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this post was submitted on 26 Jul 2026
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My take:
The cube's mass is x (eg one gram) and its velocity is zero, therefore its momentum (m*v) is zero. Objects in motion tend to stay in motion, while objects at rest (e.g. momentum of zero), tend to stay at rest.
The thing which is moving, is the portal. At best, the event horizon of it reaches the leading-end then the trailing-end of the cube at the speed the portal is moving over the depth of the cube. If the portal moves faster, that distance is covered faster, and the full cube is entirely on the other end of the other portal faster [than a slower moving portal].
Since going through the portal is a frictionless and forceless action, the speed at which the portal fully envelops the cube would neither add nor remove momentum upon it. And if the cube did have its own momentum it would be maintained, but since it does not, the cube would not move until another force, like gravity, acted upon it.
So, A.
I don't think this can work. Think through it in several steps rather than just one. Once the cube just started entering the portal, a part of it will already be on the other side, as close to the 2nd portal as possible. When the portal continues moving, more of the cube needs to be on the other side, so it "pushes" the first part of the cube further away. The cube imparts momentum onto itself.
Another way to look at it is using reference frames. It really doesn't make a difference whether the cube is moving towards the portal or the portal towards the cube. It only matters what the difference of velocity is. The difference of velocity must be maintained on the other side of the portal, and since the 2nd portal is stationary, the cube must move.
Hmm, maybe? But two things to (re) consider:
Saying "
the cube imparts momentum onto itself" sounds like it would violate the law of conservation of energy. The 'energy can't be created or destroyed' theory, so where did it actually come from? Did the cube suck force going at one vector (the portal) and convert it into the opposite vector? That feels like it violates inertia. And since momentum is mass*velocity, that means the mass of the portal would play into the math changing the cube's own momentum. And I'd honestly assume the portal's mass is zero, which still maintains it wouldn't impart any energy onto the cube, right?Secondly, regarding inertia from another standpoint, I have a thought experiment: let's say the portal is coming at the cube blazingly fast, then immediately stops halfway through enveloping the cube, so it's just chilling 50/50 on each side. By your statement of "
it pushes the first part of the cube..." That would mean one half would have momentum at the same time the other half, not yet through the horizon, would still have none. Maybe so much so that (1) the cube would be pulled the rest of the way through even through the portal had stopped because half the atoms of the cube suddenly had momentum for some reason, or (2) the cube would rip in half (brittle cube I guess, but let's ignore the material considerations for simpliciry) because half the cube now suddenly has momentum while the other half still has none.So I think I understand your perspective, but I can't yet get around what seems to be a few lingering problems. All the same, I'm enjoying the discussion, thanks net-neighbor!
Or, this, but mind blown --> The exit portal would be the thing that moves backwards to maintain the zero momentum of the cube 😅