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I'm imagining taking the "circle hovering in air" image of a portal into the third dimension; a portal shaped like sphere, about halfway sunk into the ground. When you step through the shell from the outside at location A, you set foot "inside" the portal at location B. When you step out from the portal, you step into location B. As an aside, if light obeys these rules, the portal may be invisible from the outside and it always looks like you are stepping into the "wrong" end of the portal.

The problem is that blood circulation will be forcing some of your blood back towards the "exit", and a similar issue would arise with flat one-way portals. Similarly, trying to pull one's leg back out of the portal would be ill-advised as it would end up on the wrong side.

Is there a way to enter the portal without a gortesque and fatal spray of blood from your veins, as well as lymph and other bodily secretions falling out?

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    $\begingroup$ I don't understand why you are fixated on blood specifically. The same phenomenon will happen with your hair, your teeth, your eyes, your toenails, your tonsils and the tip of your tongue. (And actually blood is the least to suffer, as it is confined in vessels. Humans can easily, for example, roll forward, and the momentary switch of the force of gravity on their blood is inconsequential.) $\endgroup$
    – AlexP
    Sep 26, 2021 at 11:47
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    $\begingroup$ @AlexP The problem is that part of the vessel containing the blood is on the "wrong" side of the portal; when the blood goes towards where the vein should be on Side A, it actually exits the portal on side B where there is no vein to carry it back. $\endgroup$
    – HAEM
    Sep 26, 2021 at 11:50
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    $\begingroup$ if your portal teleports from A to B and from B to C, then passage of solid matter is the least of your issues. The biggest issue is perfect vacuum at A, and overpressure and the continuous fizzle of nuclear reactions at B/C, as molecules of air get warped into each other. Your second biggest issue is the amount of energy pumped into the system. And if it's water getting warped into water - or anything solid - the pressure at B/C will rise fast. $\endgroup$ Sep 27, 2021 at 5:27
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    $\begingroup$ @Hermann The arm thing was also done in SG-1, it worked as part of the Stargate's safety mechanism. The "space between" is a buffer in the Stargate, not physical space and not part of the wormhole physics (see the episode Teal'c gets stuck inside the gate, for example); more relevant to this question, it's explained that an object is only transferred from one Stargate to another once the entire object is in the buffer, so that nothing gets split in half. His arm re-entering the wrong way is iffy on consistency because it was at least the correct side of Stargate, just the wrong direction. $\endgroup$
    – Izkata
    Sep 27, 2021 at 22:00
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    $\begingroup$ A one-way portal is mathematically & physically indistinguishable from a black hole (to an outside observer). That's a good starting point $\endgroup$
    – neph
    Sep 28, 2021 at 6:37

13 Answers 13

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Speed infrastructure

Making special infrastructure can make this doubly safe. First of all, you make sure that controlled infrastructure will prevent/seriously reduce the chance on any collisions or otherwise dangerous situations. Secondly you can add the speed required so little to no loss is happening. If you move with the highest speed of blood in any direction (or even faster), you can prevent such things from happening.

If you want to have some cool transition with stepping in and out of the bubble you can use techniques. Blood goes on average 4,8 to 6,3 km/h, or about walking speed. At max maybe 10km/h. Running (on average 10km/h) and jumping, or even just falling through the portal can already reduce the amount of loss to a minimum or fully eliminate it for most people. This is by simple virtue of not being long on the edge of the bubble and speed. Keep in mind that minor blood loss isn't a problem at all in most situations. With the speed of the body any other accidentally moving backwards parts will not exceed the threshold of the whole body. If you move 20km/h one way, you can't 'go back' if you move only 5km/h the other way.

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    $\begingroup$ I read "minor blood loss isn't a problem at all" in Cave Jonson's voice from portal 2, and feel this is a very "Portal" explaination - I kind of like it because crossing portals is likely to suck - causing some bruising and blood loss, and in some cases, major hemorrhaging. Grabbing someone halfway through a portal and pull them back would effectively bisect them $\endgroup$
    – lupe
    Sep 27, 2021 at 12:06
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There actually is no problem. Imagine having a 2D world, and the "portal" is a tube lifting out of the 2D world-sheet in one point, and landing in another. When seen from the 2D perspective, you see that the portal tube is always locally 2D - there are no discontinuities. You are distorted in the third dimension while you travel, but you are unaware of this because the light rays and everything else in your local space is distorted the same way:

enter image description here

If you are Mr. Green Circle, and look around you along the green lines, you see everything as usual. When you start looking towards your friend Mr. Pink Circle though, the light arriving at your 2D "eyes" is the light coming out of the portal from Mr. Orange Circle.

In 2D, you see the world in the direction of the portal becoming distorted hyperbolically, you can actually see infinite more and more distorted and thinned-out reflections of yourself, and then the distortion reverses and in the middle you can see, clearer and clearer, a different place.

Exactly the same thing happens in 3D. In this image you see a distorted Milky Way, and in the middle the opposite section of the same galaxy - the "inside" and "outside" of the distortion are 30,000 light-years apart.

The nearer you go to the portal, the more distortion you see far from you and the less distortion you see near you. Wherever you pass through, the local distortion is effectively nil (for a large enough portal of course - at least, say, one hundred times your own size).

enter image description here

All this assumes that there is a continuous isotropic distortion around the portal; otherwise, the behaviour of matter and light on the boundary is anyone's guess. The portal might even be impassable (or its borders could act like an atom-thin razor, in which case, in 3D, any object entering would effectively be instantly shredded at the atomic level - not even freezing would be enough to protect biological entities; some ultra-cooled crystals could make it through reasonably unscathed, with just their lattice defects rearranged).

If the distortion is anisotropic, then there is a "backwash" effect. Taken to the extremes, the distortion becomes one-way - particles can only move in one direction. In that case, traversing the portal would deliver a nasty hydrostatic shock when blood finds itself incapable of moving in those vessels that run in the wrong direction. For very small times - one hundredth of a second or less, the shorter the better - you would receive the equivalent of a coin sock to the head. Confusion, perhaps loss of consciousness. The longer the time (and passage repetitions), the more damage - up to brain haemorrhage and death. A short passage, or a very thin anisotropic portion of the interface, would reduce the damage down to nothing.

Incidentally, this implies that the danger of a portal (if more than one kind exist) may be visually estimated by measuring the distortion and comparing it against its aperture. A large portal with wide, continuous distortion would be innocuous except to very large objects; a narrow one with irregular distortions would be deadly).

Also, portals leading to areas with lower barometric pressure (higher up, or warmer air, or the eyes of hurricanes) would suck air, possibly at a considerable rate. If there is nothing to "hold to" near a portal, you might have trouble avoiding going in, or in the other direction, attempting to go back.

The "continuous unidirectional matter transmitter" (i.e., whatever intersects the portal plane gets instantly trasmitted from A to B, but not from B to A) unites the problems of unidirectionality to those of discontinuity (to the barometric pressure problem). This kind of "portal" is in practice a matter shredder, and blood coming out the wrong way is the least of the problems; the real killer is that the portal disrupts molecular bonds, or at least long-chained molecules, like DNA, which all continuously vibrate due to Brownian motion. It would be a horrible way to die, but since the brain gets shredded too, it ought to be almost painless, provided the entry is fast. It would probably be, since pressure isn't transmitted, so the portal acts like a perfect vacuum, sucking with a pressure of about ten tons per square meter.

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    $\begingroup$ It worth to add that any complex body will experience squashing and tearing forces while moving through, proportional to its speed and local distortion gradient. In some cases a moving body may bump or even bounce back due to material resistance. In other words, moving fast is opposite of safe. $\endgroup$ Sep 27, 2021 at 2:09
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    $\begingroup$ I think you misunderstood the question. Your portal works both ways, OP's portal works one-way. If you step into the sphere at A and immediately step back, you end up in area B. If you step halfway through the portal and then step back, half of your body remains in A and the other half ends up in B. This is the problem: even if you walk straight forwards some of your blood is moving towards the A side, which the portal does not allow and sprays the blood trying to get to A into the B side instead. $\endgroup$
    – Demigan
    Sep 27, 2021 at 11:00
  • $\begingroup$ Here's a nice video that visualizes how a 3d wormhole going through would look like. youtube.com/watch?v=SZDOKtT_QZE $\endgroup$ Sep 27, 2021 at 11:37
  • $\begingroup$ @Demigan I've tried to model this particular mechanism adding it to the answer. $\endgroup$
    – LSerni
    Sep 28, 2021 at 12:56
  • $\begingroup$ @LSerni thanks. Unfortunately the OP realizes something is wrong and asks how the portal could work despite these conditions, rather than "oh no it is actually worse". $\endgroup$
    – Demigan
    Sep 28, 2021 at 13:21
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a TRUE one-way portal would be instantaneous death. .
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for the whole planet it exists on.

The moment matter passes over the boundary, it will stop interacting with any matter attached to it. Chemical bonds will fail. nuclear bonds will fail. Even subatomic bonds will fail between that part of an atom that have passed through the barrier, and that part that i still in the originating location.

The inevitable result of this is an explosion that will very much resemble an antimatter reaction.

The only way to prevent this from happening is to either allow two-way transmission, or to allow the portal to only send intact objects. Intact protons. Intact atomic nuclei. Intact molecules.
But some molecules are huge! A diamond is a single molecule. A single DNA strand is formed from all covalent bonds, thus is a single molecule that is TWO METRES in length if unfurled. So your portal must be able to send macroscopic items through, intact.

So why, exactly, is it choosing to shred the poor bloodbag person that is trying to step through??

The usual way the technobabble doubletalk works around this is for the portal to accept incoming matter into a 'holding space' within itself, and only complete the transmission when it figures that the transmitting object is complete. Example the Stargates in the series of the same name.

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    $\begingroup$ This. If you try to apply physics to it, it will be unsuitable for typical stories that depict humans passing through a portal. For example even if you only allow whole molecules to pass, air is going to escape through the portal, as there is no pressure acting on the air from the direction of the portal. $\endgroup$
    – user31389
    Sep 27, 2021 at 21:53
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    $\begingroup$ @user31389 "air is going to escape".. umm, yes. the portal would act effectively like hole to perfect vacuum. There might be a slight breeze towards it. Should be subsonic, but more than hurricane force. fun! $\endgroup$
    – PcMan
    Sep 28, 2021 at 6:15
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There are a few potential safety rules depending on exactly how it works.

I think the simplest solution, is that the machine isn't constantly running. You enter it, then it teleports the entire contents of the machine to the recieving sphere one way. If any limbs were outside the sphere, they'd be severed, but anything inside the sphere stays intact.

If the portal must always be running, and maintain a constant connection to the receiver, a useful guideline will be "machine must be entered under constant velocity."

Any people who wish to teleport will sit in a large chair mounted to a track, that ramps them up to such a fast speed, that minor variations in internal movement don't have enough time to affect your body since you enter and exit near instantaneously.

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  • $\begingroup$ Requiring activation is key to avoid losing the atmosphere in A. A one-way spherical portal useful for human passage would be a sphere at location A that when activated, teleports its contents to location B. (If you want, you can make the activation only possible from the inside of sphere A.) Before the swap, force field at A would push particles away from the boundary and force field at B would push all particles away from destination sphere, making a spherical void. Then the sphere is teleported to B (now A has a spherical void held by a force field) and the force fields slowly relax. $\endgroup$
    – user31389
    Sep 27, 2021 at 22:06
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The portal is one-way, but there is no fixed surface which particles can only travel one way through. It's more like an airlock: you go in, the portal closes behind you, the portal opens in front of you, and you go out the other side.

At no point is your blood ever restricted from flowing from the frontal direction toward the backward direction; while the entrance and/or exit surfaces are closed, particles cannot pass through them in either direction. As long as you are fully inside the portal while the entrance closes, you won't get cut in half by it. But it's a one-way portal because the exit can only open after the entrance is already closed.

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  • $\begingroup$ what happens if a fly flies in while the person goes out and then the exit closes and the entrance opens to accept the next person? $\endgroup$ Sep 28, 2021 at 12:05
  • $\begingroup$ @user253751 The portal could be single-use, or the entrance might only open if the space inside is empty; or, like an airlock, the space inside might be (literally) evacuated before the entrance opens. $\endgroup$
    – kaya3
    Sep 28, 2021 at 12:09
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A thicker edge to the portal which functions as a teleport area.

You step in from A and into the shell-side, this shell-side is essentially a pocket dimension similar to a bag of holding that surrounds the actual portal. If you step back now you return to A.

If you move forwards to the center of the sphere you meet the boundry between the shell and B-side. The moment you touch the barrier, the shell-area directly around you collapses into B-side (without affecting anyone else still in the pocket dimension). If you step back now you remain in B-side without re-entering the pocket dimension. You can now walk out safely out of the portal area.

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One potential problem with a portal is the edge/surface. Imagine a circular portal perpendicular to the surface of the Earth. Push something halfway through and then drop it. What would happen? It’s an imaginary situation so this is not definitive, but one possibility is that the circle itself is a very sharp one dimensional edge between two different 3D spaces so the object might simply be cut in half on contact with the edge with one half falling into each side of the portal.

The same could also be true with spherical portals. You would want a fairly large portal and want to jump cleanly into it. What would not be good would be trying to use a small portal. Someone walking towards it might find only part of themselves transported through the portal. with the rest left behind. Could be a bit gruesome.

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    $\begingroup$ I think this 'answer' is the exact premise of the question? $\endgroup$
    – Trioxidane
    Sep 26, 2021 at 18:42
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You're looking at it in a 3D realm, as such there's a big problem with any reverse flows (air, also.) Suppose the "portal" is actually in a 4D realm, though. LSerni came close but that's a wormhole, not a portal. The portal actually moves you in a fourth spacelike dimension. The instant you touch the surface of the portal you have the reverse flow--but instead of being a gruesome mess the body is intact, as Newton says there's an equal and opposite reaction--the reverse flow presses against the portal, the result is the body part that went through the portal is being pushed away from the portal--thus pulling the rest of the creature through it. The creature, being a mere 3D being is incapable of exerting any force in this 4th direction, it's impossible to resist this tug.

Thus the result is anyone and any thing (normal vibration of the atoms will do the same thing, just slower) that touches the portal is inevitably drawn through it.

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The problem is that blood circulation will be forcing some of your blood back towards the "exit",

This problem of fluids alone is easily overcome. Blood moves at about 1.5 meters per second, i.e. walking pace. So at worst, some of your blood vessels will be carrying blood back through the barrier at walking pace when you stand still half-in half-out of the portal. To avoid backward spray, maintain a forward velocity greater than walking pace: then the resultant motion of the blood relative to the barrier will be positive throughout your circulatory system.

A bigger problem might be effects that propagate through your body at faster speeds. Nerve impulses can go at up to 100 m/s (230 mph or 360 km/h): unless you go faster than this, a significant portion of the information that keeps your brainstem working in synchrony will get lost in the transition. One missed impulse is nothing to the brain—but if a significant portion of the neural messages that are being transmitted at a particular instant all get lost together, across the whole brain, all bets are off as to what will happen.

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Such a portal would be useful for communication.

Photons will not be put out by these rules. I could easily see you on the other side of the portal and you could see me. Radio communication would be equally easy because photons are not obliged to turn around. It is an ansible and you can use it to violate causality! Solid objects might be ok and we can play catch thru the portal. I do not see a way around the problem of a heterogenous object in motion like our bodies. If you want to get into the weeds, even the ball we are playing with is in heterogenous motion at a molecular level. You could make it so only photons can safely pass thru.

Unless you wave your hands, which you need to do anyway when it comes to portals generally. For example there will be pressure differences and so it would be windy next to the portal unless you decree it not be. A place close to the pole of the planet will be moving slower relative to one on the equator, and speed will need to be normalized to the new place via vigorous handwaving to avoid being hurled against the wall. Points on different planets pose an even greater problems. Places that are moving very fast relative to one another would red- or blueshift light traversing the portal which would be a nifty special effect.

You can assert these are nonproblems and broaden your fiat to let people poke appendages into the portal without coming to harm. Or you could keep the limitations you have invented which could be useful for a story.

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Situation A:

  • Can you pause time inside a box?

If time has been paused inside the box, everything inside the box is not moving, then there wouldn't be any problem with things going back or forward (being the box the point of reference) and that way the subject/thing inside the box would be freezed in time.

Situation B:

  • Can you completely freeze somebody at 0 Kelvin for 1 microsecond?

I'm saying freeze him completely to make all those fluids you mentioned completely static and completely solid. That would again prevent the fluids from spilling in an explosion of gore.

  • Can you make someone's speed 0.9c just before entering the portal?

The most dangerous thing is passing through the portal as an being with fluids that may spill or parts of your brain that may be disconnected. The faster you get through the portal, the lower the possibility that something goes wrong while any part of yourself is at two places at the same time.

Those situations are some solutions to your answer.


Why haven't I adressed the mechanics of the wormhole itself?

I can´t compare, in good conscience, that type of portal to the conception of a wormhole since I was convinced by this answer that mathematically, a 3D space is the only stable configuration of time and space that can exist. And if you got through a wormhole, you would be going through a way different than the usual 1Time-3Space so you will need to use another dimension of space, and that would mean that it is an unstable universe, and thus:

@4.12.22.4.18.0. You misunderstand the nature of the of the "unstable" here. It's not that a planet or whatever is unstable and would fall apart or something. It's that there are no stable orbits. The fact that there are stable orbits in 3d is directly tied to the fact that there are 3 dimensions and the gravity equation falls off like 1/R^2. In 4 dimensions, all paths through space with relation to another gravitational body either collide into that body or the fling off never to return. There are no stable circular orbits. – Shufflepants May 20 at 19:27

Also, there is this:

A much bigger problem is what happens to the electron orbitals of the atoms in your body. Unlike planets, unstable (open) electron orbitals would continuously lose energy (as photons) and would quickly spiral into the nucleus turning all protons into neutrons, ending all chemistry and with it all life, – RBarryYoung May 20 at 20:30

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    $\begingroup$ could you elaborate on how these answers can be solutions to the problem $\endgroup$ Sep 26, 2021 at 21:20
  • $\begingroup$ @PostlimFort Yup, already done, and extended a bit about why I don't adress the similarities of your portal with a wormhole. $\endgroup$ Sep 27, 2021 at 12:19
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It's quantum

In the macroscopic world, we are used to all things acting effectively as continuous. This portal treats things as quantum, such that the border is iffy. A human body passes over it and is treated as if all outside until, abruptly, it is all inside. A quantum leap. Nothing can be half in and half out.

Why it treats bodies and other large objects like subatomic particles would win the discoverer the Nobel Prize.

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Method 1: The Amoeba Effect

You need some sort of process to happen at the portal. As I step toward the portal, my mass causes pseudopods of altered space to come from the boundary and encompass me. Up to the point where the field meets behind me can I can back out. Once they meet, I am merged into the B side of the portal.

Two references that can clarify this imagery: Watch an amoeba ingest a particle of food.

Look at simulations of the magnetic field disconnecting and snapping back during a solar storm.

Difference in Energy.

Suppose that in going forward, you gain about 30 joules per kg. This is equivalent of about a 3 meter drop. Coming back, as you walk toward the surface you have to do the equivalent of lift yourself 3 meters during the distance it takes to go one step.

If I push my arm into the field, the part of my arm on the other side is effectively 3 meters higher. You will feel tingling, as your heart can't pump blood thing high, and blood would drain quickly back through your veins.

If you pushed your head through, you would pass out.

Coming through, you pick up about 7.5 m/sec of velocity in a direction perpendicular to the surface = 22 ft/sec = ~16 mph. This means that a person running a 4 minute mile pace can run into the barrier and get through it. Or a man on a horse.

Usually coming through you would trip and roll. This would mean you could send notes through backward by putting them on the end of a stick, and pushing them through.

I think sound would cross the barrier too.

Air molecules coming in would gain energy, so you would have a warm zone by the barrier on the receiving side, and a cold zone onthe sending side. Since air moves both ways, I'm not sure what the net effect would be.

I think I've just violated some thermodynamics laws...

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