What is Vacuum Impedance? In electronics, I would assume that it means some ohmic resistence to alternating EM signals. In vacuum, I understand it is necessary property, elasticity, for vacuum to be able to transfer energy waves. Is it some universal constant? In wires, conductor needs to be ~3 times lower resistence than insulator to become main carrier of electric current. What puzzles me and I can't find, what would be implications of vacuum impedance change in some specific direction, say tube shaped, would this create prefered path for photon EM waves, creating sort of virtual conductor tunnel into sea of insulation? Is there any reason to suppose impedance changes can occur? And if yes, then does some matter in the path of photon have any influence on vacuum impedance on that path? Basically, can matter in the path between source and target affect imedance in such a way as to result preference of alternate path between source and target where matter is no present?