Larry Niven covered a lot about this topic on his book, "All the Myriad Ways". He commented on how you can gain and lose energy by teleporting up and down and also about the orientation and speed that you have on each part of the planet. He got a lot further with the infinite loop, though. He wrote of an hipotetical situation, where an iron sphere is put on an infinite portal loop, confined in a vacuum. This way, the sphere could break the air resistance speed limit, and go towards the light speed given enough time. Because of Relativity Theory, it would gain mass by doing so, and reach a big fraction of Earth's mass in a matter of months.<br><br>I don't want to spoil all of it. Have fun reading the book if you want. It has other short stories and scientific arguments on it.