In the video game Portal, there are often puzzles which must be solved by gaining a large amount of momentum. Typically, this is accomplished by putting one portal on the ground and another directly above it on the ceiling, then jump into the portal in the ground, and the person would appear at the ceiling and started to fall off. When the person fall off, he/she gain kinetic energy and reach a certain speed and jump into the portal in the ground again. After that, he/she appear at the ceiling again. Thus you create an infinite loop of falling.
Although the idea of a portal is absurd to common sense, let's discuss its possibility to existence.
The portal would seem to be creating potential energy. Typically when an item falls, its potential energy is converted to kinetic energy by virtue of it losing height, and additional energy would need to be expended to raise it up again before it could fall again.
What is the possible source of the potential energy, if portal would exists? Does a portal provide potential energy in order to push people to higher position?
And this has lead me to wonder another point, though, whether the portals should also transmit gravity. This might remove the whole problem of conservation of energy; if the gravity is transmitted through the portal, then the object would be pulled back up with equal force, and would actually reach an equilibrium floating within the portal, rather than falling continuously.