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In Weaponizing the return-energy channel of a one-way portal, I describe the function of a hermes portal, which 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.

Now, following the suggestion given by BMF in the accepted answer, my characters have set up twenty capitol Fulminium laser-blasters pointed into the portal, ten each side, mounted so that they will jitter in random patterns across the mouth of the circular portal.

A Fulminium laser-blaster of this type emits ten (10) 8.3 MJ one-millisecond (8.3 GW) pulses of orange and UV laser light per second, each pulse carrying an energy equivalent to nearly two kilograms of TNT. In addition, these weapons emit copious amounts of electrons, which are typically discharged down the ionised channel of air that the laser beam creates in the air between the weapon and the target. The Fulminium rods in the lasers are the weapons' atomic power sources, and contain enough energy to fire continuously for years.

Since the hermes portal which is the target of these lasers is receiving an incoming connection, the light can pass through the portal but the electrons from the blasters cannot, but instead remain on the surface of the portal field, accumulating a charge of up to a billion volts. However, the effects of the charge can pass through the portal's return energy channel, repelling negatively charged objects, and attracting positively charged objects. Any positively charged objects, such as ionised dust or gas, can pass from the source side to the destination side, capturing some of the electrons on the destination side of the portal field, resulting in free electrons on the source side.

On the source side of the portal is a large walled city, much like Carcassonne in that it is entirely land-bound, and sits on a large hill-top, with the palace about 500 meters from the outer wall, with suburbs outside the outer wall:

Carcassonne

However, the city is much larger, on the order of the size of Byzantine Constantinople, roughly six kilometers across:

Constantinople

The following diagram shows the position of the portal and the ranges to the relevant structures. The portal is placed 500m from the city wall, on level ground in a square in the city's outer suburbs, one side of the portal facing directly toward the palace. The diagram shows a scale representation of the hill upon which it is built. The hill is composed of earth and granite.

City Diagram

The portal is about 3m in diameter. The lasers are placed from 0.1m to 1.5m from the bottom of the portal. The lowest lasers are set to sweep about 1­° below the horizontal for a small portion of their movement, but all the lasers are set to sweep up to a maximum of 6.5° above the horizontal. The lasers will sweep an arc of up to 179.75° to either side of the centre of the portal (so that they will miss the edges of the portal device, which is quite thin) on both sides of the portal, ten lasers on each side, each pivoting randomly and independently within these limits.

The instant the portal is opened, the lasers begin firing, and continue to fire continuously for the ten minutes that the portal remains open, sweeping the area around the source portal as described above. The lasers are suitably heat-sinked, and won't burn out or reduce their rate of fire due to their own waste heat.

In the vicinity of the portal, waiting to pass through, are 5000 infantry soldiers wearing late medieval plate armour, and six solid steel automata ranging in volume from 100 to 500 liters, which may simplistically be considered to be ovoids twice as high as they are wide.

The buildings in the vicinity of the portal are late medieval timber, stone, lath and plaster structures with slate roofs. The buildings inside the walls of the city are dressed stone or brick, also with slate roofs, while the palace is heavy fortified stone with copper roofs, like the keep of a late medieval castle.

The city has a population of several hundred thousand people.

My question is: What will be left of the army, this city and its population after the laser-blasters have fired for ten minutes?

Don't forget that the laser-blasters charge the portal up to a billion volts as well as shooting 8.3 MJ / 2kg TNT equivalent laser pulses 10 times per second for each of 20 lasers, for a total of 200 pulses per second emitted in a fan from both sides of the portal.

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    $\begingroup$ That weapon doesn't work in an atmosphere? At those energy levels, lasers will make the air opaque. You'll get flash-bangs of plasma near the barrel with a yield of a few kg of TNT per second? From what I can tell with napkin math, it isn't energetic enough to clear its own path, but it is energetic enough to turn air into opaque plasma. (This matters, because I was trying to work out what it would do when it crossed the portal, and I got, well, it couldn't go anywhere) $\endgroup$
    – Yakk
    Feb 28, 2023 at 17:58
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    $\begingroup$ Your lasers will go offline much earlier than in ten minutes. Once any dust or debris passes through the portal, you'll start hitting them on your own side. I hope you have some high-speed cameras to monitor this and turn the lasers off immediately. Otherwise you bring all that hell on your side of the portal. $\endgroup$ Feb 28, 2023 at 19:22
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    $\begingroup$ @chasly-supportsMonica The wire isn't actually being hit directly by this energy, and it isn't conducting the electrical charge... and it's magic. $\endgroup$
    – Monty Wild
    Mar 1, 2023 at 6:27
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    $\begingroup$ This isn't a good question. I shouldn't need to read 13 paragraphs and look at pictures and diagrams to understand what you're asking. $\endgroup$
    – Tony Ennis
    Mar 1, 2023 at 19:37
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    $\begingroup$ @TonyEnnis The recent trend is closing questions unless they are very specific, down to the finest details, so I think he's pretty well justified, even if much of the detail was not used in the answers. The thin line of what's acceptable is thin indeed these days! $\endgroup$
    – JamieB
    Mar 1, 2023 at 22:50

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With the amount of energy we are talking about - everything in the immediate path of the laser is pretty much vaporized.

The humans don't do anything particularly interesting - and they are the lucky ones - but the walls of the Palace? This is where it gets fun.

The stone doesn't just vaporize - the tiny bits of water inside the stone are super-heated and turn to steam instantly - the stone walls explode. The bits of stone that were near the laser beam (but not close enough to explode) are melted into a magma state - which subsequently gets exploded everywhere.

This super-hot liquid rock that has been blown everywhere is now starting fires for everything that's combustible.

So we have raging fires.

But wait! There's More! All that charged energy on the surface of the portal? Well, that disperses into the atmosphere causing a current of charged air to start moving.

Really moving - causing a localized wind that fans the flames in an unnatural way.

The heat generated from the firestorm is now causing flash ignition throughout the city and anything that doesn't ignite melts - Those copper roofs? They are now dripping death drops of molten copper.

Let's also remember that all those electrons that caused our winds have a charge - so something approximating sheet lightning is ripping through the streets in the carnage.

This emanates out from the epicenter at a frightening rate - driven by the combination of exploding molten wall rock splattering everywhere and fire winds ripping through the city.

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    $\begingroup$ This is... just a bit worse than I anticipated. I suppose it'll give the guy who keeps sending troops through portals reason to worry about something else for a while... and maybe not use a portal again. $\endgroup$
    – Monty Wild
    Feb 28, 2023 at 3:02
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    $\begingroup$ Admitedly, when I wrote that I did think you put 2 tonnes of TNT for each pulse, I see now it's 2 KG - however, 2 KG of TNT 10 times a second is still a lot of energy... However re-reading it - 20 KG of TNT equivalent of energy being dumped into a stone wall is still going to cause havoc. Maybe not quite as severe as that - but you're the writer - you can tone it up or down as you see fit :) $\endgroup$ Feb 28, 2023 at 4:01
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    $\begingroup$ I figured that it'll chew through the wall in a minute or so, and start slagging it down, blast hot fragments everywhere... you'd still get the firestorm and lightning. $\endgroup$
    – Monty Wild
    Feb 28, 2023 at 4:30
  • $\begingroup$ sasuga maou sama, jokes aside, the extent of damage that you described was on par to that planet destroyer hype during Quora's "golden" ish age $\endgroup$ Mar 1, 2023 at 2:00
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So, you tag it as "Medieval" and then you go and invent lasers? You could cause some pretty serious damage to a city without anachronistic lasers if your portal device exists. Place your entry portal at the bottom of the sea, and your exit directly above the royal palace or whatever. 10 solid minutes of sea-floor pressure water suddenly exploding over the center of a city will be plenty destructive enough to wipe out most medieval period construction. You also get a secondary hit after the portal closes, as the surrounding water will rush in to fill the resulting void and could cause tsunami level waves to all nearby shorelines. So much for your enemy's naval power.

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    $\begingroup$ The heroes of the story are only in control of the exit portal, not its entry. $\endgroup$ Feb 28, 2023 at 18:47
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    $\begingroup$ Like the stargate from the TV series, water won't go through... it's magic, not strictly scientific. Basically, anything already there when the portal forms is ignored. $\endgroup$
    – Monty Wild
    Mar 1, 2023 at 6:44
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    $\begingroup$ [Medieval] was intended as a description of the fortifications and troops at the business end of the lasers. $\endgroup$
    – Monty Wild
    Mar 1, 2023 at 6:45
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TheDemonLord gives a good explanation of the type of damage that your setup could potentially do, but you'd only see that sort of destruction if the enemy was completely stupid (which is very unlikely if they're using the portal for military purposes). Here's a more realistic view of what would happen.

The enemy opens the portal to your world. They see a brief glimpse of some machinery they don't recognize, and then something terrible starts erupting to the portal. The abort signal is given, and the portal is shut down. Total portal open time would be on the order of 5-10 seconds. There will be significant damage to anything immediately in front of the portal, but it's not open anywhere near long enough to do the level of destruction described above.

This plan is not entirely without risk to yourself. When the portal abruptly closes, there will be laser blasts that are in progress that will go flying past where the portal used to be and slam into whatever is behind it. If you're really unlucky, someone on the attacker's side might notice that one of your death beams reflected off of a mirrored surface and figured out that they were made of light. After a bit of planning, the attackers re-open the portal with a large, thick mirror immediately in front of it. They could wait for the reflections to destroy your defenses, or push forward using the mirror as a shield.

The portal is placed 500m from the city wall, on level ground in a square in the city's outer suburbs, one side of the portal facing directly toward the palace.

What idiot would put a portal there? Even if you're not expecting nasty energy weapons to come back at you, simply opening the portal is enough to show the other side who opened it, giving them a nice free view at your palace and its defenses. Not to mention, a spy stealing that portal's activation code means that an invading army can simply dial into your portal, launch trebuchet volleys through it, and demolish your palace from afar. A portal is a massive liability that could possibly destroy your kingdom, and it's unreasonable to think that "nobody has figured out the activation code yet" (security by obscurity) would be considered adequate security.

No, any sane person would keep the portal far away from anything they remotely cared about. The view through the portal would reveal as little information about the other side as physically possible, perhaps overlooking a large blank expanse of desert or steppe, preferrably with a natural barrier (like a giant cliff) behind it. You'll have lots of wide open space off to the sides in which to stage any invading forces. You'd open the portal, peek through it from the side using a periscope or expendable crewman, and only send anything through after the "all clear" is sounded. If you see that you're in over your head, the only thing the enemy might see is a brief glimpse of a pair of scared-looking eyes before the portal disconnects.

the light can pass through the portal but the electrons from the blasters cannot, but instead remain on the surface of the portal field, accumulating a charge of up to a billion volts.

1 gigavolt is enough to arc across an air gap of over 1,100 feet. You won't be able to isolate your portal far enough from a source of ground to accumulate anywhere near that much static charge. Also, remember that voltage is a relative measurement. The voltage you measure on the surface of your portal is relative to your local ground potential. The ground potential on the other side of the portal is not necessarily the same as your local ground potential, so the surface charge they see on their end could be completely different. If this portal was built to any remotely sane building standards then its chassis is connected to an earth ground for safety reasons, which means you likely won't be able to build up much surface charge at all. A non-grounded portal would be terribly unsafe even in normal use cases due to the lack of a common reference voltage described above (a harmless 2V charge on your side could be a lethal 2TV on the other).

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  • $\begingroup$ The enemy isn't expecting anything like this. They're still operating on a renaissance-era mindset at best, and the tactics they're using have worked before. Also, it isn't possible to shut down the portal if the person operating it is dead or incapacitated. No, it isn't smart, but people aren't always smart. I very much doubt that they'd try anything this stupid ever again... once they recover from their mistake. $\endgroup$
    – Monty Wild
    Mar 1, 2023 at 6:39
  • $\begingroup$ As for activation codes, they can be set to something like "You have to be an authorized person present when the portal device was made", or even more specific and complex things. It'd be easier to create your own portal and infiltrate it than to crack someone else's. $\endgroup$
    – Monty Wild
    Mar 1, 2023 at 6:51
  • $\begingroup$ The enemy still might shove a shield through the portal as a means of precaution, and should your lasers vaporize that shield on their side... ha ha. $\endgroup$
    – Vesper
    Mar 2, 2023 at 4:28
  • $\begingroup$ @bta if the portal is unable to be moved, I'd encase it withing a house, with a door on the blind side and no windows facing the portal, so whoever opens the portal from that side will only see a wall with some space before it, maybe backlit or whatever, period. $\endgroup$
    – Vesper
    Mar 2, 2023 at 4:36
  • $\begingroup$ The portals are portable when inactive but become fixed when active. They can be suppressed by burying/encasing them as in the Stargate method - basically by putting them in a strong box. $\endgroup$
    – Monty Wild
    Mar 2, 2023 at 8:14
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The portal room would blow up, but not serious damage in the city

The problem is that your charge buildup on the destination side would cause attraction of ionized particles from the source side inward, thus causing wind directed towards the portal. That wind would carry inward whatever dust and molten rock that arises from your lasers when they blast open the closest wall, and clog the portal from destination side, blocking the energy flow from destination to source. As an additional problem, the charge buildup would also cause electrostatic discharge on the destination side, potentially breaking your lasers' electronic parts causing them to stop.

The closest location on the source side would suffer of course, 8.3 MJ pulses would blow up holes in whatever they meet, vaporize dust particles, incinerate humans, yet since they are not striking the single direction, they will only break up the innermost area and not travel further, the excess heat on the outside would turn air opaque effectively raising local temperature, and also some of that heat energy, together with expanding air, would travel to the destination side. The outside area would suffer a big blast of molten rock, but then matter buildup on the destination side of the portal would start obscuring the incoming energy flow. Note that plasma is positive ions mostly (mass wise), thus it would get pulled into the portal by your electrostatic field at the destination side.

As a side note, since incoming particle shower (aka wind) would not be able to propagate backwards, the temperature on the destination side would rapidly rise, proportional to the amount of energy not delivered through the portal. Even if the lasers won't overheat, the room would, potentially breaking the destination side apart, together with whatever complex where it's installed. After that point, the connection would break, leaving source side city pretty much unharmed.

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