only problem is if you make a portal loop like that you have to remember that it wont keep going forever air resistance and as gravity is passing through the portals you have to remember that the one on top has gravity pointing up, the one bellow has gravity that is pointing down. so eventually the object will freefall until it runs out of initial momentum and freeze in the center, or at the point where both the g's are equal to zero... if you place a coil around it, it will slow it down much faster...