Does the universe have characteristic impedance? If so, what is its value? Answers begin to emerge as we consider these facts:
1. Characteristic impedance is an attribute of any medium that can support the transmission of an electromagnetic wave regardless of whether or not it is connected to a power source at one end and a load at the other. Characteristic impedance modifies the current or signal flowing through a conductor. But if there is no moving energy at a given point in time, that does not alter the fact that the medium has a specific characteristic impedance. A reel of coax in the warehouse has the same characteristic impedance as when it is wired into an active, functioning network.
2. The universe is composed of vast amounts of empty space between relatively small stars widely spaced apart. It is thought that when our Milky Way and the Andromeda Galaxy meet four billion years from now, not a single star from either galaxy will actually collide. Scientists say if stars were the size of ping pong balls, they would be spaced on the order of two miles apart. (Though there will probably be no collisions, both galaxies will be profoundly affected by the interactions of their immense gravitational fields.) As an electrical media with characteristic impedance, the universe was probably manufactured with better quality control than our best coaxial cable.
3. Characteristic impedance, unlike conventional impedance, is independent of length or distance over which signals propagate. Accordingly, the characteristic impedance of the universe is the same as the characteristic impedance of a circuit traversing (let us say) 50 feet of free space.
4. The characteristic impedance of a vacuum is substantially the same as that of dry air, which has virtually no effect on conductance.
5. The characteristic impedance of free space is equal to the square root of the ratio of permeability of free space (in henrys per meter) to the permeability of free space (in farads per meter). It works out to about 377 Ω, and that is the characteristic impedance of the universe.
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