An Introduction to Information Theory: Symbols, Signals and Noise, by John R. Pierce, says the following:
While linearity is a truly astonishing property of nature, it is by no means a rare one. All circuits made up of the resistors, capacitors, and inductors discussed in Chapter I in connection with network theory are linear, and so are telegraph lines and cables. Indeed, usually electrical circuits are linear, except when they include vacuum tubes, or transistors, or diodes, and sometimes even such circuits are substantially linear.
Because telegraph wires are linear, which is just to say because telegraph wires are such that electrical signals on them behave independently without interacting with one another, two telegraph signals can travel in opposite directions on the same wire at the same time without interfering with one another. However, while linearity is a fairly common phenomenon in electrical circuits, it is by no means a universal natural phenomenon. Two trains can’t travel in opposite directions on the same track without interference. Presumably they could, though, if all the physical phenomena comprised in trains were linear. The reader might speculate on the unhappy lot of a truly linear race of beings.
Thinking about this from a physical perspective, I was wondering how it is that telegraph wires are linear, in the sense that two telegraph signals (in other words, two electric currents) can travel in opposite directions on the same wire, at the same time, without interfering with each other?
I was naively thinking about the wire as a single-lane, two-way road. In this analogy, the cars would be able to travel in either direction, but not at the same time. As I understand it, in solids, movement of electrons produces an electric current, so the electrons would be the cars. Given the author's explanation of linearity, what is going on here with the electrons that allows this concurrent, two-way flow of current?
I didn't find anything on the Wikipedia page for linear circuits that clarifies this physical property of linearity.
I would greatly appreciate it if people could please take the time to clarify this.
P.S. I do not have a background in electrical engineering, so a basically-worded explanation is appreciated.
