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I want to make a world where mages can search for and establish portals between a planet and another, but in a way that only one portal can exist between planet A and planet B, one between A and C, etc ... Portals can be created from A to B even while B is yet inhabited.

I have the idea of a story where this is used as a 100% certain fact to ensure security (i.e check people coming from a planet), but at some point someone manage to open a second portal between planets, bad things ensue.

So the solution should be plausible enough to be admitted, yet breakable with another condition.

I thought about portals needing a large amount of magic from the planet, but then it would also prevent portals one a single planet to several other planets, which is not what I want.

Other solutions I could think about are more Sci-Fi related, while I'd like to keep my world Fantasy oriented.

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    $\begingroup$ Why bother explaining it at all? Maybe the mages themselves haven't figured it out yet. "Mages have tried for centuries - it's part of their exam. None ever succeeded. None ever got close while a portal was already open. Their attempts just fizzled." $\endgroup$ – user535733 Feb 12 at 17:10
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    $\begingroup$ Heh heh. Portals are Fermions. en.wikipedia.org/wiki/Pauli_exclusion_principle $\endgroup$ – puppetsock Feb 12 at 17:42
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    $\begingroup$ Larry Niven & Jerry Pournelle have a SciFi novel series where "ships" can jump in zero time between "Alderson points" around stars. The calculations to find such are well known but finding the actual "jump points" is somewhat experimental. The conditions for a jump link to open or close vary with certain astronomical factors. In some cases a jump link may open only every N years and somewhat uncertainly if the factors are complex. If a star flares or .... a new link may form. All the above factors are used in their stories. (The mote in God's eye, The gripping hand, ...) You could create a .. $\endgroup$ – Russell McMahon Feb 13 at 1:42
  • $\begingroup$ ... planet and moon based system that worked in similar manner. Perhaps a large enough asteroid or comet close enough, or certain planetary conjunctions or .... may alter link formation. The links may stay or again vanish or .... to suit your desire. $\endgroup$ – Russell McMahon Feb 13 at 1:43
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    $\begingroup$ @user535733: “Don’t explain it” may well be the best approach in telling the story, but still it’s good for the author to have an explanation in mind that goes a bit further. Generally, an author’s worldbuilding background notes should go several layers beyond what’s ever shown/explained in the books, to make sure that what is shown is reasonably internally consistent and stands up to readers’ imaginative scrutiny. $\endgroup$ – Peter LeFanu Lumsdaine Feb 13 at 13:06

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The portals are natural phenomena and already exist. They must be accessed.

Like ley lines or dragon currents on and in the Earth, energy lines already connect every mass in the universe to every other mass; it is part of the interaction by which mass molds space and vice versa. Your mages discover the portals and then build the means to access and use them.

If there is an existing portal between A and B, but I wish to have my own different portal between A and B and I somehow make it, it may turn out that I have actually hijacked the existing but undeveloped portal between A and R. Possibly B is "on the way" to R? Or this hijacking may have consequences for B and R which are entirely different celestial bodies. Or were entirely different celestial bodies.

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  • $\begingroup$ I accept this answer because the idea and the terms you used (lines, connect and bodies) got me to think about my final idea I described in my own answer, thanks ! :) $\endgroup$ – ClmentM Feb 13 at 9:22
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Make your portals (to borrow a term from physics) 'magnetic'. In other words, two magic portals that are too close to each other in space will attract each other and merge into a single portal, and because portals between planets require tremendous amounts of magical energy, they have tremendous 'magnetic' attraction. Two portals that have their origins and destinations on the same two planets will be drawn together into a single portal no matter how far apart they are on their respective planets.

Just like physical magnetism, that rule could be broken by proper construction or shielding, though for the sake of the story you'd want to make that more complicated and difficult than it is for electrical circuits.

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    $\begingroup$ For something that is just magic anyways, this is definitely the coolest idea as a rule to have in your world $\endgroup$ – doe Feb 13 at 3:17
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    $\begingroup$ It could also turn into a plot, I'm liking this idea. I might just use this in my own world for a few D&D games :D $\endgroup$ – John Hamilton Feb 13 at 7:29
  • $\begingroup$ Another concept from physics you could borrow - electron orbit "shells" around atomic nuclei. $\endgroup$ – arp Feb 13 at 14:42
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Ideas:

  • The target of a portal is somehow related to its location. If you want to create a portal on A that leads to B, there is exactly one place on A that such a portal can be created. (Create a portal anywhere else, and it will lead somewhere else.) There is some fudge factor here, but the fudge factor is less than the size of the portal. Similarly, when you create the portal, because you are creating it on A, there is only one possible location on B that the other end can appear.

  • Portals resonate. You could create two portals between A and B, but if you did, something awful would happen. This could range from the second portal causing the first to break, to destabilizing both of them, to causing both A and B to explode.

Now... how do you break this? A cop-out answer would be that someone finds a way to open portals from A to X and X to B with the portals on X close enough to each other that they can be used "almost like" a portal from A to B. Maybe they're practically back to back such that you have to be really careful if you actually want to stop at X and not just go from A to B or vice verse.

Alternatively for the first case, someone figures out something that lets them change the rules for portal locations. Alternatively for the second case, someone finds a way around the resonance, or (depending on what sort of chaos you want) exploits it for their own purposes.

p.s. Recommended viewing: Stargate SG1. Each planet normals has exactly one gate, and funny stuff can happen if you introduce a second.

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  • $\begingroup$ Hadn't seen this answer somehow. Good one! $\endgroup$ – LSerni Feb 12 at 18:04
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    $\begingroup$ The issue on Stargate:SG1 was that portals (wormholes) weren’t permanent but rather had to be temporarily created from one useable gate to another one. However the addressing necessary for the sending gate to identify which target gate to open a portal to could not distinguish between multiple gates on the same planet. So if a planet had two or more gates, you could never be sure which one would receive an incoming portal. $\endgroup$ – RBarryYoung Feb 13 at 1:18
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    $\begingroup$ Thanks for the Stargate recommandation, it's a great plus in your answer $\endgroup$ – ClmentM Feb 13 at 9:19
  • $\begingroup$ @RBarryYoung, right, but I could imagine something similar happening with permanent portals... open more than one pair, and you aren't sure which one you'll exit... or if all of you will exit the same one. (A somewhat more benign version, which could be played for comedy and/or drama, would be if your body exits one portal, but your clothes exit the other...) $\endgroup$ – Matthew Feb 13 at 16:20
  • $\begingroup$ "If you want to create a portal on A that leads to B, there is exactly one place on A that such a portal can be created." Ha! thats exactly the idea I had in mind $\endgroup$ – Julian Egner Feb 14 at 10:50
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Stargates and IP Address Conflicts

Locking an interplanetary portal to a single apparently-stable position is actually a massively complex understanding that Mages simply don't understand very well. If the portal is truly stationary, then it would instantly zip up into the sky (or through the ground) as its planet of origin rotated and revolved and fell on its path around the galaxy. But that's not what happens: Portals remain fixed where they're created, relative to the planet on which the open.

This happens due to a strange force that mages do not understand, yet is unique to each planet. The "signature" (IP Address) of any given portal is made of two parts: The Source, and the Destination (somewhat similar to the gateways from Stargate). Unknown to the Mages, these correspond to the gravity wells created by various celestial bodies (this can double for an explanation of why most gateways open onto similarly-sized planets; opening onto a celestial body with a vastly different gravitational pull could be more difficult).

This means that two portals with the same Source and Destination would be considered, from the Universe's perspective, to be the same portal. It's essentially an IP address conflict, except it just means that any energy which goes into creating the second portal is instead dumped uselessly into the first portal--possibly reinforcing it but having no other effect.

Hack the Planet('s gravitational signature)!

If someone learns enough about why duplicate portals fail, they may learn to "spoof" the signature of an existing portal, disabling the original at will (which could be bad if someone is in the middle of moving through it).

Alternatively, they may learn how to "refine" the Source or Destination portion of the the Signature, allowing a second portal to open to the same destination, but still be considered "distinct" by universal law. This would be like converting from IPv4 to IPv6.

The benefit of this is that you can keep the details of it as vague as you want, as the Mages don't need to know about gravity wells etc, but you can include little details that show you do have a consistent set of rules that you're following. This can give the reader the benefit of being able to guess the underlying rules, which makes the worldbuilding seem more robust and coherent.

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    $\begingroup$ Good idea, but I really can not think of a way to make it "Fantasy" like, with some kind of magic / mythic explanations :/ $\endgroup$ – ClmentM Feb 13 at 9:24
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    $\begingroup$ @ClmentM thanks; you could have them refer to each world having its own "soul", and portals are formed by calling upon the soul of the world on which you stand, and the world to which you travel. People who can create second portals have found a way to know a World Soul more deeply. $\endgroup$ – Liesmith Feb 13 at 17:53
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Thanks to all the answers given here, and mostly the ones about having some sort of mana connection between planets, I had this idea that was not mentioned yet :

Planets are connected by a mana circulatory system, with "arteries" and "veins"

Just like the circulatory system in our body, made of blood vessels, the universe has a mana circulatory system between all planets.

For the known portals in place, they are all using the "arteries", still full of mana. The fact that these vessels are full of mana is what made it easier for mages to create portals, and to consider it's the only path that could exist.

Mana depleted vessels, the "veins", have not been discovered yet, as mana has to be injected to have portals to go through them.

This also open the idea of having an "heart" managing all the mana of the universe, could be a star / the sun, with some more story to tell.

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    $\begingroup$ This is an interesting answer! This could easily incorporate the notions from my answer and others that a particular pair of portals can only be opened in specific places, and it's a unique take on your requirement for "unexpectedly" being able to open a second pair. $\endgroup$ – Matthew Feb 13 at 16:24
  • $\begingroup$ This is a fun concept, would the possibility of having a portal network such as A > B, B > C and C > A break this however? If it is like an interplanetary circulatory system you don't really see closed loops like my example that don't include the heart along the path $\endgroup$ – BKlassen Feb 13 at 23:16
  • $\begingroup$ @BKlassen If the portals work both directions, the flow direction in the artery doesn't matter. A <-> B <-> C and A <-> C can simply be a split and merge in the system $\endgroup$ – Suppen Feb 14 at 12:37
  • $\begingroup$ Such a circulatory system would also imply there is less mana in the arteries the further from the heart they are. How this affects the system could be interesting. Does it become harder to create the portals? Do they transport things slower? Does it limit the size of things which can go through the portals? $\endgroup$ – Suppen Feb 14 at 12:38
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    $\begingroup$ For more inspiration on this topic, John Scalzi wrote a sci-fi trilogy based around a very similar concept: en.wikipedia.org/wiki/The_Collapsing_Empire The author explores the economic consequences of the effect, which you might also find interesting. $\endgroup$ – John Walthour Feb 14 at 14:10
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The portal between two planets can be opened only establishing a connection between the mana of planet A and planet B.

Once the connection is established, the two manas act like one, thus it's impossible for a mana to connect with itself.

If you force this process to open a second portal, you are actually splitting the mana of a planet. Bad things are granted to happen.

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    $\begingroup$ That would seem to impose a much stronger restriction, not merely that there can only be a single connection between any pair of planets, but that the portal network must be a strict tree structure. $\endgroup$ – Logan R. Kearsley Feb 12 at 19:10
  • $\begingroup$ Your suggestion was a good inspiration for my final idea, I'm just not "splitting" the mana, but having two different flows, full and deplete. $\endgroup$ – ClmentM Feb 13 at 9:27
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Part of the fantasy answer will depend on the background of your magics, and of your people.

At the core of the issue, if people do not delve into the interplanetary portals and actively discover that a second pair is possible, then it will be accepted knowledge that it isn't possible to do so. Whether this is because of a propagated lie by ruling parties, a rule of magic that has not been proven breakable by current mages, or just a simple matter of complacency by the masses does not matter.

Thus at the root of the answer is somebody challenging the accepted theories of interplanetary portals for better and/or worse. It likely involves research or inspiration, and will almost assuredly be an untested theory when tried the first time.

Some ideas include:

Spell Research

Any form of portal and/or permanent transportation spell requires a certain level of calculations so that the ends of them are stable. Because Space is Big, there is only the precision in the magical math to only guarantee one safe portal per pair of planets. The research needed is not in the creation of the portal, but in the calculations needed to link them.

A way around this is to determine that the math to make those calculations has not been invented yet or not widely known by mages, which can be plausible if that kind of math are not needed in everyday magic.

Resonance

It's been touched on that connecting planets by portal creates a link between them. In this case, a resonance in the mana of the planets though the "line" that the portal travels in. Like a vibrating string, there is a specific frequency that it vibrates at. Of course, it's fantasy, so it'll be called a hum or something.

For whatever reason, it is generally accepted that interfering with the resonance between interplanetary portals is a bad thing. But there are mages that theorize that it is plausible to create another pair, though it has never been done and is exponentially harder to do because of the already established mana resonance.

But all it takes is for one person to get desperate enough to polish up and try an untested theory, and be lucky enough to succeed even through the risks.

Alternate Solutions

This method of portals between world is the most reliable way to do so. Perhaps at its inception, the fact that only one per planet pair could exist was a bug that they could not work out, but over time that became a feature thanks to the rulers who recognized that by only having one way through, it could be controlled.

But this is not the only way, just the most reliable.

Ancient research could be unearthed where mages were researching an alternative method to make the interplanetary trip and for whatever reason that line of spell research was abandoned. A bit of polish, and some new theories applied to old principles, and a new solution exists. Only when the new portal is active are the reasons discovered.

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Same reason as why they don't have multiple stargates on a planet in the Stargate franchise: your portals use wormholes, and those wormholes are attracted to each other.

You see, the wormholes have a huge amount of energy involved. As a result, every mage has it drilled into his head at a young age that it is of the utmost importance that you "don't cross the streams."

If, by some unfortunate mishap you do cross the streams, a huge explosion occurs on both ends, causing H-bomb level damage (including the radiation part) in a world that has never even heard of the atom. This happened a few times when the first portals were being developed, and has left whole continents uninhabitable. As a result, you mages make absolutely sure to never let there be more than one portal extant.

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    $\begingroup$ That's..not why there's only ever one gate on a planet in stargate. It's because the programming for the gates is built around the assumption that there's only one gate per world. Insufficient address granularity rather than any mechanical problems with it. Still, otherwise a good example, and sets up the real tension behind why two gates on a planet is an "oh crap" discovery. $\endgroup$ – Ruadhan Feb 13 at 8:43
  • $\begingroup$ @Ruadhan That is part of the reason; however, in one of the SG1 episodes set in Russia they discuss that, should the two gates ever be operating at the same time, there would be a catastrophe of the "both gates blowing up" type. Also, it really isn't a matter of address granularity, as the Russians are able to use their gate without accidentally ending up at the SGC. $\endgroup$ – The Daleks Feb 13 at 16:04
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    $\begingroup$ @Ruadhan Sorry for the vitriolic nature of my response, I thought the last part of your comment was about the OP's portals part of the answer, not the stargates. $\endgroup$ – The Daleks Feb 13 at 16:07
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    $\begingroup$ (OT, but...) The Russians avoided winding up at SGC because they had a DHD. Connecting the DHD caused all incoming wormholes to go to that gate rather than the SGC gate. They had to carefully arrange to only come back at pre-determined times (when Russia would connect the DHD), or they would have wound up at SGC. $\endgroup$ – Matthew Feb 13 at 18:04
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    $\begingroup$ @Matthew I wasn't disputing that; I was just saying that it says in the series that activation of both gates at the same time would result in a major explosion, hence the "can't activate a second gate if one is already going" macro that the Ancients built into the stargates. $\endgroup$ – The Daleks Feb 13 at 20:51
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Tangled Portals

Portals have a "thread" of magic in a straight line from each other that the traveller moves down.

The problem is that the thread oscillates and waves around loosely (depending on the precision of the portal-spell) and can catch and even merge with other threads if there are any.

Over short distances, this isn't really a problem, nobody generally needs two parallel portals from the same locations, so portals work perfectly fine for lots of users travelling around the same planet. Accidents are pretty rare, but a known quantity.

The big problem is when you have two portals parallel over long distances.
The threads wave and tangle and the result is that there's an extremely high likelihood of the travellers emerging mangled and dead for both portals.

The solution is to conduct the portalling spell with greater precision, but precision is the enemy of power, and you need a lot of power to portal between worlds. (Alternate explanation: When creating the thread, you need to judge the distance right. Either you get the length too short and the portal doesn't connect, or too long and it works but has excess length flapping in the aether causing a hazard. A perfect portal is a taut line between two locations)

So many portals can be used on a single world, and portals can be sent to anywhere, but if you have two portals going to the same rough destination they'll be dangerous in proportion to how good at spell-casting the mage who produced the portal is.

For your story, you have quite a few explanations for the relatively safe use of a second portal.
* Experienced or lucky spellcasters producing a stable "taut" portal.
* The portal came from somewhere relatively nearby and isn't as unstable.
* Some way is discovered to shield or guide the threads to keep them apart

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  • $\begingroup$ The practical upshot of this approach is that the best system of portals on a planet is that the portals be physically arranged in a web across the surface of the planet. Direct portals from one side of the world to the other risk meeting in the middle and tangling with portals from literally any other place. so a network of portals with short stable journeys is much less likely to intersect another portal's path. You wouldn't jump directly to the other side of the world, instead you'd walk through a chain of portals, crossing a few thousand miles at a time at most. $\endgroup$ – Ruadhan Feb 13 at 9:46
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One spot only

For reasons that are too complex to explain, a portal from planet A to planet B must be constructed in one exact spot on Planet A, and lead to one exact spot on Planet B. A portal constructed anywhere else will not lead there.

So, by controlling and guarding the spot, you control the traffic between A and B.

Mages can feel potential portal spots, but not know where they lead without actually opening one. Opening a new portal is a high-risk job.

Since new portals are random and the universe is very very large, there are no loops. All new portals lead to a new previously unknown world. (Thus, no A->X->B links)

Potential loopholes:

It turns out the altitude of the magic spot is not fixed. By digging a tunnel under an existing portal, you can make a second portal to the same planet. You need to coordinate with somebody to dig a tunnel on the other planet too.

By pure chance, a loop is created in system. Instead of going A->B, you can go A->X->Y->Z->B. At least one of these links are not known to the opposition.

A smart mathemagician figures out how these spots are located. When knowing the exact coordinates of the link spots from A to B and from A to X, they can calculate how to get from X to B. Finally a A->X->B link!

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The Dark Materials trilogy have this exact setup. A boy finds a knife that can cut portals to other worlds, but in the end they realize that each open portal allows some kind of soul-sucking creature to roam around and, well, suck souls.

An angel tells the boy at the end that only one portal can remain open, and they decide it should be the one in hell to let all the people escape, something like that.

So, some kind of portal-creature would fit your idea of someone finding a way to open a second portal and bad things happen

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The portals could depend on physical conditions and eg maths, but the wizards don't tell people that. As John noted in a comment "They could also depend on maths too complex for the wizards to figure out, a lot math had to wait for computers before it became possible to calculate, especially if there are many compounding factors."
So you could posit that the system is explicable but the wizards find the links by certain rules which they do not understand. Harder to find, more complex links may exist but the wizards have almost never stumbled upon them.

A useful parallel scenario: Larry Niven & Jerry Pournelle have a SciFi novel series where "ships" can jump in zero time between "Alderson points" around stars. Most of the time taken to travel interstellar distances is spent travelling across star systems to reach the next jump point. So most distance is covered instantaneously but the in-system parts set the trip time.

The calculations to find "Alderson points" are well known but finding the actual "jump points" is somewhat experimental. The conditions for a jump link to open or close vary with certain astronomical factors. In some cases a jump link may open only every N years and somewhat uncertainly if the factors are complex. If a star flares or .... a new link may form.

All the above factors are used in their stories. ("The mote in God's eye", "The gripping hand", ...)

You could create a planet and moon based system that worked in similar manner.
Perhaps a large enough asteroid or comet close enough, or certain planetary conjunctions or may alter link formation.

The links may stay after formation, or fade away after a certain period, or under certain conditions to suit your desire.

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  • $\begingroup$ They could also depend on maths to complex for the wizards to figure out, a lot math had to wait for computers before it became possible to calculate, especially if there are many compounding factors. . $\endgroup$ – John Feb 13 at 2:36
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There is a huge cost to build another portal. Like the souls of 10'000 virgins, or 10'000'000 acorns etc. It is due to this barrier that no one bothers to make another portal

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Each world has a different 'frequency' or 'flavour' of mana, and these are incompatible. At a portal, you get some minor mingling and diffusion of mana between the worlds - kept in check by the mana pressure of each world - which causes local plants and animals to die off or sicken in the vicinity, and allows some from the other world to slip through.

Highly magical creatures (such as a well-trained mage) can survive for a reasonable time in the other worlds, because they carry their own mana store with them, like a diver with an aqualung carrying their own air.

However, once you open a second portal, you can develop a turbulent mana stream, flowing in from one portal, and out the other. This wrecks the local thaumic field, and has adverse affects on the wildlife and environment. Fortunately, these streams cannot cross worlds - that is, if you have portals between worlds A, B and C, then the stream cannot form from portal AB to BC to CA and back to AB, only from AB to AB' in event of a second portal being opened.

This is a bit like having tiny holes in a bottle of water - just one hole, and the vacuum pressure inside the bottle holds the water in. As soon as you open a second hole though, air can enter through one hold and fill the vacuum, which allows water out the other.

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Making a closed loop with portals slams one or both of them closed. (either limited to 3 portals on 2 locations, and more indirection's fine, or any loop of any size)

Like leaving your front and back-doors open at the same time, and having a light breeze smash it closed.

You could circumvent this by creating some kind of portal-based revolving door. (which is how the door slamming shut in buildings thing is handled IRL)

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Identity (peronality,ego,..) of planets

tl;dr; Planets have self-identity, portals connects such identities, so their are unique for each pair, but there are more theory around and some special cases...


Each planet is unique so it is its self-identification (or ego, or personality or call it as your magic tradition is used to call it).

From magical point of view it is somehow based in the "inner heart of planet", which is usually somewhere near the centre of its mass.

And potencial portals between planets can be represented as distance between their hearts - which naturally give only one way for any pair of planets. (And any two planets can be connected this way.)

There is the problem, that each planet more or less consider itself as "center of universe" and feels other planets as far as their personalities differs. Which makes total mess of what is from magic point of view about "straight" "close or far" and "at which direction" - something like if you would like map relations in large group, whith respect to their affinity - just not on paper but in 3D.

Basically the planet "feels" its ouside as its more important part so potential for portal is placed usually at landside, or at see level, while some where found in mountain, even in caves and there are theories, that some may be underwatter (more often between sea worlds with fish-like inhabitants) or even underground inside rocks or lava (for planets, that hates each other?).

Usually planets with closer identities allow for larger, shorter and relatively cheaper portals.

Magican and senitive beings can feel, where the potential portal end is placed, say in similar way as for magnetism or electricity ("my hair ticks here and sometimes even rise and flows as in the wind"). A lot can be found from carefully observing such place. Still it takes a considerable effort to perfectly pinpoint the right place and then open actual portal there - which takes also some time (like days or even months or years) from the the point when entrances starts to manifestate on both sides to the point when portal connect those ends and is wide enought to be usable.

Some tests was done, but the place is 3D restricted, so there is given the size of the portal (usually omething between narrortunnel, which must be crawled thru, to really big gates, where even two warships could flow side by side) a well as its position (and magic of such force cannot be controlled so preciselly to prevent the portal entrancess to span over their full size before reaching the other planet.

Even trying to create two (or more) portals at the same time, from the same size or from opposite sides, resulted only to merge of magic sources and faster opening.

The same goes for eventual closing of portals, which shrinks "the tube" slowly.

Opening and closing works against each other, deducting the manas used from each other - so final effect takes longer, if one side can pump sufficiently larger quantity of mana into it.

Small quantity of mana does not affect portal at all, as it have some natual magic inertia, which dissipates some amounth of mana trying to affect it - so if there is not enough mana to open portal fast, it would not open at all and dissipate the invested mana to emptiness while it shrink to itelf old "potential" state. But once a "critical mass" is reached, it will open to its fullness and stay so nearly indefinitively, dissipating eventualy small amounts of closing mana.


There are some problemss, as the identity of planets changes over time (say from nearly empty world to overcrowded one, river and seas may dry out or change substantially, some major pecies can die-out or even big enough space body can hit the planet) - which affects the identity of planets as well as their relations.

Small changes, as wars and plague and such does not matters for planets sso much, so the change of portals is usually not even measurable, but bigger and long term changes may start to move portal places, so the portals may move too. As magic constructs, they just ignore terrain, buildings, restrictions and move thru, eventually diplace/tranport obstacles to their other end. If the change in planet identity so too big and fast (at planet terms, not human ones), then the portal cannot follow it and is destroyed, resulting in eventual disasster around old gates (a lot of magic released uncoordinated).

There is even way to create two portals between the same pair of planets, in the theoretical case, when one planet get "split personality", while it is not clear, what could have such dire result and not destroy all live on such planet, or at least half of it.

Also is theoretically posible for two large space bodies with high enought personality for portals (so probably small moon or larger) to colide in way, that they till keep it personalities, while effectively merged in one space body - like crash big moon to Saturn and let it survive as single body, mainly enclosed in other - such two-planet could have two sets of portal on one space body. (But usually one or both planets are so damaged, that their personalities are destroyed and eventually new personality is borned) - probably noone would survive such impact, but portals to such double-planet D1-D2 could be found from outside O and while extremly rare, another portal on the other part of D1-D2 may even be not so far from the first - which could shorten travel on planet O by shortcut O--D1-D2--O.


Still more probable shortcuts are A-B-C-A (or longer chains), where A-B and A-C portal are on A far away and A-B is near of B-C on B and B-C and C-A is near on C.


Also I would suggest, that transport is not entairly for free (to avoid tornados, flows etc.. from different conditions on different ends), but some "magic" had to be performed to allow somebody to enter the "tunnel" - and it would open one side of the "tunnel", let enter as many entities and cargo as could fit in, then close the gate and open the other, with slowly growing force to empty the "tunnel" before it closes after some time again.

Who was strong enough to stay inside, or was not able leave the tunnel, he was lost forever and never heard of him again - so portal ending undeground are really feared by explorers. (yes, execution by dead-end tunnel is posible. And sending slaves this way with tools to make a small chamber at the other end and then return (if they are able pay "magic toll") is possible too)

The "magic toll" is same for both direction and can range from hard rituall and plenty of magic for too different "peronalities" to simple spoken pray from mundane for really close peronalities (like "Oh mother earth, please let me step inside and travel safely" from atheist and not the same phrasing is required, just the intensity of will/mana)

Also while tunnel is open, anybody can entry - so there may be "porter", who will open it for you, but not entry at all and wait for other travelers. Some skilled magican may be able even formulate toll for "way back" which would open portal from other side.

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The end result isn't stable. Or pretty.

Maybe it's the one interstellar leyline like Willk's answer. Maybe the portal paths merge like Ted Wrigley's answer. Magi-scientists don't know. The issue is if the portal from Earth to Minbar is has only one entrance/exit on each planet, it is fine and stable. But attempt to open a second portal, and you just get one portal with two exits. And that is bad.

Why?

Sure, you step into portal A on Earth. But that doesn't mean you will arrive at portal A on Minbar. You might exit via portal A. Or you might exit via portal B. Or your top-half might exit portal A will your bottom half exits portal B. Or left side/right side. Maybe you just lose a hand. Or your luggage. Whatever the outcome, it is unpredictable, uncontrollable, and generally not worth the risk.

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There is an analogy with electric current. I you have two wires with current going the same direction, they are attracted to each other.

So if you have two portals going from A to B, then through most of their length they are congruent. This means that anything going through them is split in half between the two destination portals. This can be used to create dwarfs, but the dwarfs are rather stupid due to smaller brains.

You get around this by finding a third planet C, and creating two portals A to C and C to B. This keeps the portal 'tunnels' far enough from each other they don't interact.

It's normally not done because creating a portal takes a lot of infra-structure. Doing this will require the sort of effort it would take today, but build a launch infra-structure for Mars on the moon.

If you have free and easy passage to B, you could build the BC link from there -- your infra-structure is mostly in place.

Extending this notion, it's relatively easy to add a connection internally to your portal network, but expensive (difficult) to expand it into new territory.

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A simple model:

Portals are not simple tubes. They expand until the halfway point and then contract. (Think of a huge American or Rugby football). Anything transiting them is similarly expanded and contracted, but it's a transient and conservative process that somebody transiting the portal doesn't even notice (or maybe does, as "transit shock" which might be a useful feature if you want portal users to arrive seriously disorientated for a few seconds/minutes which guards can take advantage of).

Overlap doesn't matter normally. Technical or magical details make sure that you remain "coherent" going from A to B even if there is another co-linear portal from A to C. The one going to the further destination has a very large "diameter" compared to the tiny size of the closer exit portal.

But try to create two portals A to B and A' to B' starting and terminating on the same planets, and the technical or magical details fail because of excessive overlap for the entire transit. The result is that half of whatever enters A or A' comes out of B, and the other half out of B', randomly on a sub-millimeter scale. Messy.

And you can't do a portal from A to A' on the same planet because of insufficient distance (wave hands and mention wavelengths in portal space far too short for science or magic to control, coupled to interference from normal matter in large quantities along the entire length). Portals work only over interplanetary (or only over interstellar) distances.

Incidentally, you might allow that if A and B are allies and A' and B' are allies, an interesting sort of warfare is possible, starting with the "rebels" "blockading" the portal with a parallel one, so nothing can transit between these two planets. The blockade lasts until one of the blockading portal-pairs is shut down. It might be possible to get from planet A to plant B via some other destination C, but that will inevitably be construed as C taking sides ....

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The planet with the extra portal is actually two planets that somehow merged into one. The “fluke” portal is on the other planet from the “normal” one.

OR

The fluke was made by someone who found a different method of creating it, so it doesn’t violate the only-one rule of the other method.

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Portals can repel each other along their entire length, but not enough that it matters for two portals each with one end on the same planet but their other ends on different planets.

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Adding to your original idea of having a certain amount of magic in the planet that’s used to make portals, I think you can still use it:

Each planet has a limited amount of magic, enough to create one portal to another planet. But think of it as a different kind of magic/mana that’s not to be mixed with the mana used by wizards to do their regular magic - something that’s depleted once the portal is made but wizards can still cast fireballs or whatever, because it’s a different fuel. Perhaps a planet “soul” - the soul is consumed to build a portal. Bear with me.

Then, you still have the problem of having a planet with portals to multiple planets, and this is the catch: when creating a portal, one of the two planet souls can be used. So A used its soul to build to B. But C’s soul was used to build from A to C. And D’s from A to D. And the portal from B to E? E’s soul.

This is a bit limited as it still has the problem where in some cases it’s not possible to build portals: if an A-B-C triangle portal is built then D can only have a portal to only one of them.

The wizard that hacked the universe to create a second portal did that by using the soul of a nearby planet. Or he discovered that the big moon on that planet also had a soul that he could use to build the second portal in that planet. This could also be used to resolve the A-B-C + D problem I mentioned above.

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