I was thinking about this last night as I laid in bed.
The cube has no movement vector, and the portal is not exerting any sort of pressure on the cube, it has no reason to move; There is simply no way for the momentum of the portal to translate to the cube.
It's like swinging a hula hoop underwater, you might disturb the water but you are not ejecting the water through the other side of the hula hoop. A portal is like a hula hoop with a wall exerting through it's outer-border towards infinity. Portals are also
sort-of non-euclidean* but this particularity (and the in-game footage**) do not suggest any further abnormalities such as unconventional physics.
*In normal Euclidean space, two points can't occupy the same space or be instantly connected without the space between them, but in a portal they can. A better description would be 'a portal is a spatial anomaly whose effects are non-euclidean', but this is a mouthful.
**From what we observe in the games, the movement vector and acceleration of a person going through a portal remains the same no matter how many times she goes through. All the proof we have when it comes to portal physics is that they pretty much obey conventional physics, they are just a doorway.
I find this a fascinating problem, you would initially assume A, since if you have an object pass through something like a moving door frame, obviously the frames momentum will not transfer to the object, however if instead of just the cube passing though you also think about what would happen if the platform underneath it also moved through the portal with it you would think B. As it would be kinda silly to have an object on a spire jiggle around without being launched or moved. This, however would mean you could basically multiply the force you are capable of exerting using portals, you would essentially be able to ignore mass. So for instance, if you have an incredibly heavy block that you could fit a portal around and placed another object over the portal exit, you would be able to exert a ton of force on the exit object by just moving the portal over the heavy block as fast as you could. So this doesn't make any sense either.
Thus I propose an option C, in order for an object to move through a moving portal, the entrance and exit portal must move in tandem with each other, thus properly balancing out all forces involved.
By extension this means the video game Portal's unmodified in engine way of handling this, just not working, would be the most correct.
I think this is probably the most likely answer though, portals just not working is the simplest explanation and it is satisfying.
But I must dispute some of the claims:
if instead of just the cube passing though you also think about what would happen if the platform underneath it also moved through the portal with it you would think B.
I think that would be right, the cube would have a movement vector and an acceleration that will remain constant when the cube goes through the portal. The speed of the portal would not be added to the current momentum of the cube, as the cube would be launched once the two platforms 'clap' onto each-other. Kind of like a car crash.
On the other hand if the platform holding the cube was small enough to go through the portal, the acceleration of the moving portal would be added to the movement of the platform: The platform would be propelled through the other side of the portal with a speed of Portal + Acceleration of platform.
So for instance, if you have an incredibly heavy block that you could fit a portal around and placed another object over the portal exit, you would be able to exert a ton of force on the exit object by just moving the portal over the heavy block as fast as you could.
I think I don't understand this, so I am going to assume the big object is covering the portal door and the heavy object is about to fall onto an accelerating portal.
If this were the case, the heavy object that fits through the portal would hit the heavy block with a force of it's mass + acceleration, but the speed of the portal would not translate into the mix, the heavy block is on the other side of the portal.
The only way for the big block to feel the portal's acceleration + the cube's acceleration would be if the big block had a 'peg' or something going through the portal. If the heavy block's peg was 'halfway through' in this manner, this peg would accelerate at the speed of the portal moving, thus hitting the block at portal speed + heavy block speed.
Of course, this is all nonsense and we are talking about something that does not exist, but it is fun to think about.
