It is possible to design networking systems that only use two wires: one for data and one for a common ground. Examples include 1-wire and Pin&amp;Play. These are called single wire systems because the requirement for an earth wire is implied too. But you can also get systems to extend home networks that use the home earth to connect network points, like Power Line Communication. How is this possible over just one wire?

== EDIT ==

From the answers (thanks!) I think I failed to word this question clearly. Let me try again.

Can you send data usefully over one wire, literally one wire? Radio is zero, 1-wire is two, but is it possible with one? "No, and here's why" or "Yes, here's how it is done in X" are the kind of answers I am hoping for.

(N.B. I'll also change the question title from "Single wire systems need two wires; so how does ethernet over ground work?" to "Can you send data usefully over one wire, literally one wire?")


Question: Photons and Relativity
Answer: You have completely mixed the modern and classical concepts of relativity. If you're talking about mass increment, you shouldn't calculate speed of Sun based on absolute time &amp; space notion.

For you as a photon, space will be contracted to zero and time will be dilated to infinity. So, you can't calculate a speed (which is a time-like spacetime event) of Sun.

While its a nice satisfactory explanation, its not the real one.
Real Answer:
Relativistic physics doesn't allow you to take position of a photon. In other words, relativistic physics doesn't allow photons to be an observer. Its because a photon can see itself stationary which breaks the framework of relativistic physics. Relativistic physics doesn't allow photons to be at rest in any reference frame.