Another Portal Paradox

Portal’s core rule—objects entering one portal exit the other with the same speed, with direction determined by portal orientation—creates room for paradoxes once the portals behave in ways physics would normally forbid. The most striking “halfway” scenario asks what happens if a portal stops partway through an object. With physically realistic portals, the cleanest way to reason it out is to replace the portal with a mechanical analogue: two cubes connected by a rope, where the “orange portal” ends between them. The upper cube shoots out of the “blue” portal at full speed; the rope then yanks the lower cube upward. If the rope is weak, it breaks. If it’s strong and the cubes have equal mass, momentum sharing forces both cubes to emerge from the blue portal at half speed. Translating that back to a single cube yields a general rule: if the portal stops at 1/3 of the cube’s length, the cube exits at 1/3 the speed; if it stops at 1/2, it exits at 1/2 speed. For an extremely fragile cube, the “stopping” would instead tear it apart, but for a uniform, intact object the momentum-transfer picture predicts partial-speed emergence rather than a true paradox.

A second paradox gets subtler: what if a cube is “sandwiched” between two portals at once? The analysis again swaps the portal for a more physical stand-in. Imagine a piston extending through the blue portal; because there’s no meaningful “inside” to a portal—everything on one side is effectively outside on the other—there’s no place for the piston to keep extending without interacting with itself. When the piston returns and hits its rear face, three outcomes emerge depending on rigidity and mass. A weak piston would crumple like it hits a solid wall. A rigid piston with extremely heavy portals would stop expanding because the portals can’t separate further. But if the portals aren’t heavy, the momentum bookkeeping implies something stranger: the piston’s push on the orange portal drives the orange portal left, while the blue portal absorbs the incoming piston momentum and gets pushed right. The net result is that the portals themselves get pushed apart.

The same logic applies to the sandwich-cube case. The blue portal moving toward the cube acts like a piston, driving the cube out of the orange portal until the cube collides with itself. If the cube is weak, it crumples. If it’s strong, the collision forces the portals to stop or even bounce—effectively the portals recoil from the self-impact.

The paradoxes aren’t all original. The “sandwich” challenge came from someone testing a sandbox version of Portal, while the “halfway through the cube” question was posed by a “portal physics & rendering” programmer from the team behind Portal 1 and 2. The solutions, however, are presented as the video’s own hypotheses: since moving portals weren’t implemented in the games, the physics remains open to interpretation—making paradox analysis the point.