with small capacitor, then when they detect your coordinates are outside boundaries, they release small pulses that would cause pain, discomfort and uncontrollable spasms, maybe even death, if you feel like it*.

In reality, this would require a ammount of energy a single nanobot probably can't generate/contain


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