So I have a character who uses various portals to store and deploy his weaponry and tools. These portals are 2 dimensional portals like those of the game Portal. I have had little trouble with this idea until a situation popped in my head. So my character has a weapon, like a pistol but the barrel is replaced with a square plate with a portal where a barrel of a big railgun would stick through and that railgun is anchored to the ground in some remote location, if someone or something were to push on the railgun barrel or whatever object was poking through (the portal can move to different equipment to poke through my character’s side), would it be perceived as pushing against the ground or would my character feel the force? The railgun which pokes through the portal is big and heavy and is attached to the floor itself with only the muzzle poking through the portal.

I would imagine that the weight of the handheld device would increase only by the portion of the barrel sticking through the portal. The railgun has little recoil since it is a rail gun but any recoil it does experience wouldn’t translate to the character right, since it is distributed into the floor from the base? If it doesn’t then does an external push on the muzzle translate to the character or does my character have an immovable object if the portal connects to an immovable concrete wall?

I don’t know if this is breaking the rules in terms of asking to many questions or if it isn’t Worldbuilding or if the only good answer is to make the rule up myself for whatever seems right but my main question is what should happen if someone pushes on the muzzle of the gun, if my character would feel any force, based on physics.

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    $\begingroup$ This looks like something that is entirely up to you. You could say all the force is transferred, some percentage of the force is transferred, or none of the force is transferred. Pick one that seems coolest to you, and make sure to stick with it to prevent inconsistencies. $\endgroup$
    – sphennings
    Commented Oct 25, 2021 at 15:09
  • $\begingroup$ Others may disagree, but I think your questions all relate to the same topic, which is basically the physics of 3D objects interacting around & across a 2D portal. You could easily eliminate the stigma of all those question marks by editing this into a single question, perhaps of character perception and by turning those sub-questions into points you'd like considered by the respondent. You might also want to write a clearer / more focused marquee question. This is a good worldbuilding question! $\endgroup$
    – elemtilas
    Commented Oct 25, 2021 at 15:27
  • $\begingroup$ Yeah I think it’s mostly my say, I was just curious if there was some major aspect that I was missing which would make the answer obvious. Is there any way to answer my own question? I feel like I should post my own answer in case anyone else has a similar problem and wants inspiration $\endgroup$ Commented Oct 25, 2021 at 15:27
  • $\begingroup$ You're certainly welcome to answer your own question, but really the point here to get help & perspective from others. Answering your own question kind of defeats the purpose. And anyway, I at least would much rather read your answer to the question by reading the story where this happens! $\endgroup$
    – elemtilas
    Commented Oct 25, 2021 at 15:29
  • $\begingroup$ Gotcha, I’ll try to write the answer in a very scenic way. Should I also try to rewrite the question to be more like a movie scene that leads up to the problem where I ask what happens? $\endgroup$ Commented Oct 25, 2021 at 15:33

6 Answers 6


The gun would be weightless and have no recoil

Portal break a lot of things in physics. Conservation of energy is the obvious one (I mean, just put 2 portals at different heights and fall forever. The energy comes from nowhere). However, one thing that doesn't break is that momentum is consistent across the portal*. Force is just a change in momentum, so forces pass through portals too.

The forces acting on your railgun barrel are gravity (downward) and, if you just fired a shot, recoil (backward along the barrel). However, the railgun barrel isn't in contact with you (it's only in contact with the rest of itself through the portal), so those forces aren't transmitted to you. So the recoil and the weight will act on the railgun barrel on your side, and on the railgun base, but not on you.

[D]oes an external push on the muzzle translate to the character or does my character have an immovable object if the portal connects to an immovable concrete wall?

This is an excellent extension of the question. You're correct that if the recoil isn't felt by the character, then they could use the portal as a sort of portable immovable wall. Let's go into some specifics. I'll assume that the portal on the square plate at the end of the gun is connected to another portal just 1 mm away from (and facing) a durable, static concrete wall.

If someone punches their hand through the plate, for example, they'll hit concrete (painfully) but your character holding the gun-plate will not be pushed backward at all.

In a less contrived case, your character could attach a battering ram to the wall through the portal plate and run toward something. Upon impact, they'll feel no force, and the ram will effortlessly plow through whatever they ran toward. Think of it as though they had loaded that concrete wall and battering ram onto a truck and sent it rolling forward at running speed. It's a terrifying amount of momentum.

  • Side note: unstoppable force vs. immovable object - If their target is actually solid enough to not be pushed aside or crushed, the wall and ram will be pushed back through the portal, but the character will not feel any resistance (in fact, they'll slam face-first into whatever they were running toward as the battering ram sinks/crumples into the portal plate).

All of this reasoning assumes that the object poking through the portal is not attached in any way to the gun-plate your character is holding. If struts attach the railgun barrel to the plate (for instance, to keep it level so that the recoil doesn't slice it against the portal edges), then some small of the recoil and weight will be transferred through that to your character.

*There are some subtleties related to how moving portals work that are tangentially related to your question, but my answer works regardless of how you choose to resolve them


The user would feel nothing

Let's not look at air pressure differences, moving and rotating planets, shockwave of the shell and all that stuff. Lets just look at the person, an artillery piece fixed to a tiny planet and two portals in a complete vacuum.

Now let's follow where energy flows. The barrel poking through the portal is connected to itself and the gun. It doesn't push against the portal or the person. If you push against the barrel, it would go along the barrel to the artillery piece into the ground. As no physical connection is made with the barrel and the portal, no energy is transferred to the portal. The portal is just as good as a hole in the wall, except infinitely thin and leads to somewhere unexpected.

Moving the muzzle against a wall is different. The problem here is inertia. In the first example, there is someone actively pushing on it. In the second, both the barrel and the wall aren't moving. If you move the hand towards the wall, two non-moving objects impart no 'active' energy towards each other.

That doesn't mean nothing happens. A big rock on a table doesn't 'do' anything, yet is full with potential. It's weight is still there, pushing on the table's surface. The table's surface pushes back with equal force, making it look like nothing is happening.

The crux of your problem is then when you move the portal against the wall, will it let you? In one way, the portal and man are completely unrelated to the barrel and wall. In another, they push two inert objects against each other. This is unlikely to be resolved by physics until it actually happens.

What I personally think is that you can compare it with is gravity. Being inert doesn't mean without energy. Pushing against the wall like that would prove much like gravity and inert objects. The objects will push against each other like gravity. The amount of 'gravity' is difficult to determine, but in some ways it's definitely infinite if you just press your hand against the wall. This is because you're in essence pushing a universe against another/itself. That means both the wall and barrel will interact with infinite energy, probably leveling the universe.

A thought experiment with portals often leads to one sure answer. It breaks the universe in more unexpected ways than you can imagine. Infinite energy that is often proposed by having portals is very detrimental in this case. I think I read once that if a portal would exist, it would require infinite energy to move. My guess is that this is why.


portal physics are wonky anyway. think air pressure, rotational speed of the planet, gravitational potential. so it will be up to you to decide the answer.

I would imagine that the weight of the handheld device would increase only by the portion of the barrel sticking through the portal. The railgun has little recoil since it is a rail gun but any recoil it does experience wouldn’t translate to the character right, since it is distributed into the floor from the base?

i would pick that the weight and the recoil both are transferred through the portal into the railgun. as that feels more consistent since this way the portal feels more like a normal hole. i may consider air pressure if it does not mess up the other weapons to much. as for the other complications, i would just ignore and forget about them for the sake of the story.


TL;DR answer

would it be perceived as pushing against the ground or would my character feel the force?


2 dimensional portals like those of the game Portal

= The guy holding the weapon wouldn't feel a thing. The guy or thing touching the actual weapon is just pushing it weapon through the portal.

If you own both the original Portal game and Garry's Mod (a "game" sold by Valve which is actually a sandbox for all its Source engine based games), you can play with these concepts to your heart's content.

If you wanted realistic physics instead of the game's physics, though, Portal violate science so much that you're bound to get an answer where the Earth ceases existing. Very probably from me.


I love this idea, but it makes portals incredibly powerful... maybe too powerful... And there might be some problems...

"Back home" I have an artillery piece mounted to the ground. It's big. Call it a rail gun, call it a tactical nuclear delivery system, call it what you want. It doesn't actually matter.

Me? I'm hauling around your standard ACME model X2147 (because market analysis has proven that having an "X" and the number "47" in any model number increases sales by 17%) portal. And when that sucker turns on, it's reference point is about six inches down the barrel of that artillery piece! Yeah, baby! You mess with me, you mess with my whole country!

Now, the reality of my portal is that it's just a window. I don't feel the weight of the artillery piece anymore than I would if I was holding a picture frame over the barrel. But there are some things I would feel, and some things I need to worry about!

Problem #1 How do I pull the trigger? OK, I have a radio on my head that sends a signal back to the artilleryman and that dude pulls the trigger.

Problem #2 What happens when the shell leaves the barrel? If I survive the moment at all, I get knocked flat on my backside, an infinitely thin 2D portal slicing through me like a hot knife through butter. Oh, my nemesis took that four-inch shell right to the chest! But I'm probably dead right along with him. The blast from the barrel is next to my chest.

Problem #3 What happens with the recoil? A whole lot of bad, I'll tell you! When any kind of gun (including a rail gun, just to a more limited extent) lobs something forward, there's a kick. Equal actions and reactions, right? That means the barrel just touched my infinitely thin 2D portal and was probably sliced off. Now, the reference portal generator could be mounted on the barrel, so it would move along with the barrel... but that means my side of the portal is trying to deal with the near-instantaneous change in relative positions — and the excess energy from that experience made the air around the portal turn to plasma!

Problem #4 Finally, I'm a bit concerned about the weather at the portal interface. I mean, it's sunny and bright here on my end — but the barometer's gone crazy and it's Hurricane Esmeralda on the other side! The wind being pushed back and forth through the portal isn't just turning my local weather (as in within 50 feet of me) into a raging cyclone of death, it's also freezing my Rastafarian Ney-Neys off!

Man! Why didn't the user manual explain all this? It would be easier to use the portal as a melee weapon, slicing through everything in its path (as an infinitely thin 2D object would do) and leaving debris back on that artillery piece. Serves them right for not reminding me I needed ear protection when they pulled the honking trigger!

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    $\begingroup$ As I understand it, the portal is small, say 10 cm x 10 cm, and attached to the end of a small gun. A huge railgun barrel sticks out of the portal and fires a shot. No portal is being fired, no 2D infinitely-thin anything is hitting anyone. And there certainly isn't enough airflow or pressure differential through the portal to cause hurricane-force winds. I'm not sure whether you misunderstood the question, the physics, or both, but I have to downvote, sorry. This isn't at all related to what would happen. $\endgroup$ Commented Oct 25, 2021 at 16:08
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    $\begingroup$ @NuclearWill depending on the force of the railgun, you'll certainly die. The bullet is forced at such high speeds that when it slams against the air a shockwave and terrible noise still rips through the air. Imagine the wind and noise of a truck as it drives past you. Now decrease weight by a factor or ten and increase the speed by a factor of a thousand. Figures aren't close at all, but it might give an idea what is happening. Something is so energetic that when it sails through the air it will definitely make a noise that will stick with you for the rest of your life if you're that close. $\endgroup$
    – Trioxidane
    Commented Oct 25, 2021 at 22:52
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    $\begingroup$ @elemtilas I might not have been clear. The point of #3 is that things that recoil move. They kick both back... and up.... If the portal on the handheld end is "stable" then the recoil can kick the weapon right through the top of the portal, cutting off the barrel. Oddly, as I re-read it, I didn't mention feeling the recoil at all (I think...). $\endgroup$
    – JBH
    Commented Oct 25, 2021 at 23:34
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    $\begingroup$ Wow, so many comments. I definitely should have mentioned that the portal is fixed to a solid surface however the inner barrier of the portal is interesting, as silly or cool as it would be, I don’t want my character using the portals as a super high tech guillotine $\endgroup$ Commented Oct 26, 2021 at 12:09
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    $\begingroup$ @JoinJBHonCodidact -- Definitely so! I guess I'm neither an 11 year old nor a scientician, because it didn't headache me at all! It's just like shoving a tessaracty peg into a spherical hole, only with fewer dimensions. As for an infinitely thin edge as in the X2147, I suspect there would be quite a lot of activity what with air molecules & photons & gravitons being sliced & hacked into wee strings, there'd probably be some kind of weirD Engergy force generated into a croissant coil all the way around which might actually either prevent one from touching the edge or else cause one's... $\endgroup$
    – elemtilas
    Commented Oct 26, 2021 at 13:56

The answer to this will depend entirely on how your portals function. The portals in the game act as if they are the interface of a spatial fold. The portals themselves don't interact with the objects passing through the fold.

In that model your railgun isn't going to apply any force to the portal itself at any time. The projectile will still be moving at some crazy velocity through the air when it exits the portal, which is something you don't want to be standing right next to without serious protection, but it's not going to apply any force to the portal itself just because it moved through.

What if the portal is interacting with the material and fields though? Perhaps it's a crazy complex matter transmission system that can recreate the matter sent through it, with all of the same properties at the quantum level. (Yes, that's impossible, but we're talking portals here.) In that case then there may be some field interactions between the projectile and the portal that could result in the portal experiencing some force. It could probably be adjusted to compensate for these effects though. If it can fabricate the quantum states of the projectile then recoil compensation should be a trivial problem. In that case, recoil or feedback or whatever becomes either a choice or a poor calibration.

There are a few other ways we can hand-wave... I mean explain portals, but they all boil down to either matter transmission or space warps, just like teleporters.

I'd focus more on the effects of having a hyper-velocity projectile suddenly appear in the vicinity. That's going to be one hell of a trip.


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