The Portal Paradox


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This video is about the Portal Paradox – a paradox in the video game Portal (and Portal 2) regarding whether or not a companion cube passing through a moving portal plops out of the other end with no speed (velocity, momentum), or shoots out at high speed. It’s a question of conservation of momentum, relativity of velocities, wormholes, 3D printers and quantum teleportation, glitches, and more.


Conservation of momentum

Principle of Locality

YouTube video of testing the Portal 2 game engine on the portal Paradox

Tutorial for how to program the Portal portals on your own:

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  1. Another paradox:
    Consider one portal is on the ceiling and it’s counterpart directly beneath it. Now put a solenoid that extends from the bottom portal to the top portal and attach this solenoid to some electrical circuit. Dropping a magnet would change flux (due to acceleration by gravity) but since the magnet falls through the second portal, it never stops accelerating due to gravity. Thus this results in an infinite ♾ induced EMF breaking the law of conservation.

  2. Personally, I think that if a stationary object were approached by a portal, the first layer of particles of the object would manifest in the other end of the portal, and then the next layer would come in. However, since that space is already occupied, the first layer would be pushed aside in a random direction. Except, since the particles are connected, they'd want to stay connected, thus forcing the layer to move away from the portal the object is exiting. Kind of like if you have a car, and want to move a second car to that exact location. The space is already occupied, so the thing that occupies the space is moved aside.

    The same can be applied to the bonus question: If the object enters the orange portal, its particles are moved to the blue portal. Then, the next layer of particles enters the orange portal, but in a slightly different position relative to the orange portal, thus resulting in them being arranged in a different position when exiting the blue portal. So basically, the object would come out slanted, and pop out at an angle.

  3. That B option wouldnt make sense. That would mean that the cubes energy is changing without any force affecting it and that goes against the energy conservation law

  4. What if a portal in a wall have it's other end in the wall.
    And the movie portal 1 will always have an exit moving portal 2.
    Ah idk what I'm saying

  5. What if the portal is really a box or cylinder on the inside instead of a slice. That would make more sense for a 4D plane

  6. I would say B because the cube is traveling some speed relative to the orange portal therefore the cube must fly out of the other blue portal

  7. If the cube is staying still and the portal hits it, the blue portal would basically just be a solid wall at that point and the cube would fall over (if at an angle) or not move at all (if flat); doesn't seem that hard if you really think about it

  8. I would like to offer up Option C. The theory of a "pocket dimension" between the portals in which an object with no velocity can enter and remain until the original entrance portal reverses away from the object. However, while in this "pocket dimension" the player may at any time retrieve the object and pull it trough the exit portal.

    The "pocket dimension" would be an area between the two portals in which all matter remains the same only stretched over a larger area as a result of passing through the enhanced gravity of the portal. Hence explaining how the area perceived between the portals is ~.000001 centimetres and still be able to contain the item until received by the player or until leaving the entrance portal.

    While the player passes through the portals, their matter is affected the same way but happens at such a high speed (the same applies with all objects) that the player would not be able to notice the change and would be able to leave the portal before an outside observer can notice.

  9. I personally think that it is A.
    If are standing next to the blue portal, it'll look like the cube is moving towards you. Since the platform isn't moving you'll see that the cube just sits on the platform until part of it is through the portal. The new orientation in relation to gravity will just pull the cube downward and it'll just sorta "fall out" of the blue portal as more of it is exits the portal.

  10. Here is a question, if you put a portal on a ceiling and on a parallel floor, then feed a rope through the portal and tie both ends together, where it appears to be a straight line from one portal to the other, is that rope infinitely long?

  11. I'd say it's option A, because the cube itself doesn't have any momentum that would keep it going in a certain direction. Once a certain part of it (more than a half?) has went through the portal, it would enter free fall.
    The interesting thing then would be to observe what happens to the part that didn't enter the portal yet, as the cube would basically be exposed to the force of gravity in two different directions. Does that part "fly up" or does the cube get torn in half?

  12. What paradox baffled me is why gravity changes instantly across the plane of the portal. Like wouldnt the gravity fields interact causing worldbending shit upon the formation of the portal if they were different

  13. A: If it Was slow.
    B: If it was fast
    C: Regular Portal. Nothing happens. You cant put portal on moving walls. Thanks

  14. Rewatching this forever later…
    It might shoot out of the blue portal straight up but mishapen, skewed in the direction of movement of the orange portal at an angle based on the speed of the movement. If it fell back into the blue portal with the same orientation and the orange portal didn't accelerate, it might even return to its original shape!
    I'm imagining light refractions or objects being skewed in dimensions as they approach C.

    This also causes its center of gravity to change and force the now rhombo-hedron to tumble. It might not land back in the blue portal ir in the same orientation, so it could be twice-skewed on two (slanted) axies of entry.

  15. But, if u had a portal on the floor, and the other end on the wall, gravity will pull you from the portal on the wall as well as from the portal on the floor, so… gravity will pull you diagonally if you were standing on the floor away from the floor partal and on the wall portal at the same time.(the floor portal is directed to the floor)

  16. It feels really weird. But I'm going to disagree with a minute physics video. The theory of relativity and Einstein say that you CAN treat objects velocities as relative to each other IF they are in a nonintertial reference frame. Earth is an inertial reference frame which is why we use both the special and general to calculate gps satellites. So this is undoubtedly not a situation where you can treat the velocities as relative. And the true velocity of an object physically distorts and warps the fabric of reality and space time. The cube has no velocity so it falls out due to the shifting of gravity and we have option A

  17. IMO it should work that the system sums up the space velocity of the cube with space velocity of portal and if the second portal doesn't move, the sum will be applied on the cube but if the second portal move, velocity of the second portal will be substracted of the sum and the rest of that total will then be applied on the cube (or any object).
    (Sorry if my english is choppy but i think this idea solves the problem very well)

  18. What i believe people aren't understanding is that there is force acting on the object. We are not in a vaccum, gravity is what keeps the box on the floor if the portals acted in accordance to theory B, then air would be displaced along with the box and create a semi-vaccum. But since they are more likely to act as a gateway, then that means when it envelops you more than half way, the opposite portal's force of gravity would overtake the moving portals, causing you to be pulled out the other side with the force of if you fell from that position, in this case, being plopped out.

  19. Surely as momentum of items passing through is conserved and the mass isn't changing there must be no change in velocity

  20. That last question.
    I would think that the cube would come out with its faces shaped more like a parallelagram, in the game you shoot out of whatever side youre closest to, so the right side of one portal shoots you out to the right side of the other

  21. Here’s my hypothesis for anyone (no-one) who cares to hear about the paradox in the thumbnail.

    Within Portal it is generally known that objects retain their momentum through portals. While the portal may be moving in this example, The companion Cube has no momentum. So when that portal comes crashing down on the cube, the cube would simply sit flat directly where it was as if the portal was just a hole in the ceiling that came down around it. If it appeared out a wall through the portal it would simply be affected by gravity.

  22. I think it's A, because imagine this: You're standing on the platform where the cube is, looking directly at the portal. As the portal moves on top of you, your body exits the other end of the portal, your body slowly rising out of the portal as if you're on an elevator. Then, you drop to the floor, since the portal is angled so that you exit diagonally from the portal. It's the speed of the object that affects how fast it goes out of the portal, not how fast the portal ITSELF is moving

  23. I believe option A is more logical. If you shove a hole onto an object and by going into that hole gravity changes, then it would just hit the new ground on the other side.

  24. So the way portals in the game are programmed is that there is a duplicate object made near the portal, so there is the cube you are holding which phases through the wall the portal is on, and the cube on the other portal is linked inversely to the first cube. So if the first cube is pulled away from the portal, the second cube moves away from its respective portal but relative to the global space, and is unloaded to preserve continuity.

    So, speedy thing goes in, speedy thing comes out. The portals get caught when you try to smash them together because the two cubes are colliding, not because of theoretical portal physics.

    There was a YouTuber who recreated the portals from portal and explains the programming, it’s very helpful. I don’t remember the video title, though.

  25. what is this shit? did you ever played the games at all?
    it is literally impossible for a portal system to sustain itself while on moving objects.
    how portals work is that they basically fold reality to where both ends meet, if you move one end then the other has to move the other way, making it no longer a portal.
    Portal just disallows portals to move, it could have occurred but would just unlink the portals until they moved back in place.

  26. whenever i see these types of questions i always end up thinking of how it would look like through one portal, i act as if the other side of the pirtal is a whole diferent place and the portal is just an entry way like if i stared at one of the portals and saw the cube moving, how would it move once it comes through, sorry im not good at explaining myself but i tried

  27. personally i feel like for the first paradox the answer would be A and for the one you gave us the answer would be C im not going to explain why here bc i am GARBAGE at explaining myself


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