It's the scene with the lasers innit? So many "paradoxes" are solved with the answer of "portals cannot move relative to one another," and then they do the bit with the laser. -_-

The thing is, movement is relative. Everything on earth is constantly in motion if you're observing from any other celestial body, so motion itself can't be what breaks portals. What it might be, though, is acceleration. Those panels in the video seem to be moving at a constant speed, so aren't experiencing any substantial acceleration, making a portal on them possible

Just came here to check whether someone already posted the minutephysics video.

I think their explanation is rather good for passing portals through themselves. The box doesn't add much to the equation: it is just physical objects smashed into eachother...

You can't place a portal on anything the game physics would allow to go through another portal.

That doesn't work because gravity makes the game's reference frame non-inertial. One of the big takeaways of general relativity is that idea a reference frame that's inertial except for gravity is meaningless. Even ignoring relativity, everything is subjected to centripetal acceleration due to the Earth's rotation (and the story canonically takes place on Earth).

I think the real answer is probably the least satisfying: the game's physics just don't correspond to real physics. Most portals appear to exist in a privileged reference frame that can be said to be motionless, but even that isn't the real rule; the real rule is that portals can exist where the level designers want to allow them to exist. They try to make it feel like there's a certain logic behind it, but they'll bend the rules as necessary to make a cool puzzle work, and they keep the everything consistent within a single puzzle, but some subtleties of how portals appear to work are subject to change between puzzles.

Duh, that makes sense

Oh, okay. It has a scene where you have a portal go into a portal.