You put a metal rod halfway through a portal and open another portal so that the end of the rod that went through the first portal touches the other end of the rod. If you weld the two ends of the rod together, how will you remove the rod from the portals? Assume that the portals cannot close if there is something in the way.

Since you can't put the rod through the first portal before opening the second one, both portals are already opened but spaced far apart and on movable surfaces so that once the rod is halfway through the first portal, the surface with the second portal is moved closer to bring the ends of the rod together.

A friend asked me this question and I've been thinking about this for a long time now. I apologize for not giving more details before.


closed as unclear what you're asking by f'', Deusovi, CodeNewbie, Regan, xnor Feb 12 '16 at 7:00

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  • $\begingroup$ It would depend on how portals work. Are these the portals from the game Portal, or some other type of portal? $\endgroup$ – f'' Feb 12 '16 at 2:48
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    $\begingroup$ How do you even put it halfway through the portal when the other 'end' isn't up yet? $\endgroup$ – DrunkWolf Feb 12 '16 at 6:49
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    $\begingroup$ maybe you can't... $\endgroup$ – Ben Feb 12 '16 at 8:01

Well, since portals (like the ones in the Portal video game and other science-fiction entertainment) don't exist, it is difficult to answer the question definitively. It all depends on whatever fake physics are involved in making the portal in the first place. Here is one simple answer:

Shut off the portal(s). In most science fiction, this will sever the objects that are midway through the portal in the plane of the portal.

  1. Duplicate the situation with a second rod and two more portals (C & D), ...

... completely apart form the original rod and the first two portals (A & B).

  1. Open yet another entry portal (E) ...

... that loops through portals A & B, parallel to the original rod along its entire length.

  1. Open exit portal F for portal E so that ...

... portal F loops through portals C & D, parallel to the second rod along its entire length.

  1. The original rod's local space, which portals A & B had turned into a 3-dimensional “surface” of a 4-dimensional cylinder, is now ...

... part of a 3-dimensional “surface” of a 4-dimensional torus, with both rods on the same surface. You can now slide the original rod completely through portals E & F to be next to the new rod, without touching portals A & B.

  1. After doing that, close portals E & F so that the original rod is completely independent of portals A & B, even though it is now ...

... still linked to itself, but through portals C & D just like the second rod

(The second rod wasn't actually necessary, just the portals.)


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