In the first scene of Indiana Jones and the Raiders of the Lost Ark, we see a booby trap where a series of spikes are thrusted out of a wall, and are triggered by "walking into the light". I think in the story of the film, this was meant to be something related to superstition (or the beliefs of the tribe) that triggered it. The Indiana Jones Wikipedia page says that this was a temple built by a Chachapoya tribe(s).

With ancient technology of any civilisation, would it be possile to develop a light trigger mechanism, where the sudden absence of light could cause something to happen? I guess it is possible over a long period of time with photosynthesis, but I'm looking for something near instant.


  • The system must not contain any living animals to function. Any types of plants/fungi, if can work, are alright
  • I think for the moment, it may be best to not worry about day/night cycles (just assume that when the sun goes down, depending on how the system works, it will be triggered)
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    $\begingroup$ There's a pretty big problem with this kind of trigger - how would it not trigger at night? If it triggers each night, then you put a serious limitation on how long it can last without maintenance (it would be tricky to make it work for a month, let alone centuries). $\endgroup$ – Rob Watts Apr 9 '18 at 21:59
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    $\begingroup$ A pressure plate you step on inadvertly when walking somewhere you block the light? $\endgroup$ – Alberto Yagos Apr 10 '18 at 5:46
  • $\begingroup$ @AlbertoYagos if you haven't seen Raiders of the Lost Ark, what happens is that when Indiana Jones moves his hand in front of a beam of light (like a God ray), the spikes suddenly come out of the wall. So it had nothing to do with him touching anything. $\endgroup$ – PlanetAlexanderProjects Apr 10 '18 at 7:54
  • $\begingroup$ @RobWatts - even if one (somehow) accounts for dusk, playing of OP's suggestion that the suddenness of the shadow is relevant, I don't see why other sudden shadows (from storms, clouds, falling branches, etc) wouldn't trigger it as well. Tricky. $\endgroup$ – Megha Apr 11 '18 at 7:07
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    $\begingroup$ Indy faked that scene, it was a misdirect. He put his hand in front of the light while sneakily pressing down on a pressure plate with his foot, so that his untrustworthy companion, Satipo, would get himself skewered by trusting that light (not pressure plates) was the trigger mechanism. TL/DR: This was a bogus trap even within the Indiana Jones universe, which contains all kinds of supernatural phenomena, so don't pull your hair out trying to figure out how an ancient tribe could design a trap like that, it was a simple misdirect on Indy's part. $\endgroup$ – Ghoti and Chips Apr 11 '18 at 9:15

The light enters a lens which heats mercury in a glass thermometer changing its weight distribution causing it to turn on an axis opening a valve in a water-wheel powered pneumatic system which retracts the spring loaded spikes. When the beam of light is interrupted the mercury cools, the thermometer returns to its rest position closing the value and the pneumatic system suddenly releases the spring loaded spikes.

Alternatively: https://www.youtube.com/watch?v=6Qq7754ZZlE

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    $\begingroup$ You need a second ray that arms the mechanism in the day and disarms it at night. $\endgroup$ – xpy Apr 10 '18 at 13:16
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    $\begingroup$ For the infrared from the sunlight to be conducted as heat through the glass, then the mercury, enough to meaningfully change the volume (weight distribution) could take a long time (matter of minutes). The elephant in the room, too, is the assumption that the room temperature is going to remain stable enough to facilitate a trap that relies on the tiniest of temperature changes thanks to sunlight. During the winter you will need an entirely differently calibrated apparatus than in the summer, to put it concretely. $\endgroup$ – Ghoti and Chips Apr 10 '18 at 13:46
  • $\begingroup$ Differential heat detection that works when part of the beam is obscured may resolve some issues. $\endgroup$ – KalleMP Apr 16 '18 at 9:30

A radiometer could offer sufficient sensitivity to light. Here is a demonstration of a modern device that seems to react in a few seconds.

To stop it reacting at night and to make it react to the absence (not presence) of light, one could arrange it as a balance with mirrors. Instead of the leafs being colored silver on one side and black on other, they would be black on both sides. Mirrors would be arranged so that sunlight directly hits one side and light to the other side goes through the trap area. As long as equal light hits both sides, the balance remains in place, so when the sun sets it wouldn't activate.

However this still requires a quite bit of technology, which is not impossible to imagine to have existed in e.g. ancient Egypt, but has not been demonstrated to have:

  • Ability to blow glass to an optically transparent bulb. Glass blowing existed, but results were not very transparent.
  • Ability to pull a slight vacuum to the bulb - this would probably be possible by just heating the air and melting the vent.
  • Mechanism to transmit movement from inside the bulb to the outside. Perhaps a weight could drop inside the bulb and break the glass.
  • Ability to make mirrors that last through the centuries - this might be the hardest part.
  • $\begingroup$ The torque provided by a radiometer is pretty small. $\endgroup$ – KalleMP Apr 16 '18 at 9:31

If you go with the difference of two light rays, it might work. You need one light-ray that gets evenly split into two. A big quartz will do that and even preserve the IR that you'll later need (no glass optics, they'll absorb the IR; quartz and mirrors all the way!). Then you need to funnel both rays towards a target, on different paths. They get focussed onto bimetal strips, that bend equally and thereby balance some mechanism. Nightfall or cloudcover will not trigger the trap as the bimetal strips get evenly relaxed, but both events will deactivate it, so you'll need some mechanism that only allows or encourages access on cloudless days (Map only readable in sunlight (UV-active inks?).

When one ray gets interrupted, the corresponding bimetal strip will relax, unbalancing the mechanism, and triggering something. Depending on the smallness of the bimetall relative to the heating power of the rays, the trap is more or less fast. Depending on the intricacies that your ancient trapsmiths were capable of, the mechanism may even be able to re-balance in the face of very slow unbalancing (for instance because one ray-path or mirror slowly builds up dust over the years) - look into the way car-breaks self-calibrate to adjust for different wear on the discs.

For the unbalancing mechanism, i thought about a sort of spoon filled with mercury or very fine sand - the spoon is balanced on a point halfway. The bimetal-strips act on the left and right side of the spoon handle end, respectively, so as long as they equally tense and relax, the spoon-cup just goes up and down a little. If they are unbalanced, the spoon will be "twisted", and the mercury can drop onto some real trigger.


Absence of light is the trap

You could design a trap where the sudden lack of light is the danger. For instance, imagine a path where you have to jump from one pillar to the next one. The pillars are big enough so that, if you can see where you step, there is no big risk of falling.

However, the only light source is coming from one of these pillars, probably the one in the middle. When you step on it, your weight closes the light conduct (and you can’t reopen), and it’s all dark. You don’t know exactly how far are the next pillars. You can’t go back either, for the same reason.

If you try and jump, you might get lucky for the next pillar, but not for the fifteen others after it. So eventually you are going to fall and die because it’s totally, 100%, dark now. You can’t even see your hand before it touches your face.

If you decide to stay on the pillar, you'll die when you eventually fall asleep/dehydrate/lose balance because of total darkness.

Sidenote 1

I know… light is not really the trigger of anything. But still, it's fine with your condition:

would it be possible to develop a light trigger mechanism, where the sudden absence of light could cause something to happen?

Yes: the trap is going to kill you because you shut the light.

Sidenote 2

A flashlight might save you. But maybe you did not pack one. Or the designer of the trap constructed a short underwater passage before the trap to make sure no fire torch (nor any electric non waterproof flashlight) made it to it.

  • $\begingroup$ An intriguing trap design, and I guess while it might fit with what I said in one sentence, unfortunately, it's still not a trigger. $\endgroup$ – PlanetAlexanderProjects Apr 16 '18 at 21:15

Well it's an illusion. The sunlight could be hiding all sorts of triggers: ropes hanging down, blowing wind (yeah this one's a stretch), other hidden objects, etc. Remember in the Last Crusade that the bridge was invisible until you shifted your perspective. Maybe the sunlight adds to the illusion?

  • $\begingroup$ Welcome to the site Mr Anonymouse, when you have a spare moment please take the tour and read-up in our help center about how we work. Your question has been flagged as low quality because of it's length and content. That being said, what you have conceptualized is potentially a very valid answer. What you would need to do is flesh it out a bit and explain how active booby-traps might be set-off as a result of optical illusions - not just the passive "there is no path across this chasm" illusion. You can edit to elaborate upon your idea. $\endgroup$ – Tantalus' touch. Nov 12 '19 at 17:45

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