Conservation of energy is already broken by portals. Consider the portals in a frictionless tube (it could be frictioned, but that would be less fun) that use the earth’s gravity to accelerate the cube to the speed of light. If we are take the cube’s effect on the earth as non-negligible, then the earth has also accelerated a significant bit in the ‘upwards’ direction that the tube has. Simply run to the other side of the earth, put the ‘exit’ portal aiming towards the same direction as the ‘upwards’ of the tube, in this case directly at the earth, and now you have accelerated the earth twice as much in that ‘upwards’ direction with no apparent gain or loss of energy (and also have obliterated the earth, but let’s not talk about a cube’s mass hitting the earth at c.
As for the portal suddenly stopping while moving blazingly fast, sure, it’s a good quandary. It seems insane to think of the cube suddenly gaining momentum, but it’s also insane to think that a particle (in this case the portion of the cube’s atoms that are through the portal already) that is moving at speed somehow has no momentum.






Let’s make sure you feel the same about your eyes: https://e621.net/pools/20220