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A common feature in Scientifiction and Fantasy works are portals, doors / holes / oversized spinning stone rings which allow characters to get from point A to point B instantly (or after 3.2 seconds).

These are often one-way; once characters pass through, they can't go back.

Why is this?

NOTE: In my story, portals are a bit of a crossbreed between traditional fantasy portals and those in the Stargate-verse. They are magical in nature, but a specific type of oversized stone ring square is required to create them. Also, while the "entry" is always wherever the not-ring is located, the "exit" can be created anywhere within a circular area of 100 miles from the "entry".

EDIT: In response to @JBH's very helpful comment, here is some more background information to narrow things down.

  • The way my magical system works is that there is a small amount of background magic which well-trained people (wizards and sorcerers, with the distinction between the two being as per this answer) can manipulate via spells (also per that answer).

  • Portals require significantly more energy than the normal background amount, and thus can be only created by sorcerers. Wizards technically have the ability, but they lack the necessary expertise in spell-scripting and manipulation of magic capacitors.

  • The range of portals is limited by the amount of magic required to generate and sustain them. It is conceivably possible to extend their range by drawing power from multiple crystal balls; however, nobody has been able to successfully do this.

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  • $\begingroup$ Can you tell us about your magic system? What are its rules for engagement? What are its limitations? Etc. How will you judge a best answer? I just finished explaining to a new user that open-ended questions like this must be converted from an off-topic infinite list of things to an on-topic finite list of things. $\endgroup$
    – JBH
    Dec 24, 2020 at 1:17
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    $\begingroup$ I wouldn't use discworld magic rules, because fundamentally there aren't any apart from avoiding magic ever really being useful. $\endgroup$
    – Separatrix
    Dec 24, 2020 at 10:50
  • $\begingroup$ @Separatrix I meant the "background magic generated by natural processes, which only certain people (wizards and sorcerors) can use" part. $\endgroup$
    – The Daleks
    Dec 25, 2020 at 13:44
  • $\begingroup$ These are often one-way; once characters pass through, they can't go back. Why is this? Every instance of such a McGuffin has its own handwavium to explain it. This seems like an unfocused question. Are we to collect every instance of such a magic ring teleporter and produce a report on the author's particular reasoning for how and why they decided on a unidirectional plot device for every such case? $\endgroup$
    – J...
    Dec 25, 2020 at 15:27
  • $\begingroup$ In the Stargate series, radio signals and certain types of radiation can travel both ways. Matter only goes in one direction out of a conscious design decision--the source gate turns matter into energy and the destination gate reassembles it. $\endgroup$ Dec 25, 2020 at 19:31

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Your portals are explicitly set up to be one-way!

They are Sending Portals. Half-duplex.

You have a complex and large "device" at the sending side. An "oversized stone ring square is required to create them", and "require significantly more energy than the normal background amount, and thus can be only created by sorcerers"

Well, OBVIOUSLY the reason you cannot get back easily from your destination is the appalling lack of "oversized stone squares attended by Sorcerers" on the other side.

I'm quite sure if you bother to install a suitable oversize stone square with attendant Sorcerer on the other end that you would be able to get back quite easily.

Or, of course, you could pay the extra subscription fee and use a full-duplex system. Last I heard the subscription fee was a mere 5 virgins per month over the base Portal licensing fee. Then your portal can be used to "fetch" as well as "throw" without needing all that extra equipment at the other end.

Of course, if you could "fetch" anything and anyone within a 100 mile radius, you would be the Master Thief of AlKaBah, and richer than the gods. So maybe there is a good reason why only the half-duplex version of the Portal is licensed out for public usage.

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    $\begingroup$ half-duplex, or simplex? $\endgroup$
    – StephenS
    Dec 25, 2020 at 5:35
  • $\begingroup$ Thank you. I think I will use a combination of your answer and jamesqf's in my story. $\endgroup$
    – The Daleks
    Dec 25, 2020 at 13:39
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    $\begingroup$ @TheDaleks Thank you, that's what we are here for. I am glad we served to guide your Muse. $\endgroup$
    – PcMan
    Dec 25, 2020 at 14:40
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    $\begingroup$ 5 virgins per month? What in madness? $\endgroup$
    – Galaxy
    Dec 25, 2020 at 17:36
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The portals function similar to catapults. A catapult has one place where the projectile is launched from, and multiple different places where that projectile can land, within a certain limited range. Obviously a projectile cannot go the other way from a landing site back into a catapult.

Similarly, your portals have a single place where the "launcher" is at. This maybe a stone square platform. Objects are placed onto the platform, and when the launcher is activated, it "throws" the objects in an arbitrary direction within a certain range, but without the painful impact of a catapult, and it does this instantaneously. One second the objects are on the platform, and another second the objects are magically at their destination and gravity instantly takes over.

So your portal is not a passive door through which objects can cross both ways. Rather it is an active "machine" which "launches" matter into a circular area 100 miles from the entry. This maybe a mechanical machine, a high tech relic left by an ancient advanced civilization, or simply a magical enchanted fantasy platform. The portal being a "launcher" explains two things, why it can only teleport objects within a circular range of about 100 miles, and why objects cannot go back from the landing site to the "launcher".

Things would be interesting if the "launcher" cannot be precisely aimed. So heroes or just supplies could be dumped a yard or two from their intended location, and end up in trees or bodies of water.

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  • $\begingroup$ variation to "launcher" would be... flow... like the flow of a river taking you down stream. Portals flow from A => B but going B => A would be like going up stream. $\endgroup$
    – WernerCD
    Dec 24, 2020 at 10:06
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Metaphorical currents. It is, for instance, far easier to sail north on the western coast of Africa than south -- so much more so that ships took to sailing almost to South America, then going south, and crossing back over when they had gone far enough south. If you try to go back, it's the equivalent of trying to fight the current. It can be hard to impossible.

Or perhaps the entry is the real thing. The exit can move because it's only a coordinate. All the actual working elements have to be on the entry side.

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  • $\begingroup$ 2nd part, similar to the catapult idea answer allows for heroes (or the villian) to have a better understanding of the stones than is common and be able to move the target location. $\endgroup$
    – Jontia
    Dec 26, 2020 at 22:28
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So they don't shred you into a million pieces:

Two-way portals are literally causing you to be in two places at once. The atoms of your body may be separated from each other and go through one at a time, or in spurts and platters. So what happens to the blood pumping backwards in your veins? It would materialize back as you move forward. And what if someone was on the other side trying to go the opposite direction? It could meld the two of you together like something out of The Fly.

So instead of having a portal that might rip you apart if you don't go through it fast enough, it only goes in one direction. You enter, and it flings you off to the new destination. Possibly it scans you in and throws you whole once you are fully in. That way, there are no messy mistakes. The portal on the other end doesn't need to compensate for air and other things going back and forth. It only has to remake you at the other end. It receives and spits out one full object at a time, not a continuous stream of meat as the portal sends a person through in parts.

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For the same reason we have divided highways, and lane markings on most well-travelled roads. You could build a bi-directional portal, but that would open the possibility of having things travelling in opposite directions at the same time.

This would mean that at some point they would try to occupy the same space, which would cause something like a nuclear explosion. The energy and high-energy particles would come shooting out both ends of the portal, vaporizing everything for a considerable distance around.

After the first few such incidents, wizards & sorcerers learned that building bi-directional portals was not a good idea :-)

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  • $\begingroup$ I wish that SE would allow people to accept multiple answers! In any event, I think I will use an admixture of your answer and PcMan's in my story. Thank you. $\endgroup$
    – The Daleks
    Dec 25, 2020 at 13:42
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Each end of the portal has a different energy state

The portals move their targets from a high energy state to a lower one, kinda like rolling a ball down a hill. It's possible to go through a portal backwards but it requires extra energy to do so which most travellers don't have and so for them, the portals are one-directional. As a side effect of this, travellers can find that they are warmer and/or have more momentum on exiting a portal than when they entered it, or maybe there's a flash of bright light when you exit a portal while going down an energy state.

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    $\begingroup$ Or, the energy goes into displacing air at the destination. $\endgroup$
    – wizzwizz4
    Dec 24, 2020 at 13:43
  • $\begingroup$ Couldn't you use the energy you've liberated to instantly go back through the portal? Or store it, allowing you to go back later. Also, if the energy requirements are so large that travel becomes prohibitive, shouldn't the person going the other way find themselves heated up to an impossibly high temperature? $\endgroup$ Dec 24, 2020 at 16:39
  • $\begingroup$ @HarryWilson If you can find some way to capture the released energy then sure, I guess. But getting a ball to go back up the hill it just went down requires doing a 180 on its momentum while maintaining speed, thermal dynamics (or entropy, I forget which) is as cruel to magical portal travellers as it is to hill descending balls. $\endgroup$
    – Lupus590
    Dec 24, 2020 at 18:08
  • $\begingroup$ Are you saying it's momentum then, not energy, that they gain going through the portal? $\endgroup$ Dec 24, 2020 at 18:44
  • $\begingroup$ Maybe not all of the energy gos into momentum, but into things that are harder to recapture.Ultimatly it's up to the question asker as it's their world, or whoever else wants to take this idea. $\endgroup$
    – Lupus590
    Dec 25, 2020 at 11:22
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They work in manner analogous to black holes (or Star Gate).

A doorway or portal only ever opens one way. To travel from point A to point B albeit to a different location on the same planet or to a different location in another plane/dimension (whatever) involves matter or information traveling in that direction and that direction only.

Upon arrival at your new location to return you have to open a portal in the opposite direction. The key point is the door only swings in one direction at a time.

The 'return' portal or door can occupy the exactly same physical and temporal location as the one you passed through upon arrival (a stone circle, doorway or whatever) in fact it could be exactly the same portal in a different time period. But the point is to return to your original location you have to 'about face' and open the same door again. And again it also only works one way. No-one can enter a door as you leave through it.

Think of it like a telephone call. Only one person at a time can talk or otherwise no useful information is exchanged. Information goes one way and response comes back, never both at once or it doesn't work.

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