In my world, portals are magical devices that, when activated by some command, create a connection to a linked portal device. Portals allow objects to pass from the source (the portal device that was activated) to the destination device. However, objects cannot travel back from the destination device to the source device without the destination device being activated as a source. The activation command is made specifically for the portal when it is created and is often effectively impossible to derive from a portal (active or inert) without destroying it. It is possible for an object or person part-way through a portal to retreat, as long as it has not passed completely to the other side.

When a portal is activated, energy can travel in both directions through the portal, both electromagnetic radiation and sound energy, though the gases that carry the sound energy do not actually pass through the portal. This allows the person activating the portal to more easily determine if it is safe to proceed through the portal prior to actually doing so and allows a person on the source-side of a portal to communicate bidirectionally with a person on the destination side. This is the return-energy channel.

Physically, a portal device is often a physical doorway or tunnel, or perhaps a ring, maybe like a stargate, but it could be much more lightly constructed, possibly as little as a stiff, shaped wire. A portal functions from both sides simultaneously without collision of transmitted objects within the portal.

In my story, an inactive portal device left behind by an enemy has been discovered, but its activation command cannot be determined. Its discoverers wish to leave a trap for the enemy should the enemy ever attempt to use the portal again.

It would be easy enough to place the portal device in an unsafe location, but the enemy would likely be able to see that the location was unsafe and would decline to send anything through it, thus avoiding the trap.

I am currently thinking that some sort of energy weapon trap could be placed near the destination portal device so that when the enemy activates the portal, the energy weapon would be activated and its energy would pass through the portal in reverse using the return-energy channel, and cause harm to facilities and personnel in the vicinity of the source portal.

My question is: What would be the most appropriate weapon or harmful effect known to present-day science to use as the source of harm in this trap, considering the caveats below, in order of importance?

  1. The weapon/effect should cause as little harm or destruction as possible on the destination side of the portal (save to enemy personnel/material coming through the portal who get in the way).

  2. The weapon/effect should cause the greatest possible harm and destruction to enemy personnel and facilities on the source side of the portal.

  3. The weapon/effect should maximize the time that the portal remains open, either by incapacitating the source portal's operator quickly enough that it cannot be closed, or by being sufficiently subtle that it is not immediately noticeable that harm is occurring.

  4. The weapon should function for as long as possible and should be able to be activated and deactivated automatically when the portal opens or closes.

  5. Collateral effects passing through the portal from the source to the destination side should be minimized as far as practical.

  6. The weapon should cause as little damage as practical to either the source or destination portals so as to maximize the potential for damage to other personnel and facilities on the source side.

  7. The monetary cost of the weapon/effect, if a cost can be determined at all, should be minimized.


  • Answerers need not concern themselves with how portal activation is detected. Assume an appropriate detector exists and can activate and - if possible - deactivate the weapon/effect.

  • The total energy emitted from the weapon/effect should not exceed 1 terajoule.

  • Where the delivery of energy occurs over the period during which the portal is open, assume that the portal will remain open for ten minutes unless closed earlier by its operator.

  • A physical modern-day device capable of emitting a suitable harmful effect would be preferred, but a suitable harmful effect which cannot be generated with current technology may also be considered - the effect can be generated magically.

  • Assume that the enemy uses human soldiers in late medieval plate armor and small numbers of automata that are effectively robust solid steel objects that do not have electronics susceptible to EMP and range in volume from 100 to 500 liters. Construction of facilities on the source-side of the portal should be considered to be similar to that of late medieval fortifications.


As additional context, the destination portal device is in a large garden area, and cannot be moved from its location without potentially giving away that it has been discovered. Any weapon placed in the garden and aimed at the inactive portal must not harm the garden. Nukes are right out, as there isn't enough space for the massive amount of shielding required to protect the garden from the nuke. The weapon is going to need to be some sort of directed energy.

Additionally, please describe the effects of the proposed weapon on the personnel and structures near the source portal.

  • $\begingroup$ Is the portal surface optically transparent, hence your statement that "... but the enemy would likely be able to see that the location was unsafe", or more like Stargate's flavor of a metallic watery surface obfuscating the other side? $\endgroup$
    – BMF
    Commented Feb 24, 2023 at 19:25
  • $\begingroup$ @BMF The activated portal appears transparent from both sides. The transparency from destination to source is the return energy channel. $\endgroup$
    – Monty Wild
    Commented Feb 25, 2023 at 0:40

3 Answers 3



This is the more expensive but long-term option. When the portal activates, arrays of megawatt-scale, pulsed lasers aimed at both sides of the portal burn anything and everything. The array would likely need its own nuclear power plant for a continuous assault. Dynamic mirrors and optics can divert the beam to various points and angles at and through the portal, effectively choking the entrance with random blazing light. If the medieval army somehow fashions a mirror capable of bouncing back the beam, the rapid and random nature of the optics should prevent the backscattered beam from hitting the laser itself. If the entrance is door-sized, there's a good chance nothing gets through intact.

Why pulsed? The microsecond pulse trains take advantage of mechanical beam-target interactions which are 1-2 orders of magnitude more efficient at causing damage than thermal interactions from continuous laser fire, for the same input energy. The laser flicks on and off very rapidly, millions or billions of times a second, drilling small cavities filled with vaporized material which rapidly expands (aka explodes), creating shocks which weaken the material for the next fire pulse.

The lasers themselves don't necessarily have to be "near" the portal. They could potentially be miles away, with infrastructure to divert the beam to the portal.


To comply with the edit to the question asking for the effects of the weapon, heat from laser shots would scorch/set the immediate environment on fire. The point where the beam makes contact would be far too bright to look at and would blind anyone in proximity to the target (regardless of laser wavelength). If the target is on the other side of the doorway, the majority of the flash might be blocked from the home side by merit of small viewing angle, but if the shots are at targets already penetrating the doorway, most of the flash will be on the home side.


Go subtle, go with radioisotopes

The problem with any booby trap is it is likely to just kill the first poor guys to open it. You want to play a longer game. So, first thing you do is to get some landscapers in. You build what looks like a regular house basement around the portal, a concrete box around that, and then a nice grassy hill in the garden to hide it. You can even plant a tree on top.

You line the concrete box with lead

When the enemies open the portal, it'll seem like stuff has changed, but a basement isn't a massive threat. Someone built a house over the portal, no problem. They'll bring in people, someone will try the door, it won't open because it's basically just attached to the concrete. The commander might show up to oversee what they're dealing with.

Everything possible in that basement is packed with radioisotopes. Basements have paint cans, all that kind of stuff in them, and all of those should be packed to the brim with as much radioactive material as they can scrounge up. Put a bunch next to the portal. Scatter radioactive dust everywhere to make it look like a disused basement

They'll drag radioactive dust back into their place. Make sure you've got some nice gamma sources angled at the portal, too.

No one drops dead immediately, but once they do figure out what happened, there's a bunch of casualties. The enemy base needs a massive, massive cleanup operation.

It'd also be one of those situations where germ warfare is unlikely to come back and bite you, if you're really looking to breach the Geneva convention in as many ways as possible. Basically, there's a whole terrifying category of things that look like normal basement dust. Your goal is as much foot traffic into their base as possible, carrying as much dust with them as possible.

  • $\begingroup$ This doesn't answer the question... but it is an interesting idea. However, in this scenario, the portal would open, people and automata would come through, the portal would be closed, and the people on the destination side would look for a way out. Maybe they die of radiation sickness before they find one, but the automata are radiation-proof, and maybe they blast their way out... releasing the nasties where the nasties are not wanted. Maybe they open a return portal and report what they've found... and the people on the other side get suspicious and cut the expedition off. $\endgroup$
    – Monty Wild
    Commented Feb 25, 2023 at 15:12
  • $\begingroup$ The leaders of the enemy are suspicious and uncaring. They'd likely throw away the entire expeditionary force if things didn't appear to be as they expected, on the mere suspicion of a trap. $\endgroup$
    – Monty Wild
    Commented Feb 25, 2023 at 15:16

2 thoughts:

1: Booby Trapping the exits is still going to be effective

Consider a claymore mine, it's silent, can be disguised and is very effective against infantry. Someone advancing through a portal who can hear/see what is on the other side is going to struggle to detect a Claymore.

Any IED type threat on the exit is probably going to be your cheapest, most effective solution - but that requires people to come all the way through and targets them on the other side.

2: Neutron Bomb in a sealed lead Chamber

So in this scenario - the exit gate is placed in a large, sealed Lead chamber - with a Neutron Bomb hidden - once the portal is opened, it detonates - large amounts of lethal radiation floods the area - and as per your rules, radiation can travel back through the portal. Bonus points for the Neutron bomb being 'focussed' to direct most of it's radiation back towards the portal.

The poor saps on the other side upon opening it up get a literal face-melting amount of radiation blasted through to them, irradiating the area and killing lots of people.

The purpose of the sealed Lead Chamber is so you don't irradiate/kill your own people.

  • 3
    $\begingroup$ OP said energy (I'm guessing EM) can travel through the portal. Matter is one-way, energy is bidirectional. Neutrons don't fit the bill. $\endgroup$
    – BMF
    Commented Feb 24, 2023 at 20:28
  • $\begingroup$ @BMF it's called a neutron bomb, but the radiation released definitely includes EM radiation. $\endgroup$ Commented Feb 24, 2023 at 21:59
  • $\begingroup$ I'm afraid that a nuke fails criterion #1: minimal damage to the destination side. At the very least, the people setting the trap would be stuck with a large, heavily irradiated lead box. $\endgroup$
    – Monty Wild
    Commented Feb 25, 2023 at 1:09
  • $\begingroup$ An explosive on the destination side fails criteria #1: minimal damage to the destination side, and #2: maximise damage to the source side, as pressure and shrapnel won't pass the return energy channel. $\endgroup$
    – Monty Wild
    Commented Feb 25, 2023 at 1:11

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