Portal Physics

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Conservation of momentum rules, unless the portal draws some of it's energy from the kinetic energy of an object moving through it.

`without knowing excactly what happens at a physical level it holds little meaning,

that being said since the momentum of the cube is 0 and only its location changes when the portal is forced over it i would say it would be A,
it would be affected by gravity and be pulled to the ground with the bottom plate stopping it from entering back through the portal.

A seems right.

But it is possible that "portal physics" does not actually make consistent sense. This might be a counterexample that shows that there is no mathematically consistent portal physics that matches our intuitions.

The people who know the most about portal physics would be Valve, I guess. I wonder, did they purposefully not include certain types of portal interactions because they figured out that it would break the illusion?

I'm going to say B, but I can't really justify it in a 'hard' way. I'm just imagining being a player and having a moving portal drop over me. It makes the most intuitive sense (to me) from a first-person POV that you'd aquire the relative velocity when you popped out the other side.

I suspect that that is the case. We already know that Portal violates conservation laws by allowing infinite freefall by putting a portal on a floor and on a ceiling, and other examples. Given that, it becomes a free-for-all -- it's like saying, ok, 1+1 = 5, so what is 1+2 ?

Really interesting to think about, though.

i agree we have no clue and all this is wild speculation,

as i see it the only way to preserve the laws of conservation in that case would be if the portal exterted an equal reaction on the opposite face the players enter and exit from, though that in itself is pure guesswork.

**edit** shouldnt this almost be in the gaming forum, also has anyone tried it in portal, also can one move the object a portal is attached to with the portal following that, or would it destabilize?

In the context of a game engine, I posit A would occur.

In the usual interpretation of logic, if 1+1=5 then every possible statement about numbers is both true and false.

Maybe that resolves our portal problem. The correct answer is A, B, not A, and not B!

I can't really substantiate my choice, but I'd intuitively say plop. I mean, A.

B, unless the kinetic energy was absorbed somehow. I understand that we don't know what actually happens in there, but I'll give my reasoning: People have been saying that the conservation of momentum rules don't apply, as the cube isn't moving, but thats not strictly true. Unless dealing with things moving at relativistic speeds (and I assume this isn't) there is no frame of reference from which to say that one thing is moving and the other is stationary. Thus, the cube is moving towards the portal, relative only to the portal, so if conservation of momentum holds true, it should fly out of the second portal at the speed at which the first portal moved towards it, not taking into account air resistance of course.

Thats my wild speculation. Of course, if the portal does absorb the kinetic energy (perhaps to power the portal itself), a plop, or a decrease in momentum could be expected. But I expect a small amount of KE wouldn't be nearly enough to power the portal, so B seems more likely to me.

^ What if the portal comes down onto the cube at high speed but then stops abruptly when the cube is halfway in?

From what I know about the physics in the game Portal, the standard rules of conservation of momentum are conserved for objects moving through the portal. Therefore, my guess would be that A will occur because it's the portal that's moving, not the box that get's sent through the portal.

Movement is something relative, so I would say B.


Then the cube stop halfway through the portal.

Speedy thing goes in, speedy thing comes out.

but the thing that goes in doesnt move

But relatively to the portal, it move.