Alright, this episode was a little complicated. If I understand correctly: Peter found out that he was from an other dimension because a man from an other dimension appeared on a bridge from an other dimension, in the dimension Peter currently is in. And this happened because three shape-shifters from another dimension set up three harmonic rods in the dimension our charters are in (lets just call it the Fringe dimension, from now on) that corresponded with three harmonic rods in the other dimension. When the radio waves from the other dimension correspond with the solar from the Fringe dimension, the area between the triangle created by the three points is switched. So, Peter saw the man from an other dimension on a bridge from an other dimension when this was happening and the body of an other man that Peter was with disassembled. Peter and the man from the other dimension lived through the dimension switcher-roo, so Peter came to the conclusion that he was from an other dimension. Alright some of that seems a little far-fetched for Fringe even, but how can I say that? I’m watching Fringe after-all.