Imagine a portal. This portal is two-dimensional and circular, with a diameter of one meter. It is completely above the ground. It leads from Planet A to a planet across the galaxy (Planet B) . An object that passes through the portal in its entirety ends up on the surface of Planet B. When the portal is closed, its circumference shrinks toward the center. Anything sticking partially through the portal when it is closed is severed, with a portion of it stuck on each planet. Think Doctor Strange in Infinity War.

Now, imagine sending a train (which is a lot wider and taller than one meter) through the portal. What will logically happen to the train?


My first though was that the portal would cut a clean cylinder through the train and transport this cylinder to Planet B. This means that any object that touches the rim of the portal is severed (imaging waving your hand through the rim and losing some fingers to the edge).

The portal is strictly one-way, meaning that a person on Planet A can travel to Planet B but not vice versa. Also, a person on Planet A is able to use either "face" of the circular portal to reach Planet B.

Edit: Answer criteria

Answers should provide an explanation of the fate of the train. Answers that incorporate mechanisms of quantum physics (observed or hypothetical) will be preferred. Only local damage can be done to either planet (ie. both planets won't explode, but they might each see a small crater formed around the portal).

The structure of the portal isn't set in stone, so the fundamental nature of the portal can be changed for an idea to work (but please explain why). The physics in this universe is the same as that of our universe (with the exception of this hypothetical portal).

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    $\begingroup$ I am afraid that unless you define the physics of how this portal works at its boundaries we cannot answer to this question. It's your world, you decide $\endgroup$
    – L.Dutch
    Sep 1, 2018 at 4:43
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    $\begingroup$ I'm not sure the question can be answered with the detail provided, as it depends entirely on what's going on at the rim of the portal. If it's just a perfect two-dimensional hole then, yeah, the edge of the portal is effectively an infinitely sharp cutting edge. However, if there's energy transfer and stuff going on creating a lightshow like Dr Strange's portals do, then there's something there that's going to affect an object that's straddling the rim. $\endgroup$ Sep 1, 2018 at 4:43
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    $\begingroup$ I would say that all of the necessary physics is visible from the behavior already observed. When the portal closes, it severs anything which lies across its boundary. That implies that the boundary rim is stronger than the molecular bonds of normal matter. If a train hit the boundary rim, it would either be hollowed out like the original poster surmised, or it would crumple in on itself as if hitting an immovable, indestructable wall which happens to look like a one meter ring. The speed of the train and the maximum severing speed of the rim would decide which result would occur. $\endgroup$ Sep 1, 2018 at 4:56
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    $\begingroup$ Hello Axiom. What @RonJohn means by primarily opinion-based is that you haven't provided enough information for us to know how you'll judge our answers. In other words, "perfect cylinder ends up on planet B" is no more or less the best answer as "the portal explodes, destroying both planets." When asking "What happens next?" questions, you need to give a clear explanation of how you'll judge or what the best answer would look like. $\endgroup$
    – JBH
    Sep 1, 2018 at 7:03
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    $\begingroup$ Hi @JBH, thank you for the explanation. I went back to refine my question and hopefully it now fits all of your criteria. But if not, would you mind providing further advice on how to improve it? Thanks! This is something that I want answered, and VTCs won't help me learn anything. $\endgroup$
    – Axiom Sage
    Sep 1, 2018 at 13:14

4 Answers 4


Your choices are

A: Clean cut cylinder

The portal causes material bonds to release wherever they are separated by the portal. However, the portal is one way, so it may treat matter passing through it as a series of single molecule wide sheets (As each layer of molecules passes through, it is no longer connected to the previous sheet). But because covalent bonds share electrons, diamonds and quartz and similar materials may be treated as a single object, allowing you to make "perfectly" round diamond rods. Metals, which are what most trains are made of, share a looser bond which may either be treated as a single material or as sheets, your choice as far as this portal goes.


B: It compresses the train into a viscous fluid where it touches the portal and ejects it as liquid hot jets of train through the other side.

So this option implies a lot of quantum super-positioning and stuff on a large scale, which is not yet really achievable with real life technology. Basically the principle is that any material that passes through becomes quantum entangled with itself.

  • Gases are not connected, so the portal treats them as individual molecules whenever one passes through.
  • Liquids are identified as a liquid, so the portal acts much like a gravity siphon, where the liquid will be drawn through the portal at a fixed rate depending on gravity changes between portal ends, pressure differences between portal ends, and surface tension of the liquid.
  • Metallic Solids are classified by a kind of shared electron situation, where as when an electron shared by the entirety of the solid is passed through the portal, it treats all molecules associated with the electron as part of the transferred material. The entirety of the material will end up on the other side of the portal, I'll describe it more below.
  • Covalent/Ionic Solids also pass through the portal in their entirety, but have a more eloquent way of doing it that doesn't involve turning into superheated goo.

Okay, so lets describe some interactions that you could have take place with your portal:

  • You poke it with a graphite rod: The rod will sink into the portal as far as you push it, but be unable to be pulled out. The quantum wave function of your portal lets it sit there indefinitely, treating the bonds in the material as existing independently on either side of the portal. You can break the rod on either end with no problem and keep the piece you broke off on that side of the portal. If someone on the far side pushes they will be unable to move the rod, but if they pull it can pass through the portal. It's impossible to break the rod at the exact point where it is passing through the portal unless you drag the rod through the edge of the portal, which will sheer the rod off at that exact point. This works for any material with covalent bonds.
  • I poke the portal with a graphite rod wider than the portal is: It treats the rod as a single object, and entangles the molecules that pass the portal. Then the molecules react to the quantum state of the molecules that are passing through the portal, and will reassemble themselves on the other side of the portal, in a kind of quantum teleportation that isn't actually teleportation, but the replacement of information encoded on one side of the portal with the information on the other side, which recreates the object exactly the same, but doesn't move the physical atoms. Point being you can transport slabs of marble easily, you just have to push it all the way through yourself.
  • You poke it with a metal rod: the metal rod will be slowly sucked through the portal at whatever speed you originally touched the portal with. You can cut off a part of your rod in order to save it, but part of the rod touching the portal is unretrievable. Of course, this assumes that your portal is using the same amount of energy to transport objects as there is energy in those objects themselves and therefor not violating the second law of thermodynamics. So what happens if I don't have enough energy to transport the whole rod? Then the portal collapses and the rod will not go through. Unless you add more quantum shenanigans which define conservation of energy as an average, and by adding energy to a system (through teleportation) it simply decreases energy in other systems by manipulating probability. (Basically you can't make a perpetual motion machine out of a teleporter.)
  • Finally, the fun one. You poke it with a metal rod wider than the portal: The portal treats the object as a whole, as opposed to a covalently bonded material that it treats as a series of interconnected molecules. So as the train passes through the portal, any material not directly inside the radius of the portal is converted into a wave function and collapsed back into the inside of the portal, because the portal views it as the same object as the metal already passing within the portal. This, unfortunately for the train, means that the molecules which originally took up a greater area than the portal, are now super compressed into the size of the portal. Now, compressing solids is very hard, so I couldn't get a hard estimate for the temperature of iron compressed to half its original size, but for the sake of simplicity lets just assume it won't convert the iron into some kind of exotic plasma. Instead, we give it a temperature of a couple thousand degrees. Actually, at this point the metal would vaporize into a gas from being too hot, but I'm guessing the high pressure would keep it in a liquid state. Again, it's really really hard to find information related to the compression of solids because it's really hard to do. Anyways, if you look at what happens to a pressurized liquid in a tube, (which the portal is basically an infinity narrow tube, in layman's terms) you can see that higher pressure equals higher speed. So what was originally a train driving through a portal, is now a liquid that has been under enormous pressure being shot out the other end at a high speed. Part of the liquid may return to gas as it passes through the portal and becomes free of the high pressure, which will cause a blast and a heatwave that will scour clean the ground on the other side of the portal before oxidizing into iron oxide and raining down as an extremely fine powder. The other part of the metal will reman liquid for a while before slowly hardening into a solid.
  • What if we add more material? Well if you added a large enough amount of material all at once, you may be able to create a black hole. I don't know the calculations to calculate how compressed matter would have to be to do this, but it will not end well for the planet the black hole comes out on. if the black hole is small enough it will immediately radiate too much matter for it to maintain itself and will explode, again, not good for the planet.

Either way it's a very bad day for the conductor, and anyone on the wrong end of the portal.

And that's a moderately accurate portal that speculatively builds on actual quantum mechanics. Do not take this as like, a scientific study; this level of thought only goes as far as writing and not much farther. If you showed this to a quantum physicist they might stab you in the eye. But for anyone who doesn't study quantum mechanics in a institute of higher learning, this is passable, probably.

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    $\begingroup$ I suppose a third option would be that it simply stops at the portal, as if one were trying to pound a train-shaped peg through a portal-shaped hole. $\endgroup$
    – Dubukay
    Sep 1, 2018 at 5:30
  • $\begingroup$ @Dubukay That's an option, but it's so outside my knowledge of physics and spacetime that it gave me headaches trying to describe it so I gave up. $\endgroup$ Sep 1, 2018 at 5:37
  • $\begingroup$ @ClayDeitas You could describe it as a kind of surface tension across the portal that resists large objects. Since it can't pass through, either the object will push the portal back, or it will stop as if it hit a solid wall. $\endgroup$
    – Cadence
    Sep 1, 2018 at 5:45
  • $\begingroup$ @Cadence But then you would have to write in extra laws of physics to justify what forces keep the tension, and how far they interact outside of the portal, and a lot of other stuff. If it reacts as if it hit a solid wall, you have to formulate the thickness of the portals edge, how much wrap around force effects it, again, how far the field extends. Clean cut or vacuum pressurised tube make more sense to me. $\endgroup$ Sep 1, 2018 at 5:52
  • $\begingroup$ Thanks for your answer, @ClayDeitas . If possible, would you mind elaborating on how option B might come about? It's an interesting concept, and I'd like to hear about how the train and portal could interact to yield this result. $\endgroup$
    – Axiom Sage
    Sep 1, 2018 at 16:48

That's one way you could do it, since portals are mostly products of the imagination. If you're trying for a small, traversable einstein-rosen bridge (the entrance will be three-dimensional, save you break general relativity), then the most likely scenario in my eyes is the train getting squashed into the portal via the curvature of spacetime and becoming "unsquashed" on the other end. But since your portal is 2d, virtually all known types of teleportation or instantaneous conversions don't apply (quantum entanglement may-it's a bit too weird for me to fully figure). If you want cutting at the edge, make the edge cut; you're working with magic here, like Doctor Strange.


Because the portal is one-way, you essentially have an entropy pump as proposed by James Maxwell in 1867. It allows for violation of the second law of thermodynamics. Maxwell’s Demon The fast moving particles will more commonly encounter the portal and vanish from Planet A. Planet A gets cooler. Planet B gets hotter. If this portal has been open long enough, the air around the portal may be super chilled, and thus the metal of your train may be chilled enough on approach to shatter when it hits the ring (not all metals shatter when super chilled, but iron does). That would allow the core (still warm) to pass on through the ring on pure inertia while the outer shell cracks away.

On planet B, the front of the train flies out of the tube. As long as it is above the ground, it comes out clean, but as soon as it dips enough to drag on the ground, the front of the train will slow down. You’ll get a classic pile up of a train wreck at that point.

If the outside of the train doesn’t have time to chill, you’ll probably get serious crumpling at the front. And you’ll get dust on Planet B. Why? Vibrations. Consider a vibrating object going through portal... it moves forward 1 millimeter then back 1 millimeter. When the front edge hits the portal, it goes to Planet B. But the back edge pulls away, slicing the object. Then it vibrates forward again. The object goes through in slices. That’s a danger always when going through the portal, but if the train is decelerating because it is colliding with the ring, it’ll get really serious and dust the train.

  • $\begingroup$ Rather than slicing when it vibrates, it'll put pressure on the cross section and pull the still connected molecules of the train through, causing internal stresses but unlikely to cut the train. $\endgroup$
    – Demigan
    Sep 2, 2018 at 20:55
  • $\begingroup$ @demigan At small vibrations, yes, but as vibrations increase, it can’t work like you describe. Since the pull-back force is not transferred at all to Planet B (as that would require force-carrying fields to go bidirectional), there has to be snapping on every pullback if the vibrations become severe. Sure, atoms and molecules would be pulled through intact, but the bonds between molecules are pretty fragile compared the speed of vibrations on train. Ramp those up, substances will come apart easily. $\endgroup$
    – SRM
    Sep 2, 2018 at 21:10
  • $\begingroup$ Thats actually a good point: if no forces can be send the other way then any molecule moving in a different direction than straight through would likely break it's bonds. Since the train is likely not absolute zero all it's molecules will vibrate anyway and a spray of molecules happens at the other end of the portal. Did I say molecules? They'll rip apart their electrons and possibly their neutrons and protons if they move even slightly while one part travels through. The resulting spray of plasma and possibly nuclear fusion happening on the other side isnt going to be pretty (up close) $\endgroup$
    – Demigan
    Sep 2, 2018 at 21:42
  • $\begingroup$ @demigan I was sort of hand waving a general “momentum transfer” that keeps the bonds together as long as the object moves coherently through. So solid matter would make it. I hadn’t really addressed biological entities... those get splashed even on regular trips unless moving through really fast. (Basically, I’m assuming the portal generally works and then trying to find special cases.) $\endgroup$
    – SRM
    Sep 3, 2018 at 0:21

The edges of the portal are in all likelyhood not a matter, but just an edge separating the space on planet A from planet B.

This means that the answer is actually very very simple: the train has one part of itself going in one "direction" and the other in another "direction". If you threw a bar at the edge like this:


Legend: ------ = the bar | = the portal edges

Then the bar will try to bend around the edges of the portal while the middle part tries to pull the outer edges of the bar into the portal, just like it being thrown at a doorpost only the doorpost is infinitely thin and cannot be collided against. kind of like this:


Where \ = the left side of the bar trying to bend around that edge. ^ = the middle part of the bar being bend around the doorposts as its momentum tries to pull it on but the outer edges keep it from going / = the right par of the bar trying to bend around that edge.

And that's what would happen to the train: it wouldnt be "cut", it would basically use it's own momentum to pull itseld apart around the edges of the portal. You could simulate this basically by having an invisible force grab a circular center of the length of the train and and simply stopping that center instantly, then see what the outer edges of the train would do (depending on it's speed it could rip itself off). Then you do the exact same but stop the outer edges of the train and see what the middle does. This is almost exactly what the portal would do simultaneously, only it would start at the front of the train and go down the back.

The only reason a portal would "cut" anything would be that as it shrinks, the molecular bonds suddenly have a galaxy of space between them and stop being bonded. Hope you dont cut some atoms or electrons or you might have some trouble!


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