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Ah, if that’s how you’ve been interpreting the explanation, I can see why we’ve been at an impasse. Portals as shown absolutely do seem to violate conservation of momentum in the more global sense.

You don’t even need a complicated set up to do it. Put a portal in empty space, rotate the plane of the exit 90 degrees, then send a box through at a simple constant speed. Assuming the universe isn’t compensating in other ways*, the total momentum of the universe will be different before and after the box went through, so there’s no environment encompassing the portal interactions that will satisfy the zero condition. Only each object’s local experience will keep the condition.

*I can think of a few options for this bit:

The entire universe somehow accelerates the opposite way in response, a fun idea, but it has no meaningful consequences.
Portals behave more like wormholes as scientifically defined. They have mass themselves and, if free-floating, move in reaction to mass sent through them in a way that preserves total momentum. This is enough to run with though. The games’ portals always have to be anchored to other mass. This leaves room for the idea that the portals actually do want to move in response to objects passing though them. They just transmit that force into the mass they’re anchored to, and that force propagating out is where the momentum conservation actually resolves.

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Bytemeister@lemmy.world ⁨3⁩ ⁨days⁩ ago

Put a portal in empty space, rotate the plane of the exit 90 degrees, then send a box through at a simple constant speed. Assuming the universe isn’t compensating in other ways*, the total momentum of the universe will be different before and after the box went through, so there’s no environment encompassing the portal interactions that will satisfy the zero condition.

Pretty sure the momentum from the perspective of the box stays the same. The game doesn’t explicitly say that momentum for the entire system stays the same, just that "Momentum, a function of mass and velocity, is conserved between portals.” so rotating the exit of the portal in a different direction changes the momentum of the entire system (room, portals, box, etc…), but the box never actually experiences a change in momentum. Maybe a better way to put it, is that the absolute momentum of the box never changes when it passes through a portal.

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- ericwdhs@discuss.online ⁨3⁩ ⁨days⁩ ago
  
  Pretty sure the momentum from the perspective of the box stays the same.
  
  How exactly are you defining monentum here? “The box sees the entire universe move at velocity V backwards both before and after passing through the portal.” This is probably at least approximately true, but total momentum of the universe minus the box would have to be exactly zero to make this technically correct. “The box never perceived a change in its own momentum.” This is 100% true.
  
  The game doesn’t explicitly say that momentum for the entire system stays the same
  
  Then why are we still talking about this? Lol. I thought that was one of the main points of your position.
  
  the box never actually experiences a change in momentum
  
  Correct, the box never experiences a change in momentum. The box exists in one inertial reference frame and, in going through the portal, is deposited into a different reference frame. Two inertial reference frames that differ in orientation are equally as distinct as two inertial reference frames that differ in velocity. The same logic applies to both.
  
  Maybe a better way to put it, is that the absolute momentum of the box never changes when it passes through a portal.
  
  You’re describing what I labeled as “inherent,” so I’ll take it.
  
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